AN ANALYSIS OF TRACE METALS IN AN ABANDONED MINE DRAINAGE PASSIVE TREATMENT SITE, NORTHUMBERLAND COUNTY, PENNSYLVANIA, USA

Abandoned mine drainage (AMD) causes harmful environmental effects because of the low pH and high concentrations of iron and toxic metals. Passive treatment centers mitigate contamination from these toxic metals in local watersheds. Scarlift 15, a passive treatment site in the Shamokin Creek watershed in the Western Middle Anthracite Field of Pennsylvania, is designed to raise the pH and remove iron by precipitating it as iron oxy-hydroxide or iron sulfide through 2 different pathways. The treatment site has one oxidation pond and three vertical flow wetlands (VFWs), allowing water to flow through the ponds or through the mushroom compost and limestone in the VFWs. These two pathways facilitate reactions to happen both anaerobically and aerobically. The anaerobic reactions take place because the mushroom compost induces reduction conditions and then the limestone-produced alkalinity causes the pH to increase, where the iron could precipitate as iron sulfide from both the increased pH and lack of oxygen. The oxidation reactions that take place in the ponds above the VFWs precipitate iron as iron oxyhydroxides.

While passive treatment systems are designed to decrease the concentration of dissolved iron, the fate of the trace metals in these systems is not well understood. To determine the fate of the other metals, samples from Scarlift 15 at the influent, the final overflow under aerobic conditions, the final set of standpipes discharging VFW water, and the final effluent were analyzed for iron and trace metal concentrations in dissolved and colloidal phases. The iron primarily precipitates in the aerobic reactions, while aluminum precipitates in the anaerobic conditions from the increased pH. The trace metals tend to follow the same precipitation pattern as aluminum, perhaps co-precipitating out in the VFWs. In the overflow, in oxidizing conditions, the pH decreases because of hydrolysis of Fe³⁺, but increases in the VFWs and effluent from the alkalinity added by the limestone. The main removal of Fe from solution by oxidation generating a lower pH may inhibit the adsorption of other metals. The increased pH in the VFWs may drive trace metal adsorption onto aluminum precipitates rather than iron oxyhydroxides by increasing the negative charge on precipitated aluminum.