Analysis of coal combustion by-product disposal practices in an arid climate : leachate water quality

Coal combustion by-products (CCBs), produced from a large power plant in New Mexico, have been disposed of in a nearby coal mine since 1973. These CCBs consist of fly ash, bottom ash, and sludge from the flue gas desulfurization process and have elevated concentrations of hazardous constituents. There is concern that disposal of these materials in an unlined surface mine may cause ground and surface water contamination. This research project focused on determining the leachate characteristics from fresh unburied and old buried CCBs, and investigated the geochemical transformations that occur with time. Samples of CCBs ranged in age from less than a year old to over 30 years old. These samples were subjected to batch leaching tests as well leaching in unsaturated column tests. The mineralogy and physical characteristics of the CCBs and spoil were determined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The analyses found that unburied and buried ash contained elevated concentrations of arsenic and barium relative to spoil material while all other constituents analyzed had similar concentrations to those found in the native soils/spoil. Concentrations of chemical constituents in ash samples varied with sample depth to which suggested geochemical evolution of CCBs as a function of time. These changes are attributed to age, water chemistry, process changes within the power plant and other factors. Of the chemical constituents analyzed, barium and arsenic demonstrated the largest change in concentration over depth. The remaining constituents showed relatively no change over depth and were in trace amounts. As depth of sampling increased, the age of recovered ash was likely to have increased as well. Mineralogical examination found evidence of dissolution of some mineral phases on buried CCBs. Older ash samples showed evidence of secondary mineralization leading to formation of calcite. Dissolution of aluminosilicate minerals was found in unsaturated column leach tests as increasing concentrations of aluminum, silica, lithium and vanadium. Barium showed the highest concentration when leached both with ground and deionized water. Although present in underlying groundwater, boron was not present at elevated concentrations in any of the ash sample leachates or column tests.New Mexico Energy, Minerals and Natural Resources DepartmentCivil EngineeringMastersUniversity of New Mexico. Dept. of Civil EngineeringThomson, BruceStormont, JohnHart, Kar