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: Heather Golden Department of FNRM SUNY-ESF 18 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 ha 1 st order catchment 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 NO 3 -N

Increased K during October 1991 event – more than likely because of decreased plant uptake and increased leaching NO 3 -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 NO 3 -N formation Chemical changes – pipe network: K and NO 3 -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 NO 3 -N only during summer months Little temporal and spatial variations in Cl, SO 4, 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 NO 3 -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 A procedure for the fractionation of aqueous aluminum in dilute acidic waters. Int. J. Environ. Anal. Chem. 16: Muscutt, AD, Reynolds, B, And Wheater, HS Sources and controls of aluminum in storm runoff from a headwater catchment in Mid-Wales. J. Hydrol. 142: Seip, HM, Andersen, DO, Christophersen, N, Sullivan, TJ, and Vogt, RD Variations in concentrations of aqueous aluminum and other chemical species during hydrological episodes at Birkenes, southernmost Norway. J. Hydrol. 108: