Quantifying annual groundwater recharge and storage in the central Sierra Nevada using naturally-occurring 35S

Identifying aquifer vulnerability to climate change is of vital importance in the Sierra Nevada and other snow-dominated basins where groundwatersystems are essential to water supply and ecosystem health. Quantifying the component of new (current year's) snowmeltin groundwaterand surface water is useful in evaluating aquifer vulnerability because significant annual recharge may indicate that streamflowwill respond rapidly to annual variability in precipitation, followed by more gradual decreases in recharge as recharge declines over decades. Hydrologic models and field-based studies have indicated that young (<1 year) water is an important of component of streamflow. The goal of this study was to utilize the short-lived, naturally-occurring cosmogenic isotope sulfur-35 (35S) to quantify new snowmeltcontribution to groundwaterand surface waters in Sagehen Creek Basin (SCB) and Martis Valley GroundwaterBasin (MVGB) located within the Tertiary volcanics of the central Sierra Nevada, California. Activities of 35S were measured in dissolved sulfate (35SO42-) in SCB and MVGB snowpack, groundwater, springs, and streamflow. The percent of new snowmelt(PNS) in SCB streamflowranged from 0.2 ± 6.6% during baseflowconditions to 14.0 ± 3.4% during high flow periods of snowmelt. Similar to SCB, the PNS in MVGB groundwaterand streamflowwas typically <30% with the largest fractions occurring in late spring or early summer following peak streamflow. The consistently low PNS suggests that a significant fraction of annual snowmeltin SCB and MVGB recharges groundwater, and groundwatercontributions to streamflowin these systems have the potential to mitigate climate change impacts on runoff.