CONTAMINANT STORAGE IN STREAM SEDIMENTS IN A STREAM IMPACTED BY ACID MINE DRAINAGE

West Little Sugar Creek is impacted by acid mine drainage (AMD) from the abandoned Green Valley coal mine site in western Indiana. AMD that discharges from the site has a pH <3.5 and contains elevated iron, aluminum, and trace elements. The acidic effluent enters the stream channel adjacent to the site, creating iron-precipitates that coat the channel bottom. To assess the amount of contamination temporarily stored in the stream sediments and in the underlying channel substrate material, 33 core samples (3 cm diameter) were collected from channel and point bar deposits approximately every 150 m over a distance of 0.8 km from the mine site. Samples were also collected from the zone of mixing where acidic effluent discharges into the stream and from an uncontaminated section upstream from the site. Stream water pH in the channel during the sampling period ranged from pH 3.7 in the zone of mixing to pH 6.0 at the most distant sample location.

Sediment cores varied in length from 15.5 to 101.5 cm and consisted of upper gravelly sand underlain by clay. The upper unit ranged in thickness from 7.6 to 68.8 cm and was composed of approximately 50-80% medium- to coarse-grained sand intermixed with about 20-50% granule- to cobble-sized gravel, all of which was iron stained to varying degrees with the most intense staining occurring in the top 10 cm. The lower clay horizon was present in 29 of the 33 cores and was gray to dark-gray in color with no discoloration from iron oxides. The contact between the overlying sandy unit and underlying clay was relatively sharp, suggesting that the sandy material represents recent transported stream sediment and the underlying clay older material derived from glacial outwash. Samples, approximately 2.5 cm in length, of the upper gravelly sand, the stream sediment-clay interface, and underlying clay were extracted from each core and analyzed by ICPMS for Fe, Al, and trace elements.