Studies of several coal-mining sites in western North Dakota have resulted in the development of a hydrogeochemical model which accounts for the observed chemical characteristics of subsurface water in undisturbed settings. Critical hydrogeochemical processes include sulfide oxidation, gypsum precipitation and dissolution, carbonate mineral dissolution and cation exchange.

In the semi-arid climate of western North Dakota, the near-surface several meters of the landscape is constantly subjected to alternate wetting/drying. This mechanism is the key to hydrogeochemical evolution of both pore water in the rooting zone and subsurface water that eventually reaches the water table.

Recent refinements in the hydrogeochemical model have focused on the sulfur cycle and determination of the applicability of the model to postmining (spoils) landscapes. Field activities have included detailed groundwater instrumentation of undisturbed and spoils areas at two geologically distinctive mine sites in western North Dakota. Refinement of the model included analyses of the texture, bulk mineralogy, clay mineralogy, sulfide abundance, and sulfur concentration in overburden core samples. Laboratory experiments were designed to determine the source and mechanisms of sulfate salt production from overburden samples.

From the study we conclude 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. This research indicates that the major source of sulfate is sulfides in the overburden and that the solubility of sulfate in groundwater in these settings is largely controlled by the sodium/calcium ratio.

We also conclude that negative hydrogeochemical impacts resulting from surface mining in the Northern Great Plains will include increased mineralization of groundwater and sodic and salt effects on plants. The degree of these impacts will be dependent upon site· specific hydrogeological, geochemical, and mineralogical variables. A consideration of these non-soils aspects of surface-mined lands thus is essential to proper surface-mine reclamation design in this region.



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