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  Characterization of Coal Combustion residues from Electric Utilities - Leaching and Characterization Data
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This report evaluates changes in composition and constituent release by leaching that may occur to fly ash and other coal combustion residues (CCRs) in response to changes in air pollution control technology at coal-fired power plants. The addition of flue-gas desulfurization (FGD) systems, selective catalytic reduction, and activated carbon injection to capture mercury and other pollutants will shift mercury and other pollutants from the stack gas to fly ash, FGD gypsum, and other air pollution control residues. The objective is to understand the fate of mercury and other constituents of potential concern (COPC) in air pollution control residues and support EPA’s broader goal of ensuring that emissions being controlled in the flue gas at power plants are not later being released to other environmental media.

This report includes data on 73 CCRs [34 fly ashes, 20 flue gas desulfurization (FGD) gypsum, 7 "other" FGD residues (e.g., scrubbers without oxidation or with inhibited oxidation), and 8 blended CCRs "as managed" (e.g., scrubber sludge mixed with fly ash and lime prior to disposal)]. Each of the CCRs sampled has been analyzed for a range of physical properties, total elemental content, and leaching characteristics for mercury, aluminum, antimony, arsenic, barium, boron, cadmium, chromium, cobalt, lead, molybdenum, selenium and thallium.

The leach testing methods that were used in this research consider the impact on leaching of management conditions. These methods are intended to address concerns raised by the National Academy of Science and the EPA's Science Advisory Board with the use of single-point pH tests. Because of the range of field conditions that CCRs are managed during disposal or use as secondary (or alternative) materials, it is important to understand the leaching behavior of materials over the range of plausible field conditions that can include acid mine drainage and co-disposal of fly ash and other CCRs with pyrites or high-sulfur coal rejects. The methods have also been developed into draft protocols for inclusion in EPA's waste testing guidance document, SW-846, which would make them available for more routine use. (http://www.epa.gov/osw/hazard/testmethods/sw846/index.htm)

The major conclusions from this research include:

  • • There is great variability in both the range of total constituent concentration values and in leaching values (orders of magnitude). In comparing there results to health indicator values such as the maximum concentration limit or toxicity characteristic, there are multiple COPCs of potential concern.
  • Distinctive patterns in leaching behavior have been identified over a range of pH values that would plausibly be encountered for CCR management.
  • Total constituent content is not a good indicator of leaching which has been found to be a function of the characteristics of the material (pH) and field conditions in which the material is managed.
  • The maximum eluate concentration from leaching test results varies over a wide range in pH and is different for different CCR types and elements. This indicates that there is not a single pH for which testing is likely to provide confidence in release estimates over a wide range of disposal and beneficial use options, emphasizing the benefit of multi-pH testing. Furthermore, for CCRs, the rate of constituent release to the environment is affected by leaching conditions (in some cases dramatically so), and that leaching evaluation under a single set of conditions will, in many cases, lead to inaccurate conclusions about expected leaching in the field.

The intended use for the data in this report is to support future risk and environmental assessments of the CCRs studied. A follow-up report is planned which will use these data in conducting a probabilistic assessment of mercury and other COPCs release rates based on the range of plausible management scenarios for these materials in either disposal or beneficial use situations. The data summarized in this report will also be made available electronically through a leaching assessment tool (LeachXS Lite®) that can be used to develop source-term inputs needed for using groundwater transport and fate models. The leaching assessment tool will also provide means for data management in viewing data resulting from the of the improved leaching test methods.

Note: The appendices could not be made Section 508 accessible. For further information, please refer to contact listed below.


Susan Thorneloe

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