SEDIMENT PORE-WATER EQUILIBRIUM INTERACTIONS ASSOCIATED WITH ARSENIC AND URANIUM TRANSPORT WITHIN A HISTORICAL URANIUM MINING IMPACTED WATERSHED, HARDING COUNTY, SD

The purpose of this study was to determine how localized changes in sediment redox behavior influence remobilization of As and U within impacted stream sediments of the Cave Hills uranium mining region. Five pore-water equilibration samplers (peepers) were spatially and temporally deployed, and As and U sediment pore water concentrations and speciation were determined and correlated to iron reduction, ORP, and pH behavior. Soil cores collected adjacent to pore water sampler locations were analyzed for solid-phase metal concentration using XRF. For conditions within the sedimentation pond adjacent to existing mine tailings, redox was consistently +200mV, and As concentrations increased as a function of depth for both the solid (53 mg/kg maximum) and aqueous phases (108 μg/L maximum), with As(V) existing as the predominant arsenic phase at depths greater than 5cm. Approximately 2 km downstream of the sedimentation pond at a wetlands-dominated deposition zone, oxidizing condition again existed throughout depth, and As(V) was the predominant As specie. Surface water concentrations of As (490 μg/L) were 25x background concentrations and were significantly greater than measured at the same site 3 months prior (<10 μg/L) during high-seasonal flow, suggesting seasonal conditions influence As transport. Pore-water U concentrations (781 μg/L) were 3.5x greater than the surface water, and approximately 40x background concentration. Solid phase As (53 mg/kg) and U (32 mg/kg) were consistent throughout depth, indicating that solid phase dissolution is actively occurring. For the sampling site near the Bowman-Haley reservoir backwaters, reducing conditions began directly below the sediment-water interface, and Fe(II) concentrations increased with depth as a result of iron reduction. Pore-water As concentrations peaked (41.6 μg/L) below the sediment-water interface while As(III)/As(V) ratios decreased with depth, promoting the formation of mobile As(III) under these iron reducing conditions. The study results suggest that localized redox conditions, especially those dominated by (bio)geochemically-influenced iron reductive processes, appear to influence both As and U behavior from within these contaminant watershed sediments.