HISTORICAL GEOCHEMICAL AND BIOLOGICAL CHARACTERIZATION OF A SOUTHERN APPALACHIAN WATERSHED IMPACTED BY COAL ACID MINE DRAINAGE
AMD enters CC mid-drainage from Black Branch, a tributary with pH ~3.3 and elevated dissolved Al (10,000 ug/l), Fe (800 ug/l), Mn (4,000 ug/l), Pb (17 ug/l), Cu (27 ug/l), and Zn (170 ug/l). Mixing occurs with pH ~7 CC water precipitating Fe and Al oxyhydroxides. Analyses of both natural and artificially mixed water samples coupled with geochemical modeling document the extent of precipitation. Calculations predict sorption of Pb, Cu, As and Zn onto precipitated Al and Fe oxyhydroxides. Al and Fe precipitation on bed sediments as well as cation sorption predictions are confirmed by sequential chemical extractions. The extractions show an increase in Al, Fe, Mn, Zn, Pb, Cu and As in the labile fraction (hydroxylamine-HCL soluble) compared to sediments upstream from the mixing zone. Bioavailability of Al, Fe, Mn, Zn, Pb, Cu and As may adversely effect biota. Chemical analyses of Corbicula and Odonata, compared with Corbicula data from a national database, show increased Al, Mn, Fe, and Cu concentrations within and downstream of the mixing zone suggesting bioaccumulation in aquatic fauna.
Since 1965, mines in the headwaters have been successfully reclaimed based on a ~2 pH increase in CC, which has greatly improved water quality. The data show that current downstream mining has had negligible adverse effects on water quality. Past mining along Black Branch continues to adversely impact the drainage both chemically and biologically.