CONTROLS FOR SEDIMENT COMPOSITION VARIATION IN A STREAM IMPACTED BY ACID MINE DRAINAGE

Lake Harris is a man-made lake in Tuscaloosa County, Alabama that serves as the tertiary source of drinking water for the county. Coal-rich deposits attracted mining companies to the area and a coal strip mine operated within the watershed of Lake Harris between 1968 and 1976. The process of mining coal exposes the bedrock to weathering and releases harmful chemicals into the streams, soil, and groundwater - a process known as acid mine drainage (AMD). The mined area was abandoned until it was reclaimed in 1986. Lake Harris Stream is a first-order stream that originates within the strip mine and empties into Lake Harris. Thirty years after reclamation, the stream continues to show effects of AMD. Studies have been conducted to observe the water chemistry of the stream but not the sediment chemistry. This study examines spatial and seasonal changes in the chemistry of Lake Harris Stream sediment to determine how it is affected by AMD.

Samples of sediment were collected monthly from five locations along Lake Harris Stream for a 12 month period beginning in Nov 2015. Samples collected in Feb, May, Aug, and Nov were selected for microwave-assisted acid digestion to represent temporal changes in sediment chemistry during the sampling period. The sediment samples were air dried and sized through a 60 mesh sieve before partial digestion using EPA Method 3051A, which extracts only the environmentally available fraction. After digestion, the aqueous samples were analyzed using ICP-OES.

The data were plotted to identify spatial and seasonal changes in sediment composition. Sediment Fe and Al concentrations generally increase upstream and have higher concentrations in May/Aug compared to Nov/Feb at a particular sample site. However, unusually high concentrations of Fe and Al were observed in sediment collected from site LH2 in Nov. Site LH2 sediment had highest Mn concentration in Aug. Sediment Ca and Mg concentrations decreased and then increased upstream, with lowest concentrations observed at site LH2 in Feb/May or LH3 in Aug/Nov. The sediment data will be statistically analyzed to show element correlations and indicate possible trace element concentration controls. Sediment samples are also being analyzed by XRD to identify their mineral compositions and help explain the observed spatial and temporal element distributions.