SOIL GEOCHEMISTRY AND GEOLOGY IN THE LOWER RIO GRANDE VALLEY, TEXAS

142 surface soils, sampled on a grid covering 1600 km² in the Lower Rio Grande Valley (LRGV) of Texas, were analyzed for chemistry and mineralogy as part of the US-Mexico Border Health Initiative project. Soil samples were collected from the top 5 cm of the soil profile, which is considered the soil interval most likely to affect human health. The soil mineralogy and geochemistry is characterized by the geologic setting. Soils were collected from Pliocene sediments (Goliad fm.), Pleistocene fluvial deposits (Lissie and Beaumont fms.), Holocene floodplain deposits along the Rio Grande and a Quaternary sand dune unit (Qds) that stretches from SSE to NNW across the grid. Soils were analyzed by ICP-MS for 42 elements and mineralogy was determined by X-ray diffraction using Rietveld methods for the < 2 mm and the < 2 µm size fraction.

Holocene soils have the highest average concentrations for all 42 elements analyzed with the exception of Si and Zr. Pliocene soils have the highest Si and Zr due to the presence of residual minerals quartz and zircon. Holocene soils exhibit a mean Si/Ca ratio of 2.0 and illite/smectite clay ratio of 5.0, while the older Pliocene soils average ratios of 20.0 for each. Pleistocene soils fall between these end members. Both ratios show positive trends with increasing soil age. Soils with high clay content correlate with abandoned fluvial channels. Although trace element concentrations were highest in the Holocene deposit adjacent to the Rio Grande, these values were not considered anomalous.

Statistical results indicate the elemental concentrations of Si, Zr, Ca, Al and Mg are distinctive and can be used to aid delineation of the underlying geologic units. Soils developed on the Qds however, display a wide array of elemental concentrations indicating that they are not a chemically homogenous unit, consistent with data from ASTER image processing results and airborne gamma-ray survey analysis. Geochemical correlations with geologic contacts are observed, however some lateral soil geochemical boundaries appear blurred, possibly a product of agricultural processes. GIS analysis combining mineralogy, geology and geochemical thematic layers identified areas for further environmental studies. Future work will focus on the impact of elevated trace metal concentrations on environmental and human health.