Quantifying Apparent Groundwater Ages near Managed Aquifer Recharge Operations Using Radio-Sulfur (35S) as an Intrinsic Tracer

The application of the cosmogenic radioisotope sulfur-35 (S-35) as a chronometer near spreading basins is evaluated at two well-established Managed Aquifer Recharge (MAR) sites: the Atlantis facility (South Africa) and Orange County Water District's (OCWD's) Kraemer Basin (Northern Orange County, CA, USA). Source water for both of these sites includes recycled wastewater. Despite lying nearer to the outlet end of their respective watersheds than to the headwaters, S-35 was detected in most of the water sampled, including from wells found close to the spreading ponds and in the source water. Dilution with S-35-dead continental SO4 was minimal, a surprising finding given its short similar to 3 month half-life. The initial work at the Atlantis MAR site demonstrated that remote laboratories could be set up and that small volume samples-saline solutions collected after the resin elution step from the recently developed batch method described below-can be stored and transported to the counting laboratory. This study also showed that the batch method needed to be altered to remove unknown compounds eluted from the resin along with SO4. Using the improved batch method, times series measurements of both source and well water from OCWD's MAR site showed significant temporal variations. This result indicates that during future studies, monthly to semi-monthly sampling should be conducted. Nevertheless, both of these initial studies suggest the S-35 chronometer may become a valuable tool for managing MAR sites where regulations require minimum retention times.