THE EFFECTS OF FLY ASH AND FLUE-GAS DESULFURIZATION WASTES ON GROUNDWATER QUALITY IN A RECLAIMED LIGNITE STRIP MINE DISPOSAL SITE (SOLID WASTE, HYDROGEOLOGY, HYDROGEOCHEMISTRY, NORTH DAKOTA)

Coal conversion wastes were generated at a lignite-coal-fired, steam-electric power station near Center, North Dakota, and were buried in unsaturated and saturated settings within the nearby stripmine, typical of settings used by the industry. The study ran for 8 years beginning in 1978. Approximately 240 piezometers were used for collecting groundwater level and groundwater chemistry data in the saturated zone. Pressure-vacuum lysimeters were installed to sample the unsaturated zone. Interstitial water was extracted from Shelby tube and auger samples at otherwise uninstrumented points.;More than 12,000 chemical analyses for major cations and anions, As, Se, Mo, Ba, Pb, Cr, Hg, and other constituents were performed on groundwater samples from selected settings and strata during this study. The hydrogeochemical data indicate that both the fly ash and the FGD waste initially generate highly mineralized leachates from entrained water. Leachates are characterized by relatively high Na, Mo, Se, As, and SO(,4) concentrations. Fly ash generates a high pH leachate. FGD wastes produce normal pH values.;Leachate pH is quickly buffered to the 6 to 9 range within the sediments. Arsenic and selenium are simultaneously attenuated to acceptable levels. Iron hydroxide coatings on the sediments resulting from previous alternate wetting and drying cycles may provide a mechanism for alkaline pH buffering with concurrent arsenic and selenium adsorption. Lead, cadmium, and barium are maintained at low levels by sulfate and carbonate precipitation reactions. Molybdenum remains mobile in this hydrogeochemical setting. The cation exchange capacity of Na-montmorillonite clays, may account for decreases in calcium and increases in sodium concentration levels with depth. Sulfate concentration levels decline with depth where bacterially-induced sulfate reduction apparently occurs. Pozzolanic activity within the fly ash deposits, in both saturated and unsaturated settings, apparently reduces the mobility of all constituents.;A disposal site well above the reestablished groundwater table, protected from recharge, is considered the best setting for disposal of these wastes because very little leachate forms. The native sediments attenuate As, Se, Pb, Cd, and Ba to acceptable levels and quickly buffer pH of any leachate that may leave the waste deposit. Lined disposal sites are therefore unnecessary to control regulated trace elements, but would be necessary to contain sodium, calcium, sulfate, and molybdenum migration in a saturated setting.;More than 15,000 water-level recordings allowed precise definition of groundwater flow and occurrence at the study site. Evaluation of the hydrogeological data from the Center Mine and other surface-mined areas in ND indicates that the hydrogeological regime stabilizes after reclamation near the pre-mining levels, and that the flow systems are not homogeneous. The base of the spoils commonly constitutes the major aquifer in the disturbed portion of the post-reclamation setting.