Geochemical processes in the Gascoyne Lignite Mining Area, Bowman County, North Dakota

Abstract

A study was made of the geochemistry of shallow ground water and the unsaturated zone in the vicinity of the Gascoyne, North Dakota lignite mine. The strip mine currently (1984) produces about 3 million tons per year from the Harmon lignite bed, an extensive, economically important unit in the Tongue River Member of the Paleocene Fort Union Formation. Test wells were drilled near and in the mine area for water-level measurements and collection of water samples. Ground and surface waters were analyzed for major, minor, and trace constituents; tritium and carbon-13 isotopes; and dissolved gases. Gas samples from several different levels in the unsaturated zone were obtained from six sites, two of which are in reclaimed mine spoils. The gas samples were analyzed for composition, carbon-13, and carbon-14. Mineralogic and cation-exchange data were obtained from core samples, highwell samples, and miscellaneous surface solids. Chemical compositions of three Harmon lignite samples were determined. Reactions that control ground-water chemistry in the Gascoyne area include oxidation of organic matter and ferrous iron minerals in the unsaturated zone and in the upper water table; dissolution of carbon dioxide, carbonate minerals, gypsum, and sodium-magnesium sulfates in unsaturated-zone water; cation-exchange reactions on clays and on lignite; sulfate reduction and pyrite deposition in lignite below the water table; leaching of halides and organic matter from the lignite; and dissolution of silicates, which produces highly alkaline ground waters near the lignite outcrop. Carbonate-mineral dissolution apparently is sufficient to maintain groundwater pH values near or above 7 throughout the mine area. Development of strong mineral acidity in the waters by pyrite oxidation is unlikely in the present hydrogeologic environment. Continued mining of the lignite will increase the total amount of sodium sulfate in ground-water discharge to the North Fork Grand River drainage system.