USING WATER STABLE ISOTOPES TO UNDERSTAND THE HYDROGEOLOGICAL DYNAMICS OF AN ALPINE WATERSHED DURING SNOWMELT

In the western United States, alpine watersheds often serve as “water towers”, providing most of the water for down valley water supplies and reservoirs. Despite the relative importance of alpine watersheds, relatively little is known about the seasonal dynamics of mountainous groundwater systems. In this study we investigate Sagehen Creek Basin, a snowmelt-dominated watershed in the north-central Sierra Nevada, over the course of the 2022 snowmelt season. Monthly stable isotope samples were collected from perennial springs, ephemeral springs, headwater streams, and an adjacent lake from May through September to investigate how summer snowmelt interacts with the local groundwater system. We paired stable isotope sampling with measurements of in situ physical and chemical measurements including temperature, pH, dissolved oxygen, specific conductivity, and spring discharge. The samples collected at the perennial springs reveal distinctive stable isotope compositions compared with other sample types. We observe that individual spring 𝛅18O compositions were nearly constant over the summer and did not vary by more than the analytical uncertainty. These findings were complementary to the in situ water quality observations that were also invariant over the snowmelt season. We anticipate that if seasonal snowmelt was incorporated into spring waters in significant amounts, we would expect spring waters to experience declines in pH and specific conductivity. Spring discharge, in contrast, increases significantly during the summer melt season (a median increase of 200% in flow). Our combined results suggest that snowmelt recharge mobilizes deep groundwater storage, as evidenced by springs discharging a pulse of near constant geochemistry during the spring-summer melt season. Actively monitoring groundwater discharge points during the summer melt season may provide a critical window for water managers to better assess aquifer storage.