GROUNDWATER CONDITIONS AROUND A COAL-COMBUSTION RESIDUE SITE IN SOUTHERN ILLINOIS

Abstract

Coal-combustion residues, predominantly composed of scrubber sludge, served as backfill to an abandoned surface-mine pit. A low-relief valley now trends across the area that was once the old pit, replacing the former hummocky terrain left by the mining operations. Twelve wells, installed within and adjacent to the fill, provide data on hydraulic head at the base of the spoil, in the coal, and in the coal-combustion residues. The bases of the screens ranged in depth from 5.6 to 15.9 m. Three of the wells were sampled for groundwater chemistry. Hydraulic conductivity of the spoil from nine falling-head (slug) tests ranged from 1.9X10{sup -7} to 4.1X10{sup -3} cm/s, with a geometric mean of 1.8X10{sup -4}, somewhat greater than the 9.8X10{sup -6} cm/s hydraulic conductivity of the coal-combustion residues. The mine spoil is heterogeneous, composed of disturbed surficial sediment, including loess and diamicton, blended with fragments of bedrock. Prior to reclamation, groundwater flowed away from the pit lake to topographic lows to the north, east, and south. Reclamation has not altered groundwater flow significantly. A two-dimensional numeric random-walk contaminant-transport model, based on (1) properties of the coal-combustion residue as determined from column studies, (2) data from a finite difference model of groundwatermore » flow at the site, and (3) conservative estimates of dispersivity and porosity, suggests that leachate generated by the fill poses little threat to ambient groundwater quality.« less