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

•Developed sulfur-35 (35S) method for dating managed aquifer recharge (MAR).•35S successfully measured in MAR source waters and groundwater.•35S travel times comparable to previous well-established tracer studies.•35S shows potential as a groundwater tracer for MAR systems. 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 2–6month 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 (35S). The 87.5day half-life of 35S is ideal for investigating groundwater travel times on the