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Introduction

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  1. Effect of Geochemical Processes on Selenium in Soil and Water, Central South Dakota Md. Salah Uddin Sharif, Phil Gerla, Scott Korom (Department of Geology and Geological Engineering, University of North Dakota) and John Finley, (Grand Forks Human Nutrition Center, USDA Agricultural Research Service) Soils and Sampling Soil Geochemistry Soil Water/Groundwater Selenium, an essential element in humans, can be fatally toxic to humans, livestock, and waterfowl, depending on the amount and chemical form present in the environment. Most early reports focused on Se toxicity problems, although more recent research indicates the necessity of Se in both animal and human nutrition and benefits to human health (e.g. Clark et al. 1996, Finley 2000, and Davies et al. 1999). Some of the highest concentrations of Se worldwide occur in the central Great Plains of the United States. The sedimentary rocks that make up the Cretaceous formations, most notably the Pierre Shale, contain the greatest concentrations. These shales were deposited under typically anoxic conditions in a shallow, warm sea that covered the present area of the Great Plains during the Mesozoic Era. The selenium that occurs in the shales has been mobilized, transported, and redistributed through the on-going processes of soil formation. The movement of soil water, groundwater, and surface runoff further modifies the original distribution. Plants growing on these soils absorb Se, which can then be transported into animal tissues through the food chain. This report describes the results of soil and water sampling at the Hovland Ranch in South Dakota. Earlier work showed that areas of the ranch have high concentrations of Se in soils, crops, and native range vegetation. The objectives of the work reported here is to show (1) the correspondence of Se and other geochemical parameters in soils and waters at the ranch, and (2) reveal the spatial distribution of Se and associated elements to better understand the geochemical cycle of Se in the natural range and cropland environment at the ranch. A rough correlation can be seen between Se and Na+, Mg2+, and SO42-, but not Ca2+ (not shown) suggesting that more soluble sulfate minerals may act as a temporary sink for Se. west transect Soil water chemistry also shows a correlation between EC and TOC, although there does not appear to be much similarity between soil water and saturated paste/hot water extractable Se in soils adjacent to the sampler. Acid-extracted Se shows some correlation with total organic carbon (TOC), but not with Se released by the saturated paste extraction. east transects Groundwater sampled from seeps and shallow wells shows a weak correlation between EC and sulfate, but no clear trend to organic carbon. Introduction Abundant Astragalus racemosus (Pursch) along transect C revealed significant uptake of Se (unpub, J. Caton, NDSU). Previous work by South Dakota State University also showed elevated concentrations of Se in wheat (ppb) at the ranch: Nimbro Series (Nc): very deep, well-drained soils formed in alluvium on bottomlands and low stream terraces. The surface layer is gray silty clay loam 8” thick. The substratum is grayish brown to light brownish gray stratified silty clay loam and clay loam. Opal Series (Od): moderately deep, well-drained soils in residuum weathered from clay shales on uplands. The surface layer is dark grayish brown clay 2” thick. The subsoil is dark grayish brown, very firm clay 24” thick. Pierre Series (Pe): moderately deep, well-drained soils formed in residuum weathered from clay shale on uplands. The surface layer is dark grayish brown and brownish gray clay 7” thick. The subsoil is light brownish gray clay 20” thick. Promise Series (Pr): very deep, well-drained soils formed in clayey sediments on uplands, colluvial fans, and terraces. The surface layer is dark gray clay, 8” thick. The subsoil is grayish brown and light brownish gray very firm clay 17” thick. Samsil Series (SbF): shallow, well-drained soils formed in residuum weathered from shale. The surface layer is light brownish gray clay 2” thick. The layer below is 5” of light gray clay. The substratum in all but the Nimbro Series consists of 10”-20” of light gray or light brownish gray clay that overlies shale. The Pierre-Samsil (Pk) and Opal-Promise (Oe) associations are also shown on the maps. Conclusions • Se concentration in soils and waters vary greatly, suggesting that near-surface geochemical processes strongly modify the original distribution of Se in unweathered Pierre Shale. • Hot water and saturated paste extractions generate about 50 times less Se than that recovered by soil digestion, and show generally poor correlation. This indicates that only a small fraction of the total Se is mobile and labile. • Se concentrations roughly increase with both increasing saturated paste EC and total organic carbon. Some correlation is observed between Se and Mg2+, Na+, and SO42- suggesting that non-gypsum sulfate minerals may act as a temporary sink for Se. • The greatest concentrations of Se occur in areas of groundwater seeps. Leaching on upper slopes and evaporative precipitation at discharge points is likely an important process that redistributes Se at the ranch and in the region. • Future work at the ranch will focus on coupling soil geochemistry with plant and animal uptake of Se. Soil Geochemistry The following Eh-pH diagram shows that selenate (SeO42-) (+6) constitutes the most typical form of Se in most soils. Selenite (SeO32-) (+4) occurs under mildly oxidizing conditions, but was detected in only a few soil samples. Seo can occur in reducing environments. Microbes are known to catalyze reactions and generate organic Se. Acknowledgments We thank Mr. Allen Hovland for not only his permission to work at the ranch, but for his hospitality, time, and unfailing interest in the effort. Results are based upon work supported by the U.S. Department of Agriculture, under agreement 59-5450-1-9034. Any opinions, conclusions, or recommendations expressed in this poster or accompanying abstract are those of the authors and do not necessarily reflect the view of the U.S. Department of Agriculture. Hot water and saturated paste extraction compared to soil digestion suggests that less than a few percent of the total Se is mobile and labile. Seeps, swales, and water courses tend to have elevated concentrations of mobile Se.