Sediment Pore-Water Interactions Associated With Arsenic and Uranium Transport Within a Uranium Mining-Impacted Watershed in South Dakota

Abstract

The extent of historical uranium mining impacts is well documented for the North Cave Hills region of Harding County, South Dakota. While previous studies indicate watershed sediment and surface water exhibit up to 90x established background concentrations for arsenic (As) and uranium (U), it was unclear whether or how localized changes in sediment redox behavior influence contaminant remobilization. Five pore-water equilibration samplers (peepers) were spatially and temporally deployed within the study area to evaluate seasonal solid-liquid equilibria as a function of sediment depth. At a sampling site 2 km downstream of the mine site within a wetlands-dominated deposition zone, seasonal variations of pore-water geochemistry were observed. Summer conditions promoted strongly reducing conditions, resulting in the remobilization of sediment-bound As(III). Fall conditions promoted oxidizing conditions within the sediments, resulting in decreased As (5x) and increased U (10x) concentrations within the sediment pore-waters. Peak pore-water U concentrations (781 μg/L) were 3.5x greater than observed within the surface water (226 μg/L) immediately above the sediments. Pore-water As(V) concentrations peaked directly below the sediment-water interface, suggesting As(V) was scavenged and accumulated by strong interactions with surficial iron (hydr)oxides. The study results suggest that localized redox conditions, especially those dominated by (bio)geochemically-influenced iron and sulfur reducing processes, may influence seasonal As and U behavior within these mining impacted alluvium sediments.