In-situ metal speciation in soils using synchrotron based techniques David McNear 1 , Markus Gräfe 1 , Maarten Nachtegaal l , Paras Trivedi 1 , Matthew Marcus 2 , and Donald L. Sparks 1 1 Environmental Soil Chemistry Group, Dep. Plant & Soil Sciences, 152 Townsend Hall, University of Delaware, Newark, DE 19717-1303. 2 Advanced Light Source (beamline 10.3.2), Ernest Orlando Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, USA INTRODUCTION Contamination of the environment resulting from anthropogenic activities such as mining, smelting and agriculture is of great public and political concern. As such, a greater understanding of the mobility, bioavailability and long-term fate of the materials resulting from these activities is needed in order to make more accurate decisions when devising remediation and legislative strategies. To provide an accurate description of the fate of metal contaminants in heterogeneous systems, it is essential to understand their speciation. Macroscopic chemical analyses are not sufficient and hence must be complemented with advanced molecular scale techniques. Synchrotron based micro x- ray fluorescence and extended x-ray absorption (!-SXRF, !-EXAS) spectroscopies provide the needed selectivity by allowing one to probe, in-situ, the local coordination environment of a species over a wide range of concentrations in homogeneous and heterogeneous environments. The resultant structural information provides a mechanistic understanding of the processes controlling the fate of contaminants under environmentally relevant conditions. Current research conducted at the ALS by our group has focused on the application of !-EXAS and !-SXRF techniques at beamline 10.3.2 to investigate the speciation of zinc (Zn), nickel (Ni), and arsenic (As) in contaminated soils. CHANGING ZINC SPECIATION IN A REMEDIATED SMELTER SOIL Zinc is an essential trace element in plants; however, at higher available concentrations it can become phytotoxic. In the topsoil around former Zn smelters and on sewage sludge sites, weight percent levels of Zn can be present often leaving little or no vegetative cover. In situ remediation (immobilization/ inactivation) of these large contaminated sites using soil amendments, which modify the physicochemical properties of the contaminating heavy metals, is a valuable alternative for more expensive and complicated civil engineering techniques, i.e. excavation and landfilling of the contaminated soil. Regulatory acceptance of in situ immobilization as a secure reclamation method depends on the ability to predict the long-term stability of such remedial treatments. As such, we probed the changes in Zn speciation resulting from reclamation procedures in an effort to predict the long-term stability of the metal. Soils were obtained from a former zinc smelter (Belgium), where 135 ha of land were heavily contaminated. Three hectares of soil were successfully remediated by adding beringite (an aluminosilicate by-product from coal burning) and compost to the soil. Twelve years after remediation, healthy vegetation has developed on this site.