干旱区生态环境知识资源中心

Groundwater resources can be impacted by contamination from geogenic and anthropogenic inputs but it can be difficult to disentangle contaminant sources. In this study, we investigated the sources and distribution of NO3 and As in Goshen Valley, UT, a semi-arid alluvial basin in the western USA that contains geothermal waters, playa soils, agriculture, and legacy mining. Surface water, springs, and wells were analyzed for As and NO3 concentrations in relation to major ions, trace elements, and stable isotopes in water (delta O-18 and delta D), and other isotopic tracers. Major ion concentrations showed high spatial variability ranging from freshwater to brackish water, with the highest salinity found in geothermal springs and springs discharging from playa sediments (Playa Springs). Radiogenic Sr-87/Sr-86 ratios in the Playa Springs suggest that Sr is sourced from crystalline basement rocks. The highest NO3 concentrations were found in groundwater beneath agricultural areas, particularly dairy farms, with isotopic values indicating manure, not fertilizers, as the major source. Many of the NO3-contaminated wells contained old groundwater (based on C-14 and H-3), suggesting that reinfiltration of pumped groundwater may be a source of NO3 pollution. The Playa Springs also had the highest As concentrations, with moderate As concentrations found in other geothermal springs. Wells containing moderate As concentrations were found in areas where the groundwater interacts with alluvial sediments or carbonate rocks. Surprisingly, nearby mining and mineral processing seems to have minimal effect on As contamination in the alluvial aquifer. This study has implications for understanding water quality in regions that are impacted by multiple potential contaminant sources.