SOURCES, PATHWAYS AND FATE OF METALS IN GROUND AND SURFACE WATERS AT THE ABANDONED RAMSHORN MINE NEAR CHALLIS, IDAHO

The Ramshorn mine near Challis, Idaho was listed on the CERCLIS by the USEPA in 2003 due to concerns of metals contamination in Bayhorse Creek, which flows adjacent to the tailings and slag piles. Discharge measurements and water samples were collected once per month for four months during the summer and early fall. Both filtered and unfiltered ground and surface water samples were analyzed for metals to quantify the dissolved and total metals concentrations and loads. Samples were collected from locations adjacent to suspected areas of contamination. One set of sediment, tailings and rock samples was also collected and analyzed for metals, and compared to the metals loads in the creek.

The chemical analyses indicate that there are only six metals of concern: arsenic, copper, iron, lead, manganese and zinc. Of these six metals, iron was detected at the highest concentrations, while manganese and zinc were the only other metals detected in significant concentrations. Furthermore, the loads of these three metals were high only during the first month when the creek discharge was highest.

The results suggest that the tailings and waste rock piles at the Ramshorn mine site in the upper portion of the drainage basin, and the slag pile at the Bayhorse town site further downstream are the main sources of the metals contamination. It also appears that the chemistry of the hydrologic system is controlled by pyrite-poor sphalerite-galena veins and replacements in carbonate sedimentary rocks. The lack of pyrite combined with the carbonate bedrock lead to a more alkaline environment. Thus, the iron and manganese are transported in the less toxic suspended form, while only zinc is transported in the dissolved form. In conclusion, the relatively low iron, manganese and zinc loads in Bayhorse Creek do not appear to pose a significant risk to aquatic life or human recreation.