PRELIMINARY ASSESSMENT OF METALS PARTITIONING IN SOIL AND VEGETATION IN A SUBALPINE WETLAND (BISMARK MINE, SOUTHWEST MONTANA)

The Bismark Mine is located near the headwaters of the South Boulder River in Madison County, Montana. The bedrock of the area is predominantly metamorphic rocks of the Archean Pony series, intruded by a tongue of the Cretaceous quartz monzonitic Tobacco Root batholith. The gneiss and the granite are both weakly fissured and mineralized near the contact. The mine is located on several of these fissure veins near the granite-gneiss contact. Additionally, numerous small veins of pyrite, chalcopyrite, and molybdenite with quartz are present. During its operation, over 58,000 lbs of copper, more than 7,000 lbs of lead, and almost 1,200 oz. of silver and smaller amounts of gold were removed between 1913 and 1917. The mine was abandoned by 1963.

During the mining process mine tailings were discarded downslope from the mine adits. A wetland naturally developed on top of the tailings due to diffuse seepage of seasonal snowmelt from the surrounding area and discharge from the lower adit of the mine. Multiple unmapped seeps contribute flow to the wetland. The water emerging from the adit and seeps has a pH between 6.78 and 8.05 and contains measurable concentrations of trace elements, including lead (up to 17.4 µg/L), zinc (15 – 36 µg/L), copper (up to 116.7 µg/L), arsenic (up to 3.66 µg/L). The purpose of this study was to investigate metals retention and accumulation within the wetland soils and vegetation and is part of a more extensive wetland characterization study.

Soil trenches were dug at various location across the wetland during the summers of 2014 – 2017. Samples were collected from each horizon and from any interesting features observed in the pit. Texture, color, pH, loss on ignition, organic matter content and total metal concentrations were determined. Wetland vegetation samples were collected during the summer of 2018, digested and analyzed for total elemental concentrations. Preliminary results indicate the accumulation and concentration of metals in the soils with concentrations of trace elements 10 to more than 1000 times greater in the soils compared to the water. Higher concentrations in the soil appear to be associated with the presence of organic matter and/or iron oxides in the soils. Preliminary results also indicate the accumulation and concentration of certain elements within the vegetation samples.