A multi-tracer and well-bore flow profile approach to determine occurrence, movement, and sources of perchlorate in groundwater

Highlights: • Perchlorate in groundwater at concentrations above health-based thresholds. • Well-bore flow and depth dependent water quality data provides detailed distribution of tracers and perchlorate in aquifers. • Age dating, stable isotopic and chemical tracers used to determine source and movement of perchlorate in aquifer systems. • Multiple sources appear to have contributed to perchlorate contamination. • Perchlorate fluxes from the unsaturated zone appear to be an important contributor to contamination. The purpose of this study is to determine the occurrence, movement, and sources of perchlorate in groundwater using a comprehensive set of environmental tracers coupled with discrete borehole data. Potential sources of perchlorate to groundwater at the study site have been attributed to waste disposal and industrial activities as well as to past agricultural operations. Perchlorate concentrations in samples ranged from <1 to 40 μg/l, with a median of 6.1 μg/l. Concentrations were relativity consistent with depth except at one site where dilution may be occurring due to the infiltration of surface water from Pyrite Creek. Well-bore flow profiles indicated that perchlorate redistribution was occurring via intra-well bore flow at one site where up to 14,000 mg/year of perchlorate could be moving from the shallower to the deeper zones of the alluvial aquifer. Natural attenuation processes of perchlorate do not appear to be widespread in groundwater but do occur in portions of the aquifer adjacent to the Santa Ana River, likely limiting the mobility of perchlorate from the southernmost extent of the mapped plume to areas farther down-gradient. Age dating tracers indicate that the advective transport of perchlorate originating from the waste disposal ponds has largely moved through the zones of the aquifer sampled. Age distributions, noble gas temperature, delta neon values and stable isotopes of water indicate that a substantial fraction of perchlorate in groundwater may have been mobilized from the unsaturated zone and/or is from the infiltration of storm water runoff originating from Pyrite Canyon.