Hydrogeochemistry of the upper part of the Fort Union group in the Gascoyne lignite strip-mining area, North Dakota

Abstract

Hydrogeochemical investigations were initiated in 1973 in response to public concern about the effects of lignite strip mining on the quantity and quality of shallow ground water. The Harmon lignite bed is a local aquifer supplying water to livestock and domestic wells in the Gascoyne area. The lignite aquifer is recharged mainly by local precipitation and discharges mainly to the underlying basal Bullion Creek-Slope aquifer which discharges to the principal surface-water drainage system in the area, Buffalo Creek and its tributaries. The sandstone aquifer is recharged by leakage from the overlying lignite aquifer. Laboratory experiments with natural-aquifer sediments and waters and equilibrium geochemical modeling indicate the controlling processes are: Dissolution of soil gases generated by oxidation of organic compounds; oxidation of iron sulfide minerals; dissolution of carbonate minerals; precipitation and dissolution of gypsum; cation exchange on clay minerals and cation exchange and adsorption on lignitic materials in the unsaturated zone; and cation exchange, sulfate reduction, and calcite precipitation within the aquifer. Most of these reactions occur in the natural environment but a are accelerated by mine disturbances. Because oxidation of reactable iron sulfides to sulfate salts proceeds to completion during the overburden-stripping process itself, the effects of the process onmore » aquifer water quality cannot be alleviated by any reclamation activity. Selective replacement of near-surface overburden above water table, however, can decrease the introduction of soluble salts into postmining aquifers.« less