Quantifying groundwater travel time near managed recharge operations using 35S as an intrinsic tracer

Summary Identifying groundwater retention times near managed aquifer recharge (MAR) facilities is a high priority for managing water quality, especially for operations that incorporate recycled wastewater. To protect public health, California guidelines for Groundwater Replenishment Reuse Projects require a minimum 2–6 month subsurface retention time for recycled water depending on the level of disinfection, which highlights the importance of quantifying groundwater travel times on short time scales. This study developed and evaluated a new intrinsic tracer method using the naturally occurring radioisotope sulfur-35 ( 35 S). The 87.5 day half-life of 35 S is ideal for investigating groundwater travel times on the 35 S found in water as dissolved sulfate ( 35 SO 4 ) were measured in source waters and groundwater at the Rio Hondo Spreading Grounds in Los Angeles County, CA, and Orange County Groundwater RechargeFacilities in Orange County, CA. 35 SO 4 travel times are comparable to travel times determined by well-established deliberate tracer studies. The study also revealed that 35 SO 4 in MAR source water can vary seasonally and therefore careful characterization of 35 SO 4 is needed to accurately quantify groundwater travel time. More data is needed to fully assess whether or not this tracer could become a valuable tool for managers.