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.