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Fate and Transport of Heavy Metals and Radionuclides in Soil: The Impacts of Vegetation

Principal Investigators
A.P. Schwab, M.K. Banks, and L.E. Erickson, Kansas State University


Goals: The overall objective of this research is to determine whether establishment of vegetation in heavy metal and radionuclide contaminated soil will significantly affect retention of metals in soils and to mathematically predict the results using a calibrated model.

Rationale: Vegetation is often the primary method of reclamation in mining areas to stabilize waste with respect to wind and water erosion and to minimize downward translocation of contaminants. Plants reduce the possibility of metal leaching through decreased water infiltration, adsorption of metals to root surfaces, plant uptake of metals, and stimulated microbial immobilization in the rhizosphere. Plants may increase metal leaching through complexation with rhizosphere organic acids exuded by roots, produced by microbial activity, or generated by decomposition of soil organic matter. Field and laboratory determinations are needed to quantify effects of vegetation on the leaching of metals. Models that attempt to predict the fate of heavy metals in soils have focused primarily on the geochemical aspects of the problem and have not considered the effect of a plant's geochemistry. The difficulty associated with using models to simulate the fate of a heavy metal in the root-soil environment is properly accounting for all interactions between water movement, contaminant transport, uptake of water and metals by plant roots, and geochemistry.

Approach: Impact of vegetation and revegetation schemes on the mobility of metals (lead, cadmium, zinc, barium, etc.) is being investigated on contaminated soil and/or mine waste from zinc and lead mining regions of southeast Kansas, lead mines of Montana, and a paint-producing industry in southern Kansas. The following series of experiments will be employed to pursue the objectives: a sequential extraction procedure for determination of various fractions and mineral associations of the metals; batch (laboratory-scale equilibrations) and column experiments to directly assess impact of organic acids on heavy metal mobility; large soil columns to determine effects of vegetation overlying soil depth on mobility of metals and metal uptake by plants; sorption/desorption and determination of potential or existing solid phases of the metals to quantify the soil chemical aspects of metal retention; and integration of geochemical and solute transport modeling to predict and analyze the fate of metals as influenced by the presence of vegetation.

Status: Soils and mine tailings have been thoroughly characterized for their important chemical and physical properties as well as the chemical fractions of the metals. Investigators have used x-ray diffraction to identify predominant minerals. Conclusions drawn from this aspect of the study are that high concentrations of readily mobile metals exist in all the soils and tailings, but the predominant fraction depends upon the source of the material. The southeast Kansas mine tailings and soils tend to have the metals present as carbonates and/or sulfides. The Montana mining waste has significant amounts of heavy metals in organic and "unclassified" or residual fractions. Highly contaminated industrial soil from southern Kansas has lead present as oxides and carbonates. These results suggest that metals present in the mining wastes tend to be a long-term threat to the environment as the sulfides and carbonates weather. Lead in the industrial soil can be controlled if the soil is stabilized. Initial batch and column experiments coupled with geochemical modeling have determined that very few naturally occurring organic acids have the capability to mobilize heavy metals, and these acids must be present in concentrations not usually found in soil. For example, citrate will solubilize zinc and lead at concentrations of 3.0 mmol/L or greater, but citrate is usually present at < 0.05 mmol/L. There is also evidence that the presence of organic acids in concentrations typical of the rhizosphere will decrease metal mobility. Larger column experiments testing interaction between plants and metal mobility as affected by soil cover have been initiated. This project is in its first year.

Clients/Users: This project is intended to be of interest to any group that is involved in restoration of land contaminated by heavy metals, including U.S. Environmental Protection Agency, U.S. Department of Energy, Kansas Department of Health and Environment, and U.S. Department of Defense.

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