We examined a large (240 km2) northern Appalachian bituminous coal basin (Irwin Syncline, Westmoreland County, Pennsylvania) comprising 27 mine complexes with nine major (> 2.5 x 10(3) L/min) discharges. The synclinal basin was divided into seven subbasins based on equilibrium hydraulic relationships established during the past 25 years. Recharge rates, mine pool velocity, and residence times respond to hydraulic changes in the overburden induced by mine subsidence. The estimated maximum depth for subsidence fractures is 60 m (30 times mined thickness) with recharge rates decreasing significantly in subbasins with thicker overburden (> 75 m). Calculated subbasin recharge rates range from 2 to 6 x 10(-4) L/min/m2 and are significantly lower than the previously used rate for the basin. Residence time of ground water in the Irwin subbasins calculated using average linear velocity ranged from one to five years and were more consistent with field observations than estimates obtained using discharge and basin volume area. A positive correlation (r2 = 0.80) exists between net alkalinity of the mine water-impacted discharges and residence time in the mine pools. Our results for the Irwin coal basin suggest that use of a subbasin approach incorporating overburden depth, mining methodology, and the extent of postmining inundation will lead to improved determination of ground water flow parameters in mined watersheds in northern Appalachia and elsewhere.

Abstract. The Irwin Syncline bituminous coal basin has been extensively underground mined with numerous complexes (>95% mined over a 240 km 2 area; Pullman-Swindell, 1977). Earlier work in the Irwin Syncline demonstrated the benefit of dividing the basin into smaller sub-basins based on equilibrium flow conditions established over the past 30 years (Winters et al. 1999; Winters and Capo, in press). MODFLOW numerical modeling was undertaken to quantify the hydraulic relationships within the basin and to corroborate sub-basin delineation over time (~ 5-15 yrs) as post-mining equilibrium hydraulic conditions develop. Boundary conditions are imposed by (1) the coal outcrop, which limits hydraulic influence, (2) the low hydraulic conductivity of the coal seam floor (typically clay, K~10-8 cm/sec), and (3) large surface water bodies. Because of these constraints, recharge can be assumed to emanate primarily from infiltration through the overburden rocks. Basin discharge can be directly measured from the large discharges that developed following basin flooding.In the Irwin basin, the overburden rocks range in thickness from 0 m at the outcrop to 200 m in the interior. Overburden units were modeled as four distinct hydraulic conductivity zones that correspond to classic mine subsidence profile models (Singh 1992). Initial model results indicated that mine water is discharging through the intervening overburden to the Youghiogheny River, which overlies the southwestern portion of the basin. To calibrate the model, 15 mine pool monitoring points from the 1970's were used to establish known hydraulic head elevations in the northern 2/3 of the basin. Hydraulic head elevations in the southern 1/3 were determined from current pumping elevations at two treatment plants in the area. Preliminary results from the calibrated model demonstrate the hydrologic impact of interior coal mine barriers on the flow regime and confirm that the largest discharges (Q >0.18 m 3 /s) are the dominant influence on the flow system. Future modeling efforts will concentrate on sensitivity analysis of recharge and other hydraulic parameters and on refinement of methods used for modeling the mine-void aquifer system.