GEOCHEMICAL ANALYSIS OF TRACE ELEMENTS IN SEDIMENT CORES OBTAINED FROM A STREAM IMPACTED BY ACID MINE DRAINAGE IN INDIANA, USA

Concentrations of trace elements were evaluated in stream sediment samples collected from a stream impacted by acid mine drainage (AMD) emanating from the abandoned Green Valley coal mine site in western Indiana. Despite reclamation efforts at Green Valley, AMD containing elevated concentrations of Fe, Al, and trace elements discharges into West Little Sugar Creek, which flows adjacent to the site. As the acidic effluent mixes with stream water, trace elements adsorb to or coprecipitate with very fine-grained Fe-oxyhydroxides and Al-hydroxides that form as pH increases with dilution. These precipitates aggregate and settle into the sediment along the channel bottom. This study evaluates the degree to which contaminants are stored in stream sediment. Twenty-eight sediment core samples were collected along a 1.5 km stretch of the stream and analyzed via hand-held x-ray fluorescence spectrometer. Two of the cores were collected upstream from the mine site and represent uncontaminated stream sediment for natural background comparison. Core samples adjacent to the mine were elevated in Fe, Mn, Zn, Sb, Sn, Cd, and Ni, in order of abundance, compared to background levels. Three of these elements (Cd, Ni, and Zn) are toxic to most aquatic life at elevated concentrations. Overall, most elements displayed a decrease in concentration with distance from the site due to downstream dilution and dispersion. Exceptions to the trend occurred for Mn, Zn, Te, and Ba, which initially decreased in concentration away from the site and then increased at the most distal end. These elements are commonly found in agricultural applications, and their increase may be associated with agricultural run-off via a small channel that flows into West Little Sugar Creek in this area. Three sediment cores were also analyzed for vertical changes in trace element concentrations with depth. In most cases, trace elements were more highly concentrated in the upper part of the core (0-30 cm), with deeper core samples testing near background levels representative of pre-mining conditions. The stream sediment data indicate trace element contaminants are temporarily stored in the stream sediment. Any remobilization from erosion during flooding may disperse these fine precipitates into the water column where they may negatively impact aquatic life.