THE IMPORTANCE OF GROUNDWATER DISCHARGE TO THE HYDROCHEMISTRY OF A SMALL SIERRA NEVADA STREAM

Most streams receive water from many different sources, including direct precipitation into the stream channel, overland flow, shallow subsurface flow, and groundwater discharge. Water from each of these sources have unique chemical and isotopic fingerprints as well as varying residence times within a watershed. Monitoring changes in stream water chemistry through time provides a basis for determining the influence of each water source on the chemical composition of streams.

Sagehen basin is a high elevation watershed in the Sierra Nevada near Truckee, California. Recent studies in Sagehen Basin revealed a positive correlation between residence times of shallow groundwaters and their chemical compositions. This relationship between residence times chemical compositions provides a framework for interpreting chemical changes in Sagehen Creek. Chemical and isotope composition data collected weekly from Sagehen Creek suggests that the delta O-18 of Sagehen Creek during the snowmelt period responds rapidly to snowmelt pulses. The delta O-18 ratio of Sagehen Creek is more negative and similar to isotope ratios measured in the snowpack early in the snowmelt period during the peak discharge. The subsequent increase in the delta O-18 during the baseflow recession is concurrent with the decrease in discharge. Changes in major ion chemistry, including calcium, sodium, magnesium, silica, potassium, occur simultaneously, however, the response lags approximately two weeks behind changes in delta O-18.

During late summer and fall, the chemical and isotope composition of Sagehen Creek reflect that of shallow groundwater with a residence time of approximately 20 years. The stoichiometery of solutes in Sagehen Creek is related to the primary weathering reactions occurring the glacial till, and is consistent throughout the year, suggesting that the composition of shallow groundwater and dilution by snowmelt are the primary controls on the solute chemistry and delta O-18 composition of Sagehen Creek. Additionally, Sagehen Creek is more concentrated, and thus being fed by groundwater with longer residence times during periods of baseflow than other similar Sierra Nevada streams, suggesting a substantial groundwater system is supporting this watershed.