The objective of this study is to provide a better understanding of the natural geochemical cycling of arsenic and selenium in soils. This information is fundamental in evaluating soil quality and managing the fate and availability of chemicals in soils. We used synchrotron-based x-ray fluorescence (XRF) spectroscopy and micro x-ray absorption spectroscopy to study soil matrices. These tools are ideally suited for evaluating trace element distributions and chemical speciation in heterogeneous materials, such as soils, without disrupting the natural soil structure. The soil used in this study was collected from the eastern side of the California Coast Range, and is the parent material for the alluvial soils of the western San Joaquín Valley.

Using m-XRF spectroscopy, the elemental distributions of Fe, S, K, Ca, As, Se, Zn, P, Si, Ti, Mn, Cu, Cr, and Ni were determined in thin sections of soil cores. Two unique mineral aggregates in the soil were distinguished: an iron oxide region and an iron-potassium-sulfur region. These distinct aggregates occur throughout the soil horizon, range in size from 10-500 mm m in diameter, and can be located within only a few microns of each other. The soil matrix consisted of fine-grained clay and weathered shale material. Mineral identification of the Fe-O and the Fe-K-S aggregates was verified using micro extended x-ray absorption fine structure (m-EXAFS) spectroscopy. Semi-quantitative analysis of the m-EXAFS results suggests that the Fe-O aggregate consists of the minerals ferrihydrite (Fe₅HO₈·4H₂O) and goethite (a-FeOOH), and that the Fe-K-S aggregate is comprised of jarosite (K Fe(OH)₆(SO4)₂). The concentration of As and Se were much higher in the Fe-O aggregates than in any of the other regions in the soil. Using x-ray absorption near edge structure spectroscopy (XANES), it was determined that As exists in the soils primarily as As(V), a less mobile form of As, and that Se exists as both Se(IV) and Se(VI), with Se(VI) predominating. Our results provide the first direct evidence of elemental distributions and oxidation states of As and Se in uncontaminated and unaltered soils. Combining this kind of information about the solid phase with that gathered from traditional chemical analyses will allow for a better understanding of soil geochemical processes and their influence on soil quality.