HYDROGEOCHEMICAL CONTROLS ON ARSENIC DISCHARGE TO A HEADWATER STREAM NEAR AN ABANDONED ARSENOPYRITE MINE

At an abandoned arsenopyrite mine in southwestern Virginia, we are conducting field, laboratory, and numerical experiments to identify the hydrogeochemical and biological processes that control arsenic fluxes to a shallow headwater stream adjacent to the mine. Surface water, spring channels, hyporheic water, and groundwater have been sampled quarterly for up to 2 years. In contrast with other mine drainage sites, the stream pH is circumneutral (6-7) and iron concentrations are generally below 0.1 mg/L due to the formation of scorodite, FeAsO₄.2H₂O, which weathers to iron hydroxides and dissolved arsenate. Monitoring results demonstrate significant spatial and temporal variations in arsenic speciation and concentration in stream, spring, and hyporheic water at the site. The variations do not appear to be controlled by the location of tailings piles but instead by the location of spring channels and groundwater discharge areas, which contain extremely high (> 5000 ppb) arsenic concentrations. Stream gaging indicates that the stream receives groundwater discharge along the reach adjacent to the mine; however, the extent to which the stream functions as a barrier to transport of arsenic-contaminated ground water is unknown.

Continuous collection of hydrologic and geochemical data, in addition to seasonal and stormflow monitoring of cations, anions, and arsenic speciation of all hydrologic components, are being conducted to (1) establish baseline hydrogeochemical conditions and (2) examine correlations between arsenic, water levels, discharge, and key parameters such as pH, DO, and other geochemical parameters. We also plan to conduct stream and subsurface tracer experiments, which will yield information on flowpaths across the ground water-surface water interface and will also aid in delineation of the stream's capture zone. Because streams are more accessible and thus more amendable to remediation than groundwater, determining the function of the stream for retaining or transporting arsenic is a first step in evaluating possible remediation strategies.