PROCESSES AFFECTING DISSOLVED, LABILE, AND TOXIC METAL CONCENTRATIONS DURING MIXING OF ACID-MINE DRAINAGE AND AMBIENT SURFACE WATER DOWNSTREAM OF THE ELIZABETH COPPER MINE SUPERFUND SITE, VERMONT

A combination of laboratory, field, and modeling studies was used to determine concentrations of dissolved elements in a river that is impacted by acid-rock drainage, to evaluate the dominant physical and geochemical processes that control those concentrations, and to assess metal speciation and toxicity. Values of pH increase from 3.3 to 7.6 and the sum of dissolved base metal (Cd + Co + Cu + Ni + Pb + Zn) concentrations decreases from 6270 to 100 mg/L in the mixing and reaction zone that is downstream of the river's confluence with the acid-rock drainage. Physical and thermodynamic modeling using PHREEQC and the diffuse layer model successfully describes the behavior of dissolved elements in this system. Mixing and dilution affect the concentrations of all dissolved elements in this dynamic reach, and are the dominant processes controlling Ca, K, Li, Mn, and SO4 concentrations. Dissolved Fe concentrations also decrease throughout the reach due to precipitation of schwertmannite and ferrihydrite, whereas dissolved Al concentrations decrease by precipitation of gibbsite when pH > 3.7. Only mixing and dilution affect dissolved concentrations of Pb for pH < 3.1 and dissolved concentrations of Cd, Co, Cu, Ni, and Zn for pH < 4.5-5.6. For higher pH values, newly formed Fe precipitates adsorb the metals. Although precipitation and adsorption are critical in regulating the concentrations of dissolved metals, the uptake of metals by aquatic organisms is highly dependent on the solution speciation of the metals and on the kinetics of solution and solid complexation reactions. The Biotic Ligand Model and the Diffusive Gradients in Thin Films technique (DGT) were used to evaluate the speciation and acute toxicity of dissolved Cd, Cu, and Zn in the mixing and reaction zone. Dissolved and DGT labile metal concentrations are generally equal at pH > 4.9. Labile concentrations of Cd and Zn do not exceed LC 50 concentrations of the metals for fathead minnows and water fleas. In contrast, labile concentrations of Cu at sites closest to the confluence (~8-30 m downstream) exceed LC50 Cu concentrations for the organisms. These results are in good agreement with previously conducted toxicity tests done by U.S. EPA that indicate minimal to no survival of the organisms at a site 16 m downstream of the confluence.