Geohydrologic evaluation of a proposed coal-ash disposal site near Mandan, North Dakota
Montana-Dakota Utilities Company (MDU) operates Heskett Station, a 100 megawatt coal-fired power plant, located near Mandan, North Dakota. In the process of generating electricity, MDU's Heskett Station produces approximately 62,000 tons of coal-ash annually. The ash has been disposed of by above-ground stockpiling on the plant site for the past 35 years. Limited storage space and new solid waste regulatory restrictions have necessitated the search for a new ash disposal site. This investigation focused on selecting a disposal site that would be suitable for long-term disposal of coal-ash generated at Heskett Station. Specifically, the objective was to locate a site that would require minima] engineering redesign and use in situ materials for leachate containment and trace element attenuation. Several areas were considered and ultimately one site was selected for detailed geohydrologic evaluation. The site which is being proposed for ash disposal is located approximately one-quarter mile west of Heskett Station. A total of 27 monitoring wells were installed at the proposed site. Monitoring well water levels indicated the presence of a static water table which is generally 30-40 feet below the ground surface. The groundwater beneath the proposed site is flowing north-northeast. Potentiometric levels indicate that there is a strong downward component of groundwater flow over the entire proposed disposal site (recharge area). Chemical analysis of water samples obtained from the various monitoring wells at the proposed site indicate that the shallow ground water is highly mineralized with total-dissolved-solids (TDS) concentrations commonly exceeding 8,000 mg/L. The groundwater existing in the pre-disposal setting is of similar quality to the simulated ash leachate generated during EPA-EP toxicity testing. Cadmium, arsenic and lead were the only trace elements leached from the ash which slightly exceed primary drinking water standards. During the operational phase of ash disposal the objective should be to minimize leachate production by continual reclamation of ash filled "cells". Covering the cells with low permeability excavated materials, along with the high climatically induced evapotranspiration, will minimize infiltration and thus leachate production. A dry disposal setting can be further assured by maintaining a 5- to 10-foot buffer/ attenuation zone between the pit bottom and the elevation of the highest recorded water table. If these disposal site design criteria are followed, coal-ash emplaced in this proposed facility will have a minimal impact on groundwater quality and, in general, on the geohydrologic flow regime.
Coal-fired power plants--North Dakota--Waste disposal; Coal ash sites--North Dakota; Groundwater--Sampling--North Dakota, Geology
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