Metal Levels in Vegetation Growing on in Situ Treated Acid Metalliferous Mine Wastes in Montana

Abstract

The purpose of in situ or in-place treatment of metal mine wastes is to immobilize contaminants within the soil/plant complex, reduce metal and arsenic movement to groundwater, reduce metal and arsenic movement to receiving streams, stabilize the landscape from wind erosion, and to provide a functional ecosystem compatible with current and future land uses. In-place treatment includes the incorporation of chemical and biological amendments to provide a hospitable rootzone, and to select plant species that can thrive in the newly amended environment. During in-place treatment, metals are chemically precipitated, and/or sequestered by complexation and sorption mechanisms within the mine wastes/ contaminated soils. Metal availability to plants is minimized, and metal leaching into groundwater is reduced. Metals and arsenic that remain in soil solution are immobilized via chemical reactions at plant root surfaces. The selection of plant species for in-place treatment is based on availability of seed or seedlings, the species' relative lack of ability to translocate (or move) metals and arsenic from the roots into the above ground biomass of the plant, and land use and management considerations. In-place treatment does not remove contaminants from the soil. As a consequence, short-term and long-term effectiveness of in-place treatment has been and continues to be debated. One concern is the potential toxicity to livestock and wildlife from the contaminants in soils and/or plants. This paper provides vegetation metal data from several Superfund and AML sites at which in-place treatment has been used as a remedial strategy. Interpretations of these data based on plant toxicity and residual risk to terrestrial receptors including grazing animals are presented.