Disposal of Western Fly Ash in the Northern Great Plains

Leachates from western fly ashes are typirally alkaline. Our studies indicate a strong correlation between alkalinity of western fly ash leachate and trace element concentrations. Elements of particular concern include As, Se, and Mo. A base neutralization mechanism is operative in all of the overburden types found at mine disposal sites in western North Dakota. Regional geological similarity suggests that this mechanism is operative throughout the Northern Great Plains. Although the mechanisms of neutralizatioti are speculative, laboratory experiments indicate significant neutralization at all levels of base above background levels. Long-term monitoring of fly ash disposal-sites indicates that alkaline neutralization of fly ash leachate is occurring. Further, field data indicate that toxic trace elements (particularly As and Se) in disposal site leachates decrease in concentration as the pH of the learhate is neutralized. Thus, the intrinsic corditions at Northern Great Plains fly ash disposal sites appear to promote significant attenuation of critical toxic elements found in fly ash leachates. Regardless of the pH, leachates in those settings have high concentrations of sodium and sulfate. Western fly ashes are commonly cementitious. Our studies indicate that fly ashes comnoniy develop significant strength after several months of burial, particularly if emplaced in an unsaturated disposal setting. Once cementitious reactions have occurred, the fly ashes show little potential for leaching. Thus, a combination of intrinsic disposal-site conditions and the cementitious behavior of the fly ashes suggests that surface-mine disposal of western fly ashes in the Northern Great Plains, assuming proper disposal-site selection, may not cause long-term environmental problems associated with toxic trace elements.