The current drought crisis in California highlights the vulnerability of freshwater resources, particularly groundwater reservoirs, which supply up to 60% of California’s water during drought years. Understanding the potential impacts of climate change on groundwater recharge and storage is critical as drought periods become more frequent in the state. Groundwater residence times provide insight into groundwater recharge and transport mechanisms and storage capacities. This study developed and evaluated a new intrinsic tracer method to quantify groundwater recharge and transport using the occurrence of the naturally-produced radioisotope sulfur-35 (35S, half-life 87.5 days) in water as dissolved sulfate (35SO4). Improvements made to established analytical techniques expand the analytical range of 35SO4, which broadens the utility of 35SO4 as a hydrologic tracer. The 35SO4 tracer method was applied to two distinct hydrologic settings: 1) high-elevation Sierra Nevada basins, and 2) low-elevations basins containing managed aquifer recharge (MAR) facilities.

In the Sierra Nevada study, the new 35SO4 method was used to empirically constrain annual groundwater recharge in Sagehen Creek Basin (SCB) and Martis Valley Groundwater Basin (MVGB). Compared to relatively high 5SO4 activity in seasonal snowmelt (5.5 ± 0.3 to 52.9 ± 3.4 mBq/L), groundwater and surface water consistently yielded low 35SO4 activities resulting in a calculated percent new snowmelt (PNS) of <30%. The consistently low PNS suggests that recent (<1 year old) snowmelt represents only a small fraction of the larger aquifer system. As snowpack continues to decline due to climate change, streamflow and springs may respond in a two phase manner: rapid response in discharge followed by more gradual decreases over decades due to declines in groundwater recharge.

The MAR study used 35SO4 to quantify groundwater travel times near MAR operations. MAR sites divert excess surface water, imported water, and reclaimed wastewater into surface-spreading ponds or direct injection wells to replenish groundwater in heavy-usage areas. Identifying groundwater travel times near MAR facilities is critical for determining the fate and transport of potential contaminants, especially for facilities that incorporate a significant portion of reclaimed wastewater. Successful application of the 35SO4 tracer method near MAR sites is dependent on careful characterization of the 35SO4 activity in source waters. Relative to established deliberate tracer experiments, which require extensive field and laboratory effort, the less intensive 35SO4 technique showed comparable groundwater travel times at two MAR facilities located in southern California. Both the Sierra Nevada and MAR studies demonstrate that 35SO4 is a valuable, yet underutilized tracer in hydrologic studies.