DEVELOPMENT OF A REGIONAL MINE AQUIFER IN POST-CLOSURE PORTIONS OF THE PITTSBURGH COAL BASIN

The synclinal basin of the Pittsburgh coal (Upper Pennsylvanian Monongahela Group) is one of the oldest districts in the Eastern coal fields. Reserves have been 50-60% extracted; mining continues in about 12 predominantly deep mines in the center of the basin. However, more than 1000 mines have been closed, and most of these along the 220-km perimeter outcrop are either flooding (water level still rising) or flooded (new hydraulic steady state achieved). Most large mines in the basin have flooded within the period 1980-2003, turning what had once been an active mining district into a large regional aquifer in portions of three states. Extent of flooded mine areas have been identified based on monitoring well data, groundwater flow modeling, and GIS mapping to create a map history of flooding progress at decadal scale. The timescale for flooding of individual mines is generally a decade or less, depending on local infiltration rate, rates of barrier leakage between mines, and water management activities. The water chemistry of mines soon after flooding is complete is generally acidic and high in iron, but within 10 years after flooding, water becomes net alkaline due to low oxygen levels caused by flooding. Several older (ca 50 years) discharges from fully-flooded mines in select portions of the basin are not only alkaline, but have negligible iron. If these discharges are indicative of the rate of chemical ameliorization of flooded Pittsburgh mine discharges, then the Pittsburgh mine aquifer may become a water resource of considerable extent and capacity within a foreseeable timeframe.