TEMPORAL AND SPATIAL VARIATIONS IN STREAM WATER CHEMISTRY IN A WATERSHED IMPACTED BY ACID MINE DRAINAGE

Lake Harris, a man-made reservoir constructed in 1929, is a source of water for industry and the tertiary source of domestic drinking water for Tuscaloosa County, Alabama. Coal-rich deposits hosted by the Pottsville Sandstone in the Black Warrior Basin attracted the coal mining industry to the area during the 1960s and 1970s. Between 1969 and 1976, shallow coal seams were removed by strip mining and the site was abandoned without reclamation. During a decade of exposure to weathering and erosion, increased rates of sedimentation and impacts from acid mine drainage (AMD) destroyed the aquatic ecosystem of a first-order stream feeding the lake. Lake Harris Stream still shows the effects of AMD, 30 years after reclamation. This study examines the temporal and spatial variations of AMD conditions in Lake Harris Stream and identifies AMD impacts on stream water chemistry. Aqueous stream samples were collected monthly from Lake Harris, the study stream, and a control stream in an adjacent watershed. The samples were analyzed by ICP-OES and IC for cation and anion concentrations, respectively. Temperature, pH, and conductivity were measured in the field and alkalinity/acidity titrations were conducted in the laboratory within 48 hours. The pH of Lake Harris Stream water samples ranged from 6-7 between December through February, and dropped to 3-5 during the remainder of the year. In contrast, the control stream showed pH in the 6-7 range throughout the study period. For a given month, acidic stream water conditions resulted in elevated levels of sulfate, Fe, Mn, Al, B, Ba, Co, Cr, Ni and Sr at those sample sites. In addition, Fe, Mn, Co, Cr, Ni, and Sr concentrations showed strong positive correlations with one another. Temporally, for a specific site, Fe and Mn concentrations decreased with pH, while Al, B, Ba, Co, Cr, Ni, and Sr concentrations increased. During base flow conditions, stream water Fe concentrations spiked in the mixing zone at the mouth of the stream. PHREEQC models are being run to determine mineral sources/sinks for stream water solutes and to perform mixing simulations between stream and lake water. Ultimately, NETPATH modeling will be used to determine the relative importance of water sources as a function of seasonality, and the potential impacts of continued AMD discharge on lake water quality and the food web.