EVALUATING ARSENIC TRANSPORT WITHIN ANTHROPOGENIC AQUIFERS IN MINED WATERSHEDS

In a watershed in southwestern Virginia, mining of arsenopyrite (1903-1919) from a quartz sericite schist resulted in formation of waste piles consisting of host rock and roasted ore. Over time, mass wasting of these piles, which are located adjacent to a headwater stream, has generated an anthropogenic aquifer that has vastly different properties from the surrounding clay-rich regolith. Results of chemical extraction of sediment from the waste piles and aquifer indicate spatial variability in Fe, As and S concentrations, and also demonstrate preferential loss of As and S from the sediment. Monitoring results demonstrate significant spatial and temporal variation in As speciation and concentration in ground, surface, and hyporheic water, which are strongly controlled by chemical and physical weathering of these piles.

We are using a multi-method approach to identify the hydrogeochemical processes that control arsenic flux to the stream with focus on delineating groundwater flowpaths from the mine through the aquifer and across the ground water-surface water interface. Electrical resistivity surveys combined with tracer dilution gauging are being used to discern patterns of groundwater discharge and arsenic flux to the stream. Results indicate that the stream receives groundwater discharge along the study reach throughout the year, and also reveal the presence of discrete zones of higher hydraulic conductivity in bedrock underlying the stream. These patterns are being compared to geochemical data to assess whether high arsenic groundwater is being transported along preferential flowpaths. Determining the pathways of arsenic from the mine and sediment to the stream is a first step in evaluating possible remediation strategies for the site.