TRACKING THE SOURCE: BACKFILL AND BACKFILL WATER QUALITY AT A LARGE POWDER RIVER BASIN COAL MINE

摘要

Due to poor water quality in the initial backfill well constructed in 1981 at the Caballo Mine, the Wyoming Department of Environmental Quality, Land Quality Division required that the mine drill more backfill wells in the small area surrounding the initial well. The water quality results from these wells led to a number of landmark studies on backfill water quality and the behavior of selenium, nitrogen, and organic carbon in backfill waters. Although it was thought at the time that these water quality results would be typical for the Powder River Basin (PRB), the benefit of experience with other backfill wells in the PRB and a review of the history of the backfill surrounding these original backfill wells, indicates that the high total dissolved solids (TDS), selenium, and nitrogen concentrations observed in the early backfill area are not typical of backfill wells at Caballo. These high concentrations are a product of the backfill material used near the wells and their location. Selenium and nitrogen concentrations in the backfill waters are now very low in the initial backfill area. Moreover, the water quality in the initial backfill area and other resaturated or partially-resaturated backfill areas at Caballo is also similar to or better than premining water quality in the area surrounding these wells. Moderate amounts of selenium have been detected in more recently constructed wells at Caballo, but selenium concentrations have decreased to below the detection limit or are expected to rapidly decline as more saturated and reducing conditions develop.