Analysis of Coal Combustion By-Products Disposal Practices at the San Juan Mine: Hydrologic and Water Quality Issues

Coal Combustion By-Products (CCBs) are generated by the burning of coal for electric power generation and as waste streams from emissions control systems. They consist of bottom ash from the furnaces, fly ash from electrostatic precipitators, and flue gas desulfurization sludge from air pollution scrubbers. Depending on the characteristics of the coal, CCBs may contain high concentrations of metals or other constituents that present a threat to ground or surface water resources if not properly managed. This report describes results of an investigation of CCBs generated by a power plant in northwestern New Mexico. Coal Combustion By-Products from this plant are disposed in mined out areas of a nearby surface coal mine. The objective of this project was to determine the potential for leachate from CCB disposal to contaminate underlying ground water. This was done by: 1) measuring the physical characteristics of fresh and buried CCBs that affect their unsaturated hydraulic properties; 2) determining the chemical, mineralogical, and leaching characteristics of these materials, and 3) through development of a numerical simulation of water migration through an unsaturated column of cover material and buried CCBs. The CCBs were found to have a dry bulk density of about 1100 kg/m3 and 800 kg/m3 for fly and bottom ash respectively, however, both are highly compressible and their density varies with effective stress. The saturated hydraulic conductivity of these materials was about 1x10-4 cm/s for fly ash, 5x10-3 cm/s for bottom ash, and less than 8.5x10-6 cm/s for spoil material used for cover. Soil moisture characteristic curves were measured to permit calculation of unsaturated hydraulic conductivities. Acid digestion and subsequent elemental analysis of the fly and bottom ash found them to consist primarily of aluminum, barium, calcium, potassium, sodium, and silicon. The mineralogy of these samples is dominated by amorphous glass along with mullite (Al6Si2O13), quartz (SiO2), calcite (CaCO3) and clay minerals. Flue gas desulfurization sludge primarily consists of gypsum (CaSO4 .2H2O). Batch and column leach tests of fresh and aged CCBs suggest some dissolution of amorphous materials occurs, but elemental concentrations in the leachates were moderate. Arsenic was present in both the ash material and subsequent leachates but at very low concentrations. Numerical simulation of water flow through 2 m of cover material into buried CCBs predict very low infiltration rates, less than 0.4 mm/yr, due to low hydraulic conductivity of the cover and water uptake by vegetation. At the bottom of the pit, infiltration from the CCBs into underlying sandstone was predicted to be near zero, and some modeled scenarios actually predicted an upward flux of water. The very low (or possibly zero) downward flow of ground water through the unsaturated waste predicted by the modeling study, together with the low concentrations of contaminants in leachates from buried CCBs provides evidence that the potential for contamination of the underlying regional aquifer at this site is small