christchurch-west melton groundwater qualitydocs.niwa.co.nz/library/public/ectru02-47a.pdf · a...

Christchurch-West Melton Groundwater Quality: A review of groundwater quality monitoring data from January 1986 to March 2002 Report No. U02/47

Prepared by

S A Hayward July 2002

Report No. U02/47

58 Kilmore Street P O Box 345 CHRISTCHURCH Phone: (03) 365 3828 Fax: (03) 365 3194

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Fax: (03) 688 9067

Christchurch-West Melton Groundwater Quality: A review of groundwater quality monitoring data from January 1986 to March 2002

Environment Canterbury Technical Report 1

Executive Summary

Groundwater quality data for the Christchurch-West Melton area were analysed to determine spatial, seasonal and long-term patterns, and were compared to New Zealand drinking-water standards. Inorganic and microbiological data were available for 3000 samples collected from 438 wells for the period January 1986 to March 2002. The quality of groundwater in the Christchurch-West Melton area generally reflects the quality of the recharge source, aquifer geology and land uses. The Waimakariri River is the dominant source of recharge of the unconfined groundwater immediately south of the river near Halket, and is the dominant recharge source for the confined groundwater system. The Waimakariri River is an alpine-fed river and as such has a low ionic content resulting from little mineral weathering, low nutrient inputs and minimal influence from coastal sea spray. Consequently, groundwater recharged by the river had low concentrations of ions, as indicated by conductivity values less than 15 mS/m, chloride values less than 10 mg/L and nitrate-nitrogen concentrations less than 1 mg/L. Rainfall-derived infiltration is the dominant recharge source in the southern part of the unconfined zone, and extends eastwards into the confined zone. Rainfall recharge also appears to contribute to the deep groundwater in the northeast part of the confined aquifers via deep underflow beneath the Waimakariri River. Rainfall-derived groundwater generally contained higher concentrations of the major ions, with conductivity values typically within the range of 15-30 mS/m and concentrations of chloride ranged from 10-20 mg/L. Nitrate-nitrogen concentrations were highly variable, reflecting local land use patterns. The evolution of the groundwater chemistry from the recharge zone to coastal confined groundwater showed only subtle changes in groundwater quality. Despite estimated ages of groundwater in the deep confined aquifers to be hundreds to thousands of years, little modification of the groundwater chemistry had occurred. This reflects the relatively inert nature of the greywacke gravels that comprise the aquifer material. A distinction in groundwater quality between the confined and unconfined zones was observed. Groundwater in the unconfined zone was considerably more vulnerable to contamination from land use activities than in the confined zone. Microbial contamination was found to occur in groundwater from a number of shallow (less than 50 m deep) wells throughout the unconfined zone. This occurred in both river-recharged and rainfall-recharged areas. Of the 14 wells yielding groundwater with nitrate-nitrogen concentrations above the maximum acceptable value (MAV), 13 were located in the unconfined zone or at the confining boundary. The quality of the groundwater in the confined zone was generally very high. Only 2 samples from 47 wells collected from the confined zone of Aquifer 1 contained faecal coliforms. In both cases the detections occurred only once. No faecal coliforms have been detected in groundwater from the deeper confined aquifers. Elevated nitrate-nitrogen concentrations occurred in some areas of the southern confined zone of Aquifer 1. Groundwater from confined zones of Aquifer 2 and deeper generally contained very low concentrations of nitrate-nitrogen. Trace concentrations of arsenic were detected in groundwater from a number of wells, mostly in the confined zone. Generally, the concentrations were well below the MAV. In most cases, the arsenic was naturally derived and was associated with reducing conditions within the aquifer mobilising arsenic from sediment deposits. Groundwater from three wells contained arsenic at concentrations above the MAV. For two of these wells, the arsenic originated from former discharges from a timber treatment operation; the concentrations of arsenic in more recent samples from these wells were below the MAV. The arsenic in groundwater from the third well is likely to have originated from natural sources.


2 Environment Canterbury Technical Report

The main transgressions of aesthetic-based guideline values were for low pH values in the shallow unconfined groundwater zones. Samples with high pH values only occurred occasionally. Iron, and to a lesser extent manganese, occurred at concentrations that may affect the aesthetic quality of the groundwater in a number of wells located throughout the area. Saltwater contamination of groundwater in the Woolston-Heathcote area has resulted in localised areas of groundwater containing chloride and hardness values exceeding aesthetic guidelines. Statistically significant seasonal patterns were detected for conductivity and concentrations of chloride and nitrate-nitrogen from three of nine wells sampled monthly or quarterly. Seasonal variations in groundwater quality were generally related to seasonal variations in recharge as indicated by changes in groundwater levels. In general, peak concentrations of anions including nitrate-nitrogen coincided with high water levels occurring during late winter and spring. For some wells, a lag of about two to three months occurred between high water levels and peak ion concentrations. Groundwater from a well located at the confining boundary margin showed a negative correlation between water levels and concentrations of chloride and nitrate-nitrogen. This was attributed to a dilution effect from increased upward flow of deeper groundwater during winter and spring. Statistically significant long-term trends were predominately of decreasing concentrations. This commonly occurred in areas where contamination of groundwater had occurred from previous land use activities, in particular from industrial discharges, which have now ceased or were no longer having an impact on groundwater quality. Some trends of increasing concentrations were detected. This was most notable in the Woolston-Heathcote area, where groundwater quality had deteriorated as a result of saltwater intrusion. Outside of the area affected by saltwater intrusion, groundwater from three wells in the Woolston area showed trends of increasing concentrations of a number of ions. This may indicate the lateral movement of contaminated groundwater eastward from the unconfined zone to the confined zone of Aquifer 1. Trends of increasing nitrate-nitrogen concentrations were found in data from five wells. There were no obvious sources of the increasing nitrates in the immediate area of the wells, and the increases may reflect changes in land use in the wider area. However, trend analysis of nitrate-nitrogen concentrations showed predominately decreasing concentrations or no change. The influence of land use activities on groundwater quality was apparent in some areas. In particular, the legacy of old landfills and some industrial activities have resulted in poor quality of parts of the groundwater system. The areas affected tended to occur where groundwater was more vulnerable such as the unconfined groundwater in the southern part of the study area where recharge is predominately rainfall-derived infiltration. The influence of groundwater quality on spring-fed streams in the Christchurch-West Melton area is primarily through enrichment of inorganic nutrients, especially nitrate-nitrogen. Phosphorus concentrations in groundwater were generally low, especially relative to nitrogen concentrations, indicating phosphorus is the limiting nutrient for plant and algal growth in the streams.



Table of Contents

Executive Summary ................................................................................................... 1

1 Introduction ...................................................................................................... 7 1.1 Christchurch-West Melton groundwater system............................................................7 1.2 Christchurch-West Melton groundwater quality monitoring programmes ......................8

2 Methods .......................................................................................................... 14 2.1 Sample collection .......................................................................................................14 2.2 Sample analyses ........................................................................................................14 2.3 Data analysis ..............................................................................................................14

3 Results and discussion................................................................................. 16 3.1 Hydrochemistry of the aquifers ...................................................................................16

3.1.1 Springston Aquifer .........................................................................................18 3.1.2 Aquifer 1 ........................................................................................................18 3.1.3 Aquifer 2 ........................................................................................................20 3.1.4 Deep aquifers.................................................................................................20 3.1.5 Chemical evolution of groundwater ................................................................44

3.2 Comparison to drinking-water quality standards .........................................................46 3.2.1 Determinands of health significance ..............................................................46 3.2.2 Aesthetic determinands..................................................................................65

3.3 Seasonal variation in water quality .............................................................................76 3.4 Temporal trends .........................................................................................................85 3.5 Risks to groundwater quality.......................................................................................95

3.5.1 Over-pumping - saltwater intrusion ................................................................95 3.5.2 Land use contamination .................................................................................98

3.6 Effects of groundwater quality on spring-fed streams ...............................................103

4 Conclusions ................................................................................................. 104

5 Future monitoring and recommendations ................................................ 105

6 Acknowledgements ..................................................................................... 105

7 Glossary........................................................................................................ 106

8 References.................................................................................................... 108

Appendix 1 .............................................................................................................. 111

Appendix 2 .............................................................................................................. 113

Appendix 3 .............................................................................................................. 115

Appendix 4 ............................................................................................................. 119

Appendix 5 .............................................................................................................. 131

Appendix 6 .............................................................................................................. 133

Appendix 7 .............................................................................................................. 137



List of Figures Figure 1.1 Location of wells included in groundwater quality monitoring programmes for the

Christchurch – West Melton area (2000/2001 year) ......................................................11 Figure 1.2 Stratigraphy of the Christchurch – West Melton groundwater system (Brown and

Weeber, 1992) ..............................................................................................................12 Figure 1.3 Recharge sources to the upper aquifers of the Christchurch-West Melton

groundwater system......................................................................................................13 Figure 3.1 Distribution of median values of various determinands for each of the Christchurch-

West Melton aquifers (Note: log scale on the y-axis for all graphs except for the pH graph) ...........................................................................................................................17

Figure 3.2 Pie charts of median anion concentrations for groundwater from the Springston aquifer ...........................................................................................................................22

Figure 3.3 Pie charts of median cation concentrations for groundwater from the Springston aquifer ...........................................................................................................................23

Figure 3.4 Pie charts of median anion concentrations for groundwater from Aquifer 1...................24 Figure 3.5 Pie charts of median cation concentrations for groundwater from Aquifer 1..................25 Figure 3.6 Pie charts of median anion concentrations for groundwater from Aquifer 2...................26 Figure 3.7 Pie charts of median cation concentrations for groundwater from Aquifer 2..................27 Figure 3.8 Pie charts of median anion concentrations for groundwater from Aquifers 3, 4 and 5 ...28 Figure 3.9 Pie charts of median cation concentrations for groundwater from Aquifers 3, 4 and 5 ..29 Figure 3.10 Median conductivity values for all samples collected from Springston aquifer and

Aquifer 1 wells...............................................................................................................30 Figure 3.11 Median conductivity values for all samples collected from Aquifer 2 wells ....................31 Figure 3.12 Median chloride concentrations for all samples collected from Springston aquifer ........32 Figure 3.13 Median chloride concentrations for all samples collected from Aquifer 1 wells..............33 Figure 3.14 Median chloride concentrations for all samples collected from Aquifer 2 wells..............34 Figure 3.15 Median chloride concentrations for all samples collected from Aquifer 3, 4 and 5

wells ..............................................................................................................................35 Figure 3.16 Median sulphate concentrations for all samples collected from Springston aquifer

and Aquifer 1 wells........................................................................................................36 Figure 3.17 Median sulphate concentrations for all samples collected from Aquifer 2 wells.............37 Figure 3.18 Median silica concentrations for all samples collected from Springston aquifer and

Aquifer 1 wells...............................................................................................................38 Figure 3.19 Median silica concentrations for all samples collected from Aquifers 2, 3, 4 and 5

wells ..............................................................................................................................39 Figure 3.20 Median nitrate-nitrogen concentrations for all samples collected from Springston

aquifer and Aquifer 1 wells ............................................................................................40 Figure 3.21 Median nitrate-nitrogen concentrations for all samples collected from Aquifer 2 wells ..41 Figure 3.22 Median nitrate-nitrogen concentrations for all samples collected from Aquifer 3, 4

and 5 wells ....................................................................................................................42 Figure 3.23 δ18O values versus chloride concentrations...................................................................43 Figure 3.24 Chebotarev geochemical sequence of the relative abundance of anions for

groundwater ..................................................................................................................44 Figure 3.25 Faecal coliform detections in groundwater from Springston aquifer wells .....................53 Figure 3.26 Faecal coliform detections in groundwater from Aquifer 1 wells ....................................54 Figure 3.27 Faecal coliform detections in groundwater from Aquifer 2 wells ....................................55



Figure 3.28 Relationship between faecal coliform detections and well depths .................................56 Figure 3.29 Concentrations of faecal coliforms, total coliforms and E. coli in groundwater

sampled monthly from wells M36/0279, M36/0271 and M35/1003................................57 Figure 3.30 Concentrations of faecal coliforms, total coliforms and E. coli in groundwater

sampled monthly from wells M36/4227, M36/04655 and M36/1059..............................58 Figure 3.31 Concentrations of faecal coliforms, total coliforms and E. coli in groundwater

sampled monthly from wells M36/4151, M36/5248 and M35/1051................................59 Figure 3.32 Nitrate-nitrogen concentrations exceeding the MAV in groundwater samples from

Aquifer 1........................................................................................................................60 Figure 3.33 Nitrate-nitrogen concentrations exceeding the MAV in groundwater samples from the

Springston aquifer and Aquifer 2 ...................................................................................61 Figure 3.34 Maximum concentrations of arsenic detected in groundwater in the Christchurch –

West Melton area ..........................................................................................................62 Figure 3.35 Maximum concentrations of arsenic detected in groundwater in the Johns Road area .63 Figure 3.36 Median iron concentrations for all samples collected from Springston aquifer and

Aquifer 1 wells...............................................................................................................68 Figure 3.37 Median manganese concentrations for all samples collected from Springston aquifer

and Aquifer 1 wells........................................................................................................69 Figure 3.38 Median iron concentrations for all samples collected from Aquifer 2 wells ....................70 Figure 3.39 Median manganese concentrations for all samples collected from Aquifer 2 wells........71 Figure 3.40 Median iron concentrations for all samples collected from Aquifer 3, 4 and 5 wells ......72 Figure 3.41 Median manganese concentrations for all samples collected from Aquifer 3, 4 and 5

wells ..............................................................................................................................73 Figure 3.42 Median hardness values for all samples collected from Springston aquifer and

Aquifer 1 wells...............................................................................................................74 Figure 3.43 Median hardness values for all samples collected from Aquifer 2 wells ........................75 Figure 3.44 Seasonal variations in water levels, rainfall and calculated recharge values.................80 Figure 3.45 Location of wells sampled monthly and quarterly, and water level monitoring wells......81 Figure 3.46 Monthly groundwater quality data for wells M35/1051 and M35/1003 ...........................82 Figure 3.47 Monthly groundwater quality data for wells M35/1059 and M36/0271 ...........................83 Figure 3.48 Monthly groundwater quality data for wells M35/1382 and M35/6791 ...........................84 Figure 3.49 Significant trends in nitrate-nitrogen and chloride concentrations for the Christchurch

– West Melton area (based on at least 8 years of data). .............................................88 Figure 3.50 Significant trends in sulphate and bicarbonate concentrations for the Christchurch –

West Melton area (based on at least 8 years of data). .................................................89 Figure 3.51 Significant trends in calcium and magnesium concentrations for the Christchurch –

West Melton area (based on at least 8 years of data). .................................................90 Figure 3.52 Significant trends in sodium and potassium concentrations for the Christchurch –

West Melton area (based on at least 8 years of data). .................................................91 Figure 3.53 Significant trends in conductivity values for the Christchurch – West Melton area

(based on at least 8 years of data). ...............................................................................92 Figure 3.54 Temporal concentrations of nitrate-nitrogen in groundwater from three wells in the

Hillmorton area..............................................................................................................93 Figure 3.55 Annual conductivity and anion concentrations for three wells located in the Woolston

area (* indicates a significant temporal trend detected for that determinand) ................94 Figure 3.56 Chloride concentrations in groundwater from Aquifer 1 wells in the Woolston-

Heathcote area (the blue lines represent the position of subsurface volcanic ridge (Charteris and Ettema, 1999) ........................................................................................99

Figure 3.57 Chloride concentrations in groundwater from Aquifer 2 wells in the Woolston-Heathcote area............................................................................................................100



Figure 3.58 Water levels and chloride concentrations in groundwater from M36/1159, located in the Woolston-Heathcote area......................................................................................101

Figure 3.59 Median chloride concentrations in groundwater from the Springston aquifer and Aquifer 1 in relation to location of landfills, excavation pits and industrial zoned areas...........................................................................................................................102

List of Tables Table 1.1 Current ambient groundwater quality monitoring programmes for the Christchurch-

West Melton area (2000/2001)..................................................................................... 10 Table 3.1 Maximum acceptable values for micro-organisms of health significance (MoH, 2000). 47 Table 3.2 Maximum acceptable values for inorganic determinands of health significance (MoH,

2000)............................................................................................................................ 47 Table 3.3 Guideline values for aesthetic determinands (MoH, 2000) ........................................... 47 Table 3.4 Summary of transgressions of the drinking-water standards maximum acceptable

values for the Christchurch-West Melton groundwater system.................................... 48 Table 3.5 Summary of detections of trace constituents in the Christchurch-West Melton

groundwater ................................................................................................................. 64 Table 3.6 Transgressions of the Drinking-Water Standards 2000 aesthetic-based guideline

value ............................................................................................................................ 66 Table 3.7 Summary of Kruskall-Wallis test for seasonality for determinands collected quarterly

(alpha = 0.05)............................................................................................................... 78 Table 3.8 Spearman rank correlation analysis of monthly and quarterly data .............................. 79 Table 3.9 Summary of temporal trends in groundwater quality data .............................................86 Table 3.10 Chemical composition of seawater, and contaminated and uncontaminated Aquifer 1

groundwater ..................................................................................................................96



1 Introduction Christchurch is reputed to have one of the highest quality, lowest cost sources of untreated drinking-water in the world. The confined aquifer system below the city supplies water to a population of over 300,000 people. Groundwater is also the major source of industrial and irrigation water supply for the Christchurch-West Melton area. For the year ending June 1999, an estimated 104 million cubic metres of groundwater was abstracted in this area, with just over half of it being used for public reticulated supplies (ECan, 2001). Maintaining the high quality of the groundwater has been identified is a priority for Environment Canterbury. This report presents an analysis and interpretation of groundwater quality data gathered by Environment Canterbury and its predecessors from 1986 to 2002. The aims of the report are to describe the current state of the groundwater quality and identify factors, natural or otherwise, that influence the quality of the groundwater, and determine if changes are occurring. The study area for the Christchurch-West Melton groundwater system is shown in Figure 1.1. The area is about 70 000 hectares and is bounded to the east by the coastline, to the north by the Waimakariri River and to the south by the Port Hills. The area extends westwards to include Halket and Burnham. Both confined and unconfined parts of the groundwater systems are included in the study area.

1.1 Christchurch-West Melton groundwater system

The Christchurch-West Melton groundwater system comprises late Quarternary deposits of postglacial and interglacial fluvial gravels. Towards the coast, these gravels are interbedded with fine sand, silt, peat and clay deposits, together with marine, estuarine and lagoon sediments which accumulated during fluctuating climatic periods of the last 1 million years. The principal aquifers are in outwash and reworked gravel deposits. The intervening silt, sand, and peat layers confine the groundwater (Taylor et al., 1989). Figure 1.2 shows the sequence and nomenclature of the

late-Quarternary deposits underlying Christchurch. Flowing artesian aquifers underlie the area from the coast extending inland to Papanui, Fendalton and Riccarton. Five known aquifers are present to a depth of 200 m. The western limit of groundwater confinement in the first confined aquifer (Riccarton Gravel) is indicated by the 3 m isopach (thickness) line in Figure 1.1. The overlying confining sediments (Christchurch Formation) increase in thickness eastwards of the line and are about 30 – 40 m thick at the coast. Confinement of the deeper aquifers extends only a few kilometers west of this line (Taylor et al., 1989). Figure 1.1 shows the piezometric contours of the unconfined and first confined aquifers. The direction of groundwater flow is perpendicular to the contour lines, and is in a general east to south-east direction. Less is known about the flow directions of the deeper aquifers but they are likely follow similar lines of flow. Within the confined zone, upwards flow of the deeper groundwater into the shallow aquifers is thought to occur. Several isotopic and chemical tracer studies of groundwater in the Christchurch-West Melton area have significantly contributed to the understanding of the source and age of the groundwater system (Taylor et al., 1989; Taylor and Fox, 1996; Stewart, et al., 2002). Figure 1.3 provides an interpretative summary of groundwater recharge sources based on these studies and chemical data presented in this report. Recharge of the Christchurch-West Melton groundwater system occurs in the unconfined areas primarily from drainage from the Waimakariri River and rainfall on the plains. River-derived stockwater races also contribute to groundwater recharge. Flow gauging of the Waimakariri River (mean flow 120 m3/s) showed flow loss to groundwater occurs mainly between Halket and Crossbank, and is in the order of 7-10 m3/s (Talbot et al., 1986; Scott, 2000). Though it is extremely difficult to accurately measure the flow loss, indications are that it is relatively steady.



Studies using oxygen-18 (18O) isotopes in groundwater show the dominance of river-derived recharge in the northern part of the upper unconfined aquifers (Figure 1.3). As groundwater travels southeast, rainfall-derived infiltration becomes an increasingly significant recharge source. Rainfall is the dominant recharge source of groundwater in the southern unconfined zone. Groundwater in the confined zone is predominately recharged by Waimakakiri River water, although rainfall-derived infiltration contributes to groundwater in the southern part of the first confined aquifer. A contributing source of groundwater in the northeast part of the confined zone appears to be deep flow beneath the Waimakariri riverbed of rainfall or Eyre River drainage (Stewart et al., 2002). The source of deeper groundwater of the Christchurch-West Melton system is predominately derived from Waimakariri River drainage. Some contribution of rainfall-derived infiltration is indicated in the northeast part of the deep confined aquifers as discussed above. The ages of groundwater in the upper aquifers ranged from 0-10 years old immediately downgradient of the Waimakariri River recharge zone to greater than 30 years old at the coast. Groundwater ages tended to increase southwards with distance from the Waimakariri River. Residence time of the deeper unconfined groundwater ranged from 20 years immediately downgradient of the Waimakariri River to greater than 50 years in the southern unconfined area (Stewart et al., 2002). Carbon dating of groundwater from Aquifer 4 gave estimates of residence times ranging from 800 years for groundwater just outside the confining zone to 2000-3000 years within the confined zone (Taylor and Fox, 1996). Discharge of the shallow groundwater occurs by groundwater abstractions and spring discharges to the Avon, Heathcote, Halswell and Styx rivers at the confining boundary margin (Figure 1.3). It had previously been suggested that offshore discharge also occurred from the first confined aquifer at a considerable distance from the coastline (Talbot et al., 1986). However, more recent modelling studies have presented an alternative view (Scott, 2000). These consider the groundwater system to be dominated by

shallow circulation with most of the recharge re-emerging as spring-flow, and off-shore discharge from the uppermost confined aquifer occurring closer to the coast than previously thought. These studies suggest that groundwater abstractions result primarily in reduced baseflow to spring-fed streams rather than reduced off-shore flow (Scott, 2000). While less is known about the deep aquifer system, it is thought that the deep aquifers (Aquifer 2 and deeper) do not have significant direct connection to the sea, instead discharging upward into the first confined aquifer (Talbot et al., 1986; Taylor et al., 1989). Land use in the Christchurch-West Melton zone includes the residential, industrial and business zones of Christchurch City, and agricultural areas of northern, southern and western parts of the study area. Agricultural activities include moderate intensity of irrigated and non-irrigated grazing and cropping including dairying, and some areas of intensive horticulture.

1.2 Christchurch-West Melton groundwater quality monitoring programmes

Groundwater quality monitoring of the Christchurch-West Melton area dates back to the 1970s when groundwater samples were collected from a number of wells at various sampling frequencies. In the mid 1980s a routine groundwater quality monitoring programme was established in which groundwater was sampled and analysed from about 50 wells within this area on an approximately yearly basis. The objectives were to monitor the main chemical and microbial indicators of groundwater quality in the Canterbury Region, to assess suitability for use, and to determine spatial and temporal patterns in quality. Subsequently the number of wells and types of analyses included in the programme have expanded. In addition, a number of separate programmes have been established to address specific groundwater quality issues such as saltwater intrusion monitoring and seasonal variations in groundwater quality.



The combined objectives of the groundwater quality monitoring programmes are: • to characterise the baseline quality of the

main aquifer types in the Canterbury Region

• to determine seasonal trends in groundwater quality

• to determine temporal trends in groundwater quality

• to monitor the effectiveness of plans • to identify risks to groundwater quality. Table 1.1 summarises the current routine monitoring programmes undertaken in the Christchurch-West Melton area. The location of wells included in these programmes is shown in Figure 1.1. The annual ion survey involves sampling from about 250 wells annually throughout the region. The samples are collected during the spring months to coincide with high water tables and corresponding high nitrate concentrations. During the same period, groundwater from a smaller number of wells is collected as part of a hydrocarbon monitoring programme and are also analysed for a limited range of inorganic determinands (Short chemical survey). Monthly monitoring of groundwater from seven wells in the Christchurch-West Melton area for a limited range of inorganic and microbiological determinands aims to assess temporal and seasonal variations in groundwater quality. Environment Canterbury contributes to a national groundwater quality monitoring programme undertaken by The Institute of Geological and Nuclear Sciences (GNS) by sampling groundwater from 6 wells throughout the region four times a year (Rosen, 1997). The samples are sent to GNS’s laboratory for analysis. Two of the six wells are located within the Christchurch-West Melton area. An annual saltwater intrusion monitoring programme includes 15 wells in the coastal parts of Christchurch. In addition to the monitoring programmes above, there have been a large number of groundwater samples collected over the previous 16 years as part of small one-off investigations. In most cases these data have also been included in this report. A small amount of data were excluded from this dataset; this included data with obvious

analytical or transcription errors, or data for wells abstracting from Christchurch formation lithology. Christchurch formation wells are mostly less than 5 m deep, and are only used for site-specific monitoring purposes such as leachate monitoring of landfills. They are not considered representative of the general groundwater resource. The Christchurch City Council (CCC) also kindly provided groundwater quality data collected as part of their public water supply monitoring programme. Only data for samples taken directly from the wells were included in this report. This additional CCC data usually involved only one or two samples per well collected over the last six years, but has provided very valuable additional information on groundwater quality, especially for the deeper groundwater for which Environment Canterbury has limited data. The data presented in this report include inorganic chemical and microbiological data collected between January 1986 and March 2002 for the Christchurch-West Melton groundwater management area. The report does not include pesticide or hydrocarbon analyses. Hayward and Smith (1999) reviewed of the hydrocarbon data collected from groundwater throughout the Canterbury region, including data for the Christchurch area.



Table 1.1 Current ambient groundwater quality monitoring programmes for the Christchurch-West Melton area (2000/2001)

Monitoring programme

Number of wells in the CHCH-WM area

Start of monitoring programme

Sampling frequency Range of analyses

Annual ion survey

32

1986 Annually – September to November

pH, conductivity (cond), nitrate-nitrogen (NO3N), chloride (Cl), sulphate (SO4), bicarbonate alkalinity (HCO3), reactive silica (SiO2), sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), iron (Fe), manganese (Mn), total ammonia-nitrogen (NH3N), arsenic (As) (1999 only), faecal coliforms (FC), total coliforms (TC), E. coli (ECOLI) (1999), total hardness (THD), sum of cations (CATION), sum of anions (ANION), ion balance (IONBAL)

Short chemical survey

20

1989 Annually – September to November

pH, cond, NO3N, Cl, SO4 (hydrocarbon screen)

Monthly ion survey

7 1992 Monthly pH, cond, NO3N, Cl, SO4, FC, TC, ECOLI

Saltwater intrusion survey

17 1993 Annually - January to February

pH, cond, NO3N, Cl, SO4, HCO3, Na, Ca, Mg, K, Fe, Mn, NH3N, THD, SiO2, CATION, ANION, IONBAL, lithium (Li), fluoride (F), soluble phosphate (SOLP), free CO2 (CO2), bromide (Br), boron (B), strontium (Sr)

Quarterly GNS 2 1995 March, June, September, December

Samples are analysed by GNS. pH, cond, NO3N, Cl, SO4, HCO3, Na, Ca, Mg, K, Fe, Mn, NH3N, THD, SiO2, F, Br, soluble phosphate

Christchurch-W

est Melton G

roundwater Q

uality: A review

of groundwater quality m

onitoring data from January 1986 to M

arch 2002

Environment C

anterbury Technical Report

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Christchurch - West Melton groundwater study areaRoadsDistrict boundariesRiversThree-metre isopach (thickness) line of surface confining sedimentsPiezometric contours for Aquifer 1 (m above msl)

# All other wells with groundwater quality data used in report

Wells included in routine groundwater quality monitoring prgrammes for 2000/2001# Annual ion survey&V Monthly ion survey%U Quarterly GNS$ Saltwater intrusion survey

0 2 4 6 8 Kilometers

OverviewSouth Island New Zealand

Figure 1.1 Location of wells included in groundwater quality monitoring programmes for

the Christchurch – West Melton area (2000/2001 year)

Christchurch-W

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12 Environm

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Figure 1.2 Stratigraphy of the Christchurch – West Melton groundwater system (Brown and Weeber, 1992)

Riccarton gravel – Aquifer 1 Linwood gravel – Aquifer 2 Burwood gravel – Aquifer 3 Wainoni gravel – Aquifer 4 Un-named gravel – Aquifer 5

SPRINGSTON GRAVEL

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Environment C


13

Waimakariri River

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Eyre River

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Deep flow beneath the Waimakariri River

" Waimakariri River - dominant sourceprobably from upwards flow of deeper groundwater

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RoadsDistrict boundariesChristchurch - West Melton groundwater study areaRiversThree-metre isopach (thickness) line of surface confining sediments


Ó*

Waimakariri River - dominant sourceWaimakariri River dominant + some rainfall Rainfall dominant + some Waimakariri RiverRainfall - dominant sourceVolcanic-derived groundwaterSpring discharge to surface streams (colour indicates groundwater recharge source as above)

Recharge source of the upper aquifers (Aquifers 1 and 2, and Springston aquifer)

Figure 1.3 Recharge sources to the upper aquifers of the Christchurch-West Melton

groundwater system



2 Methods

2.1 Sample collection From 1986 to 1996 the general procedures of Sinton (1986) for purging wells and collecting groundwater samples were followed. From 1997 onwards, samples were collected following the procedures detailed in the Surface Water Quality, Groundwater Quality, Biological and Habitat Assessment Field and Office Procedures Manual (ECan, 1999). Briefly, the procedures included selecting a clean sampling point as close as possible to the well or pump, purging the well water to waste prior to collecting the sample, sterilising the sample point by either flaming or using alcohol (for microbiological samples only), and storing samples in cooled “chilli bins” for transport to the laboratory. Samples for microbiological analysis were generally analysed within 6-8 hours of sample collection. Samples for pH, conductivity and alkalinity were stored at less than 10 °C and analysed within 24 hours. Other chemical analyses were stored or preserved as appropriate and analysed within the required time (ECan, 1999). The aim of these procedures is to standardise sampling practices and minimise the possibility of sample contamination. They also ensure samples are representative of conditions in the groundwater rather than of stagnant water in the well casing. Additional quality control practices were introduced in 1995, requiring duplicate samples for approximately 5% of all samples collected. Samples collected by CCC staff were collected in a similar manner to above.

2.2 Sample analyses Most samples collected by Environment Canterbury were analysed at Environment Canterbury’s Christchurch laboratory. However, at times samples have also been sent to a range of laboratories in New Zealand including Environmental Science and Research Centre (ESR) and it’s predecessors, Cawthron Institute, Hills Laboratory and Medlab South. Samples collected by the CCC were analysed at ESR. Samples collected for the Quarterly GNS monitoring programme are analysed at the GNS laboratory at Wairakei (Rosen, 1997).

Analytical techniques for a number of chemical and microbiological analyses have changed considerably over the last 16 years, and different laboratories have used different techniques at any one time. Details of the analytical methods were not archived with water quality data stored on Environment Canterbury’s database prior to 1990. For these reasons only generalisations can be made about the analytical methods used for each determinand. Appendix 1 summarises the main analytical methods used over the period of the dataset.

2.3 Data analysis Identification of the aquifers from which each well was abstracting was an important first step in the analysis. For many of the wells sampled within the confined zone, the aquifer from which they were drawing had been previously identified and recorded on Environment Canterbury’s Wells database, based on bore log information collected at the time of drilling. For the remainder of the wells, the aquifers were identified using ‘Layers’, a program which interpolates known lithological data over most of the Christchurch-West Melton area for any given reference point and depth (D. Scott, Environment Canterbury pers comm.). Beyond the confining zone, identification of the aquifers is generally not possible from bore logs, as there are no distinctive separating layers to identify the different gravel sequences. However, it is expected that the aquifers extend some distance westwards of the confining boundary margin (J Weeber, Environment Canterbury pers comm.). For wells with groundwater quality data in the unconfined zone, the aquifers were identified by simply extrapolating westwards the known depths of each of the aquifers (Little, 1997). The water quality data were analysed using a range of software, including Microsoft Excel for tabulation, summary statistics and graphing of data; WQStat Plus, Statistica and Systat for statistical analyses and box plots; Arcview for geographical presentation; and Groundwater for Windows for Piper diagrams. Where duplicate data were collected, the results were averaged prior to any analysis. Where concentrations of determinands were



below the analytical limits of detection, the results were reported as ‘less than’ the detection limit. These non-detect data were converted to a value equal to half the detection limit for the purposes of data analyses. The following describes each of the main types of data analyses performed. Analysis of the major cation (Ca, Mg, Na, K) and anion (NO3N, HCO3, Cl, SO4) data was used to characterise the chemistry of the different aquifers. This included construction of Piper diagrams, pie charts for geographical comparison, and box plots. The data included in these analyses were restricted to those samples collected since 1986 that had complete major ion analyses and where the ion balance error was less the 5%. This gave a high level of confidence in the accuracy of the data, which was important for interpreting subtle variations. Median concentrations were calculated for wells with more than one sample. The Kruskal-Wallis H statistic test for seasonality was performed on the data collected from the monthly ions and quarterly GNS surveys. However, monthly sampling of groundwater may be too frequent for some statistical analyses, resulting in serial correlation of the data. There is an assumption that quarterly sampling of groundwater will result in independent samples and will avoid serial correlation problems (Ward, 1999). Therefore, for the seasonality test the monthly data were reduced to quarterly data by only using data for the central month of each season, i.e., January, April, July and October. Some exceptions to this occurred where sampling for that month was missing or had been undertaken in the last week of the previous month; in these situations the closest sampling occasion to the central month was selected. Spearman Rank correlation analysis was carried out on the monthly data obtained from the monthly ion survey and on quarterly data from the GNS survey. The Mann Kendall trend analysis and Sen’s slope estimate were used to analyse the data for temporal trends (IDT, 1998). This is a non-parametric statistical analysis, suitable for water quality data with some missing values. Not all of the data were suitable for trend analysis; data for the analysis were selected using the following criteria:

• only data from wells currently included in

the regular monitoring programmes listed in Table 1.1

• at least 8 years of data available • for the annual data no more than 2

consecutive years were missing. Trends were calculated on annual data collected from the annual ion survey, the annual short chemical survey, and the saltwater intrusion monitoring programme. For the annual ion survey and short chemical survey, only data collected between the months of September to December were used. If more than one sample was collected within this period of the same year, the data were averaged. The same criteria were applied to data collected for the saltwater intrusion monitoring programme except that only data collected within the months of November to February were used. The monthly ions were also analysed for trends. As with the test for seasonality, the monthly data were reduced to quarterly data. Monthly samples were analysed for the full range of determinands once each during spring. Results obtained that particular month were taken as the spring quarter for trend analysis. A confidence level of 0.1 was used to determine statistically significant trends in the Mann Kendall test. While this determined the statistical significance of a trend, it did not necessarily indicate environmental significance i.e., whether the trends were large enough to be meaningful. Therefore, for the purpose of determining environmentally significant trends in this report, only those trends with a magnitude of change greater than 1% per annum were considered environmentally significant and presented in this report (Stansfield, 1999). The exception to this was if the median value of a determinand of health significance (e.g., nitrate-nitrogen) was greater than ½ the value of the MAV, then a trend of increasing concentration of any magnitude was considered environmentally significant. The magnitude of a trend was calculated using Sen’s slope estimator (IDT, 1998, Gilbert, 1987). The percent change per annum (relative slope) was calculated from the slope (determinand unit per year) divided by the median value for that well (Smith et al., 1996).



3 Results and discussion

A summary of the groundwater quality data for each of the main aquifers, based on all the water quality data available from January 1986 to March 2002, is provided in Appendix 2. Altogether, groundwater quality data were available for 3000 samples from 438 wells within the Christchurch-West Melton area. Including all the data may have resulted in some biases owing to marked differences in sampling frequency and non-random spatial distribution of wells. However, given the large number of samples and wells for each of the aquifers, the results adequately represent the general quality of the groundwater in each aquifer. A description of the main determinands included in groundwater quality monitoring programmes is given in Appendix 3. The chemical characteristics of the groundwater for each of the main aquifers is presented in a series of box plots (Figure 3.1), Piper diagrams (Appendix 4) and as pie charts showing geographical differences (Figures 3.2 to 3.9). The data used for these diagrams was limited to samples for which complete major ion analyses were available, as described in Section 2.3. Figures 3.2 to 3.9 and Appendix 4 also included the results of a single water sample collected from the Waimakariri River at Intake Road, Halket in February 1997, which was analysed for the major ions. Although a single analysis of river water will not necessarily represent average water quality conditions, from other data available for the Waimakariri River water, the sample collected at Halket in 1997 appears representative of typical quality of the river. The median values of various determinands for all data available are presented in Figures 3.10 to 3.22.

3.1 Hydrochemistry of the aquifers

The chemical composition of groundwater is influenced by a number of factors including; recharge source water, aquifer material, residence time, flow rates and contamination sources. The chemistry of the different recharge waters has a large influence on the quality of the groundwater in the Christchurch-West Melton area. The Waimakariri River is

primarily fed from high altitude precipitation and therefore tends to have a low ionic content with little influence of coastal sea spray or mineral weathering and with low nutrient additions. Consequently, groundwater predominately recharged from the river water generally contains low concentrations of ions. In contrast, rainfall drainage interacts with minerals and organic matter in the soil and can contained elevated concentrations of a number of soluble ions reflecting soil type and land use activities. The relationship between recharge source and ion concentrations is illustrated in Figure 3.23. The graph shows δ18O values1 for Christchurch-West Melton groundwater samples plotted against chloride concentrations. While this includes only a limited number of wells, it shows increasing chloride concentrations with increasing dominance of rainfall-derived groundwater. For groundwater recharged predominately by Waimakariri River water (i.e., with δ18O values less than –8.8‰) chloride concentrations were below 10 mg/L. For groundwater recharged by rainfall infiltration, the chloride concentrations tended to be within the range of 10 – 20 mg/L. This general pattern is evident throughout the gravel aquifers of Canterbury and can be seen in Figures 3.12 to 3.15. Because there is little chemical exchange of chloride within the aquifer, chloride concentrations provide a useful indicator of recharge sources. However, chloride concentrations above the general range suggested for rainfall-derived groundwater of 10 –20 mg/L were found in parts of the study area.

1 Oxygen-18 (18O) is a stable isotope of oxygen and is used as a conservative, naturally occurring tracer to distinguish between river- and precipitation-derived groundwater in Canterbury. This relies on the altitude effect, which results in depletion of 18O in high-altitude precipitation compared with that of precipitation on the lower altitude Canterbury plains (Stewart et al., 2002). Measurements of δ18O values in groundwaters in the Canterbury region have been undertaken periodically since 1969 (e.g., Taylor et al., 1989; Stewart et al., 2002). The δ18O data collected from 1980 to 2001 is presented in Figure 3.23. Stewart et al. (2002) provides an interpretation of recharge source of groundwater based on 18O for the Christchurch-West Melton area.



Figure 3.1 Distribution of median values of various determinands for each of the Christchurch-West Melton aquifers (Note: log scale on the y-axis for all graphs except for the pH graph)2

2 In a box plot, the centre horizontal line marks the median of the sample. The length of each box shows the range within which the central 50% of the values fall, with the box edges (called hinges) at the first and third quartiles.

Springston

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This is a result of contamination from land use activities or from saltwater intrusion (see discussion below and Section 3.5.1). Therefore, while chloride concentrations provide a useful indication of recharge sources, they are also useful indicators of contamination of groundwater. The following sections discuss the chemical characteristics of the groundwater for each of the aquifers. Overall, the greatest variability in chemical composition occurred in the Springston aquifer and Aquifer 1 data (Figure 3.1). Data for Aquifer 2 was more variable than Aquifers 3 to 5, but less than for the shallower aquifers. The variation in the data may in part be a function of the number of wells sampled for each aquifer. However, in general the small range of values for most determinands in groundwater from the deeper aquifers reflects the relatively stable and well-mixed nature of the groundwater. The large variability in the data for the shallower aquifers indicates the increased sensitivity of the groundwater quality to variations in recharge and contamination sources. 3.1.1 Springston Aquifer The Springston formation includes well-sorted postglacial fluvial channel and overbank deposits of gravel, sand and silt. This forms the unconfined ‘water table’ aquifer in the western part of Christchurch (Figure 1.2). Patchy lenses of silt and clay are also present. The Springston formation is about 5 m thick near the coast, approximately 15 m at Central Christchurch and generally over 10 m thick west of the city (Little, 1997). Groundwater quality data for the Springston aquifer was available in three main areas; each area shows slightly different chemical characteristics reflecting the recharge source and localised influences. Calcium and bicarbonate tended to be the dominant cation and anion respectively, although data from some wells located in the southern part of the area showed no dominant ions (Appendix 4). Groundwater from the group of wells located in the northern part of the study area along Johns Road had similar ionic ratios to those of Waimakariri River water reflecting its dominance as the source of recharge (Figures 3.2 and 3.3). Conductivity values and concentrations of major ions were generally low; conductivity values were usually less than 15 mS/m and chloride less than 10 mg/L.

Nitrate-nitrogen concentrations were generally less than 1 mg/L. Groundwater from wells in the Marshlands Road area contained slightly higher ion concentrations compared to groundwater from upgradient wells along Johns Road (e.g., Figure 3.12). The median sodium and chloride concentrations for the Johns Road area were 4 mg/L and 2.8 mg/l respectively and for the Marshlands Road area, 8.3 mg/L and 7.8 mg/L respectively. The higher concentrations of these ions in groundwater nearer the coast probably results from the influence of coastal sea spray. The sulphate concentrations were very low in the Marshlands Road area indicating that sulphate reduction is occurring in groundwater. This is expected given that this area has organically rich soil; groundwater from many wells exhibited reduced conditions as indicated by the presence of iron and/or manganese in samples, and low concentrations of dissolved oxygen. In the southern part of the study area, groundwater from the Springston aquifer showed the influence of rainfall-derived infiltration, with increased concentrations of a number of ions including nitrate and chloride and higher conductivity values. The relative abundance of chloride, nitrate and magnesium were higher than for wells in the northern part of the area (Figures 3.2 and 3.3, Appendix 4). Median conductivity values were generally between 20-50 mS/m and median nitrate-nitrogen concentrations of up to 12.4 mg/L occurred. Groundwater from one well, M36/3085, located in the Wigram area showed very high concentrations of all the major ions with chloride concentrations of up to 190 mg/L (Figure 3.12). The nitrate-nitrogen concentrations in groundwater from this well were low, but ammonia-nitrogen concentrations up to 32 mg/L had been found. Leachate from a closed landfill immediately upgradient of this well is the likely source of the contamination of this groundwater. 3.1.2 Aquifer 1 Aquifer 1 (Riccarton gravel) is the uppermost gravel sequence underlying the predominately fine sediments of the postglacial marine Christchurch Formation (Figure 1.2). Aquifer 1 lies 10 – 40 m below the surface and ranges in depth from a few metres thick at the coast to about 20 m thick under Central Christchurch, and is over 30 m thick west of Yaldhurst. The



aquifer becomes confined east of Riccarton and Papanui (Figure 1.1) (Little, 1997). Because of the number of wells with groundwater quality data and the complexity of the data, the relative abundance data relating to Aquifer 1 has been split into four different Piper diagrams. This split corresponds to the northern confined, northern unconfined, southern confined and southern unconfined parts of the aquifer (Appendix 4). For the purpose of this separation, confined and unconfined areas were defined as east and west, respectively, of the 3 m isopach line shown in Figure 1.1. In the northern part of the unconfined zone, cation and anion ratios and ionic concentrations were similar to those of the Waimakariri River (Figures 3.4 and 3.5, Appendix 4). The groundwater chemistry in this area was dominated by bicarbonate and calcium ions. This is consistent with isotope studies indicating that Waimakariri River drainage is the dominant source of recharge for this area (Taylor et al., 1989; Stewart et al., 2002). Conductivity values in this area were typically less than 15 mS/m and chloride concentrations ranged from 1 to 10 mg/L. Nitrate-nitrogen concentrations were less than 1 mg/L in the groundwater immediately downgradient of the river recharge area. However, within a short distance southward from the river, nitrate-nitrogen concentrations tended to increase, and were generally in the range of 1 – 5 mg/L (Figure 3.20). There was very little variability in the relative abundance of the ions in the northern unconfined zone compared to other parts of Aquifer 1 (Appendix 4). Groundwater quality in the southern part of the unconfined zone reflected the increased contribution of rainfall-derived recharge. The total ionic content increased with distance southwards, as indicated by conductivity values for this area ranging from 15 to 30 mS/m (Figure 3.10). Similarly, chloride concentrations in this area were typically in the range of 10 to 20 mg/L. Concentrations of nitrate-nitrogen were highly variable. In addition to general increases in ion concentrations, the relative abundance of the ions changed with the increased dominance of rainfall-recharge. Concentrations of chloride and nitrate increased relative to bicarbonate,

and sodium and magnesium increased relative to calcium (Figures 3.4 and 3.5, Appendix 4). In the confined part of Aquifer 1 the groundwater chemistry was more complex. As with the unconfined zone, a general trend of increasing ion concentrations occurred southwards reflecting the changing recharge source from Waimakariri River drainage to rainfall drainage (Figure 3.10). This was further indicated by increased relative abundance of nitrate and chloride in groundwater with distance from the Waimakariri River (Figure 3.4, Appendix 4). However, isotopic studies by Stewart et al. (2002) have indicated that rainfall-derived or lowland river drainage (Eyre River) flows at depth from north of the Waimakariri River beneath the river into the northeast part of the confined system (see Figure 1.3 in Section 1). The groundwater chemistry of Aquifer 1 in this area support this suggestion with small but notable relative increases in chloride and nitrate compared to groundwater derived predominately from Waimakariri River recharge (Figure 3.4). Elevated sodium and chloride concentrations relative to other ions in the southeast part of Aquifer 1 are the result of downward migration of saltwater from the Avon-Heathcote Estuary into the Woolston-Heathcote area (Figures 3.4, 3.5 and Appendix 4). While sodium and chloride are the dominant ions, the concentrations of most of the major ions are also elevated. This is discussed in further in Section 3.5.1. In the southern part of Aquifer 1, particularly between Islington and Woolston, the concentrations of chloride, sulphate and nitrate-nitrogen in a number of wells were higher than background concentrations (Figures 3.13, 3.16 and 3.20). For example, sulphate concentrations up to 130 mg/L were found in groundwater from some wells, which is over 10 times higher than the median sulphate concentration of 9 mg/L for Aquifer 1 groundwater samples (Appendix 2). Similarly, elevated chloride concentrations in the range 20 to 50 mg/L were found in groundwater from a number of wells. It is unlikely that these elevated concentrations occur naturally. The areas affected are primarily in or adjacent to industrial zones in southern Christchurch. As the concentrations appear to be decreasing over time (see Section 3.4), it is likely that



discharges from past industrial activities have resulted in contamination of the groundwater in this area. 3.1.3 Aquifer 2 Aquifer 2 (Linwood gravels) occurs at about 70 m below ground surface near the coast and becomes progressively shallower inland. At Yaldhurst, the depth to the top of Aquifer 2 is about 40 – 45 m. The thickness of the aquifer range from 25-35 m across Christchurch and increases in thickness to the south and east. The confining sediments of the Bromley Formation overly Aquifer 2. This formation is about 20 m thick near the coast and progressively reduces inland. At about the city centre, the Bromley Formation is approximately 15 m thick (Little, 1997). In the unconfined part of Aquifer 2, the groundwater chemistry showed similar patterns to that of the unconfined groundwater of Aquifer 1. The chemistry of groundwater from well M35/0925 (54 m deep), located near the river recharge zone, closely resembled that of the Waimakariri River (Figures 3.6 and 3.7, Appendix 4). The influence of rainfall infiltration was evident with distance southwards where nitrate and chloride became increasing significant anions and sodium decreased in abundance relative to calcium. The concentrations of ions were similar to those of Aquifer 1. In the confined part of Aquifer 2, low conductivity values and ion concentrations reflected Waimakariri River water as the dominant source of recharge. With the exception of the Woolston-Heathcote area, conductivity values were below 15 ms/m and chloride concentrations below 10 mg/L. Nitrate-nitrogen concentrations in the confined zone were generally less than 1 mg/L. Unlike Aquifer 1, groundwater in the southern part of the confined zone of Aquifer 1 did not appear to show any significant contamination from land use activities. In the Woolston-Heathcote area, groundwater was generally more mineralised than in other parts of the confined zone (Figure 3.11). This is primarily caused by the downward migration of saltwater from the Avon-Heathcote Estuary (see Section 3.5.1). However, saltwater contamination is not the only reason for the mineralised groundwater. Groundwater from

two wells, M36/1145 and M36/4135, contained elevated concentrations of a number of ions including chloride and sulphate and elevated silica concentrations (Figure 3.14, 3.17 and 3.19). Chloride is the dominant anion in groundwater from these wells indicating saltwater intrusion. However, the cations do not show similar ratios to those of the Aquifer 1 wells in this area known to be contaminated by saltwater. Instead the cations in groundwater from these wells plotted on the Piper diagram in similar positions to groundwater derived from Bank Peninsula volcanics (Appendix 4) (Brown and Weeber, 1994). The concentrations of silica in samples from these wells were 36.4 and 34 mg/L. Silica concentrations in groundwater for most of the Christchurch-West Melton area were usually less than 25 mg/L as would be expected for this type of aquifer system (Figures 3.18 and 3.19). Silica values above 30 mg/L can indicate the mixing of geothermal or volcanic-derived groundwater (Rosen, 2001). The elevated silica concentrations suggest that Bank Peninsula volcanic-derived groundwater is contributing to the mineralised groundwater abstracted by these wells. 3.1.4 Deep aquifers The deeper aquifers discussed here include Aquifers 3, 4 and 5. Considerably fewer water quality data are available for these aquifers than for the shallower aquifers, which is a function of both the low number of wells drilled into these aquifers and the low risk of contamination problems justifying a low level of monitoring. Aquifer 3 (Burwood gravels) ranges in depth below the ground surface from about 115 m at the coast to about 90 m at Yaldhurst. The thickness of the aquifer ranges from 5-15 m. The aquifer is immediately overlain by the Heathcote Formation, which is about 15 m thick at the coast, and thins to about 10 m at the city centre (Little, 1997). Aquifer 4 (Wainoni gravels) ranges in thickness between 5 and 25 m. The depth below the surface ranges from about 140 m at the coast to about 120 m at Yaldhurst, but is shallower in the southern part of Christchurch where it is typically encountered at about 110 m. The aquifer is immediately overlain by the Shirley Formation, which is about 15 m thick at



the coast, and thins to about 10 m at the city centre (Little, 1997). Only a few wells have been drilled into and beyond Aquifer 5. There are insufficient data to define the thickness of the aquifer. The top of the aquifer occurs at approximately 130-145 m below the ground surface across the city. Insufficient data are available to identify the depth to the aquifer west and north of the city. The aquifer is overlain by un-named confining sediments (Little, 1997). In general, bicarbonate is the dominant anion in the deep groundwater and calcium is the dominant cation except for some wells where there is no dominant cation (Appendix 4). As with the shallower aquifers, differences in the groundwater chemistry occur between the confined and unconfined parts of each aquifer. The wells located west of the confining boundary showed a distinct grouping; the relative abundance of calcium in the groundwater from these wells tended to be higher than in the groundwater from the confined parts of the aquifers (Figure 3.9). Nitrate-nitrogen concentrations were also slightly higher in the unconfined areas (Figure 3.22). This is the result of downward migration of shallow rainfall recharged groundwater, which mixes with the deeper unenriched groundwater. Overall, the conductivity values and concentrations of ions were very low in the deeper aquifers. Within the confined zone, median chloride values were less than 10 mg/L (Figure 3.15). Chloride values less than 5 mg/l indicate that little, if any, rainfall-derived infiltration is contributing to the recharge of this deep groundwater. The chloride values in the southern part of the confined zone were less than 5 mg/L indicating the dominance of Waimakariri River recharge. In the northeast part of the confined zone, however, median chloride values in the deeper groundwater were between 5 and 10 mg/L. The general chemistry of the groundwater in this area, and extending south-east to the coast, showed slightly higher concentrations of a number of other ions including nitrate-nitrogen. Monitoring of a number of production wells at the PPCS meat processing company at Belfast, northeast Christchurch, has been

undertaken by the company for a number of years. These results indicated that nitrate-nitrogen concentrations tended to be slightly higher in groundwater from the deeper wells, ranging from 1 - 1.3 mg/L from wells approximately 100 m deep. Nitrate-nitrogen concentrations in groundwater from the shallow wells (~30 m deep) were in the range of 0.5 – 1 mg/L (J Lush, PPCS Belfast, pers comm.). Environment Canterbury data for wells in the Belfast area also showed this pattern. A groundwater sample from well M35/1244 (120 m deep) contained 1.1 mg/L of NO3N, while groundwater collected at the same time from a nearby 28 m deep well (M35/1255) contained 0.8 mg/L NO3N. Concentrations of other ions including chloride showed similar differences. While these differences are small, the data supports the suggestion by Stewart et al. (2002) that rain-fall derived groundwater is travelling at depths of perhaps greater than 100 m and is upflowing and mixing with groundwater from the shallower aquifers. In general, the groundwater quality of the deeper aquifers was very similar when compared to the range of determinand values for the upper aquifers (Figure 3.1). The relative abundance of the major ions showed very little difference between the aquifers (Figures 3.8 and 3.9, Appendix 4). The actual concentrations do tend to show some minor differences, with a general overall increase in ion concentrations in the deepest aquifers (Figure 3.1). There are only a small number of wells for each of the deeper aquifers, and the wells are not evenly distributed across the study area. Therefore, it is difficult to determine whether the small differences in ion concentrations between the aquifers are a function of residence times, or relate to the location of the wells. For example, most of the Aquifer 5 wells were located on the western unconfined part of the study area and appear to show the influence of some shallower enriched groundwater entering the system because nitrate-nitrogen is present. However, most of the data for Aquifer 4 are from wells on the eastern side of the study area and are generally within the confined zones. This may explain the generally higher conductivity and ion concentrations of groundwater from Aquifer 5 wells. Overall the differences both within and between the deeper aquifers are small.

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RoadsRiversThree-metre isopach line for surface confining sediments

Springston aquifer - anions (meq/L)So4Hco3ClNo3n


SO4HCO3ClNO3N

Figure 3.2 Pie charts of median anion concentrations for groundwater from the Springston aquifer

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002 Environm


eport 23

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Springston aquifer - cations (meq/L)CaMgNaK


Figure 3.3 Pie charts of median cation concentrations for groundwater from the Springston aquifer

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002

24

Environment C


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Woolston

Heathcote

SO4HCO3ClNO3N

Figure 3.4 Pie charts of median anion concentrations for groundwater from Aquifer 1

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002 Environm


eport 25

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Figure 3.5 Pie charts of median cation concentrations for groundwater from Aquifer 1

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002

26

Environment C


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Woolston

Heathcote

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Figure 3.6 Pie charts of median anion concentrations for groundwater from Aquifer 2

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002 Environm


eport 27

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N

EW

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Woolston

Heathcote

Figure 3.7 Pie charts of median cation concentrations for groundwater from Aquifer 2

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002

28

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Aquifers 3, 4 and 5 - anions (meq/L)So4Hco3ClNo3n


Aquifer 3 and Aquifer 5 wells labelled

SO4HCO3ClNO3N

Figure 3.8 Pie charts of median anion concentrations for groundwater from Aquifers 3, 4 and 5

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002 Environm


eport 29

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S


Aquifers 3, 4 and 5 - cations (meq/L)CaMgNaK


Aquifer 3 and Aquifer 5 wells labelled

Figure 3.9 Pie charts of median cation concentrations for groundwater from Aquifers 3, 4 and 5

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002

30

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Waimkariri River

N

EW

S

RoadsThree-metre isopach line for surface confining sedimentsRivers

Springston aquifer and Aquifer 1 wells - median conductivity values (mS/m)# 7 - 15# 15.1 - 30#S 30.1 - 50#S 50.1 - 100# 100.1 - 200%U 200.1 - 520


Figure 3.10 Median conductivity values for all samples collected from Springston aquifer and Aquifer 1 wells

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002 Environm


eport 31

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N

EW

S


Aquifer 2 wells - median conductivity values (mS/m)# 8 - 15# 15.1 - 30#S 30.1 - 50#S 50.1 - 100


Figure 3.11 Median conductivity values for all samples collected from Aquifer 2 wells

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002

32

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Templeton

Prebbleton

Islington

Hillmorton

Heathcote

Waimakariri River

Belfast

Rolleston

Wigram

Waimakariri River sample Cl = 1 mg/L

Woolston

N

EW

S

RoadsThree-metre isopach (thickness) line for surface confining sedimentsRivers

Springston aquifer wells - median Cl concentrations (mg/L)# 1.8 - 5# 5.1 - 10# 10.1 - 20#S 20.1 - 50#S 50.1 - 250


Figure 3.12 Median chloride concentrations for all samples collected from Springston aquifer

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002 Environm


eport 33

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Waimakariri River

Belfast

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Wigram


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Aquifer 1 wells - maximum Cl concentration above guideline valuew

Aquifer 1 wells - median Cl concentrations (mg/L)# 1 - 5# 5.1 - 10# 10.1 - 20#S 20.1 - 50#S 50.1 - 250%U 250.1 - 17000


Figure 3.13 Median chloride concentrations for all samples collected from Aquifer 1 wells

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002

34

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Aquifer 2 wells - median Cl concentrations (mg/L)# 1.8 - 5# 5.1 - 10# 10.1 - 20#S 20.1 - 50#S 50.1 - 130


Figure 3.14 Median chloride concentrations for all samples collected from Aquifer 2 wells

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002 Environm


eport 35

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Figure 3.15 Median chloride concentrations for all samples collected from Aquifer 3, 4 and 5 wells

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002

36

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Templeton

Prebbleton

Islington

HeathcoteM36/1244

Waimakariri River

Belfast

Rolleston

Waimakariri RiversampleSO4 = 4.1 mg/L

N

EW

S


Springston aquifer and Aquifer 1 wells - median SO4 concentrations (mg/L)# 0.05 - 10# 10.1 - 20# 20.1 - 50#S 50.1 - 100#S 100.1 - 200


Figure 3.16 Median sulphate concentrations for all samples collected from Springston aquifer and Aquifer 1 wells

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002 Environm


eport 37

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Templeton

Prebbleton

Islington

Heathcote

Waimakariri River

Belfast

Rolleston

Waimakariri RiversampleSO4 = 4.1 mg/L

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S


Aquifer 2 wells - median SO4 concentrations (mg/L)# 0.7 - 10# 10.1 - 20# 20.1 - 50#S 50.1 - 100


Figure 3.17 Median sulphate concentrations for all samples collected from Aquifer 2 wells

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002

38

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Templeton

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Belfast

Rolleston

N

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S


Springston aquifer and Aquifer 1 wells - median SiO2 concentrations (mg/L)# 1 - 10# 10.1 - 20# 20.1 - 30$T 30.1 - 42


Figure 3.18 Median silica concentrations for all samples collected from Springston aquifer and Aquifer 1 wells

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002 Environm


eport 39

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Belfast

Rolleston

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Aquifers 3, 4, and 5 wells - median SiO2 concentrations (mg/L)#S 14 - 20#S 20.1 - 21

Aquifer 2 wells - median SiO2 concentrations (mg/L)# 10 - 20# 20.1 - 30$T 30.1 - 36.4


Figure 3.19 Median silica concentrations for all samples collected from Aquifers 2, 3, 4 and 5 wells

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002

40

Environment C


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Templeton

Prebbleton

Islington

Hillmorton

Heathcote

M36/4044

M36/4151M36/3670

M35/1103

M35/1839

M36/1099

M36/1244

Waimakariri River

Belfast

Rolleston

Waimkakariri River sampleNO3N = 0.09 mg/L

M35/3170

M36/2232M36/0271

M35/1883

M36/4227

N

EW

S


Springston aquifer and Aquifer 1 wells - maximum NO3N concentration above MAV

a 11.4 - 28 mg/L

Springston aquifer and Aquifer 1 wells - median NO3N concentrations (mg/L)# 0.005 - 1# 1.01 - 5.6# 5.61 - 8.5# 8.51 - 11.3#S 11.31 - 19


Labelled wells have had samples trangress the MAV for nitrate-nitrogen

Figure 3.20 Median nitrate-nitrogen concentrations for all samples collected from Springston aquifer and Aquifer 1 wells

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002 Environm


eport 41

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Aquifer 2 wells - maximum NO3N concentrations above MAVa 11.4 - 36.4 mg/L

Aquifer 2 wells - median NO3N concentrations (mg/L)# 0.005 - 1# 1.1 - 5.6# 5.61 - 8.5# 8.51 - 9.4


Labelled wells have had samples trangress the MAV for nitrate-nitrogen

Figure 3.21 Median nitrate-nitrogen concentrations for all samples collected from Aquifer 2 wells

Christchurch-W

est Melton G

roundwater Q

uality: A review



arch 2002

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Figure 3.22 Median nitrate-nitrogen concentrations for all samples collected from Aquifer 3, 4 and 5 wells

Christchurch-W

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roundwater Q

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arch 2002 Environm


eport 43

Figure 3.23 δ18O values versus chloride concentrations

-9.80

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0 5 10 15 20 25 30Chloride concentration (mg/L)

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ues Aquifer 1

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Aquifer 5

Wamakariri River recharge

Waimakariri R dominant over rainfall

Rainfall dominant over Waimak R

Inland deep wells

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Waimakariri River-derivedrecharge

Chloride enriched recharge source or contamination ?

Rainfall-derived recharge



3.1.5 Chemical evolution of groundwater Once water enters an aquifer system a range of processes can alter the chemical composition of the groundwater. Rock-water interactions and oxidation-reduction reactions are the major processes altering chemical composition of water within the aquifer over time. In general, the concentrations of total dissolved solids and major ions increase with increasing residence time. The main component of the Canterbury Plains aquifer material is greywacke, which is composed largely of silica (SiO2). This is relatively inert compared to other aquifer materials such as carbonates and volcanic rocks. Consequently, mineral interactions are limited, with the geochemical changes across the aquifer more subtle than would be expected in other types of aquifer systems. Other minor components of the Canterbury Plains aquifer material such as clays and peat result in some mineral interactions. The Cheboratev sequence describes the change in dominance of anions from bicarbonate in young, shallow groundwater to chloride dominance in very old, slow moving groundwater (Figure 3.24) (Freeze and Cherry, 1979). This rock-water type interaction depends of the availability and solubility of minerals. In some systems, especially in New Zealand, the groundwater does not evolve past the HCO3

- dominant stage because of the lack of soluble minerals available (Rosen, 2001). The groundwater chemistry of Aquifer 1 tends to show an evolution towards the coast of sulphate decreasing in relative abundance and chloride increasing, with little change in the relative abundance of bicarbonate (Waimakariri River anions HCO3 ~82%, Cl ~5% and SO4 ~13% and coastal Aquifer 1 anions HCO3 ~83%, Cl ~12% and SO4 ~5%).

The change in relative anion concentrations does not follow the Cheboratev sequence as bicarbonate remains the dominant anion throughout the aquifer, except for the groundwater contaminated with saltwater in the Woolston area. In the deeper aquifers bicarbonate also remains the dominant anion throughout the system (HCO3 ~85%, Cl ~10% and SO4 ~5%). Similar sequences for cations are more difficult to generalise because of the variety of processes that can occur for different aquifer systems. In the Canterbury gravel aquifers, the main process of cation evolution is probably cation exchange with clays. Groundwater from Aquifer 1 evolves towards the coast with increasing concentrations of sodium and potassium relative to calcium (Waimakariri River cations Ca ~70%, Mg ~15% and Na + K ~15% and coastal Aquifer 1 cations Ca ~50%, Mg ~18% and Na + K ~32%). This is a gradual change, reflecting the residence time since recharge from the Waimakariri River. In the deeper aquifers the cations tend to have no dominant cation and show similar ratios to those of the coastal Aquifer 1 groundwater. The tendency of groundwater to evolve from an oxidised state to a reduced state is known as the electrochemical evolution sequence. Oxidation-reduction reactions in water mainly involve the oxidation of organic matter by a series of oxidising agents (oxidants). In natural systems, these agents are, in order of decreasing strength; molecular oxygen (dissolved oxygen), nitrate ions, manganese (IV) and iron (III) oxides, sulphate ions, carbon dioxide and molecular nitrogen (Freeze and Cherry, 1979). In general, each oxidant must be completely, or nearly completely, consumed

Figure 3.24 Chebotarev geochemical sequence of the relative abundance of anions for groundwater

Travel along flow path HCO3

- HCO3- + SO4

2- SO42- + HCO3

- SO42- + Cl-

Cl- + SO42- Cl-

Increasing age



before the next strongest agent starts oxidising. For example, before nitrate-reduction can occur, most of the molecular oxygen must be consumed, although nitrate-reduction may occur at a micro-scale within an overall oxidised system (Freeze and Cherry, 1979). Consequently, decreases in nitrate and sulphate ions and increases in soluble iron and manganese can be expected with increasing reducing conditions in groundwater. Depending on the nitrate-reduction reaction, ammonia concentrations may increase. Bicarbonate is a product of most of the reactions involving the oxidation of organic matter, and therefore, increases in this ion also occur. As groundwater travels from the recharge source (and oxygen source) the oxidation of organic matter consumes the dissolved oxygen. The isolation of groundwater from the atmosphere means replenishment of oxygen is limited. Only small amounts of organic matter are required to result in significant depletion of dissolved oxygen. Therefore, in many groundwater systems dissolved oxygen concentrations decrease rapidly to below detection limits with distance and depth from the recharge zone (Freeze and Cherry, 1979). Although measuring dissolved oxygen concentrations in groundwater is difficult, the data available indicate dissolved oxygen is abundant in the unconfined groundwater zone of the Christchurch-West Melton area. Dissolved oxygen concentrations in this area are typically between 5-11 mg/L. Even the deep unconfined groundwater appears reasonably well oxygenated. Dissolved oxygen concentrations from a 200 m deep well located at the confining boundary margin ranged from 5.4 to 7.7 mg/L. While aeration of the water during pumping cannot be discounted, efforts were made to minimise this occurring. Within the confining zone, dissolved oxygen concentrations tended to be lower than for the unconfined zone, but still occurred in moderate concentrations. Dissolved oxygen concentrations in the range of 1 to 8 mg/l were common for groundwater in the confined zone at all depths (Appendix 2). However, very low DO concentrations were observed in some localised areas. For example, three artesian water level monitoring wells, M36/5893, M36/5894 and M36/5895, abstract from Aquifers 1, 2 and 4 respectively. They are located on the Brighton spit and are

sampled annually as part of the saltwater intrusion survey. Dissolved oxygen measurements of groundwater from these wells are likely to be reasonably representative of the groundwater conditions because no pumping is required owing to artesian pressures. The depths of the wells are 53, 84 and 138 m and median dissolved oxygen concentrations were 0.9, 1 and 3.7 mg/L respectively. The low dissolved oxygen concentration in the groundwater from the Aquifer 1 well was accompanied by high median concentrations of iron (3 mg/L), manganese (0.6 mg/L), arsenic (0.09 mg/L) and ammonia-nitrogen (3.3 mg/L). Nitrate-nitrogen (<0.025 mg/L) and sulphate (<0.1 mg/L) were not detected. This indicates very strong reducing conditions in the groundwater. Groundwater from the Aquifer 2 well showed moderately reduced conditions with concentrations of iron (0.7 mg/L), manganese (0.04 mg/L) and ammonia-nitrogen (0.15 mg/L). There was no detectable nitrate-nitrogen. Sulphate was found at a low concentration (0.7 mg/L). This indicates the groundwater is sufficiently reduced for some, but not complete, reduction of sulphate to occur. In contrast, groundwater from the deepest well, M36/5895, did not show significant reducing conditions with only trace detections of manganese (0.01 mg/L) and ammonia-nitrogen (0.01 mg/L) and low but detectable concentrations of nitrate-nitrogen (0.17 mg/L) and sulphate (3.3 mg/L). Iron was present at high concentrations (1.3 mg/L) but it is not clear whether this iron was in the reduced dissolved form or was an oxide from the well casing. The occurrence of zones of reduced groundwater is generally associated with organically rich peat or swamp deposits. These deposits are interbedded in the gravel and fine sediment sequences and occur more commonly in coastal areas, although they also occur west of the confining zone. The high organic content of these deposits result in rapid consumption of any oxygen resulting in reduced conditions. The zone of reduced groundwater is generally very localised. As indicated by the oxidised state of groundwater from the Aquifer 4 well above, widespread reduced conditions do not occur the Christchurch-West Melton area.



3.2 Comparison to drinking-water quality standards

The New Zealand drinking-water standards specify maximum acceptable values (MAVs) for determinands of health significance in drinking-waters (Tables 3.1 and 3.2). The maximum acceptable value is defined as ‘the concentration of a determinand which, on the basis of present knowledge, is not considered to cause any significant risk to the health of the consumer over a lifetime of consumption of the water’ (MoH, 2000). The standards also specify sampling protocols that must be observed if compliance with the Standards is to be established for community drinking-water supplies3. However, no general sampling recommendations are given for individual household drinking-water supplies (MoH, 2000). The standards distinguish between the terms ‘compliance or non-compliance’ and ‘transgressions’. Compliance with the standards apply to the entire drinking-water supply system including the source, treatment and distribution system. Compliance is assessed on a running annual basis and cannot be determined from an individual sample. The term transgression applies to a single sample, in which one or more determinand exceeds the relevant MAV (MoH, 2000). The groundwater quality sampling undertaken by Environment Canterbury and its predecessors involved collecting samples as close as possible to the well head, deliberately avoiding most of the distribution system because its primary purpose was to characterise ambient groundwater quality. Furthermore, the sampling frequency of most wells was considerably less than the drinking-water standards compliance requirements. Therefore, comparison of samples to the drinking-water standards only relates to whether individual samples meet or transgress the maximum acceptable values and does not demonstrate compliance or otherwise of a water supply with the Standards. The New Zealand drinking-water standards also list guideline values for aesthetic-based determinands (Table 3.3). At concentrations above the guideline value a determinand may

3 Definition of community drinking-water supplies are water supplies that serve more than 25 people for at least 60 days per year (MoH, 2000).

cause nuisance effects but is not expected to pose a health risk (MoH, 2000). Other guidelines of relevance to groundwater users include the Australian and New Zealand guidelines for fresh and marine water quality, which includes guidelines for agriculture water use e.g., livestock and irrigation water supply, and industrial water quality (ANZECC, 2000). In most cases groundwater which meets drinking-water standards and guidelines will also be acceptable for other uses such as irrigation, livestock drinking-water and industrial uses. However, for some types of industrial processes, irrigation of certain crops, or people with special medical conditions (e.g., water for dialysis treatment) additional water quality criteria may apply. 3.2.1 Determinands of health significance The number of samples and wells from each of the aquifers that have transgressed one or more of the New Zealand drinking-water health-based maximum acceptable values are summarised in Table 3.4. Significant determinands are discussed in detail below. 3.2.1.1 Microbiological Quality The New Zealand drinking-water standards specify that no Escherichia coli (E. coli) should be detected in a 100 ml sample (MoH, 2000). This replaced earlier drinking-water standards which used faecal coliforms as the indicator bacteria (MoH, 1995b). As a consequence of this change, E. coli analyses were added to the annual and monthly monitoring programmes in spring 1999. Faecal coliform analyses were retained in the monitoring programmes for the sake of continuity of the dataset, and allowed comparisons between the two types of indicators. For the purposes of assessing the microbial quality of the groundwater samples, the faecal coliform data has been used because of the small number of E. coli data.



Table 3.1 Maximum acceptable values for micro-organisms of health significance (MoH, 2000)

Micro-organism MAV Escherichia coli (E. coli) Note: If faecal, presumptive or total coliforms are measured, the counts are to be treated as though they were E. coli.

Less than 1 in 100 ml of sample

Table 3.2 Maximum acceptable values for inorganic determinands of health significance (MoH, 2000)

Determinand MAV Units Remarks

Arsenic (As) 0.01 mg/L For excess lifetime skin cancer risk of 6 x 10-4 PMAV, because of analytical difficulties

Boron (B) 1.4 mg/L Cadmium (Cr) 0.003 mg/L Chromium (Cr) 0.05 mg/L PMAV, limited information on health effects Copper (Cu) 2 mg/L ATO Fluoride (F) 1.5 mg/L Lead (Pb) 0.01 mg/L Manganese (Mn) 0.5 mg/L ATO Nickel (Ni) 0.02 mg/L Nitrate (NO3) or Nitrate-N (NO3N)

50 11.3

mg/L mg/L

The sum of the ratio of the concentrations of nitrate and nitrite to each of their respective MAVs should not exceed 1

Nitrite (NO2) or Nitrite-N (NO2N)

3 0.9

mg/L mg/L

The sum of the ratio of the concentrations of nitrate and nitrite to each of their respective MAVs should not exceed 1

PMAV – provisional MAV. ATO – Concentrations of the substance at or below the health-based guideline value may affect the appearance, taste or odour of the water. Table 3.3 Guideline values for aesthetic determinands (MoH, 2000) Determinand Guideline

Value Units Comments

Aluminium (Al) 0.15 mg/L Depositions, discoloration Ammonia (NH3) or Ammonia N (NH3N)

1.5 1.2

mg/L mg/L

Taste and odour

Chloride (Cl) 250 mg/L Taste, corrosion Copper (Cu) 1 mg/L Staining of laundry and sanitary ware (PMAV 2

mg/L) Hardness (total) (Ca + Mg)

200 mg/L High hardness causes scale deposition, scum formation; low hardness possibly causes corrosion

Iron (Fe) 0.2 mg/L Staining of laundry and sanitary ware Manganese (Mn) 0.05 mg/L Staining of laundry and sanitary ware (MAV 0.5

mg/L) pH 7.0-8.5 Should be between 7.0 and 8.0. Low pH:

aggressive water; high pH: taste, soapy feel. Preferably pH<8 for effective disinfection with chlorine

Sodium (Na) 200 mg/L Taste Sulphate (SO4) 250 mg/L Taste, corrosion Zinc (Zn) 3 mg/L Appearance, taste



Table 3.4 Summary of transgressions of the drinking-water standards maximum acceptable values for the Christchurch-West Melton groundwater system

The risk of bacterial contamination of groundwater reduces with increasing depth to groundwater. Furthermore, the presence of a relatively impermeable barrier overlying an aquifer greatly reduces the risk of contamination from downward migration of surface contaminants. It would be unlikely to find faecal or total coliform bacteria in the deep groundwater and for this reason very little microbial sampling has been undertaken of the deeper aquifers. Of the 21 samples collected from 9 deep aquifer wells (Aquifers 3, 4 and 5) no E. coli, faecal coliforms or total coliforms were detected (Table 3.4, Appendix 2). In contrast to the deep aquifers, groundwater from 29 Aquifer 1 wells and five Springston Aquifer wells had faecal coliforms detected on at least one sampling occasion (Table 3.4). Faecal coliform detections occurred mostly in groundwater from the unconfined zone (Figures 3.25 and 3.26). Groundwater samples from five Aquifer 2 wells contained

faecal coliforms. These wells were also located in the unconfined zone (Figure 3.27). The higher incidence of faecal coliform detections in the upper aquifers demonstrates the vulnerability of shallow groundwater to microbial contamination. Figure 3.28 shows the relationship between faecal coliform detections and the depth of the well. Groundwater from depths up to 55 m had occasional faecal coliform detections. However, most detections were in groundwater from wells less than 35 m deep. The detection of faecal coliforms in groundwater from any one well was generally infrequent and the concentrations were low. Given the sporadic nature of microbial contamination of groundwater, occasional or annual sampling is of limited value. More frequent sampling e.g., monthly, gives a better indication of the microbial quality of a groundwater supply. The monthly ion programme included nine wells in the

Manganese Nitrate-nitrogen

Arsenic Faecal coliformsdetected in

0.5 mg/L 11.3 mg/L 0.01 mg/L 100 ml sampleSpringston AquiferRange of well depths: 2.5 - 35.4 mPercent of samples transgressing health-based standards (number) 10% (14) 1% (5) 10.3 % (10) 2% (7)Percent of wells transgressing health-based standard (number) 2% (1) 7% (4) 4 % (1) 12% (5)

Total number of samples analysed 136 420 97 321Total number of wells sampled 43 56 24 43

Aquifer 1Range of well depths: 12 - 49.9 mPercent of samples transgressing health-based standards (number) 7% (35) 3% (55) 2.6 % (5) 9% (156)Percent of wells transgressing health-based standard (number) 4% (7) 4% (8) 3 % (2) 19% (29)


Aquifer 2Range of well depths: 33.5 - 103 mPercent of samples transgressing health-based standards (number) 0% (0) 3% (6) 0% (0) 16% (20)Percent of wells transgressing health-based standard (number) 0% (0) 3% (2) 0% (0) 16% (5)


Aquifer 3Range of well depths: 96.3 - 127.7 mPercent of samples transgressing health-based standards (number) 0% (0) 0% (0) 0% (0) 0% (0)Percent of wells transgressing health-based standard (number) 0% (0) 0% (0) 0% (0) 0% (0)


Aquifer 4Range of well depths: 103.6 - 157 mPercent of samples transgressing health-based standards (number) 0% (0) 0% (0) 0% (0) 0% (0)Percent of wells transgressing health-based standard (number) 0% (0) 0% (0) 0% (0) 0% (0)


Aquifer 5Range of well depths: 161.5 - 200.7 mPercent of samples transgressing health-based standards (number) 0% (0) 0% (0) 0% (0) 0% (0)Percent of wells transgressing health-based standard (number) 0% (0) 0% (0) 0% (0) 0% (0)


Health-based maximum acceptable values



Christchurch-West Melton area. Faecal coliforms were detected in at least one sample from eight of the nine wells (Figures 3.29, 3.30 and 3.31). These wells were all located in the unconfined zone or at the confining boundary margin. In the case of four of the wells, faecal coliforms were detected on only one or two occasions over a period of several years and at low concentrations. Faecal coliforms and total coliforms were detected frequently in groundwater from four of the wells sampled monthly (Figures 3.30 and 3.31). The source of contamination was not clear with no obvious sources nearby. The depths of the four wells (25 – 40 m) would generally indicate a low risk of microbial contamination of the groundwater. Water level measurements for wells M36/4151 and M36/5248 ranged from -12 to -28 m and -15 to -20 m respectively. Water level records were not available for the other two wells, but it is expected that they would be similar to those of wells M36/5248 and M36/4151. No clear relationship was observed with the water levels and the incidence of faecal or total coliform contamination of the groundwater samples. However, faecal coliform detections in groundwater from two of the wells (M36/4151 and M36/4655) tended to occur more frequently in summer and autumn. At times of high pumping rates, a localised drawdown effect can occur which may result in contaminants near the top of the water table being drawn into the abstraction zone of the well. This may explain the occurrence of faecal coliform contamination of groundwater during summer and autumn in samples from these wells. Faecal coliforms were detected more frequently in groundwater from well M36/5248 during autumn and winter. This is likely to occur after periods of heavy rainfall when the micro-organisms are able to survive and move more readily to the water table. The state of the well-head of all four wells may have been contributing factors to the microbial contamination. Two wells lacked secure well caps and none of the four wells had a sealed base around the well casing. Therefore, there was a potential risk of contaminants entering the groundwater directly via the well. Sources of microbial contamination may include waste disposal to land, sewage

disposal to the subsurface via septic tanks, and faecal material produced by grazing animals. Well M36/4151 is located down gradient of a piggery effluent disposal area, which may be contributing faecal contamination to the groundwater. Well M35/1051 is adjacent to a gravel extraction operation and where several large excavation pits are present. The exposure of the subsurface by excavation may have increased the risk of contaminants entering the groundwater. 3.2.1.2 Nitrate Nitrate-nitrogen concentrations in groundwater from the deeper aquifers (Aquifers 3, 4 and 5) were consistently low and did not transgress the drinking-water standards MAV (Table 3.4). The maximum NO3N concentration found in groundwater from these aquifers was 1.1 mg/L, while 75% of samples had concentrations of 0.5 mg/L or less (Appendix 2). These low concentrations are largely because the Waimakariri River is the dominant source of recharge and because of the age of the deep groundwater, which was recharged prior to any large-scale agricultural and urban developments. The loss of nitrate through nitrate reduction is unlikely to be significant because of the generally well oxidised state of the confined groundwater system including in the deep aquifers. While there has been only limited sampling of the deeper aquifers, these results indicate that nitrate does not occur in the deep aquifers at concentrations of concern at present. Groundwater from two Aquifer 2 wells (3%) contained nitrate-nitrogen concentrations above the MAV on at least one occasion. Both wells were located on the same property in the unconfined zone of the aquifer (Figure 3.21). However, in general the nitrate-nitrogen concentrations in Aquifer 2 groundwater were low, with a median concentration of 0.8 mg/L compared with 5.1 mg/L for Aquifer 1 (Appendix 2). Within the confined zone of Aquifer 2, the nitrate-nitrogen concentrations tended to be less than 1 mg/L (Figure 3.21). Groundwater from eight Aquifer 1 wells (4%) had concentrations of nitrate-nitrogen exceeding the MAV on at least one sampling occasion (Table 3.4). All eight wells were located in the southern part of the Christchurch-West Melton area (Figure 3.20).



Groundwater from four Springston aquifer wells transgressed the MAV for NO3N on at least one occasion. Figures 3.32 and 3.33 show the nitrate-nitrogen concentrations in groundwater samples from 13 of the wells where transgressions occurred. Five of the wells that yielded groundwater with nitrate-nitrogen concentrations above the MAV were located in the Burnham area. This included the two Aquifer 2 wells. Four of the Burnham wells had been sampled quarterly from June 1989 to June 1994 as part of a study on the effects of land disposal of piggery effluent (Casey and Cameron, 1995). The fifth well at Burnham was included in the monthly ion survey in September 1991. Groundwater samples from wells M36/3670, M36/0017 and M36/4044 had NO3N concentrations above the MAV on several occasions between 1986 and 1994. Nitrate-nitrogen concentrations in groundwater from well M36/0058 exceeded the MAV only once, in September 1992. Groundwater from the monthly monitoring well M36/4151 had NO3N concentrations above the MAV in August and September 1999 following a very heavy rainfall event in July 1999. Casey and Cameron (1995) suggested that the unusually high nitrate-nitrogen results in samples collected in September 1992 from the Aquifer 2 wells M36/0017 and M36/0058 were probably the result of sample contamination (Figure 3.33). They suggested that the quality of this deep groundwater was unlikely to be affected by fluctuations in groundwater levels, such as those caused by a very wet winter in 1992 including substantial snow fall in August. However, groundwater from well M36/0017 has previously had NO3N concentrations up to 29.5 mg/L. Also, given that groundwater from three other wells in this area showed large increases in NO3N concentrations in September 1992, it is likely that the high NO3N concentrations in samples from the Aquifer 2 wells reflected groundwater quality. Groundwater samples taken from well M36/4044 in September 1992 also had elevated NO3N concentrations although not as high as in some previous samples collected from that well. The mechanism by which the deeper groundwater at Burnham could have been affected by the large recharge event in September 1992 may have been by direct movement of surface run-off down the well

casing. This is a common pathway for shallow contaminated water to enter deeper groundwater systems. This occurs if a well has been poorly constructed with inadequate backfilling around the outside of the casing or there is poor protection of the well head. The degree of contamination of the deeper groundwater will reflect the volume of water moving down the well and the level of contamination from land use activities. This may explain the very large increase in NO3N concentrations in groundwater from wells M36/0058 and M36/0017 following a very wet period. Both wells were located on the site of the piggery operation where the pigs were housed and the effluent was screened with occasional problems of effluent overflowing onto the soil were reported (Casey and Cameron, 1995). In the Islington area, groundwater from two wells (M35/1839 and M35/1103) exceeded the MAV for NO3N (Figures 3.20 and 3.32). These wells were included in a study of the effects of the Waitaki NZ Refrigeration company’s discharge to land of meat processing effluent. An earlier investigation in 1981 had shown that land discharges of the main works effluent, oxidation pond effluent and pelt processing effluent were adversely affecting the quality of the unconfined groundwater immediately downgradient of the effluent disposal areas (Curtis, 1981). Following this investigation, another survey was established in February 1986. This programme continued until the freezing works plant was sold in 1988 and the effluent disposal to land ceased. Well M35/1839 was located approximately 600 m downgradient of the area where the main works effluent was flood irrigated to land. Well M35/1103 was located about 200 m downgradient of the area where the oxidation pond water was disposed to land by border dyke irrigation. Data from the 1986 to 1988 survey continued to show elevated NO3N concentrations in groundwater downgradient of the effluent disposal areas, especially from the main works effluent (Figure 3.32). Unfortunately, none of the downgradient wells were monitored after land disposal of the effluent was discontinued, so subsequent changes to the groundwater quality were not observed. The nearest downgradient well to the Islington area with more recent NO3N data is M35/1883 (29 m deep), located about 3 km downgradient



of the main effluent disposal area (Figure 3.20). Groundwater from this well has been sampled annually since June 1989. The NO3N concentrations for this well have shown a decrease from 10 mg/L in 1989 to about 6 mg/L in recent years, except for the sample collected in December 1992, which was 13 mg/L (Figure 3.33). It is possible that the elevated NO3N concentrations observed in groundwater from M35/1883 were from the discharge of the freezing works effluent and that the trend of generally decreasing nitrate concentrations is a result of the cessation of the effluent disposal. Downgradient of M35/1883, groundwater samples from two wells (M35/3170 and M36/1099) exceeded the MAV for NO3N during 1989 but not in subsequent samples (Figures 3.32 and 3.33). These wells are located over 6 km downgradient from the Islington freezing works (Figure 3.20). While the freezing works operations may have contributed to the elevated NO3N concentrations in groundwater from these wells, it is more likely sources closer to the wells existed. Nitrate-nitrogen concentrations above the MAV have occurred once in groundwater samples from each of three wells located in the southern part of the unconfined zone around Prebbleton and Templeton. In all these cases, the elevated NO3N concentrations were preceded by significant recharge events from the wet winter in 1992 or July 1999. The only well within the confined aquifer zone which yielded groundwater with NO3N above the MAV was M36/1244 (46 m deep) located in the Heathcote valley (Figure 3.20). This well has been sampled only once, in May 1994, and yielded groundwater with 19 mg/L NO3N. The reason for sampling the well is unknown. The source of nitrates is not known although fertilisers used in horticultural activities are a potential source in this area. Overall, NO3N concentrations exceeding the MAV have been detected in the groundwater from a small number of wells abstracting from the unconfined aquifers, and from one well in the confined zone of Aquifer 1. In many instances the transgressions occurred following large recharge events. In other cases, elevated NO3N concentration in groundwater occurred as a result of localised land uses, mostly from land disposal of animal wastes.

3.2.1.3 Arsenic Arsenic was included in the 1999 annual ion survey and the 2000 saltwater intrusion monitoring programme. In the Christchurch-West Melton area groundwater samples were also analysed for arsenic on some occasions in areas where localised contamination may have occurred. The groundwater quality data provided by the Christchurch City Council also included arsenic analyses. Altogether, groundwater samples from 149 wells in the Christchurch-West Melton area were analysed for arsenic. Of these, 44 yielded groundwater with arsenic concentrations above the detection limits (Appendix 2). Three of these wells yielded groundwater with arsenic concentrations above the MAV (Table 3.4). Figure 3.34 shows the maximum arsenic concentrations detected in groundwater samples. Intensive sampling of groundwater in the Johns Road area was undertaken in the early 1990s following the detection of arsenic contaminated soil on land that had historically received stormwater discharge from an adjacent timber treatment site. The sampling revealed localised contamination of the shallow groundwater in the vicinity of the timber treatment property and the adjacent property where the stormwater had been discharged (Figure 3.35) (Woodward-Clyde, 1998). Groundwater from two wells contained concentrations of arsenic above the MAV. One well was located on the timber treatment site, while the other was located on an adjacent property, downgradient of the stormwater discharge area. Site management aimed at reducing further leaching of arsenic from the site included changes to the timber treatment site operations and the stormwater discharge, and removal of some of the contaminated soil (Woodward-Clyde, 1998). Arsenic concentrations in the most recent samples from the two wells were below the MAV. In the wider Christchurch-West Melton area, arsenic was detected in groundwater from a number of wells (Figure 3.34). In most cases the concentrations of arsenic were very low. The occurrence of moderate to high concentrations of arsenic in the Woolston-Heathcote and Brighton spit areas were usually associated with elevated concentrations of iron, manganese and silica.



The source of arsenic is probably derived naturally from arsenic enriched sediments, which under reducing conditions results in mobilisation of the arsenic from the sediment into a water-soluble form. These conditions are similar to the reducing conditions in other coastal parts of Canterbury where naturally occurring arsenic has been detected in groundwater (ECan, 2002a, PDP, 2001). In general, the risk of groundwater containing concentrations of arsenic above health standards appears low, except where nearby sources such as timber treatment operations, landfills or sheep dips occur. Naturally occurring arsenic can occur at concentrations above the MAV and is associated with reduced groundwater conditions. 3.2.1.4 Manganese Groundwater from one Springston Aquifer well and seven Aquifer 1 wells had concentrations of manganese above the health-based MAV of 0.5 mg/L (Table 3.4). Three of the wells were located downgradient of disused landfills, and two were located in the Woolston-Heathcote area where contamination of the groundwater from landfill leachate and sea water has been identified (Figure 3.37). However, groundwater in parts of this area is also naturally reduced, with low oxygen concentrations resulting from peat or swamp deposits in the sediments. The detection of manganese above the aesthetic guideline value is discussed further in Section 3.2.2. 3.2.1.5 Trace constituents Trace metals and constituents are not routinely included in groundwater monitoring programmes. However, on some occasions groundwater samples have been analysed for trace metals. In some instances an intensive sampling programme has been undertaken where localised contamination has been detected, such as in the Johns Road area (see Section 3.2.1.3). However, most of the data for trace determinands for the Christchurch-West Melton area are from the monitoring of water supply wells by the Christchurch City Council. Table 3.5 summarises the number of wells sampled and the detections of a variety of trace determinands. The Springston aquifer well, M35/1463, which yielded chromium at

concentrations above the MAV was located on Johns Road. It was sampled on several occasions between 1990 and 1997. Two out of 14 samples collected from this well between 1990 and 1992 contained chromium at concentrations above the MAV. Groundwater from this well also contained elevated concentrations of arsenic. The contamination of groundwater from this well with arsenic and chromium resulted from past discharges from a timber treatment operation (see Section 3.2.1.3). The extent of the contamination appears to be limited to within a radius of 1 km from the property. The concentrations of contaminants have decreased over time; samples collected from M35/1463 between 1993 to 1997 show the concentrations of chromium to be below the MAV. Cadmium at a concentration above the MAV has been detected in an Aquifer 1 well (M36/3087) located at the confining boundary margin in Wigram (Table 3.5). The well was located close to a closed landfill, which is the most likely source of the cadmium. The detection of cadmium in groundwater from an Aquifer 4 well was unusual given its depth (Table 3.5). A follow-up sample from this well was negative, indicating that sample contamination or analytical errors were the likely causes of the detection. Groundwater from a small number of wells contained concentrations of lead, aluminium or zinc above the MAV (Table 3.5). There were no obvious sources of these contaminants. These detections probably resulted from poor well head protection or a localised contamination source. Overall, while the data are limited, they show that trace elements are rarely detected in the groundwater and in most cases where detected, concentrations are most likely to be well below relevant drinking-water standards.

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Figure 3.25 Faecal coliform detections in groundwater from Springston aquifer wells

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Figure 3.26 Faecal coliform detections in groundwater from Aquifer 1 wells

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Figure 3.27 Faecal coliform detections in groundwater from Aquifer 2 wells

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Figure 3.28 Relationship between faecal coliform detections and well depths

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Figure 3.29 Concentrations of faecal coliforms, total coliforms and E. coli in groundwater sampled monthly from wells M36/0279, M36/0271 and M35/1003

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Figure 3.32 Nitrate-nitrogen concentrations exceeding the MAV in groundwater samples

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Figure 3.33 Nitrate-nitrogen concentrations exceeding the MAV in groundwater samples from the Springston aquifer and Aquifer 2

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Figure 3.34 Maximum concentrations of arsenic detected in groundwater in the Christchurch – West Melton area

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Figure 3.35 Maximum concentrations of arsenic detected in groundwater in the Johns Road area



Table 3.5 Summary of detections of trace constituents in the Christchurch-West Melton groundwater

Trace element Health-based Aesthetic GVMAV Springston Aquifer 1 Aquifer 2 Aquifer 3 Aquifer 4 Aquifer 5

(mg/L) (mg/L)Aluminium 0.15 Number of wells sampled 0 15 11 2 4 2(Al) Number of detections 3 1 1 0

Maximum concentration (mg/L) 0.63 4.4No. wells with detections above the guideline value 1 1

Antimony 0.003 Number of wells sampled 0 0 1 0 0 0(Sb) Number of detections 0

Maximum concentration (mg/L)No. wells with detections above the MAV

Boron 1.4 Number of wells sampled 19 63 28 7 19 6(B) Number of detections 10 40 8 0 5 0

Maximum concentration (mg/L) 0.17 0.3 0.12 0.1No. wells with detections above the MAV

Cadmium 0.003 Number of wells sampled 4 26 17 5 19 6(Cd) Number of detections 0 1 1 1 2 0

Maximum concentration (mg/L) 0.005 0.0024 0.0008 0.005No. wells with detections above the MAV 1 1

Chromium 0.05 Number of wells sampled 15 54 19 5 19 6(Cr) Number of detections 1 5 0 0 0 2

Maximum concentration (mg/L) 0.08 0.05 0.03No. wells with detections above the MAV 1

Cobolt Number of wells sampled 0 0 1 0 1 1(Co) Number of detections 0 0 0

Maximum concentration (mg/L)

Copper 2 1 Number of wells sampled 17 49 17 5 19 6(Cu) Number of detections 5 16 4 0 3 4

Maximum concentration (mg/L) 0.13 0.28 0.05 0.01 0.002No. wells with detections above the guideline value

Fluoride 1.5 Number of wells sampled 5 43 24 5 17 7(F) Number of detections 1 14 5 0 4 3

Maximum concentration (mg/L) 1 0.6 0.14 0.2 0.11No. wells with detections above the MAV

Lead 0.01 Number of wells sampled 6 31 16 5 20 6(Pb) Number of detections 1 6 3 1 3 2

Maximum concentration (mg/L) 0.021 0.015 0.0017 0.001 0.005 0.0006No. wells with detections above the MAV 1 1

Lithium 0.9 Number of wells sampled 2 28 17 3 5 1(Li) Number of detections 0 20 9 1 4 0

Maximum concentration (mg/L) 0.096 0.02 0.01 0.01No. wells with detections above the MAV

Molybdenum 0.07 Number of wells sampled 0 0 1 0 1 1(Mo) Number of detections 0 0 0


Nickel 0.02 Number of wells sampled 2 27 17 5 19 6(Ni) Number of detections 0 4 2 0 4 0

Maximum concentration (mg/L) 0.0011 0.002 0.002No. wells with detections above the MAV

Selenium 0.01 Number of wells sampled 2 23 16 5 18 6(Se) Number of detections 0 2 4 2 5 2

Maximum concentration (mg/L) 0.001 0.002 0.002 0.002 0.002No. wells with detections above the MAV

Tin 1 Number of wells sampled 0 0 1 0 1 0(Sn) Number of detections 0 0


Zinc 3 Number of wells sampled 2 28 18 5 19 6(Zn) Number of detections 0 19 10 4 11 5

Maximum concentration (mg/L) 0.093 4.4 0.03 0.61 0.04No. wells with detections above the guideline value 1

Mercury 0.002 Number of wells sampled 0 1 3 0 1 0(Hg) Number of detections 0 0 0


Aquifer



3.2.2 Aesthetic determinands The number of samples and wells from each of the aquifers, which have concentrations of determinands above the aesthetic-based drinking-water guidelines are summarised in Table 3.6. 3.2.2.1 pH Groundwater from the Springston aquifer and Aquifer 1 tended to have lower pH values than the deeper groundwater (Figure 3.1). Groundwater from 84% of Springston Aquifer and 65% of Aquifer 1 wells sampled had pH values below 7 on at least one occasion (Table 3.6). Low pH water occurred mostly in the unconfined groundwater zones. However, in the southern part of the confined zone of Aquifer 1, median pH values of groundwater from a number of wells were below 7. Low pH values are common in shallow groundwater because of the acidic nature of water passing through the soil zone (Appendix 3). Groundwater from a small number of Aquifer 1 wells (3%) had pH values above 8.5. The reason for the higher pH values in these samples was not obvious and may relate to localised conditions in the aquifer or may result from inadequate purging of stagnant water in the well prior to collecting the sample. In some cases, local sources of contamination may result in high pH of groundwater. Groundwater from the deeper aquifers tended to have pH values within the guideline values of 7 – 8.5, with only 13% of Aquifer 2 wells yielding groundwater with pH values below 7. Of the deeper aquifers, only 1 well, M35/6791, abstracting from Aquifer 5, yielded groundwater with low pH values. Groundwater from some of the deeper aquifers occasionally had pH values above 8.5 (Table 3.6). 3.2.2.2 Iron and manganese Groundwater from a number of wells in each of the aquifers had concentrations of iron above the guideline value of 0.2 mg/L on some sampling occasions (Table 3.6, Figures 3.36, 3.38 and 3.40). The high incidence of iron in groundwater samples may in part be a result of the type of sample collected. In most cases, samples were not filtered in the field prior to analysis for iron and manganese. Therefore,

the presence of iron in some groundwater samples could have come from corrosion of steel casings. Where both iron and manganese are detected in samples and low concentrations of NO3N and SO4 occur, then the iron will, at least in part, be derived from soluble iron within the aquifer. A small number of wells from each aquifer yielded groundwater with manganese concentrations above the aesthetic guideline value 0.05 mg/L. In most of these cases, the manganese is likely to originate from natural peat deposits in the aquifer. These occur more commonly in coastal parts of Canterbury, but can occur inland as well. High concentrations of iron and manganese have occurred in the Brighton and Woolston-Heathcote area. These are examples of areas of reduced groundwater, resulting from buried peat or swamp deposits. Very high iron and manganese concentrations in groundwater may also be associated with discharges from landfills or cleanfills. For example, groundwater from some wells in the Wigram area downgradient of a closed landfill exhibits high concentrations of iron and manganese. 3.2.2.3 Chloride and sulphate Aesthetic guidelines for drinking-water are specified to avoid unpleasant tastes and corrosion problems associated with high chloride and sulphate concentrations (MoH, 1995a). Groundwater from five Aquifer 1 wells had chloride concentrations above the guideline value of 250 mg/L. All five wells were located in the Woolston-Heathcote area, where the elevated chloride concentrations in groundwater are primarily the result of the intrusion of estuarine water (see Section 3.5.1). No groundwater samples collected within the Christchurch-West Melton area have exceeded the guideline value for sulphate of 250 mg/L. Sulphate concentrations were generally well below this value, but elevated concentrations occurred in some industrial areas and where saltwater intrusion has occurred. The general spatial variations and sources of chloride and sulphate in groundwater are discussed in Section 3.1.



Table 3.6 Transgressions of the Drinking-Water Standards 2000 aesthetic-based guideline

value

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Sulphate Chloride Total Hardness

Iron Manganese

<7 pH >8.5 pH 1.3 mg/L 250 mg/L 250 mg/L 200 mg/L 0.2 mg/L 0.05 mg/LSpringston AquiferRange of well depths: 2.5 - 35.4 mPercent of samples transgressing aesthetic guidelines (number) 78% (368) 0% (0) 6% (8) 0% (0) 0% (0) 5% (6) 32% (43) 13% (18)Percent of wells transgressing of aesthetic guidelines (number) 84% (54) 0% (0) 2% (1) 0% (0) 0% (0) 3% (1) 40% (17) 12% (5)

Total number of samples analysed 470 470 144 403 409 110 136 136Total number of wells sampled 64 64 44 53 54 39 43 43

Aquifer 1Range of well depths: 12 - 49.9 mPercent of samples transgressing aesthetic guidelines (number) 56% (1204) 0% (10) 2% (16) 0% (3) 3% (53) 8% (42) 25% (137) 16% (84)Percent of wells transgressing of aesthetic guidelines (number) 65% (154) 3% (7) 3% (5) 1% (2) 2% (5) 8% (12) 36% (58) 19% (31)


Aquifer 2Range of well depths: 33.5 - 103 mPercent of samples transgressing aesthetic guidelines (number) 5% (11) 3% (6) 0% (0) 0% (0) 0% (0) 0% (0) 13% (15) 5% (6)Percent of wells transgressing of aesthetic guidelines (number) 13% (9) 4% (3) 0% (0) 0% (0) 0% (0) 0% (0) 23% (10) 12% (5)


Aquifer 3Range of well depths: 96.3 - 127.7 mPercent of samples transgressing aesthetic guidelines (number) 0% (0) 0% (0) 0% (0) 0% (0) 0% (0) 0% (0) 10% (1) 10% (1)Percent of wells transgressing of aesthetic guidelines (number) 0% (0) 0% (0) 0% (0) 0% (0) 0% (0) 0% (0) 10% (1) 10% (1)


Aquifer 4Range of well depths: 103.6 - 157 mPercent of samples transgressing aesthetic guidelines (number) 0% (0) 3% (2) 0% (0) 0% (0) 0% (0) 0% (0) 18% (11) 5% (3)Percent of wells transgressing of aesthetic guidelines (number) 0% (0) 8% (2) 0% (0) 0% (0) 0% (0) 0% (0) 30% (7) 13% (3)


Aquifer 5Range of well depths: 161.5 - 200.7 mPercent of samples transgressing aesthetic guidelines (number) 6% (3) 0% (0) 0% (0) 0% (0) 0% (0) 0% (0) 2% (1) 2% (1)Percent of wells transgressing of aesthetic guidelines (number) 14% (1) 0% (0) 0% (0) 0% (0) 0% (0) 0% (0) 14% (1) 14% (1)


Aesthetic-based guideline valuespH



3.2.2.4 Hardness (Total) In general, groundwater in the Christchurch-West Melton area was in the soft to moderately hard range (Figures 3.42 and 3.43, Appendix 3). Groundwater from the deeper aquifers was soft with hardness values all below 55 mg/L CaCO3 (Appendix 2). This reflects the low ionic content of the deep groundwater. Groundwater from Aquifer 2 also tended to be soft with 95% of the samples having hardness values less than 75 mg/L CaCO3 and a maximum hardness value of 134 mg/L CaCO3 (Figure 3.43). Groundwater from the Springston aquifer and Aquifer 1 also tended to be soft to moderately hard, except in some localised areas where hardness values above the guideline value were found (Figure 3.42). The hardness values for these shallower aquifers tended to increase southwards, reflecting the increasing significance of rainfall-derived infiltration, which transports soluble ions including calcium and magnesium into the groundwater. Groundwater with hardness values above the aesthetic guideline value occurred in three areas of Christchurch. In the Wigram area it is likely that the elevated hardness results from leachate from old closed landfills and current or closed cleanfills (landfills which only accept inert material). In the Hillmorton area, contamination of the groundwater from past industrial activities is the most likely source of the elevated hardness (see Section 3.5). Saltwater contamination of the groundwater in the Woolston-Heathcote area is the cause of high hardness values in groundwater from a number of wells in this area (see Section 3.5.1). The highest hardness value occurred in groundwater from well M36/1159, which had median hardness values ranging from 735 to 2400 mg/L. Groundwater from this well contained the highest proportion of sea water, with chloride concentrations ranging from 780 to 2200 mg/L. However, this does not explain the high hardness value of 280 mg/L for groundwater from well M36/1244, which is located in the Heathcote Valley but is not likely to be affected by saltwater contamination. Groundwater from this well also contained elevated concentrations of a number of ions including nitrates, indicating groundwater contamination is occurring from local land uses. Volcanic-derived groundwater may also be contributing to the elevated hardness value for this well.

3.2.2.5 Ammonia-nitrogen Groundwater from one Springston aquifer well and five Aquifer 1 wells contained concentrations of ammonia-nitrogen above the guideline value of 1.3 mg/L (Table 3.6). Four of the wells are located adjacent to, or downgradient of, disused landfills or cleanfills. Ammonia-nitrogen concentrations as high as 32 mg/L have been found in shallow groundwater immediately downgradient of an old landfill. Groundwater from two coastal wells (M36/5893 and M36/2539) contained concentrations of ammonia-nitrogen at around 4 mg/L. Hertel (1998) concluded that for one of these wells, the groundwater was being contaminated by landfill leachate leaking down the well casing from an adjacent disused landfill. However, it is unlikely that groundwater from both wells is affected by landfill leachate migrating down well casings. Also given the depths of these wells (49 and 55 m) and the thickness of overlying confining sediments (at least 40 m), it is unlikely that land-based contamination has affected the quality of this groundwater. The elevated concentrations of ammonia-nitrogen (and other ions) are probably naturally derived. Rosen (2001) reports elevated concentrations of ammonia-nitrogen in deep old confined aquifers in the Gisborne region ranging from 3.9 to 5.7 mg/L are likely to be due to ammonification of organic matter trapped in pockets of organic-rich sediments. The extremely reduced conditions of groundwater from M36/5893 (see Section 3.1.4) supports the suggestion of organically rich sediments influencing the groundwater quality. This is further supported by the bore log data for this well, which showed up to 15 m of overlying peat and blue pug.

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Figure 3.36 Median iron concentrations for all samples collected from Springston aquifer and Aquifer 1 wells

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Springston aquifer and Aquifer 1 wells - median Mn concentrations (mg/L)# 0.001 - 0.05#S 0.051 - 0.5#S 0.51 - 3.75


Figure 3.37 Median manganese concentrations for all samples collected from Springston aquifer and Aquifer 1 wells

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Aquifer 2 wells - median Fe concentrations (mg/L)# 0.003 - 0.1# 0.11 - 0.2#S 0.21 - 1#S 1.1 - 3.2


Figure 3.38 Median iron concentrations for all samples collected from Aquifer 2 wells

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Figure 3.39 Median manganese concentrations for all samples collected from Aquifer 2 wells

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Aquifers 3, 4, and 5 wells - median Fe concentrations (mg/L)# 0.003 - 0.1# 0.11 - 0.2#S 0.21 - 1#S 1.1 - 2.7


Figure 3.40 Median iron concentrations for all samples collected from Aquifer 3, 4 and 5 wells

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Figure 3.41 Median manganese concentrations for all samples collected from Aquifer 3, 4 and 5 wells

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Figure 3.42 Median hardness values for all samples collected from Springston aquifer and Aquifer 1 wells

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Figure 3.43 Median hardness values for all samples collected from Aquifer 2 wells



3.3 Seasonal variation in water quality

Seasonal variation in groundwater quality is primarily driven by variations in recharge and, to a lesser degree, by changing land uses throughout the year. Variations in recharge influence groundwater quality through flushing of soluble substances by infiltration or rising water tables causing increases in constituent concentrations, dilution of groundwater by infiltration of water with lower constituent concentrations, and the movement of groundwater of different quality in response to changes in hydraulic head (Whittemore et al., 1989). For the Christchurch-West Melton groundwater system, recharge from the Waimakariri River occurs at a relatively constant rate throughout the year. The quality of Waimakariri River water is also relatively stable4. However, rainfall-derived recharge occurs mainly during the winter and spring months, when rainfall exceeds evapotranspiration, resulting in seasonal variations in water levels. Monthly rainfall and calculated recharge for the period February 1986 to June 1999 and daily groundwater level measurements for the unconfined zone are shown in Figure 3.44 (D. Scott, Environment Canterbury pers comm.). Groundwater levels in the unconfined zone generally showed an immediate response to recharge events. The magnitude of variation in water levels was greatest in the westernmost part of the unconfined zone. Little (1997) calculated that variations in water levels up to 30 m occurred at the western boundary of the Christchurch-West Melton area, decreasing eastwards to a maximum variation of about 5 m at the boundary of the confining zone. Monthly sampling of groundwater was undertaken to determine seasonal variations in water quality (Table 1.1). Monthly data are available for nine wells in the Christchurch-West Melton area. The length of period of data varies for each well, owing to changes to the programme and difficulties accessing some wells. The wells were located in the

4 The average annual variation in conductivity for the Waimakariri River near SH1 is about ±1 mS/m. This was calculated from 11 years of monthly data collected by the National Institute of Water and Atmospheric Research (NIWA).

unconfined zone of the study area, where seasonal variations in recharge were most notable (Figure 3.45). Quarterly data were also collected from two wells as part of the GNS survey (Table 1.1). The monthly and quarterly data for these wells is shown in Figures 3.46, 3.47 and 3.48 and Appendix 6. Statistical analysis of the monthly and quarterly data was undertaken to test for seasonality and to determine whether variations in water quality data related to variations in recharge as indicated by changes in water levels. The Kruskall-Wallis H test for seasonality was performed on the data using four seasons as described in Section 2.3 (Table 3.7). A Spearman Rank correlation analysis was performed on the individual chemical determinands and water levels measured in the wells (Table 3.8). Water level measurements from nearby water level monitoring wells were used for six of the wells where measurements could not be made at the well. Neither correlation analysis nor the seasonality test were performed on the microbial data as the predominance of non-detect data made it unsuitable for these types of analyses. Section 3.2.1 discusses the microbial results in detail. Large seasonal variations in determinand concentrations were observed in data from some wells (e.g., M35/1003 and M35/1051, Figure 3.46) while very little variation was observed in others (Figure 3.48, Appendix 6). Data from the Aquifer 5 well (M35/6791, 200 m) showed little variation in determinand concentrations (Figure 3.48). This is expected given that deep groundwater tends to be well mixed and is unlikely to show short-term variations in quality. Data from M35/1382 (31 m) also showed relatively minor variations in determinand concentrations (Figure 3.48). This well is located in the northern part of the unconfined zone where groundwater is primarily recharged from the Waimakariri River. The relatively stable quantity and quality of river recharge is reflected in the relatively stable quality of the groundwater. However, while the variations in water quality were small, water level measurements from a nearby well were positively correlated with variations in conductivity, pH and concentrations of Cl and NO3N for this well (Table 3.8).



Data from three of the nine wells showed statistically significant seasonal patterns in conductivity and in concentrations of NO3N and Cl (Table 3.7). Data from one of these wells also showed seasonality in concentrations of SO4. For wells M35/1003 and M35/1051, conductivity and anion concentrations tended to be highest during the months September to October and lowest during the autumn and winter months (Appendix 7). Water levels, which were also highest during spring and lowest during autumn and winter, showed strong positive correlations with conductivity and anion concentrations for these wells (Table 3.8). This indicates a relatively rapid response of increased in anion concentrations to recharge events. Conductivity and concentrations of SO4, Cl and NO3N for M36/0271 also showed a significant seasonal pattern (Table 3.8). However, unlike the wells above, anion concentrations for M36/0271 tended to peak during November to January, while water levels in this well were highest during August to October (Appendix 7). This suggests a delayed response of increased anion concentrations to seasonal recharge. No statistically significant seasonal patterns were found in the data from the other 8 wells, although some of the wells showed large variations in water quality. This may indicate that the variations in groundwater quality were not solely related to seasonal variations in recharge. Other factors such as land use may also influence the quality of the groundwater from these wells. For example, well M36/4151 is located downgradient of an area where piggery effluent is spread, which may be affecting the quality of groundwater from this well. Variations in groundwater quality may in part relate to timing of effluent spreading. Water level measurements correlated with conductivity for many wells but not always with other determinands. pH values tended to correlate negatively with water levels. This occurs because CO2 is flushed from the soil zone into groundwater with infiltrating water, or because rising water tables intercept CO2 released from the soil zone. The CO2 reacts with water to form a weak acid, which lowers the pH value.

Data for most of the wells showed positive correlation of conductivity with anion concentrations, especially Cl and NO3N (Table 3.8). Concentrations of SO4 did not always correlate with conductivity and in the case of well M36/5248 a negative correlation between SO4 concentrations and conductivity occurred. Groundwater from two wells showed a negative correlation of water levels with conductivity values. Groundwater from the Aquifer 1 well, M36/1059, showed strong negative correlations between water levels and conductivity, NO3N and Cl values and positive correlation with pH values (Table 3.8). Peak conductivity values and anion concentrations for this well tended to occur in autumn coinciding with the seasonally low water levels (Figure 3.47, Appendix 7). The well is located in the Hillmorton area where groundwater from the upper aquifers has historically contained elevated concentrations of a number of determinands and concentrations of sulphate and chloride remain above background concentrations. Water pressures in Aquifer 1 and the deeper aquifers show similar patterns of higher pressures or water levels during late winter and spring and lower water pressures during summer and autumn. Therefore, the low anion concentrations found in Aquifer 1 groundwater during periods of high water levels probably results from a dilution effect from the upwards flow of deeper groundwater into Aquifer 1. During periods of lower water levels the relative contribution of the deeper groundwater is less and consequently there is less dilution occurring. In general, variations in groundwater quality were related to rainfall-derived recharge as indicated by water levels for many of the wells in the unconfined zone. Annual variations of up to ±3.5 mS/m for conductivity, ±3.5 mg/L of chloride and ±3 mg/L of nitrate-nitrogen were observed. However, the magnitude of variation in water quality differed for each well. While in general, recharge events resulted in higher anion concentrations and lower pH values, data from two wells showed negative correlations of water levels with anion concentrations.



Table 3.7 Summary of Kruskall-Wallis test for seasonality for determinands collected quarterly (alpha = 0.05)

n = not statistically significant at alpha = 0.05

Well Conductivity pH Sulphate Chloride Nitrate-nitrogenM35/1382 n n n n nM35/6791 n n n n nM36/0279 n n n n nM36/4227 n n n n nM35/1003 seasonality n n seasonality seasonalityM35/1051 seasonality n n seasonality seasonalityM36/0271 seasonality n seasonality seasonality seasonalityM36/1059 n n n n nM36/4151 n n n n nM36/4655 n n n n nM36/5248 n n n n n



Table 3.8 Spearman rank correlation analysis of monthly and quarterly data

M35/1003 n n n n nConductivity 0.83 ** 83pH -0.36 * 82 -0.44 ** 146Sulphate 0.55 ** 82 0.61 ** 145 -0.36 ** 144Chloride 0.82 ** 83 0.88 ** 147 -0.45 ** 146 0.60 ** 145Nitrate-nitrogen 0.81 ** 83 0.96 ** 147 -0.46 ** 146 0.62 ** 145 0.92 ** 147M35/1051 (water level data from M35/1079)Conductivity 0.68 ** 101pH -0.03 101 -0.28 ** 186Sulphate 0.39 ** 101 0.10 186 0.24 * 186Chloride 0.71 ** 101 0.86 ** 186 -0.15 * 186 0.17 * 186Nitrate-nitrogen 0.73 ** 101 0.85 ** 186 -0.12 186 0.20 * 186 0.89 ** 186M36/0271Conductivity 0.15 120pH 0.22 * 119 -0.16 147Sulphate -0.17 119 0.48 ** 146 -0.18 * 145Chloride 0.29 * 120 0.74 ** 148 -0.07 147 0.43 ** 146Nitrate-nitrogen 0.06 120 0.88 ** 148 -0.23 * 147 0.47 ** 146 0.76 ** 148M36/0279 (water level data from M36/0183)Conductivity 0.29 * 81pH 0.21 80 0.13 85Sulphate -0.22 * 80 0.47 ** 84 0.09 83Chloride 0.38 ** 81 0.85 ** 86 0.05 85 0.37 ** 84Nitrate-nitrogen 0.38 ** 81 0.84 ** 86 0.05 85 0.43 ** 84 0.82 ** 86M36/1059 (water level data from M36/4018)Conductivity -0.21 * 109pH 0.36 ** 108 -0.29 ** 139Sulphate -0.21 * 108 0.86 ** 138 -0.21 * 138Chloride -0.32 ** 109 0.87 ** 140 -0.33 ** 139 0.74 ** 138Nitrate-nitrogen -0.38 ** 109 0.74 ** 140 -0.49 ** 140 0.50 ** 139 0.79 ** 140M36/4151Conductivity -0.01 104pH 0.14 103 -0.07 116Sulphate 0.21 * 104 -0.20 * 116 -0.25 * 115Chloride 0.00 104 0.61 ** 117 0.13 116 -0.18 * 116Nitrate-nitrogen -0.07 104 0.75 ** 117 -0.22 * 116 -0.15 116 0.45 ** 117M36/4227Conductivity 0.58 ** 41pH 0.03 41 -0.30 * 60Sulphate 0.61 ** 41 0.43 ** 60 -0.01 60Chloride 0.22 41 0.70 ** 60 -0.28 * 60 0.55 ** 60Nitrate-nitrogen 0.29 41 0.78 ** 60 -0.35 * 60 0.34 * 60 0.65 ** 60M36/4655 (water level data from M36/0183)Conductivity -0.54 ** 42pH -0.49 * 41 0.30 42Sulphate -0.05 42 0.01 43 -0.01 42Chloride -0.19 42 0.64 ** 43 -0.12 42 0.08 43Nitrate-nitrogen -0.47 * 42 0.78 ** 43 0.03 42 -0.05 43 0.54 ** 43M36/5248Conductivity 0.28 * 55pH 0.00 55 0.50 ** 62Sulphate 0.11 54 -0.44 ** 61 -0.79 ** 61Chloride 0.20 54 0.46 ** 61 0.22 61 -0.05 61Nitrate-nitrogen 0.32 * 54 0.32 * 61 -0.06 61 0.29 * 61 0.55 ** 61M35/1382 (water level data from M35/3614)Conductivity 0.54 * 34pH 0.51 * 35 0.25 34Sulphate 0.07 33 0.01 32 -0.03 33Chloride 0.47 * 34 0.75 ** 33 0.37 * 34 -0.13 33Nitrate-nitrogen 0.53 * 35 0.76 ** 34 0.39 * 35 0.06 33 0.85 ** 34M35/6791 (water level data from M35/2007)Conductivity -0.08 20pH 0.16 20 -0.36 24Sulphate 0.10 20 -0.26 24 -0.11 24Chloride 0.37 20 0.04 24 -0.17 24 0.32 24Nitrate-nitrogen 0.16 20 0.16 24 -0.12 24 0.47 * 24 0.46 * 24* significant correlation at p value <0.05** significant correlation at p value <0.001

Spearman RSpearman R Spearman R Spearman R Spearman R

ChlorideWater level Conductivity pH Sulphate

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