S-35 as a Groundwater Tracer Near Managed Aquifer Recharge Facilities

Managed aquifer recharge (MAR) alleviates the stresses of the growing demand on freshwater resources by replenishing local groundwater with surplus surface water and reclaimed wastewater. Identifying groundwater travel times of reclaimed wastewater is a high priority due to water quality and public health concerns. In California, MAR facilities using reclaimed wastewater as a source for artificial recharge are required to demonstrate a subsurface residence time of six months to allow for degradation of potential contaminants. Many established artificial and naturally occurring tracer techniques for dating recharged groundwater are unable to resolve the critical subsurface time scale (six month to one year) or are laborious in both field application and laboratory analyses. These limitations are the motivation for this study to evaluate the potential of a novel groundwater tracer method using a naturally occurring radioisotope of sulfur, ³⁵S.  The short half-life of ³⁵S (87 days) is ideal for investigating recharge and transport of MAR groundwater on timescales of less than one year. The groundwater residence times at two MAR facilities in the Greater Los Angeles Region have previously been characterized using artificial tracers, which serve as a comparison for the ³⁵S tracer method.  In preliminary sampling and analysis, ³⁵S groundwater ages at both sites revealed reasonable ages ranging from 4 to 66 weeks, warranting further investigation into this method.  This study is ongoing with surface water and groundwater samples scheduled to be collected and analyzed throughout the remainder of 2012.