WATER QUALITY IMPACTS FROM SELENIUM AND OTHER METALS IN THE SOUTHEASTERN IDAHO PHOSPHATE MINING RESOURCE AREA

Nearly 40 percent of the U.S. phosphate reserves occur in the Phosphoria Formation in southeastern Idaho, northern Utah and western Wyoming. Much of these reserves are located within the Southeastern Idaho Phosphate Mining Resource Area, which covers approximately 2500 square miles. For over fifty years, large-scale open pit and contour strip operations in the Resource Area have mined surface exposures of the Meade Peak Phosphatic Shale Member of the Phosphoria Formation. In 1996, isolated livestock losses associated with excessive selenium uptake prompted concerns about potential ecological and human health impacts from current and historic mining operations in the Resource Area. Since 1997, numerous data characterizing surface water quality, soils, vegetation, and fish, bird, elk and other tissue have been collected in the resource area to formulate a human health and ecological risk assessment. Perennial waters in the Resource Area are protected by the State of Idaho for coldwater aquatic life beneficial uses and national toxics criteria apply. In 2001, the Idaho Department of Environmental Quality undertook a three-year investigation to document water quality impacts to perennial streams from phosphate mining in the Blackfoot River, Salt River, and Bear River watersheds. The objectives of the investigation were to establish a baseline for surface water quality in the watersheds, determine if numeric water quality criteria for coldwater aquatic life have been exceeded, and if water quality violations have occurred. To that end, surface water quality monitoring was performed at 31 surface water stations in 2001, 10 stations in 2002, and 13 stations in 2003. Results of the investigation suggest that the highest selenium concentrations and loads occur during the spring runoff following snowmelt. In particular streams, observed selenium concentrations have exceeded chronic and acute toxics criteria for coldwater aquatic life. Results of the investigation pertinent to the State of Idaho Total Maximum Daily Load program will be presented including a discussion of climatic conditions and other factors relevant to selenium in Resource Area streams.