ABSTRACT

A study was undertaken to evaluate the effects on groundwater of a proposed in-pit ash disposal operation at an East Texas lignite mine. An estimated 8 million tons of fly ash was to be deposited in a mined-out part of an active lignite mine during a 5-year period. A constructed bottom liner was not planned for the operation because of the low permeability of spoil materials in which the fly ash would be encapsulated and the abundance of low-permeability native sediments surrounding the mined-out area. The disposal site, according to state regulations, would be classified as a class II nonhazardous landfill.

An intensive investigation was initiated to characterize the geologic, geotechnical, hydrogeologic, and geochemical features within and surrounding the identified disposal area. The data were used in a three-dimensional numerical flow and transport computer model (swIr) to simulate the movement of fly-ash leachate from the landfill. The computer simulations indicate that the plume of leachate will travel 200 to less than 500 ft from the perimeter of the disposal area in 100 yr. Movement will not begin until after resaturation of the spoil materials, which will likely take several decades to occur. A buffer zone of mine spoils without ash will surround the disposal area. The study was reviewed by technical staffs at state agencies, and regulatory approval for the proposed landfill operations was obtained. Fly ash is currently being disposed of at the permitted class II nonhazardous landfill.

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