Reclamation effectiveness at three reclaimed abandoned mine sites in Jefferson County, Montana

Abstract

Montana has an estimated 6000 abandoned mine sites, many with associated waste rock and tailings materials contributing to the release of high levels of acidity, heavy metals, and other contaminants, creating a risk to human health and the environment. Many abandoned mine sites in Montana have been reclaimed, however, little post-reclamation monitoring has been performed, and the effectiveness of reclamation has not been quantified. The goal of this project was to quantify the effectiveness of reclamation at three sites in Jefferson County, Montana based on soil suitability for sustaining plant growth. Vegetation and soil studies were executed using a stratified random sampling design. Vegetation measurements included canopy cover using Daubenmire cover classes, above ground biomass, and species richness/diversity. Co-located soil samples were excavated in increments to a depth of 60 cm, and determinations of pH, electrical conductivity, nutrients, soluble, and total metal levels were made. Canopy cover estimates ranged from 0-120% and biomass production estimates ranged from 0-4583 kg ha-1.

Differences in species richness and diversity were observed between sample strata. The chemical properties of the soil varied greatly, with pH values ranging from 2.08 to 7.63, and soluble metal values ranging from <0.1 to1001 mg l-1 for Zn, .02 to 20.81 mg l-1 for Cu, <.01 to 7.39 mg l-1 for Cd, <.05 to 12.26 mg l-1 for As, and <.1 to 7.6 mg l-1 for Pb. Sum of total metal and arsenic (As, Cu, Pb, and Zn) concentrations ranged from 133 to 81448 mg kg-1. Associations between vegetation and soil chemistry were determined using correlation. Significant correlations between vegetation attributes and soil chemistry were found. These results indicate that reclamation at the selected sites was moderately effective in reducing human and environment risk of exposure to harmful contaminants. There are concerns with upward migration of contaminants, and the sustainability of plant communities at all sites within the study. Elevated levels of residual metals and arsenic, as well as low pH conditions may have a deleterious effect on the long-term stability of the reclamation at these sites.