The impacts of coal strip mining on the hydrogeologic system of the Northern Great Plains: Case study of potential impacts on the Northern Cheyenne Reservation

Abstract

50% of the coal production of the U.S.A. will be obtained from the western coal fields by 1990. The majority of this coal will be produced from large-scale strip mining of the Tertiary Fort Union and Wasatch Formations of Wyoming, Montana and North Dakota. The rapid escalation of coal strip-mining activities in the Northern Great Plains, where groundwater is the principal source of domestic and agricultural supply, threatens to alter significantly the local and regional hydrologic regime. At the request of the Northern Cheyenne Tribe of southeastern Montana, a study of the potential impacts of strip mining on their water resources was designed and implemented. After a basic hydrogeologic study was conducted, a hypothetical mine study site was selected for evaluation. The pre-mining hydrologic system was defined from a monitoring well network, aquifer testing, water-quality sampling and stream gaging. Saturation extract and leachate analyses were conducted on cuttings of overburden from wells in the mine area and results were used to predict the concentration of total dissolved solids (TDS) in spoil-water discharge. A material-balance model was developed which described the quantity and quality of groundwater and recharge for zones of an unmined coal seam, spoil area, clinker and alluvium. A complementary material-balance model was also developed for a stream receiving post-mining discharge. The TDS of the spoil groundwater outflow was determined to be 4100 mg/l by averaging saturation extract data. Analyses of modeling results indicated that groundwater downgradient from the mined area could be increased in TDS by 300–2070 mg/l depending on the spoil recharge rate. The quality of the receiving stream with a mean annual flow of 14.2 m 3/s could be increased in TDS by 1–26 mg/l. The site-specific mine-impact TDS changes were projected over an area of similar hydrogeology along 30 km of stream length in the Tongue River Valley. Strip mining of the entire minable area would have a major impact on the regional groundwater quality and a measurable impact on the quality of the receiving stream. Analysis of projected hydrologic properties of the post-mining system indicated that water-quality impacts will last for hundreds of years.