Depositional Setting and Groundwater Quality in Coal-Bearing Sediments and Spoils in Western North Dakota1
Gerald H. Groenewold, Bernd W. Rehm, John A. Cherry, 1981. "Depositional Setting and Groundwater Quality in Coal-Bearing Sediments and Spoils in Western North Dakota1", Recent and Ancient Nonmarine Depositional Environments: Models for Exploration, Frank G. Ethridge, Romeo M. Flores
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Studies of several active and proposed surface coal-mining sites in western North Dakota have resulted in the development of a hydrogeochemical framework which accounts for the chemical characteristics of groundwater in coal- bearing strata. Data from surface mining sites in other western states and provinces indicate much regional similarity in the hydrogeochemical characteristics of shallow aquifers. SO4= and HCO3- are commonly dominant anions. Na+ and Ca‡ are generally the dominant cations. Mg‡ is occasionally dominant. The pH of the groundwater normally ranges from 7 to 9. The electrical conductance of the groundwater ranges from 500 to 4500 /μS. Groundwater at depth is generally of the Na—H0O3 type.
Critical hydrogeochemical processes include pyrite oxidation, carbonate mineral dissolution, gypsum precipitation and dissolution, cation exchange, and sulfate reduction. The key components of this framework are mineralogical variables in the sediments. These variables are, in turn, largely dependent upon fluctuations in the fluvial depositional settings of the sediments.
During surface mining operations unweathered materials are commonly emplaced in the zone of active weathering. Hydrochemical data from postmining landscapes suggest that severe degradation of groundwater quality is possible in some settings. The degree of weathering, dissolution, and ion exchange, as in premining settings, is largely dependent upon mineralogical variables in the overburden sediments. Degradation of groundwater quality in postmining landscapes can be minimized if these factors are understood and integrated within the framework of mine design.