Arsenic and other oxyanion-forming trace elements in an alluvial basin aquifer: Evaluating sources and mobilization by isotopic tracers (Sr, B, S, O, H, Ra)

The Willcox Basin is a hydrologically closed basin in semi-arid southeastern Arizona (USA) and, like many other alluvial basins in the southwestern USA, is characterized by oxic, near-neutral to slightly basic groundwater containing naturally elevated levels of oxyanion-forming trace elements such as As. This study evaluates the sources and mobilization of these oxyanionic trace elements of health significance by using several isotopic tracers of water–rock interaction and groundwater sources (87Sr/86Sr, δ34SSO4, δ11B, δ2H, δ18O, 3H). Values of δ2H (−85‰ to −64‰) and δ18O (−11.8‰ to −8.6‰) are consistent with precipitation and groundwater in adjacent alluvial basins, and low to non-detectable 3H activities further imply that modern recharge is slow in this semi-arid environment. Large variations in 87Sr/86Sr ratios imply that groundwater has interacted with multiple sediment sources that constitute the basin-fill aquifer, including Tertiary felsic volcanic rocks, Paleozoic sedimentary rocks, and Proterozoic crystalline rocks. In general, low concentrations of oxyanion-forming trace elements and F− are associated with a group of waters exhibiting highly radiogenic values of 87Sr/86Sr (0.72064–0.73336) consistent with waters in Proterozoic crystalline rocks in the mountain blocks (0.73247–0.75010). Generally higher As concentrations (2–29 μg L−1), other oxyanion-forming trace element concentrations (B, V, Cr, Se, Mo, Sb), and F− concentrations are associated with a group of waters exhibiting lower 87Sr/86Sr ratios (0.71012–0.71503), suggesting that sediments derived from Tertiary felsic volcanic rocks are a significant source of these trace elements in groundwater. δ34SSO4 (2.0–13.4‰) and δ11B (−6.7‰ to 18.2‰) variations suggest that rain-derived B and SO 4 2 - plus marine gypsum derived from Paleozoic sedimentary rocks and B derived from felsic volcanic rocks explain the observed values. In addition to the sources of these elemental and isotopic signatures, down-gradient hydrogeochemical modification associated with silicate weathering and perhaps cation exchange are recorded by the decreasing Ca/Na ratio. Very low levels of 226Ra in groundwater relative to U throughout the basin-fill aquifer are consistent with the oxic, neutral to high-pH conditions in which mobility of U is strongly favored relative to Ra. Used in combination, multiple isotopic and geochemical tracers can provide valuable information for understanding the relative importance of sediment source and hydrogeochemical processes for observed levels of As and other naturally-occurring trace elements. These tracers, in particular 87Sr/86Sr with its sensitivity to aquifer sediment source, can be applied to other alluvial basin-fill aquifers in the southwestern USA where multiple sediment types contribute varying amounts of As and other oxyanion-forming trace elements to groundwater.