EVOLUTION OF HISTORICALLY ACIDIC MINE DRAINAGE (AMD) TO ALKALINE DISCHARGES: IRWIN COAL BASIN, PENNSYLVANIA
To determine the subsurface processes involved in the generation of natural alkalinity, we are focusing on the relationship between water chemistry, overburden mineralogy and the hydrogeology of the Irwin Basin in southwestern Pennsylvania, a 240 sq. km bituminous coal basin comprising 27 mine complexes. Acidic sulfate-, Fe- and Al-contaminated discharges are found in the northern part of the basin; the south is dominated by alkaline discharges contaminated with Fe (20-80 ppm), sulfate, and Na (100-500 ppm). Based on equilibrium hydraulic relationships developed over the last 25 years, we divided the basin into seven sub-basins. Inverse modeling (PHREEQC) indicates that the spatial and temporal change in mine water chemistry involves processes other than dilution. Acidic discharges in the northeastern end of the basin are likely the product of surface water modified by pyrite oxidation and aluminosilicate mineral dissolution. Modeling results and Sr isotopic analysis of the southwestern discharges are consistent with the development of alkaline waters as a result of limestone dissolution enhanced by cation exchange reactions with overburden clays. The data suggest that these processes occur in deeper sub-basins (overburden thickness >60 m). A positive correlation exists between net alkalinity and groundwater residence time in the mine pools, calculated to be 1-5 years. Preliminary MODFLOW results indicate mine waters are discharging through the overburden to the Youghiogheny River, and confirm that two large discharges (Q >0.2 m3/s) are the dominant influence on the flow system.