In the western United States, elevated selenium (Se) levels in soil have resulted in documented cases of ruminant fatalities. This is due to the ingestion of Se-hyperaccumulating vegetation growing on previously reclaimed phosphate mine soils. A field-scale analysis was first conducted to examine Se bioavailability to plants. Soil and plant samples were collected from transects from five study locations in Soda Springs, Idaho. Soils were analyzed for Se speciation and geochemical phases using a sequential extraction procedure (SEP). Additionally, speciation, SEP results, and Se bioavailability in the hyperaccumulator, western aster (Symphyotrichum ascendens (Lindl.)), were related using simple linear regression. Soil speciation and the validity of this SEP were then evaluated using synchrotron-sourced X-ray absorption fine structure (XAFS) spectroscopy for both whole and a sequence of extracted soils. Lastly, competitive adsorption of Se with two dissolved organic carbon (DOC) species, citric and salicylic acid, was examined on an amorphous iron oxide mineral surface. A strong relationship was identified for western aster Se and the first two combined SEP fractions, water-soluble and PO43--extractable Se (R2 = 0.85; P = <0.0001). Results also indicated a strong relationship between selenate and water-soluble Se (R2 = 0.83; P = 0.0002). This suggests that water extracts could be useful Se bioavailability assessment tools in highly contaminated systems. XAFS analyses indicated that elemental and organic Se were the most predominant phases overall in whole soils. The dominant oxidized species present was selenite sorbed onto iron oxides and calcite. Critical SEP evaluations using XAFS also indicate that oxidized Se species were underestimated by the SEP and elemental Se was overestimated. In extracted soils, XAFS results indicated partial recovery of carbonate, iron oxide and organic Se occurred. Therefore, it is suggested that researchers exert caution when employing SEPs. Additionally, sorption analyses demonstrated the highly competitive behavior of citric acid with both selenite (pH 5-8) and selenate (pH 5-6). Little competition was observed in the presence of salicylic acid for both Se species. Competition and subsequent desorption of both sorbed species in the presence of citric acid suggest a possible mechanism for Se solubilization and bioavailability in seleniferous environments. / Ph. D. / Selenium (Se) is a contaminant found in elevated levels in soils and plants in the Western United States due to phosphate mining. This has caused livestock deaths throughout the mining region following ingestion of plants with particularly high Se levels. Soils and plants were sampled from five study locations and used to assess relationships between soils and plant Se uptake. A sequential extraction procedure (SEP) was used to estimate Se in soils related to soluble, exchangeable, bound, organic, and elemental forms of Se. X-ray absorption fine structure (XAFS) spectroscopy was then used to critically evaluate the validity of these phases and the procedure accuracy. Lastly, the competitive effects of organic acids, which are naturally present in soils, on Se sorption was evaluated in a batch reactor system. From the SEP analysis, higher levels of Se were found in organic and elemental fractions, moderate quantities were observed in sorbed fractions, and smaller abundances were observed in soluble and exchangeable fractions. Relationships between soluble Se and selenate, a highly bioavailable form of Se determined from speciation analyses, indicated that simple water extracts could be used to assess Se “hotspots” in order to prevent further livestock fatalities. The critical evaluation of this SEP using XAFS determined that this procedure was under- and over-estimating bound and organic extracted phases of Se. This was possibly due to mineral and organics that were incompletely dissolved during extraction. It was determined that researchers should exert caution prior to using SEPs and have suggested several recommendations. Lastly, the batch reactor analysis indicated a form of DOC, citric acid, was highly competitive for mineral surface sites with selenite and selenate. Competition from salicylic acid was not obvious. Differences in competition were speculated to be linked to differences in molecular structure. This work suggests possible mechanisms for solubilization of both selenite, which is typically strongly bound, and selenate, which is typically soluble, in soil systems. Results offer an explanation for the exceedingly bioavailable nature of Se in the Western US.