SURFACE WATER/GROUNDWATER INTERACTIONS AND HYDROGEOCHEMICAL CHARACTERIZATION OF THE ELKHORN MINE AND MILL, BEAVERHEAD COUNTY, MONTANA

The formation of acid mine drainage (AMD) and the contaminants associated with it have been described by some as the largest environmental problem facing the U.S. mining industry. Heavy metals associated with the drainage, such as copper, cadmium, and zinc affect the water quality of streams and can cause acute or chronic toxicity to invertebrates and fish (Martin, 1992& Padrillah et al., 2018). Elkhorn Creek in the Pioneer Mountains of southwest Montana is one of these impacted creeks. The historic Elkhorn Mine and Mill complex has historically attributed to the creek’s contamination and has undergone remediation. However, the problem has not subsided. The United State Forest service partnered with the Big Hole Watershed Committee to address the next phase of remediation efforts; as part of that effort, the goals of this project include hydrological and geochemical characterization of the stretch of the creek between the mine adit and mill area. Three staff gages and stilling well pairs were installed in points along the creek to observe flow, establish a rating curve, and determine vertical hydraulic gradient. A transect of 13 piezometers were installed around the mill, and two peepers were put in the wetlands around the mill. The transects were meant to measure the groundwater table and draw groundwater samples. Results from the data collected over four months concluded that metals contamination was picked up below the confluence of the adit discharge and again past the mill. A shallow groundwater table of two feet or less was heavily contaminated with metals. The creek at this section was also shown to be a groundwater gaining reach, meaning contamination from the mill soils directly enters the creek. The peeper study revealed natural bioremediation through sulfate-reducing bacteria was present in the soil approximately 10 centimeters below the surface