The effect of soluble salt concentrations on the sorption/desorption of Cesium-137 in the soils of the lower Virgin River floodplain of southeastern Nevada, USA was investigated using gamma spectrometry, Scanning Electron Microscopy (SEM) with energy dispersive X-ray analysis (EDS), and X-Ray Diffraction (XRD). In arid environments, significant amounts of soluble salts, such as sulfate minerals and halite, can accumulate in soils. Salt concentration affects soil aggregation processes by disrupting the ionic bonding ability of clays resulting in the dispersion of clay colloids and deterioration of aggregate stability (Essington, 2004). Ion exchange mechanisms are decreased in salt-affected soils causing the release of Cs ions from sorption sites into soil solution. Cs-137 is a high-yield fission product present in soils as a result of radioactive fallout from atmospheric testing. Cesium migration is controlled by ion exchange reactions with the clay and colloid fractions of soils and sediments. Cs sorption/desorption is influenced by soil mineralogy, CEC, and competing cations, such as Na+ and K+ (Lui et al., 2000, Steefel et al., 2003, and others). The study area is located within the lower Virgin River Valley, NV, USA 152 km east of the Nevada Test Site. Geological formations in this area contain gypsum, contributing to the high sulfate salt concentrations found in Virgin River surface waters and floodplain soils (Langenheim et al., 2000). Black Butte soils occur on floodplains within abandoned meander loops or backswamps. The taxonomic class for these soils is fine-silty over sandy or sandy-skeletal, mixed superactive, thermic Typic Haplosalids in accordance with the ninth edition of USDA's Soil Taxonomy. They are very deep, moderately well-drained, and strongly alkaline with surface and subsurface zones of salt accumulation. Vegetation is dominated by halophytes, primarily pickleweed (Salicornea spp.), seepweed (Sueda moquinii), and arrow weed (Pluchea sericea). A soil profile on the Virgin River floodplain was described and sampled in 2004. SEM/EDS analyses of the salts found halite (NaCl), gypsum (CaSO4 - 2H2O), bloedite (Na2(SO4)2 - 4H2O), thenardite/mirabilite (Na2SO4; Na2SO4 - 10H2O), and eugsterite (Na4(SO4)3 - 2H2O). This is the fourth occurrence of pedogenic eugsterite in the USA. Salts occur in the profile as stage I snowballs (e.g. Buck and Van Hoesen, 2002). Cs-137 occurs to a depth of 32 cm where a texture-caused capillary barrier is present. The average activity concentration of the surface crust horizon (Az1, 0-5 cm) was 1.5 ± 0.04 Bq/kg, the second horizon (Az2, 5-21 cm) had an average activity concentration of 13.4 ± 0.14 Bq/kg, and the third horizon from the surface (Byz, 21-32 cm) had an average activity concentration of 4.9 ± 0.08 Bq/kg. These horizons also have the highest CEC (5.3 , 11.4, and 10.9 meq/100g), EC (127.4, 92.5, and 72.0 mS/cm ), and SAR (146.0, 87.0, and 68.0 dS/m ) values within this profile, thus creating favorable conditions for cesium migration within the subsurface. This site was sampled again after a 100-year flood event occurred in early 2005. Average activity concentration of Cs-137 in the silt deposited by this flood was 3.1 ± 0.08 Bq/kg. Cs-137 activity concentrations in the Black Butte Series soils are accumulating due to repeated flood events depositing sediment. Once deposited, Cs+ sorption onto smectite and mica clays present in these soils is reduced because of high salt concentrations. Factors contributing to increased Cs mobility and accumulation in the subsurface at this site are high levels of Na+ competing with Cs+ for exchange sites, alkaline soil pH, and periodic deposition of additional cesium to the site. References: Ainsworth, C.C., J.M. Zachara, K. Wagnon, S. McKinley, C. 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