Characterization of riparian wetland soils and associated metal concentrations at the headwaters of the Stillwater River, Montana

Abstract

I investigated the spatial and vertical distribution of metals in an alpine riparian wetland downstream of acid rock drainage in the New World Mining District, Cooke City, Montana. The McLaren ore deposit was discovered on Fisher Mountain in 1933, and underground and open-cut mining occurred until 1953. Fisher Mountain is the primary source of acid rock drainage in this part of the New World Mining District. Both natural and mining related processes released acidity and metals (particularly copper, lead, and zinc) into Daisy Creek and the upper Stillwater River in the form of dissolved metals and metal-rich sediment. The Stillwater River flows through the 66-hectare Stillwater wetland before entering the Beartooth Wilderness Area. This wetland has had the potential to accumulate metals beginning with retreat of the glaciers from the Beartooth plateau approximately 11,000 years ago. I investigated the spatial and vertical distribution of metals (copper, lead, and zinc) using XRF and ICP-AES laboratory analysis, and the timing of metal deposition using 14C and 210Pb age-dating techniques.

The 66-hectare wetland was sampled on a 100 by 100 meter grid (105 samples) and along 7 detailed transects (887 samples). The metal concentrations ranged from 38 to 7088 mg/kg for copper, 29 to 114 mg/kg for lead, and 43 to 775 mg/kg for zinc. The results of metal mapping and age-dating highlighted two metal deposition settings. The active floodplain had the highest copper and zinc concentrations in the 20 cm. of the soil profile and had estimated soil ages that predated the earliest mining activity. Soils in the wetland marsh had copper and zinc concentrations that increased with depth and were older than the mining activity. Radiocarbon analysis indicated a peat layer in the wetland was thousands of years old (2770 to 8710 years BP) and contained metals, suggesting a long history of metal deposition. This investigation integrates XRF plus ICP metal analysis and 14C and 210Pb age dating for characterizing metal concentrations in a riparian wetland impacted by acid rock drainage.