Dalton Hardisty

"Hydrochemical conditions at an abandoned coal mine site: implicaitons for abandoned mine lands reclamation."
BSES Senior Research 2009

Abstract

The storage of coal mine wastes on the surface for nearly forty years at the Minnehaha Abandoned Mine Lands (AML) site (Dugger, IN) has resulted in highly acidic ground water. The acidic water is caused by the reaction of pyrite from coal waste with water and atmospheric oxygen. The products of this reaction include high concentrations of water soluble sulfate and metals and low pH. At the Minnehaha site acidic water is seeping from the impounded coal waste into Mud Creek which is the principal trunk stream in the area. The purpose of this study was to evaluate the chemical characteristics of the groundwater and waste materials and to formulate a water budget for the waste deposit. Five wells were installed in the tailings/slurry section of the waste deposit. Chemical analyses of major ions and field chemistry of the groundwater revealed high concentrations of sulfate and iron within all the wells (table 1) as well as pH values as low as 3.3 (2.7 at the seep). Calculation of water budgets from individual precipitation events throughout 2008 were performed using measurements of soil moisture, water table elevation, precipitation, and weather parameters necessary for the calculation of potential evaporation (Penman). This analysis revealed that a large percentage of precipitation is contributed to recharge (31-58%). Analysis of cores for sulfide mineral content (pyritic sulfur % by weight) were used as an indicator of weathering as defined by the amount of pyrite oxidation at a series of depths down the soil profile at each well. The data from one well (MW-03) revealed that the soil is completely unweathered and the water table is near the ground surface (figure 1b).

A correlation between depth of weathering and depth of the saturated zone seems evident in the data implying that weathering does not occur within the zone of saturation due to the anoxic conditions. Similar comparisons were performed for the remaining wells where in each case the weathering zone created by unsaturated conditions was similar to the depth of weathering determined from pyritic sulfur analysis. MW-14 was typical of most of the wells on site with a deeper water table than MW-03 (figure 1a). At that location the depth of weathering extends to a depth of at least 6 feet. These findings have implications for reclamation because the weathered unsaturated zone will not likely generate new acidity if left undisturbed. To reduce pressure on the seep, the water table needs to be lowered, starving the seep. This will increase the thickness of the unsaturated zone, but by creating a network of channels to promote surface runoff, the acid generation could be reduced by decreasing the amount of groundwater recharge.