Elements present in natural materials at concentrations < 1000 mg/kg have been defined as trace elements. Most elements of the periodic table are trace elements. Although some trace elements are essential to life, many are toxic in high concentrations. Further, awareness is growing of the extent and severity of trace element contamination of soils and water resulting from the industrial and commercial use of trace elements including metals. Contamination occurs from metal mine tailings, disposal of high metal wastes in improperly protected landfills, leaded gasoline and lead-based paints, land application of fertilizer, animal manures, biosolids, composts, pesticides, coal combustion residues, and atmospheric deposition. These anthropogenic additions of trace element to the soil environment can adversely affect human, crop, and wildlife health. Trace elements that occur in some residuals of environmental concern include As, Cd, Cr, Cu, Pb, Hg, Ni, Se, Mo, Zn, Tl, Sb, and others. Contamination of soil with trace elements is a serious and common problem, both in Ohio and worldwide. Excess amounts of trace elements in soil pose threats to human health directly via inhalation or ingestion of soil, contamination of groundwater, and indirectly through the consumption of plants grown on contaminated soil. Also, excessive trace elements in soil are of great concern in soil ecosystems where organisms are in direct contact with the soil. To adequately protect or restore soil ecosystems, it is necessary to accurately characterize soils suspected or presumed to be contaminated with trace elements and define what levels of metals in these soils constitute a hazard to soil organisms. Conceptually, trace elements in soil are divided into chemical pools based on solid phase association, solubility and strength of bonding to soil surfaces. These chemical pools range from water soluble to residual forms trapped in mineral lattices. In general, trace elements in soluble or weakly adsorbed "pools" are more bioavailable than those in strongly adsorbed and sequestered forms. Most human and ecological risk associated with exposure to trace element contamination is from the biogeochemical forms of trace element that is biologically available for absorption or "bioavailable" to the humans or ecological receptors. Bioavailability is the portion of a chemical dose that enters the systemic circulation of the receptors. Therefore, risk-based soil levels should be based on trace element contaminant bioavailability not total content in soil. In this proposed project, we will develop and/or evaluate the ability of trace element speciation methods to measure and/or predict bioavailability to human and/or ecological receptors.