Coal Combustion Residuals (CCRs) and the Occurrence of Hexavalent Chromium (Cr VI)

Abstract

Each year in the U.S., coal-fired power plants generate over 100 million tons of coal combustion residuals (CCRs). Notable CCR spills in Tennessee and North Carolina drew attention to the potential hazards of CCR storage in surface impoundments, which have been reported to contribute pollutants to groundwater and vicinal surface waters. A particular public concern is focused on hexavalent chromium, Cr (VI), contamination of water resources and possible links to CCR contamination since CCRs contain elevated chromium (Cr) levels. Most of the toxicity associated with Cr is attributed to exposures to Cr (VI) compounds, which are respiratory carcinogens in humans. The common assumption that Cr (VI) is associated mainly with anthropogenic activities, and the detection of Cr (VI) in drinking water wells located near coal ash ponds perpetuates this public concern. To examine the occurrence of Cr (VI) in CCRs produced across the U.S., this study used modified Leaching Environmental Assessment Framework (LEAF) methods to provide an analysis of Cr (VI) levels and Cr speciation in leachates of CCRs obtained from coal-fired power plants sourcing coal from the Appalachian, Illinois, and Powder River Basins. Cr was found to be leachable from CCRs and existed mostly as Cr (VI), regardless of the magnitude of Cr mobilization. Increasing the liquid-solid ratio increased the Cr (VI) release from the CCRs, but decreased the Cr (VI) concentration in the leachate. Low Cr (VI) concentrations (less than 0.3 ppb) in surface waters and effluents were found at two coal-fired plants in Tennessee, which could be explained in part by dilution. Evaluation of the relationships between pH and Cr (VI) release did not show significant effects for CCRs originating from the different coal basins. However, CCRs were found to have apparent buffer capacities and to control the ambient pH conditions in the leachates. The buffering capacity of CCRs raised leachate pH values, forcing alkaline conditions (pH range: 5.36 to 12.46, mean pH: 9.79 ± 1.74). Subsequent analysis indicated Cr (VI) release increased with increases in the end-pH of leachates and that differences existed in the apparent buffer capacity/alkalinity between CCRs sourced from different basins.