Summary

Description

Elevated salt and selenium levels in groundwater and in saline seeps within the Lake Basin of northern Stillwater County, Montana have impacted water quality on Hailstone National Wildlife Refuge NWR, and have the potential to impact water quality at Halfbreed NWR. Additionally, the contribution of salts and selenium from saline seeps creates an unacceptable risk to aquatic birds using Hailstone NWR. This study was conducted to evaluate the background hydrogeologic conditions, selenium source and geochemistry, as well as the distribution of selenium and other constituents of concern, and their impacts to waterfowl and shorebirds using Hailstone and Halfbreed NWRs in southcentral Montana. This report emphasizes the Hailstone NWR because all waterbodies at Halfbreed NWR were dry for the duration of this investigation. Upper Cretaceous marine shale, along with alluvium and colluvium derived from these shales, provide the source of selenium and sulfate salts to Hailstone Reservoir. Prior to cropfallow farming practices, native prairie grasses consumed most of the precipitation. Cropfallow farming altered the hydrologic balance and precipitation unused by plants moved below the root zone and accumulated. Groundwater recharge accumulated in the alluvium, colluvium and weathered shale above low permeability shales causing the water table to rise and a groundwaterflow system to develop. Selenium, sulfate salts, and other harmful constituents were dissolved and moved downgradient with the groundwater through alluvium, colluvium and weathered shale. A groundwater transition zone developed as the water table rose. Subtle rises in the topography of the impermeable shale beds and thinning alluvium forced groundwater towards the land surface. Saline seeps develop along these transition zones. Selenium and sulfate salts are evapoconcentrated in these seeps as the groundwater reaches the surface and evaporates. In Hailstone Basin, runoff from snowmelt and rain storms dissolves these salts as it moves downslope towards Hailstone Reservoir carrying selenium concentrations as high as 3,200 gL and sulfate ions as high as 60,000 mgL. Groundwater below the transition zone has very high sulfate concentrations but relatively low selenium concentrations. Slow moving groundwater containing relatively low selenium concentrations and high sulfate concentrations also discharge into Hailstone Reservoir, although runoff appears to be the primary source of lake water, and is the source of most of the constituents of concern. Saline seep induced changes to water quality on Hailstone NWR were found to negatively impact waterfowl and shorebirds using the reservoir. Several mortality events related to high salt concentrations were noted during this investigation. In addition, a hazard assessment completed for selenium revealed that Hailstone Reservoir has the potential to cause complete reproductive failure in sensitive waterfowl and shorebirds. The Service is currently unable to manage water levels within Hailstone Reservoir in order to avoid future mortality events.