The geologic and hydrologic factors influencing acid mine drainage at a surface coal mine in East Tennessee : an environmental case history

The oxidation and hydrolysis of sulfide minerals exposed during coal mining operations can result in the production of high levels of acidity, high metal concentrations, and high sulfate concentrations in groundwater and receiving streams, a condition commonly referred to as acid mine drainage (AMD), or acid drainage (AD). The geochemical and microbiological factors influencing the production of AD are well understood, but the extent to which these problems can occur at a given mine site depends on a variety of conditions including: 1) mineralogy of the deposits, 2) surface/groundwater hydrology, 3) mining/backfill handling methods, and 4) reclamation practices. This study documented and evaluated the geology, hydrology, water quality, and reclamation activities at a large-scale (i.e., 500,000 tons/yr) surface coal mine in the coalfields of the southern Cumberland Plateau of Tennessee from the perspective of environmental impacts related to AD. Geochemical analyses, including electron microprobe (EMP), x-ray fluorescence (XRF), and x-ray diffraction (XRD) of representative overburden lithotypes found within the study area were performed to determine potential sources of AD within mine overburden, in particular sources of elevated manganese (Mn) concentrations detected in backfill groundwater. The hydrology of the study area was documented using groundwater elevations from observation wells emplaced in backfill, stream elevations, and hydrogeologic cross-sections of the mine site to determine general groundwater flow paths and points of discharge from the backfill. Groundwater chemical characteristics of the backfill during mining and following final pit closure were evaluated using graphs of specific groundwater chemical parameters. Modem mining techniques implemented in the study area, in particular a plan referred to as a Toxic Material Handling Plan (TMHP), was evaluated to determine the effectiveness of the plan in abating AD. Geochemical analyses of representative mine overburden of the study area identified two potential sources of AD: I) a siderite lithology containing nearly 1 weight percent Mn, and 2) a sandy shale unit containing an average of 1.35 weight percent sulfur. The siderite lithology could be considered as a potential source of mineral acidity by increasing Mn concentrations and the sandy shale unit capable of acid production in terms of lowering pH in backfill groundwater. Groundwater flow within the backfill is apparently to the east and southeastern portions of the study area with discharge from the backfill into Spring Branch and Glady Fork Creek most likely occurring on the east side of the study area. Discharge in the southeastern comer of the study area is not apparent at the present time due to pumping from dewatering wells in this area. It is expected that shutdown of the dewatering wells in this area would cause the previously observed seeps to reappear, possibly with negative impacts on stream water quality. Graphs of chemical groundwater characteristics suggests that the TMHP is having a positive affect on groundwater quality within the backfill of the study area in terms of increasing alkalinity, lowering metal concentrations, and increasing pH. Groundwater quality in observation wells located closest to the area where the TMHP was implemented exhibit higher alkalinity concentrations, higher pH values, and lower metal concentrations than observation wells located farthest from the TMHP area. Increasing alkalinity and decreasing metal concentrations in observation wells located farthest from the TMHP area suggests that the TMHP area is influencing water quality beyond the area where it was implemented. Evaluation of groundwater chemical characteristics suggests that the TMHP is a reclamation technique that could be beneficial in mitigating many of the adverse environmental impacts associated with surface coal mining.