Soil Aquifer Treatment Groundwater Tracer Investigation

Percolation through the unsaturated zone and subsequent groundwater transport and storage provide final polishing of reclaimed wastewater used for indirect potable reuse. This process is termed soilaquifer treatment (SAT) and is the focus of a study being conducted in Southern California and Arizona entitled “Investigation of Soil Aquifer Treatment for Sustainable Water Reuse.” The project is evaluating the efficacy and sustainability of several SAT systems, including operating field sites and laboratory soil columns. Research has been directed at assessing the fate and transformation of organics, nitrogen and pathogens in applied reclaimed water and post-SAT water. Tracer work is being conducted to establish travel and dilution rates so that fate and transport mechanisms can be characterized and confirmed. The Sanitation Districts of Los Angeles County in collaboration with the Lawrence Livermore National Laboratory and the University of Arizona, have conducted a tracer study at one of the primary field sites, the Montebello Forebay Groundwater Recharge Project in Los Angeles County, California. This site has used reclaimed water as a source of groundwater replenishment for over 30 years.The study used three specific tracers: Xenon-136 (136Xe), Xenon-124 (124Xe), and Boron-11 (11B). Each tracer had been used successfully at other sites for travel time estimations. The Xenon tracers were added in gaseous form to reclaimed water contained within a spreading basin. The boron tracer was added in the form of isotopically enriched boric acid powder. All three tracers were added on the same date. Samples were collected from monitoring wells located adjacent to the spreading basin and located over a mile away from the basin, at perforated depths ranging from 30 to 1,130 feet deep. Production wells within the vicinity have also been also monitored with perforated depths from 230 to 888 feet deep. Results from the study will be presented along with recommendations for additional tracer investigations.