SYNERGISTIC USE OF AIRBORNE GAMMA-RAY SURVEY DATA AND ASTER IMAGERY FOR MAPPING CROSS-BORDER RADON POTENTIAL IN THE LOWER RIO GRANDE VALLEY, SOUTH TEXAS USA AND NORTH TAMAULIPAS MEXICO

An airborne gamma-ray survey was flown over parts of Cameron, Hidalgo and Willacy counties in southwestern Texas. The survey measured abundances of radionuclides K40, and daughter products of U238 and Th232 within the upper ~30 cm of soils and surficial deposits. Of particular importance to human health is the spatial distribution of equivalent Uranium concentration values, which are an indirect measure of Ra226, direct measure of Bi214 and a proxy means of estimating potential indoor radon (Rn222) levels resulting from soil gas emanation. ASTER data together with geologic maps provides the necessary lithologic and mineralogic backdrop from which the predictive radon potential map can be extrapolated across the US - Mexico Border.

Using multivariate statistical methods of class separation and supervised maximum likelihood classification, 21 classes of materials with unique U, Th and K signatures were initially derived from the three-band gamma-ray data. Several of these classes were combined spatially to form seven units which correlate with the mapped distribution of Quaternary and older deposits such as the Pliocene Goliad Formation, Pleistocene Lissie and Beaumont Formations, and Holocene Rio Grande floodplain deposits. ASTER data calibrated to nine-band VNIR-SWIR (0.52 - 2.43 ƒÝm) reflectance and five-band TIR (8.125 - 11.65 ƒÝm) emissivity was used to map mineralogical differences between gamma ray classes and mapped geologic units. In order to validate the mapping results from both airborne gamma-ray and ASTER spectra, 137 samples of soils and sediments were collected within the airborne gamma-ray survey and ASTER coverage areas, with a sampling interval spaced ~3 - 5 km apart. The samples were studied in the lab using a variety of analytical methods including visible through infrared spectroscopy, x-ray diffraction, bulk density, gamma-ray spectral analysis.

Results of the gamma-ray survey show that the Holocene Rio Grande floodplain and delta deposits have the highest concentrations of uranium and thorium, and thus greatest potential for indoor radon. ASTER classification and soil analyses can identify and map these radon-prone Quaternary geologic units. Thus, using ASTER imagery, similar units can be extrapolated to the Mexican side of the study area where no gamma-ray data is available.