Heather Golden Department of FNRM SUNY-ESF 18 February 2003

1. 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

2. Presentation Outline • Background • Study objectives • Study site and methodology • Results • Conclusions • Limitations • Questions

3. 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

4. StudyObjectives • Investigate the hydrochemical changes along a stormflow path • Determine the effect of hydrochemical changes on surface water quality

5. 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

7. 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

8. 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

9. 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)

10. Results

11. 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

12. Chemical changes along pipe network

13. 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

14. 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)

15. 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

16. 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

17. Chemical changes – pipe network: K and NO3-N

19. 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

20. Chemical changes in stream head area

21. -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

22. 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

23. 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

24. 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

25. 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

26. Chemical changes along the stream channel

27. 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

28. 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

29. 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

30. 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

31. 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.