groundwater scoping report for rarotonga, …ici.gov.ck/sites/default/files/downloads/niwa...

GROUNDWATER SCOPING REPORT FOR RAROTONGA, COOK ISLANDS

NIWA Client Report: WLG2010-57 October 2010 NIWA Project: GCI11301

All rights reserved. This publication may not be reproduced or copied in any form without the permission of the client. Such permission is to be given only in accordance with the terms of the client's contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system.

GROUNDWATER SCOPING REPORT FOR RAROTONGA, COOK ISLANDS Author Dr. Mandy Meriano

NIWA contact/Corresponding author

E W Maas

Prepared for

Ministry of Marine Resources NIWA Client Report: WLG2010-57 October 2010 NIWA Project: GCI11301

National Institute of Water & Atmospheric Research Ltd 301 Evans Bay Parade, Greta Point, Wellington Private Bag 14901, Kilbirnie, Wellington, New Zealand Phone +64-4-386 0300, Fax +64-4-386 0574 www.niwa.co.nz

Contents Executive Summary iv

1. Introduction 1

2. Background 1

3. Climate 3

4. Geological and Hydrogeological Setting 4 4.1 Volcanic Rocks (Old Caldera complex and phonolitic

eruptive rocks) 5 4.2 Sedimentary Deposits (Nikao Gravels, Aroa Sands and

stream alluvium) 5

5. Assessment Of Groundwater Resources 6 5.1 Groundwater Quantity (Estimate of Water Balance) 6 5.2 Groundwater Quality 7

6. Proposed Groundwater Monitoring For The Eu Muri Project 8 6.1 First Phase: Data Collection and Analysis (Conceptual

Model Development) 9

6.1.1 Monitoring Well Location and Installation 10

6.2 Slug Tests 14

6.2.1 Groundwater Surface and Flow Maps 15

6.2.2 Groundwater Travel Times 15

6.2.3 Groundwater Discharge (baseflow) Estimate 16

6.3 Groundwater Sampling 17

6.3.1 Sample Parameters and Load Calculation 19

6.4 Recommendations for a groundwater sampling programme (summary): 20

7. Second Phase 20

8. References 22

Reviewed by: Approved for release by:

Dr E Maas Dr Andrew Laing

Groundwater Scoping Report For Rarotonga, Cook Islands iv

Executive Summary

Rarotonga’s water cycle has been modified by human activities, such as diverting mountain stream

water for drinking water supply, irrigation, groundwater extraction, alteration of run off patterns for

agriculture, infilling of swamplands for urban development, and disposal of sewage effluent from

septic tanks into the subsurface and the lagoon that surrounds the island.

The European Union (EU) Muri project has been funded to investigate ways to improve water quality

in the Muri area by upgrading septic tanks. Currently, surface water, streams and the lagoon are

monitored for nutrients and bacteria, but the quality or flow of groundwater has not been assessed.

Therefore, the aim of this study was to review what is known about groundwater in the Muri

catchment and to provide information and methods for monitoring groundwater quantity/quality.

Once this monitoring has been put in place and completed, it will allow the EU Muri project to

evaluate the potential effect of wastewater treatment changes on water quality in the Muri area.

A review of reports held in Rarotonga revealed that the geology and climate of Rarotonga were well

described, but that very little, if any, information existed about groundwater flow, quantity and quality

in the Muri/Avana area.

Therefore, two phases of groundwater investigations are described. The first phase aims to establish a

groundwater monitoring network (primary focus of the scoping project) to gather representative data

to:

• Provide baseline data to map the spatial and temporal distribution of groundwater levels and

quality

• Identify short and long-term changes to groundwater levels and flow patterns from natural

recharge and discharge

• Isolate the impact to groundwater from septic tanks and contaminant releases due to agricultural

activities

• Present early warning of potential risks and need for mitigation

• Identify and evaluate the success of mitigating measures

• Provide real-time information for compliance with guidelines

• Support collaborating efforts with surface water resource management

• Support calibration efforts when conducting computer modelling.

Groundwater Scoping Report For Rarotonga, Cook Islands v

The Ministry of Infrastructure and Planning have installed bores in the Muri/Avana area. These bores

can form part of the groundwater monitoring program, however these bores need to be widened and

loggers need to be installed. The report provides information about how to evaluate the current

position of the bores and extent this to cover the different land uses, population densities and soil types

in the Muri/Avan area. Methods for conducting slug tests, calculating groundwater surface and flow

maps, groundwater travel times and groundwater discharge are explained. A groundwater sampling

plan is provided, including which parameters need to be measured to calculate nutrient and bacterial

loads in groundwater.

Successful implementation of a groundwater monitoring programme is highly dependent on the

availability of trained individuals. Hence, suitable training and knowledge transfer opportunities need

to be provided to technicians and students.

A summary of a second phase groundwater monitoring programme is provided. However this is only a

brief description and highlights some future objectives of the groundwater monitoring programme for

water resource managers.

Groundwater Scoping Report For Rarotonga, Cook Islands 1

1. Introduction

The aim of the groundwater scoping project was to provide the water resource

managers with a suitable groundwater monitoring plan that will assist them with

the assessment and evaluation of groundwater impact on water quality and lagoon

health. The scoping report was compiled following a field visit to Muri catchment

in Rarotonga, Cook Islands, carried out between 06-10 September 2010. Dr. Els

Mass and Dr. Julie Hall of New Zealand’s National Institute of Water and

Atmospheric Research (NIWA) coordinated the visit to Muri catchment.

The following individuals are acknowledged and thanked for making available

relevant data/reports and their assistance in the field: Ms. Dorothy M. Solomona,

Acting Director, Pearl Support Division, Ministry of Marine Resources; Mr. Paul

Teariki Maoate, Integrated Water Resource Management (IWRM) Project

Manager, Ministry of Infrastructure and Planning; and Mr. Tekao Herrmann, Civil

Engineering consultant.

The objectives of the groundwater scoping project were to:

• Establish contact with EU Project staff and discuss information on sources

of contamination in the Muri/Avana catchment.

• Collate historical, current and relative data on groundwater levels and

quality.

• Identify regional estimates of water balance.

• Develop a groundwater monitoring plan for various groundwater zones

within the catchment.

2. Background

The volcanic island of Rarotonga (Figure 1) covers an area of 67 km2, and

receives an annual rainfall between two and four meters (Clement and Bourguet,

1992). The island is oval in shape and is mountainous with numerous steep ridges

and peaks of up to 600 m above sea level (a. s. l.). It is bordered by a narrow

coastal lowland alluvium and raised coral with a fringing reef (Wood and Hay,

1970). It has a north-south width of about 6.5 km and an east-west length of some

11 km and has a circumferential road (the Ara Tapu) about 28 km in length,

around the edge of coastal plain. The interior is rugged and bush-clad.

The island’s population of about 9,000 is concentrated in the coastal plain, which

is approximately 1 km wide.


Figure 1. Map showing physiographic regions of Rarotonga. Provided by the Ministry of Infrastructure and Planning.

The Muri catchment is approximately 850 ha and includes the subcatchments of

Avana, Aroko, Nukupure, Areiti, Aremango, Vaii and Maii (Figure 2).

Avana (perennial) and Paringaru (intermittent) streams define the northern and

southern boundaries of the catchment, respectively. Streams drain radially

outwards from the steep interior. In their losing segments, stream water may seep

into the streambed alluvium and recharge the shallow aquifer system where it

crosses the coastal plain. In their gaining segments, groundwater is discharged

into the stream maintaining streamflow (baseflow), particularly during drier

periods (no precipitation). This has important implications regarding water

resources. It indicates that groundwater is a contributing component of streamflow

in the catchment.


Figure 2. Map showing Muri catchment, subcatchment boundaries, and location of water intake on Avana stream. Provided by the Ministry of Infrastructure and Planning.

Water supply in the Muri catchment is provided by a distribution network, fed by

the capture of surface water at the Avana Water Intake. Figure 2 shows the

location of the water intake in the catchment. Currently, water is distributed free

of charge. The distribution system is affected by losses through pipe leakage and

difficulties are encountered during drier periods to cover demand. Presently, water

requirements include domestic, agricultural, gardening, piggeries, and cattle.

3. Climate

There are two climate seasons in Rarotonga, a rainy season (“summer” October to

April) and a dry season (“winter” May to September). The centre of the island

receives the highest mean annual rainfall (4000mm), whereas the coastal margins

receive between 2000mm and 3000mm in the north-west and south of the island,

respectively (Ricci and Scott, 1998). Mean annual rainfall is about 2100mm and

the mean annual temperature is about 24 deg C (Parakoti and Davie, 2007).


Rainfall is the only source of water on the island providing surface runoff to

streams and recharge to aquifer systems through infiltration. Evapotranspiration

removes water from the system and during drier periods impedes the water flow

into streams. Excess water from rainfall, after removal of evapotranspiration and

runoff, infiltrates and passes through lavas and breccias to reach the older

weathered volcanic formation. General groundwater flow within the aquifer

system is towards the lagoon and the sea.

4. Geological and Hydrogeological Setting

Rarotonga is a non-active volcano of late tertiary age (8 to 2 million years ago)

surrounded by quaternary sediments, gravel fan deposits, coastal terraces, mud

swamps, coral sands and reefs (15,000 years old) (Figure 1). Figure 3 depicts a

geologic cross-section showing geological relationships and groundwater flow

paths in the coastal plain. Several studies summarize the geology and water

resources of Rarotonga (see Wood and Hay, 1970; Clement and Bourguet, 1992;

Burke and Ricci, 1997; Ricci and Scott, 1998). A summary is presented below.

Figure 3. Geologic cross-section showing geologic materials and groundwater flow directions. Modified from Burke and Ricci (1997).


4.1 Volcanic Rocks (Old Caldera complex and phonolitic eruptive rocks)

The Old Caldera Complex (Late Pliocene) comprises a mixture of breccias,

pyroclastic flows, scoria and ashes. Due to its age, the formation has been highly

weathered and the volcanic rocks have been transformed into clay minerals.

Hydrogeologically, this is of great importance as these altered formations may act

as an aquitard (less permeable formation incapable of transmitting significant

quantities of water). Conversely, the more recent less weathered breccias, ashes

and fractured volcanic rocks (Late Pliocene-Early Pleistocene) act as a complex

aquifer (permeable formation capable of transmitting significant quantities of

water) system. The position of the aquifer system in the subsurface is dependent

on the local sequence and superposition of various volcanic layers. It has been

suggested that a significant portion of the groundwater in Rarotonga could be

contained within alluvium or scoria deposits in old buried stream valleys that

extend down to the impervious ancient basement rocks (Clement and Bourguet,

1992).

4.2 Sedimentary Deposits (Nikao Gravels, Aroa Sands and stream alluvium)

Nikao Gravels form the weathered volcanic gravels and sands of coastal terraces

and fans. The Aroa Sands comprise coral sand and gravel beach deposits forming

a low bench around the island between the foot of the Nikao Gravels and the inner

edge of the lagoon (Burke and Ricci, 1997). The highly permeable stream

alluvium consists of fans of volcanic gravels and debris deposited by streams.

These fans have been deposited at the foothill of the volcano below an elevation

of 30 to 40 m. a.s.l.

Hydrogeologically, the fan deposits are highly permeable and constitute the

alluvial aquifer system. Local variability in the permeability of the fan deposits

has been observed where older more weathered Nikao Gravels form less

permeable lenses (Clement and Bourguet, 1992; Burke and Ricci, 1996).

Conversely, the Aroa Sands have more uniform permeability. The alluvial aquifer

system is directly recharged by runoff and is considered to be an important source

of groundwater.


5. Assessment of Groundwater Resources

Currently little is known of the aquifers underlying Rarotonga. The Ministry of

Infrastructure and Planning has installed 10 shallow monitoring wells in the Muri

catchment area with plans to monitor groundwater levels and quality. To date,

interaction between groundwater and surface water along losing and gaining

stream segments has not been investigated. The stresses imposed on surface and

groundwater resources and the effect on water quality and quantity may not be

isolated from one another. For example, heavily polluted groundwater can

discharge into streams and lagoons, thus negatively impacting surface waters and

their dependent ecosystems.

5.1 Groundwater Quantity (Estimate of Water Balance)

The less permeable nature of the volcanic rocks in Rarotonga results in reduced

infiltration of rainfall and the emergence of streams in the high interior part of the

island. Streams can persist as far as the coast, particularly in the northern and

eastern parts of the island where large streams maintain direct channels to the sea.

The proportion of the rainfall that reaches the water table aquifer is greatest where

the rocks are more permeable (i.e., weathered volcanic alluvium of the coastal

terraces and streams as well as beach deposits in the coastal fringe). Ricci and

Scott (1998) calculated average annual recharge from rainfall to be 640 mm – or

about 30% of the average annual rainfall. The water that reaches the water table

aquifer becomes part of the groundwater reservoir (recharge) and moves through

the fractures and pores of the saturated subsurface material and reappears at the

surface at lower elevations in the form of springs and seeps (discharge) which in

turn feed the lowland streams. In addition to rainfall recharge, the aquifer also

receives recharge from streams as observed by the disappearance of streamflow

into the alluvium on emerging from the volcanic interior (Waterhouse and Petty,

1986).

Drinking water is distributed through a piped network that is known to leak

(Binnie & Partners, 1984; WMI & BURGEAP, 1992). Burke and Ricci (1997)

report unaccounted losses from the system in the order of 40-60%.

While approximately 70% of the corroded and leaking pipelines in Rarotonga has

been replaced, the effectiveness of the new distribution network remains

unquantifiable due to lack of metering (ADB Report, 2009). High water losses

from the distribution network to the subsurface provides additional source of

recharge to the shallow aquifer system and may lead to rising groundwater levels

and baseflow quantities in the catchment.


5.2 Groundwater Quality

Agricultural practices and poor sewage treatment mechanisms have been

identified as main sources of groundwater contamination in Rarotonga. Piggeries,

poultry and cattle were spotted on small family farms on the terrace at the foot of

the volcanic hills in the catchment. Septic tank systems are widely used

throughout Rarotonga and are in various state of repair (sample photos in

Appendix I). They are not connected to a sewer system and are specifically

designed for subsurface drainage of human waste at slow rates. An improperly

designed, located, constructed, or maintained septic system can leak contaminants

(i.e., bacteria, viruses, chemicals) into the ground causing groundwater

contamination. Therefore, they present a serious source of contamination to the

aquifer system.

Although, there were no groundwater quality data available during this visit, it is

generally believed that nitrate and bacteriological pollution have deteriorated

groundwater quality (Burke and Ricci, 1997). Bacterial contamination from

human and animal wastes and nitrate from intensive agriculture may be the most

likely sources of groundwater contamination. Leaching of contaminants can

occur more readily on sandy soils due to their low nutrient and water holding

capacity (i.e., Nikao Gravels, Aroa Sands). Any increase in the concentration of

these contaminants can be alarming as both contaminants present health concerns.

Excess nitrate also causes eutrophic conditions in receiving waters which may

compromise ecosystem health.

The section 6 provides an overview of a proposed groundwater monitoring and

testing programme for the Muri catchment.


6. Proposed Groundwater Monitoring for the EU Muri Project

As an essential component of water resource management, groundwater

monitoring networks are designed to optimize the collection of vast amounts of

field data. Collection, analysis, and management of groundwater levels and

quality provide fundamental baseline information necessary for identifying

potential risks and managing groundwater as a sustainable resource.

The aim of the groundwater monitoring network is to:

• Provide baseline data to map the spatial and temporal distribution of

groundwater levels and quality

• Identify short and long-term changes to groundwater levels and flow

patterns from natural recharge and discharge

• Isolate the impact to groundwater from septic tanks and contaminant

releases due to agricultural activities

• Present early warning of potential risks and need for mitigation

• Identify and evaluate the success of mitigating measures

• Provide real-time information for compliance with guidelines

• Support collaborating efforts with surface water resource management

• Support calibration efforts when conducting computer modeling.

This data will be collected and analyzed in the first instance to support the EU

(Muri) project and provide information as to the effectiveness of the changes

made to the wastewater treatment systems. However, the basic design outlined

here could be duplicated around Rarotonga for other projects and to gain an

understanding of the groundwater in that specific location.

The groundwater monitoring is divided in two phases. The first phase details the

establishment of a groundwater monitoring programme (primary focus of the

scoping project). The second phase is preliminary in its scope and is only

suggested to assist with future planning of water resources. This phase can be

carried out after enough data have been collected in phase one and is not required

for the purpose of the EU project.

It must be noted that successful implementation of a groundwater monitoring

programme is highly dependent on the availability of trained individuals. Hence,

suitable training and knowledge transfer must be provided to the designated

groundwater team.


6.1 First Phase: Data Collection and Analysis (Conceptual Model Development)

Understanding the spatial aquifer heterogeneity and behaviour with some degree

of certainty in complex volcanic islands is a difficult task. With the lack of

existing data on aquifers, the data collection phase should provide information on

aquifer properties such as hydraulic conductivity (spatial), groundwater levels and

flow directions (spatial and temporal), and groundwater quality (spatial and

temporal). Table 1 summarizes the sequential steps to develop a conceptual model

as necessary for the quantification of the groundwater quantity and quality.

Table 1: Sequential steps necessary for the establishment of groundwater monitoring in support of the EU Muri Project

Action Purpose

1. Field survey To locate groundwater monitoring sites

2. Drilling, logging of sediment cores, and installation of monitoring wells

To access distinct parts of the aquifer system (i.e., volcanic, sand and gravel) with various land use intensity for groundwater level monitoring and sampling

3. Slug testing To measure in situ hydraulic conductivity of the aquifer material

4. Groundwater sampling To develop a groundwater quality baseline against which wastewater management methods can be evaluated and future changes in quality monitored

5. Groundwater flow mapping To develop contour maps of the water table elevation and groundwater flow direction

6. Estimating groundwater travel times To estimate groundwater velocity and time of travel in the aquifer

7. Estimating groundwater recharge To quantify groundwater recharge to the catchment and to calculate nutrient loading via groundwater to surface waters

During the field visit the need for cost-effective methods for obtaining

groundwater measurements was noted. Hence, some guidance is provided below

to assist water resource managers with the establishment of a groundwater

monitoring system and instrument acquisition. The following sections outline the

steps towards achieving the development of a groundwater monitoring

programme and a conceptual model for the EU Muri Project.


6.1.1 Monitoring Well Location and Installation

A groundwater monitoring network of 10 shallow wells, between 1.2 and 5.0 m

deep, was installed in June 2010. Figures 4 (in three view panels) and 5 (in two

view panels) show the monitoring locations in the Muri catchment within the

various surficial deposits. Groundwater levels were measured following well

installation and were recorded to be between 0.1 m to 4.6 m. a.s.l. (Figure 5).

These wells have narrow opening (20 mm diameter) limiting access and hindering

well development (pumping), groundwater sampling, and instrument housing. It is

recommended that these wells be made larger and in some cases deeper (very

shallow wells risk the chance of going dry during the dry season) for easier

monitoring and sampling. Assuming that an appropriate drill rig and skilled crew

is available, it is recommended that new monitoring wells be installed in the

coastal terraces and fans (Nikao gravels) and fan deposits and terraces bordering

the volcanic hills.

Figure 4. Distribution of monitoring wells within the various soils/surficial deposits in the Muri/Avana catchment (three views). Provided by the Ministry of Infrastructure and Planning (continued on next page).


Figure 4. Distribution of monitoring wells within the various soils/surficial deposits in the Muri/Avana catchment (three views). Provided by the Ministry of Infrastructure and Planning.


Figure 5. Distribution of monitoring wells and observed water level data (two views).

Provided by the Ministry of Infrastructure and Planning.


Well locations should target major land use and various population densities on

the volcanic terrace and coastal areas (areas most likely to detect contamination)

and presence or absence of sandy (more permeable) soils. This will allow for

comparison between groundwaters across the catchment with similar land use,

soil and sediment types. A field reconnaissance survey is therefore recommended

to identify specific locations (hotspots) that warrant detailed monitoring and

sampling (anecdotal evidence from farms/residents can also be considered in the

selection). It is noted that a survey of the septic tanks in the Muri/Avana

catchment has been completed by Mr. Tekao Herrmann (personal

communication). The results of this survey should be used in the monitoring well

site selection.

It is recommended that, at minimum, three wells be installed in each targeted

geographic location (i.e., Terraces, Fans, Flood Plains, Beach Ridges, and

Depressions (swamp deposits) – please refer to Figure 1 for the location of these

five physiographic regions in the catchment) to calculate the groundwater

direction and hydraulic gradients (see section Hydraulic Head and Gradients).

In smaller non-contiguous locations (i.e., depressions) fewer wells may be

necessary to characterize the groundwater. Therefore, one must exercise judgment

concerning the number of necessary wells in these smaller locations that fall

within the greater monitoring network. For example, a single well in a small

depression surrounded by an array of wells will fulfill the monitoring requirement.

Single monitoring wells must also be installed near streamflow gauging sites (see

Section: Groundwater Recharge Estimate). It is recommended that at minimum 10

– 15 wells to be installed in the Muri catchment. It is essential that the

groundwater and surface water monitoring programmes in the catchment are

coordinated so that groundwater level and quality data can be compared and

cross-referenced with streamflow and stream water quality data.

Hollow stem augers are suitable for drilling into shallow unconsolidated deposits.

However, rotary drilling may be appropriate for wells deeper than about 30 m or

into wells in bedrock. Core samples are to be collected from bored holes and

stratigraphically logged to delineate the site geology. Presently, staff at the

Ministry of Infrastructure and Planning has the appropriate geotechnical

background and experience to produce the lithologic logs. Monitoring wells are to

be installed in the completed boreholes. There must be a sufficient working

opening inside the augers or casing. For example for a 51 mm (2-in) monitoring

well, a minimum 108 mm (4.25-in) opening is required. Monitoring wells must be

screened in the bottom within the saturated zone. It is expected that the water

table in the upper catchment areas will be found at greater depths therefore

requiring deeper bore drilling.


Currently the monitoring wells in the catchment (Figure 4) appear to be in the

correct locations - targeting the Beach and some of the older and younger Fan

deposits - but a final walk thru is recommended by MOIP with the EU project

team, to make sure that the wells cover all the different land uses, population

densities identified in EU Muri project. It is recommended that the wells

identified for monitoring are widenend to 2.5-in (63.5 mm) and deepened into the

saturated aquifer to reduce the chance of wells going dry due to seasonal water

table fluctuations and to ensure groundwater sampling during dry season when

water quality may be at its worst.

Completed monitoring wells should be equipped with pressure transducers for

continuous measurement of groundwater levels. Temperature-compensated

conductivity probes equipped with data loggers should be used to record

continuous temperature and electrical conductivity at several locations in the

catchment. Ideally, the loggers must be spatially distributed in the catchment to

representatively sample groundwater from a variety of land use.

Hourly measurements of groundwater levels, temperature and electrical

conductivity can be recorded in the data logger and should be downloaded

regularly. Manual measurement of groundwater level in monitoring wells not

equipped with level loggers should be done weekly using a water level meter.

Temperature and electrical conductivity should be measured concurrently using a

handheld probe. At minimum, one well in each ground type should be installed

with pressure transducers (see Recommended Equipment in section 1.3.1) the

remaining wells can be monitored manually.

All groundwater activities described below are to be completed using the installed

monitoring wells.

6.2 Slug Tests

These tests are to be carried out in each well once after the completion of well

installation. In situ hydraulic conductivity (K) of the aquifer material should be

determined by means of slug tests. K represents a measure of the ability of flow

through porous media (i.e., volcanic gravels, beach sand and gravels, etc) and has

the dimensions of length/time. Slug tests are particular tests where the rate of

groundwater recovery is measured after a volume (slug) of water is suddenly

displaced. A wide variety of formulas are available for the analysis of slug test

data, based on aquifer type and geometry and underlying assumptions (e.g.,

Hvorslev, Bouwer and Rice, etc). For a comprehensive introduction to various

methods please refer to Freeze and Cherry (1979). As mentioned previously, it is


essential that the groundwater technicians receive appropriate training for data

processing and interpretation of results.

Groundwater flow paths and travel times between septic systems and/or farmlands

and streams are important factors that influence both water quantity and quality.

Determining flow paths and travel times is therefore a prerequisite for interpreting

water quality changes. The following outlines a hydraulic approach to assess

groundwater flow and travel times.

6.2.1 Groundwater Surface and Flow Maps

These maps can be calculated as detailed below from the information obtained in

1.2 and can be calculated for the Muri area, this will help us to understand the

direction and magnitude of groundwater flow in the catchment.

Water in the aquifer stands at a particular level (hydraulic head). In practice,

depth to groundwater measurement is obtained and is subtracted from the top of

the well casing elevation to measure total head (datum is calibrated to sea level).

The collected groundwater level measurements are to be used to generate a

groundwater surface map. This is achieved by contouring measured head values in

the shallow aquifer (water table map).

Groundwater flow direction can be understood in the fact that groundwater always

flows in the direction of decreasing head. The magnitude of the movement,

however, is dependent on the hydraulic gradient (i) which is defined as the

change in head (∆h) per unit distance (∆l) (i=∆h/∆l). A minimum of three wells

located in a triangular pattern is required to calculate the direction and hydraulic

gradient of a geographic location. Groundwater flow direction is perpendicular to

the groundwater level contour line.

6.2.2 Groundwater Travel Times

Groundwater travel times can be calculated using the data obtained. This will help

us to understand how long groundwater stays in the ground and will help us to

predict how long it might be before changes to water quality can be noticed, that

is if groundwater stays in the ground a long time (longer flow paths) it might be a

long time before there is a reduction in the contamination of the streams and the

lagoon.


Groundwater vulnerability to pollution is best understood in relation to travel

time, which is the approximate time that elapses when water reaches the aquifer or

reaches a specific target such as a spring. This is also often referred to as

residence time. Several techniques can be used to estimate groundwater travel

times in an aquifer including use of natural or human made tracers and radioactive

and stable isotopes (see Tracer methods in section Second Phase). Most of these

methods are expensive, require a strict sampling procedure and are not always

applicable. Groundwater movement through the aquifer however, can be

expressed as its average linear velocity through aquifer material. This method uses

the physical properties of the aquifer system and has been used to estimate

groundwater travel times in many hydrogeologic projects. Since some

contaminants are assumed to travel at the same rate as groundwater (i.e., nitrate,

chloride), the estimated travel times can be used to assess the time required to see

the effects of wastewater and/or agricultural management methods upon

downstream groundwater and lagoon water quality.

The factors controlling the groundwater movement are expressed in an equation

known as Darcy’s Law: Q = KiA; where Q is discharge; K is hydraulic

conductivity; i is hydraulic gradient; and A is cross-sectional area. Darcy’s Law

can be rearranged and used to calculate groundwater velocity (v) in the aquifer.

Since groundwater moves through aquifer materials that impede groundwater

velocity, effective porosity is used to better represent the water flow through the

aquifer. The linear average velocity is expressed as: v = Ki/ne, where ne is the

effective porosity (see Freeze and Cherry for representative porosity values for

various substrates).

6.2.3 Groundwater Discharge (baseflow) Estimate

The volume of groundwater that discharges in the catchment is directly related to

the volume of water that recharges the aquifer. In other words, on along-term

basis, the system reaches a steady state where all the water that is infiltrated is

discharged through evapotraspiration and baseflow. To understand the impact of

groundwater on the streams and lagoon quality we must quantify the volume of

groundwater that discharges in the catchment as baseflow.

A simple water balance method can be used to approximate groundwater

discharge.


Streamflow at any given point in time could be considered to be the sum of

surface runoff and groundwater components. During dry periods with no

precipitation or surface runoff, the stream discharge is maintained by groundwater

discharge (baseflow). On a long-term basis, this baseflow must balance recharge.

The streamflow measurement integrates all baseflows along the river upstream of

the gauging station, and therefore provides a regional estimate of groundwater

recharge for the catchment. A baseflow recession/separation analysis (e.g., Todd

and Mays, 2005; Rasmussen and Andreasen, 1959) using a continuous record of

groundwater level and streamflow discharge (i.e., concurrent weekly

measurements of both parameters) can be used to convert complete cycles of

water table drop into volumes of water equivalent to groundwater discharge.

Local seepage from the stream channel to the shallow aquifer can also be

estimated using gauging observations. Repeated streamflow gauging along the

length of a stream can convey some information about the gaining and losing

segments of the stream and provide an upper and lower bound for recharge and

discharge. For example, the difference in discharge between two points measured

during lowflow (baseflow) and stormflow can provide spatial and temporal

estimates of recharge (losing segment) and discharge (gaining segment) along the

stream length. These values can then be used to calculate and map contaminant

loading to and from the aquifer system in the catchment. The success of this

method is highly dependent on the accuracy of flow measurements and sufficient

distance between points of measurements. Insignificant flow differences may be

due to inaccurate flow measurements and/or short stream stretches.

6.3 Groundwater Sampling

The objective of the sampling programme should be to collect representative

samples of groundwater in the catchment. The groundwater sampling method

depends on the site-specific conditions and may require equipment such as hand-

operated or motor-driven pumps, peristaltic pumps and bailers. Dedicated

sampling systems are greatly preferred since they avoid the need for

decontamination of equipment. Sampling devices (pumps, bailers, tubing) should

be constructed of inert materials (i.e., PVC, stainless steel, glass, etc.).


Prior to sampling, wells must be purged three to five times their volume or purged

dry until water-quality indicator parameters (i.e., specific electrical conductance,

temperature, pH) stabilize. Purging should be followed by sample collection to

assure samples are representative of groundwater in the aquifer and not stagnant

water left inside the monitoring well. Although the appropriate sampling

frequency is difficult to ascertain without any knowledge of prior fluctuations, it

is initially recommended that monitoring wells be sampled monthly.

Collecting a groundwater sample from an uncontaminated part of the aquifer

system, more than likely found in the upper catchment where no prior land use

has been identified, can provide information on background (pristine)

groundwater quality. Seasonal sampling is recommended.

Sampling equipment typically includes:

• Water level sensor – used to measure depth-to-water

• Measuring tape and weight – used to measure total depth of well

• Foot valves and plastic tubing – used for developing, purging and sampling

of monitoring wells

• Multi-parameter meter with flow-through cell – used to measure water

quality parameters (i.e., pH, specific electrical conductance, dissolved

oxygen, oxidation-reduction potential (Eh) and temperature)

• Decontamination supplies

• Sample bottles, sample preservation

Springs are natural groundwater discharge points. Their location in the catchment

needs to be identified and their quality monitored monthly. To estimate the

pollutant mass stored in the unsaturated zone, soil cores can be extracted below

root zone at several locations across the catchment to establish vertical profiles of

pore water concentrations. It is important to differentiate the recharge rates to

different substrates (i.e., volcanic – less permeable vs. sand and gravel – more

permeable) in the catchment since best management practices can be targeted to

land uses with shortest travel times and largest nutrient loads.

As previously mentioned, establishment of a wastewater treatment initiative can

change groundwater quality. It is therefore necessary to establish a groundwater

quality baseline (current state) in the catchment. By doing so, it becomes possible

to measure any change in groundwater quality (improvement or degradation) over

time.

A brief description on nutrient parameters and load calculation is provided below.


6.3.1 Sample Parameters and Load Calculation

Livestock and inadequate septic systems in the catchment are known to be the

primary causes of deteriorated water quality, it is therefore suggested that

groundwater samples be analyzed for their nutrient and microbiological content.

A one-time analysis of major and minor ionic constituents (Ca, Mg++, Na+, K+, Cl,

SO4-, NO3

-, total alkalinity) is also recommended to obtain an overall picture of

the groundwater system.

Quantifying nutrient concentrations, nitrate nitrogen (NO3-N) and dissolved

reactive phosphorus (DRP), in groundwater is recommended to evaluate the

effectiveness of alternative wastewater treatment and/or agricultural management

practices. Nitrates do not adsorb to soil particles and can easily move in the

aquifer system resulting in high groundwater nitrate concentrations. Permeable

and sandy deposits provide favorable conditions for vertical leaching of nitrates to

the aquifer system.

In addition to NO3-N, water samples can also be analyzed for ammonia (NH4) and

total Kjeldahl nitrogen (TKN). TKN is a measure of organic nitrogen plus

ammonia and its analysis is recommended because the TKN fraction can be added

to nitrate concentration to yield total nitrogen concentration in water. This value

along with the total nitrogen loading from surface runoff is essential for

determining the total catchment nitrogen load to the lagoon. Time series statistics

can be used to analyze and relate nitrogen and DRP concentrations to

wastewater/agricultural management and weather conditions.

Microbiological quality of groundwater samples can be determined for

enterococci (currently monitored in streams and lagoon), fecal coliforms, and

E. coli. The microbial groundwater quality can provide indicator levels to certain

catchment features and management characteristics which are likely to affect

water quality.

Baseflow loading can be calculated using stream water samples collected during

dry weather (3-5 day dry antecedent conditions) according to: L = QC, where L is

the loading (kg), Q is the discharge and C is the concentration.


Recommended Equipment

• YSI Professional Plus handheld multiparameter meter with flow-through

cell

• Wattera foot valves and tubing

• Inline disposable 0.45 micron polyethersulphone filters

• Solinst temperature-level-conductivity meter (can be used for depth-to

water measurements and conductivity and/or temperature profiling in wells)

• Pressure transducers (Aqua Troll 200) – measures and logs water

level/pressure, conductivity and temperature.

Potential supplier in New Zealand: Envco –Environmental Equipment (www.envcoglobal.com)

6.4 Recommendations for a groundwater sampling programme (summary):

The basis of the proposed groundwater sampling programme is to obtain

representative groundwater samples from established sampling points to reflect

the average properties of the system - including:

- A one-time analysis of major/minor ionic constituents (Ca, Mg++, Na+, K+,

Cl, SO4-, NO3

-, total alkalinity).

- Monthly measurement of NO3-N, DRP, NH4, and TKN.

- Monthly field determinations of pH, Eh, temperature, conductance and

alkalinity.

- Monthly microbiological quality of groundwater samples for enterococci,

fecal coliforms, and E. coli.

Following the collection, interpretation, and analysis of results in phase one, water

resource managers may find the recommendations below to be useful in

developing goals and objectives for a second phase. The recommendations below

are purely preliminary in nature and may be initiated or expanded upon as more

data become available during phase one.

7. Second Phase

The following objectives are recommended for initiation during phase two.

They include:

• Installation of deeper monitoring wells to improve the understanding of the

deeper hydrostratigraphic units and determine the nature of vertical

hydraulic gradients, physical parameters and nutrient concentrations at key

locations in the catchment.


• Hydrograph separation (see earlier section on Groundwater Recharge

Estimate) – used to isolate the groundwater component of streamflow and

calculate loadings. It ideally requires a continuous record of streamflow and

groundwater levels and water quality (i.e., nitrate, chloride) for a minimum

period of one year.

• Water balance study – used to quantify groundwater recharge and

discharge.

• Numerical modeling – used to simulate groundwater flow, surface water –

groundwater interaction, and contaminant transport. Integration of the

various datasets collected in phase one will form the basis for the

construction of a three-dimensional flow and transport model. The model

will represent an integrative view of the system and can be used to make

predictions of the magnitude and timing of the influence of land use and

wastewater management practice changes on groundwater resources. Model

calibration to steady state and transient conditions is required. A transport

model can be added to simulate contaminant transport and travel times.

Numerical modeling requires considerable expertise and extensive

knowledge of groundwater flow systems.

• Tracer methods – Environmental tracers are dissolved substances

introduced into the hydrologic cycle either by nature or anthropogenic

addition. They are able to trace water movements in unsaturated and

saturated zones over long time periods. Artificially applied tracers (i.e.,

bromide, chloride, etc) can be sued to trace waters over small spatial and

temporal scales. Tracers that undergo radioactive decay (i.e., tritium,

carbon-14) can be used to determine water ages. Stable isotopes of water

(deuterium and oxygen-18) can be used to track water movement in the

unsaturated and saturated zones. Their concentrations are affected over the

catchment area by temperature and altitude and can be used an indicators

for mixing ratios of waters from different regions. The tracer methods are

expensive, require a strict sampling procedure and are not always

applicable.


8. References

Binnie & Partners. 1984. Water resources and water supply of Rarotonga.

Burke, E. and Ricci, G. 1997. Comment on the groundwater resources surface

water resources and water supply of Rarotonga, Cook Islands. SOPAC

Technical Report 248, 44p.

Clement, H. and Bourguet, L., 1992. Outline scheme for water development and

management. Island of Rarotonga, Cook Islands Water Department.

R.1236/A.3077-192904. 53p.

Freeze, R.A. and Cherry, J.A. 1979. Groundwater. Prentice-Hall, Inc.,

Englewood Cliffs, New Jersey.

Parakoti, B. and Davie, T. 2007. Sustainable integrated water resources and

wastewater management in pacific island countries. National integrated water

resource management diagnostic report. Cook Islands. SOPAC Miscellaneous

Report 635. 47p.

Ramussen, W.C. and Anderasen, G.E. 1959. Hydrogeologic budget of the

Beaverdam Creek Basin in Maryland. US Geological Survey Water-Supply

Paper 1472: 106p.

Ricci, G. and Scott, D. 1998. Groundwater potential assessment of Rarotonga

coastal plain. SPAC Technical Report 259, 76p.

Todd, D.K. and Mays, L.W. 2005. Groundwater Hydrology, 3rd ed. John Wiley

&Sons, Inc., Hoboken, NJ, 636pp.

WMI & BURGEAP. 1992. Outline scheme for water development and

management.

Waterhouse, B.C. and Petty, D.R. 1986. Hydrogeology of the southern Cook

Islands, South Pacific. New Zealand geological Survey Bulletin 98. 93p.

Wood, B.L. and Hay, R.F., 1970. Geology of the Cook Islands. New Zealand

Geological Survey. 103p.


APPENDIX - photos

A pre-1980 installed septic tank. Note the depressed ground around the tank area.

A stream channel exists beyond the trees in the background. In the case of older

septic systems with limited or no drain field, the effluent from the septic tank can

reach the water table and be discharged into the stream just a short distance away

with minimal, if any, treatment.


A septic system covered by a concrete pad on top and an access port. No

installation date available.


A more recent (1990s) three-tank septic system.


Livestock often utilise streams.

groundwater scoping report for rarotonga, …ici.gov.ck/sites/default/files/downloads/niwa...

Documents