Condition assessment of tailings-impacted wetlands at the Carpenter-Snow Creek Mining District Superfund Site, Neihart, Montana

Abstract

In this study, I evaluate the condition of two wetlands down-gradient of mine tailings at the Carpenter-Snow Creek Mining District (CSCMD) Superfund Site near Neihart, Montana. Wetlands contaminated with mine waste are unique in that they are highly disturbed, but they still perform some level of ecological function and service. The CSCMD consists of 70 historic mine sites in the Upper Belt Creek watershed. Historic metal mining activity occurred between the late 1800s and early 1900s, with exploration activities occurring as late as the 1980s. Waste rock from mining and milling operations generated numerous tailings piles that expose heavy metals to erosion and subsequently contaminate groundwater, surface water, and stream sediments. Therefore, wetland rapid assessment tools were used to assess the capacity of wetlands to perform ecological functions. My objective was to identify which of four methods is the most effective at identifying functional loss. Rapid assessment methods included the U.S. Army Corps of Engineers - Hydrogeomorphic Approach to Assessing Wetland Function (HGM), the Montana Department of Transportation - Montana Wetland Assessment Method (MWAM), the Montana Natural Heritage Program - Montana Ecological Integrity Assessment (MEIA), and Montana Department of Environmental Quality - Wetland Assessment Protocol (WAP). The methods were performed on pairs of impacted and reference sites along Carpenter Creek and Belt Creek. My results indicate that MEIA displays the greatest sensitivity for differences between reference sites and impacted sites with a difference of 0.38 (out of 1) for the Belt Creek sites and 0.46 for the Carpenter Creek sites, while HGM displays slightly less sensitivity with differences of 0.31 and 0.40, respectively. The WAP shows the least absolute difference in index scores in the Belt Creek watershed (0.17), and the MWAM shows the least absolute difference between sites in the Carpenter Creek watershed (0.17). These results are useful for mining-related environmental cleanups where a decision regarding whether wetlands should be left in place, removed and reconstructed, or newly created where none currently exists. At cleanup sites, where long-term monitoring of wetland and riparian systems is necessary after reclamation, my study will help regulators and consultants determine whether a chosen remedial action was successful at eliminating, or at least significantly reducing, the effects of mine waste.