In 1997 and 1998, flue gas desulfurization (FGD) material made from coal byproducts (fly ash and filter cake) was formulated into grout material and placed within an abandoned underground coal mine near Conesville, Ohio to mitigate acidic mine drainage and to test the environmental impact of coal byproducts used in this way. The hydrogeologic setting of the site consists of two perched aquifers, one in sandstone and the other in a coal layer, both overlying a sandstone water-table aquifer. Groundwater enters the perched aquifers as recharge from precipitation and flows vertically downward to the water table. Some groundwater is discharged from the coal layer at seeps and former adits. A numerical groundwater flow model was constructed using MODFLOW that shows a six- to twelve-foot build-up of groundwater is likely to occur in the mine due to grouting. Particle tracking shows the head build-up increases the vertical travel time through the coal layer and into the underlying water table aquifer from 17.6 years to 15.4 years. A chemical transport model was constructed using MODFLOWT to simulate the short- and long-term transport of dissolved grout components in the groundwater that flows from the mine. The chemical signature of the FGD grout is best described by concentrations of calcium, potassium, magnesium, and boron. Boron is the least retarded and was assigned a retardation factor of 1 (no retardation), potassium, calcium, and magnesium were assigned retardation factors of 4.5, 6.5, and 7, respectively. Transport simulations show that between 1% and 4% of the major dissolved FGD grout constituents are estimated to reach the lowest aquifer layer (layer 5) over a 20-year period and that trace constituents, like boron, will also reach layer 5 but at very low concentrations, perhaps 10% or less of their source concentrations. Participant included U.S. Dept. of Energy, U.S. Dept. of Interior, Dravo Lime Co., Ohio Coal Development Office., Ohio Dept. Nat. Res., AEP, Muskingum Conservancy Dist., Ohio EPA, and Ohio State University.