NATURAL AND ANTHROPOGENIC SOURCES OF ARSENIC AND NITRATE IN A SEMI-ARID ALLUVIAL BASIN; GOSHEN VALLEY, UTAH, USA

Arsenic (As) and nitrate (NO₃) are common contaminants in groundwater that are introduced through a variety of natural and anthropogenic sources. In this study we investigated the sources and distribution of As and NO₃ in Goshen Valley, Utah, USA. Goshen Valley is a semi-arid alluvial basin in the western US that is impacted by geothermal waters, agriculture, urban development, and legacy mining. We sampled surface water, springs, and wells to analyze As and NO₃ concentrations in relation to major ions, trace elements (As, B, Ba, Ca, Fe, K, Li, Mg, Mn, Mo, Pb, Si, Sr, Zn), and stable isotopes in water (δ¹⁸O and δD). A subset of samples were also analyzed for ⁸⁷Sr/⁸⁶Sr, δ³⁴S, and tritium (³H). Major ion concentrations showed high spatial variability ranging from freshwater to brines, with the highest concentrations found in springs discharging from playa sediments. Likewise, the highest trace element concentrations, including As, were found in the playa-associated springs. Elevated NO₃ concentrations were found in springs and wells in agricultural areas of the valley. δ¹⁸O and δD values show that the valley contains old groundwater, evaporative surface water, and mixed water signatures in multiple wells. Tritium values range from 0.5 to 7.8 and further show the diversity of water in the valley by indicating old, young, and mixtures of old and young waters. Variations in ⁸⁷Sr/⁸⁶Sr were used to evaluate flowpaths of waters with elevated As. ⁸⁷Sr/⁸⁶Sr ratios suggest that the groundwater has interacted with a mixture of lithologic units including Tertiary volcanic rocks, Paleozoic carbonate rocks, and Quaternary alluvial/lacustrine fill. Correlations with As and playa affected springs indicate playa sediments as a major As source. The As found in wells has no apparent elemental correlations or spatial patterns and is likely due to the naturally occurring sources in the valley alluvium, lacustrine sediments, and/or carbonate rocks. NO₃ in the valley is concentrated in agricultural areas and is likely due to fertilizers, livestock, and alfalfa crops. Of all the potential contaminant sources, the data suggest that the major source of As is the saline playa soils and the major source of NO₃ is agricultural activities in the valley.