GROUND WATER AND SURFACE WATER CONTRIBUTIONS TO METALS LOADING IN THE BAYHORSE CREEK AT THE ABANDONED RAMSHORN MINE SITE NEAR BAYHORSE, IDAHO

In 2003, the USGS found high levels of Ag, As, Cd, Cr, Cu, Pb, and Zn in soils adjacent to the tailings pile at the abandoned Ramshorn mine near Challis, Idaho. The Idaho Department of Environmental Quality authorized remediation, which was carried out in August 2011, yet the source and fate of creek contamination remain unknown. Ground water flows first through the Ramshorn Slate via fractures, joints and bedding planes then through the tailings pile before entering the creek. This study identified the source and fate of metals contamination in Bayhorse Creek at Ramshorn mine. Understanding the state of metals contamination is imperative because it is well known that macroinvertebrates absorb metals into their tissue, which then bioaccumulate in fish and create a health risk if consumed by humans and wildlife.

Sediment and water sampling was carried out during four site visits between July and October of 2011. Metals loading calculations were done using creek and ground water seep discharge rates and chemical analyses throughout one flow season. ICP-MS analyses of water chemistry show that the toxic metals concentrations are at or below EPA limits (0.001 to 2 mg/L). Computer modeling, a diffuse double-layer surface complexation model and the geochemical program PHREEQC, were used to determine chemical controls on metals mobility and attenuation in surface and ground waters. Our model indicates that the creek is gaining ground water along the upper reach only during the month of August. Speciation models reveal the likely presence of metal oxides and buffering provided by carbonate bedrock.

High levels of As, Cu, and Zn found in a seep above the tailings suggest the Ramshorn Slate, which contains metal-rich minerals such as arsenopyrite, chalcopyrite, and sphalerite, as a potential source of contamination in the ground water and ultimately in the creek. XRD and XRF indicate high concentrations of metal oxides in sediments in lower creek reaches. Metals likely exist as metal oxides that complex with sediment and are transported as bed load. Our results suggest that toxic metals sourced from tailings and bedrock are not a health or safety concern in Bayhorse Creek and that remediation conducted was effective.