Redox conditions and pH control trace element concentrations in a meandering stream and shallow groundwater of a semiarid mountain watershed, Red Canyon, Wyoming, USA

Abstract

The interface between oxic surface water and anoxic groundwater plays an important role in trace element cycling in mountains streams. In this investigation, stream water and shallow groundwater were sampled in a semiarid mountain catchment at Red Canyon, Wyoming, USA to evaluate variability in redox conditions and trace element concentrations. Samples were collected in July 2013 during summer baseflow conditions and in May 2014 during snowmelt runoff. Field measurements showed dissolved oxygen-saturated stream water and anoxic groundwater, with relatively higher pH in surface water. Groundwater and surface water chemistry were remarkably similar during both the summer and spring sampling events, indicating overall stability of the oxic/anoxic transition and trace element concentrations. Similar stable water isotope values indicate a close connection between the surface water and groundwater (δ18O and δD ranged from − 18.1 to − 18.9‰ and from − 140 to − 147‰, respectively). Concentrations of most trace and major elements were a factor of two higher in groundwater relative to surface water. Notably, Mn, Fe, and Ce concentrations were 7–90-fold higher in groundwater relative to surface water. In contrast, As, Se, and V concentrations were 3–30-fold lower in groundwater relative to surface water, likely due to pH- and redox-driven changes in speciation and sorption. This study is important for characterizing trace element cycling in mountainous areas, where measurements are rare, and has implications for furthering understanding of the key biogeochemical processes that occur during groundwater–surface water interactions.