Project Summary (2005-2010)

Extensive mining activity in arid and semi-arid ecosystems of the southwestern United States has resulted in large numbers of tailings sites with elevated metal concentrations ranging between 0.1 and 5% (w/w). Many of these piles are barren or have minimal vegetation due to metal toxicity and low pH and thus are vulnerable to wind and water erosion. Phytostabilization has been proposed as an alternative economical remediation strategy to stabilize the metal contaminants and prevent human and animal exposure resulting from particle suspension (dust) in the atmosphere or dissolution in ground and surface waters. Although this strategy is receiving attention as an economical alternative to more permanent metal removal approaches, considerable research is needed to improve the feasibility of the approach as well as to systematically assess the longevity of revegetation as a remediation strategy. First, large amounts of organic matter (compost or biosolids) and water have traditionally been used for vegetation establishment, neither of which are feasible in many of the remote areas where abandoned mine tailing piles are located. Second, little research has been conducted to evaluate the time-dependent changes in physical, chemical, and biological properties of mine tailings during revegetation. Specifically, little information is available on the effect of organic matter addition, plant establishment, and the evolution of associated microbial communities on metal bioavailability, solubility and speciation. The overall goal of this project is to develop a feasible revegetation strategy for the phytostabilization of metal contaminants in mine tailing piles in arid and semi-arid ecosystems and to assess the impact of this approach on the physical, biological and chemical properties of the mine tailings. This strategy investigates whether the specific selection of metal- and drought-tolerant native plant species in conjunction with the controlled use of a seed inoculum developed from indigenous plant growth promoting bacteria can minimize the requirement for organic matteramendment and irrigation. Second, the bioavailability, speciation, and phase distribution of contaminating metals in the plant rhizosphere is being monitored in both greenhouse and field trials in order to assess whether revegetation results in a permanent reduction in site toxicity. In addition, the effects of specific root-microbe-tailings associations on the local biogeochemistry and metal speciation of the rhizosphere is being investigated in order to identify mechanisms involved in phytostabilization and detoxification at the micrometer and submicrometer scale. Information generated from this research defines the feasibility of plant establishment as an approach to metal stabilization in mine tailings and evaluate the potential longevity of revegetation as a remediation strategy.