EFFECTS OF INCREASED HYDROLOGIC INPUT ON AMD WATER CHEMISTRY AT THE GREEN VALLEY MINE, INDIANA

Acid mine drainage (AMD) at the abandoned Green Valley coal mine, western Indiana, flows through several constructed channels coated with up to 45 cm of amorphous Fe hydroxides. Under normal AMD conditions, effluent ranges from pH 3.0 to 3.5 with elevated levels of Fe (up to 11,800 ppm) and Al (up to 1,840 ppm) and detectable concentrations of most major, minor, and trace elements. ICPMS analyses of 59 elements in AMD sampled weekly from May through June, 2003, at three locations show a strong positive correlation between Fe and Al, with concentrations increasing during dry periods and decreasing during periods of increased hydrologic input associated with rainfall. Significant rainfall (>2 cm) generally raises effluent pH above 3.5, resulting in the formation of amorphous Al hydroxides that are suspended in the flowing water. As flow subsides, Al hydroxides aggregate and settle to the channel bottom forming an unconsolidated white layer that is subsequently coated with Fe hydroxides as the system returns to preexisting conditions. The formation of Al and Fe hydroxides suggests that the lower concentrations of dissolved Al and Fe in the effluent during periods of increased hydrologic input are likely the result of Al and Fe precipitation rather than a consequence of dilution.

Magnesium concentrations and the concentrations of many trace metals, rare earth elements, and actinides (U and Th) behave similarly to Fe with respect to wet and dry conditions. The decreased concentrations of these elements during periods of increased hydrologic input likely reflect removal by adsorption to the surface of amorphous Fe and Al hydroxides that form at this time rather than solely an effect of dilution from increased discharge. Concentrations of Ca, Ba, As, and Se, on the other hand, do not correlate well with Fe concentrations.