GEOLOGIC CONTROLS ON ARSENIC DISCHARGE TO A MINE-IMPACTED HEADWATER STREAM

At an abandoned arsenopyrite mine in southwestern Virginia, we are using a combination of monitoring and experimental methods to identify the hydrogeochemical and biological processes that control arsenic flux to an adjacent headwater stream. Monitoring results demonstrate significant spatial and temporal variations in arsenic speciation and concentration in stream, spring, and hyporheic water. Variation appears to be controlled not by the location of tailings piles but by the location of spring channels and groundwater discharge areas, characterized by extremely high (> 5000 µg/L) arsenic concentrations.

Electrical resistivity surveys combined with stream gaging are being used to discern patterns of groundwater discharge and arsenic flux to the stream. Tracer dilution gaging indicates that the stream receives groundwater discharge along the study reach throughout the year. Resistivity surveys show discrete zones of higher hydraulic conductivity in bedrock underlying the stream. These patterns are being compared with geochemical data to assess whether high-arsenic groundwater is being channeled along preferential flowpaths.

We plan to conduct subsurface tracer experiments to yield additional information on flowpaths across the ground water-surface water interface, with the goal of delineating the stream’s capture zone. Because surface water is more accessible than groundwater, determining how and where high-arsenic groundwater discharges to the stream will greatly aid in remediation efforts.