| Surface water of Burra Burra and North Potato creeks in the Ducktown Mining District, Tennessee, is significantly contaminated by acid mine-drainage (AMD). The highest measured concentrations of dissolved constituents in the AMD are: 680 mg/L Fe, 110 mg/I, Al, 160 mg/L Mn, 2000 mg/L SO4, 53000 μg/L Zn, 8200 μg/L Cu, 3300 μg/L Co, 650 μg/L Ni, and 200 μg/L Pb. The concentrations of dissolved trace metals in both creeks are mainly controlled by sorption to suspended ferric hydroxide, which precipitates as the pH increases. The water discharged into the Ocoee River by North Potato Creek is strongly acidic (pH = 3.7) and contains high concentrations of trace metals. The trace metals are sorbed to ferric hydroxide, which is ultimately deposited in Ocoee No. 3 Lake.; The chemical composition of artificial AMD produced by weathering of sulfide-bearing ore depends on the chemical and mineralogical composition of the ore. The solute concentrations of AMD during artificial weathering of ores mainly depend on: (1) dissolution of different minerals at different rates; (2) precipitation of secondary compounds such as ferric hydroxide; and (3) sorption or desorption of trace metals to the secondary ferric hydroxide.; The chemical composition of natural AMD evolves from Fe-rich to Al-rich and finally to become Mn-rich as the pH increases from strongly acidic to circumneutral because dissolved Fe, Al, and Mn are sequentially precipitated as oxyhydroxides or hydroxy sulfates from solution at pH < 4, ≈ 5, and >7, respectively. The evolution of AMD is an important geochemical process because dissolved trace metals are sorbed to the precipitates at different pH values depending on the amount and composition of the sorbent and on the chemical composition of the water. The sequence of pH-dependent sorption of trace metals is Pb > Cu > Zn ≈ Ni > Co ≈ Cd from natural AMD.; The sediment of Ocoee No. 3 Lake is well homogenized and contains significant amounts of Fe, Al, Mn, Cu, Zn, Pb, Ni, and Co in acid-leachable form. These metals are released into the water in reverse order of their pH-dependent sorption when the water is acidified. Therefore, the metal-rich sediment is a potential threat in case the water is later acidified. In addition, if the reservoir must be dredged in the future, the sediment will require remediation before it can be safely disposed of. |
