heather golden department of fnrm suny-esf 18 february 2003
DESCRIPTION
Chapman PJ, Reynolds B, & Wheater HS (1993) Hydrochemical changes along stormflow paths in a small moorland headwater catchment in Mid-Wales, UK. Journal of Hydrology 151: 241-265. Heather Golden Department of FNRM SUNY-ESF 18 February 2003. Presentation Outline. Background - PowerPoint PPT PresentationTRANSCRIPT
Chapman PJ, Reynolds B, & Wheater HS (1993) Hydrochemical changes along stormflow paths in a small moorland
headwater catchment in Mid-Wales, UK. Journal of Hydrology 151: 241-265.
Heather GoldenDepartment of FNRM
SUNY-ESF18 February 2003
Presentation Outline
• Background• Study objectives• Study site and methodology• Results• Conclusions• Limitations• Questions
Background
• Storms: change in flow paths = change in chemical concentrations
• Stormflow generation assumptions: stream water chemistry to infer dominant flow generation mechanisms– Changes along flowpaths alter water chemistry =
assumption violated
• Hydrochemical models: parameter and process identification problems
• EMMA: spatial variability ignored
Study Objectives
• Investigate the hydrochemical changes along a stormflow path
• Determine the effect of hydrochemical changes on surface water quality
Study Site• 4.125-ha 1st order catchment
• 400-509 m above sea level
• Site of long-term geochemical cycling research program
• Peat covers 30% of catchment
• Ephemeral natural network of soil pipes (5-20 cm diameter)
• Stormflow hydrograph – dominated by pipe flow and overland flow
Methods
• Automatic weather station
– 0.18 mm tipping bucket rain gauge at 5 min intervals
• Water level – potentiometer, float and weight recorded with data
logger at v-notch weir
• Pipe A (major pipe)
– 3 tipping bucket gauges
Methods
• Continuous conductivity, pH, temp at stream outlet
• Five storms (varied size & antecedent moisture):
– Stream water at outlet (LW) and head (SH) – auto samplers
– PA, A2-A5, PA1-PA5, and UW - manual
Methods
• pH: prior to filtration• Na & K: flame emission• Ca, Mg, Fe: flame atomic absorption spectrophotometry
– Ca and Mg: prior lanthanum chloride dilution• Anions: ion chromatography• Si & DOC: Skalar continuous flow autoanalyzer• Total monomeric Al & non-labile monomeric Al (Al org) –
fractionation (Driscoll 1984)
Results
Storm Hydrology
Antecedent Runoff Index, where
t = day on which event occurred
R (t-i) = total runoff on the day (t-i)
k = Coefficient between 0 and 1
Higher ARI = higher antecedent moisture and runoff potential
Chemical changes along pipe network
Chemical changes – pipe network
Al species• Inorganic Al and
organic Al ↑ between A2 and PA
• Concentrations of Al fractions highest at beginning of event with decrease through time
• Changes in Al fractions independent of Q
Chemical changes – pipe network
Al species
• Exhibited greatest spatial variation within pipe network
• No evidence of mixing with high Al waters = possibly a pipe source of Al
• Al (inorg) concentration highest after dry period = possible relationship between antecedent conditions and concentrations of Al fractions– Ex. Al (inorg) accumulates in mineral soil during dry periods and
is flushed during high rainfall events (Shoemaker 1985; Seip et al. 1989; Muscutt et al. 1993)
Chemical changes – pipe network
Al species
• Increase in Al (org) along pipe = possibly from organic complexation of Al (inorg) released from pipe perimeter
• Similar Al (org) concentrations at PA outlet for all events = suggests antecedent conditions not a factor
= Mechanisms controlling changes Al (org) and Al (inorg) differ
Chemical changes – pipe network
DOC and Fe
• Concentrations decreased along pipe network – Greater difference in summer = concentrations higher
• Concentrations of DOC & Fe positively correlated across all events (r = 0.91, p<0.001) = DOC important in mobilization of Fe
• Fe decreased through time, but DOC showed no consistent variations through time
Chemical changes – pipe network: K and NO3-N
• Increased K during October 1991 event – more than likely because of decreased plant uptake and increased leaching
• NO3-N variability: more pronounced during autumn
– Related to decreased vegetative uptake and wetting drying cycles that affect microbial activities
– Ex. Peat at head of pipe network: wetter, more aerobic in autumn inhibiting NO3-N formation
Chemical changes – pipe network: K and NO3-N
Chemical changes in stream head area
-Large increase in concentrations of Ca, Mg, and Si and decrease in H+ from pipe outlet (PA) to stream head (SH) across approximately 55 m
-Greatest change in concentrations occur over 10 m length
Chemical changes – stream head area
Ca, Mg, Si, and H+
• Changes in H+ corresponded with changes in Ca, Mg, and Si during all storms
• Largest changes in concentrations of chemicals preceded by dry period (6 weeks without pipe flow)
• Smallest changes preceded by rainfall on previous day
• Inverse relationship between ARI and magnitude of change of chemical concentrations from pipe to stream channel
Chemical changes – stream head area
Ca, Mg, Si, and H+
• Greatest change within base cation-rich drift at stream head due to:– Rapid dissolution reactions (consume H+, release base cations)– Rapid ion exchange reactions (Ca, Mg exchanged for H+)– Mixing of low-acid pipe water with high base cation storm water
Authors propose: accumulation of base cations in drift deposit between events with rapid exchange during storm events - depletion of exchangeable base cations as storms progress
DOC and Fe
Al, DOC, and Fe – decrease along pathway from PA to SH could be related to decreased solubility in base cation-rich water near stream head
Chemical changes – stream head area
Other observations:
• Substantive changes in K and NO3-N only during summer months
• Little temporal and spatial variations in Cl, SO4, and Na
Chemical changes along the stream channel
Chemical changes – stream channel
General• Large changes for some solute concentrations along 135 m
of stream channel
• K concentrations increased with Q along channel – indicative of flow path change
• K depleted along stream channel during summer events – possibly from vegetative uptake
• No substantive decrease of NO3-N concentrations along channel = biotic controls may be less important
Chemical changes – stream channel
• DOC and Fe concentrations decreased along channel during all events
• Al species: reduced concentration changes in autumn compared to summer events
– Possible summer retention of stream substrate followed by winter release
– Possibly from seasonal changes in flow sources
Conclusions and Relevance to Seminar
• Storm flow: rapid changes in solute concentrations over short distances
• Changes evident in this catchment in 3 sections: pipe network, main pipe outlet to stream head, within stream channel
• Base cation-rich (Ca/Mg) drift at hollow of stream head decreases solute dilution potential = influences concentrations of solutes affected by pH
• Highlights importance of hydrochemical changes along stormflow paths
Limitations?
• Study unique to this catchment
• Throughflow component not studied
• Peat chemistry should have a strong influence on chemical concentrations – not studied
• Need more detailed chemical analysis to infer the mechanisms driving hydrochemical evolution along storm flow paths
References
Driscoll, CT. 1984. A procedure for the fractionation of aqueous aluminum in dilute acidic waters. Int. J. Environ. Anal. Chem. 16: 267-283.
Muscutt, AD, Reynolds, B, And Wheater, HS. 1993. Sources and controls of aluminum in storm runoff from a headwater catchment in Mid-Wales. J. Hydrol. 142:409-425.
Seip, HM, Andersen, DO, Christophersen, N, Sullivan, TJ, and Vogt, RD. 1989. Variations in concentrations of aqueous aluminum and other chemical species during hydrological episodes at Birkenes, southernmost Norway. J. Hydrol. 108: 387-405.