Geological and Geochemical Controls on the Chemical Evolution of Subsurface Water in Undisturbed and Surface-Mined Landscapes in Western North Dakota

摘要

Previous studies of several surface coal mining sites in western North Dakota resulted in the development of a hydrogeochemical model that accounts for the observed chemical characteristics of subsurface water in undisturbed settings. The near-surface several meters of the landscape is constantly subjected to alternate wetting-drying, a key mechanism in hydrogeochemical evolution. The purposes of this study were to refine the hydrogeochemical model, with particular emphasis on the sulfur cycle, and to determine the applicability of the model to postmining (spoils) landscapes. Field activities included detailed ground water instrumentation of undisturbed and spoils areas at two mine sites. Refinement of the model included detailed analyses of the texture, bulk and clay mineralogy, sulfide abundance, and sulfur concentrations in overburden core samples. Laboratory experiments were designed to determine the source and mechanisms of sulfate salt production from overburden samples. It was concluded that the hydrogeochemical model is equally applicable to undisturbed and spoils landscapes and that the major species of concern in this region are sodium and sulfate. It was also concluded that the major source of sulfate is sulfides in the overburden and that the solubility of sulfate in ground water in these settings is largely controlled by the sodium-calcium ratio.