New Mexico Geological Society Annual Spring Meeting — Abstracts

A study of soil chemical and physical properties relevant to processes influencing uranium contaminant transport was conducted within Los Alamos County, New Mexico. Data from 22 soil profiles provide detailed geochemical and morphological information on each soil horizon from profiles sampled at numerous uncontaminated sites in and around Los Alamos National Laboratory (LANL). Site selection emphasized: 1) a variety of geomorphic settings and soil types representative of the region, and 2) soils with similiar properties to those located at past or potential waste disposal sites. This strategy enables data from sUch locations to provide background information for comparison with samples from contaminated sites. Data from uncontaminated areas indicate significant variability and complexity in soil properties. For example, clay content varies from <3 to >63 wt.%, pH values range from 5.15 to 8.21 , and carbonate content from <0.1 to >29 wt.%. Total uranium concentrations in uncontaminated LANL soils ranged from 2.17 to 6.73 ppm (U, (IV) and U, (VI)). Positive correlations exist between some pedogenic properties and uranium concentrations. Correlations include an association between organic carbon content and uranium concentration, and between carbonate content and uranium concentration. No apparent correlation exists between cation exchange capacity and uranium concentration nor between dithionite extractable iron and uranium content. It appears that clay content and presence of iron oxide or oxyhydroxide coatings on clays do not significantly impact uranium concentration in these soils, even though mixed-layer clays exist in at least some profiles. Organic-rich and/or carbonate-rich horizons are not present in many of the soil -profiles sampled at Los Alamos; where they are present, they are typically relatively thin and poorly developed. Nevertheless, uranium in these solis is hypothesized to be associated with solid organic material and calcium carbonate. Sorption, complexation and co-precipitation are probably the dominant processes controlling uranium distribution in these soils. We conclude that 1) any or all of these interactions may be pertinent to the ability of a given soil to retard or facilitate the migration of uranium through the profile, and 2) most, if not virtually all of the uranium in uncontaminated soils could be derived from chemical weathering of the volcanic parent material, and subsequently redistributed in the profile.