Hydrogeochemical behavior of uranium and thorium in rock and groundwater samples from southeastern of El Bajío Guanajuatese, Guanajuato, Mexico

To evaluate the hydrogeochemical processes related to the presence, mobility, and transport of uranium and thorium in the southeastern Sierra de Guanajuato of the Bajio Guanajuatense, Mexico, the concentration of these elements in volcanic rocks and in groundwater were determined. Acidic volcanic rocks contained uranium concentrations of 1.04–6.88 ppm and thorium concentrations of 3.13–15.58 ppm. A basalt rock sample contained 2.58 and 4.07 ppm of uranium and thorium, respectively, and an andesite rock sample contained 4.07 and 8.7 ppm of uranium and thorium, respectively. Scanning electron microscopy with energy-dispersive X-ray spectroscopy showed that U and Th are disseminated throughout the matrix of felsic rocks; in addition, phosphate minerals (probably monazite) containing both elements are present. The concentration of uranium in water samples ranged from 2.23 to 10.3 ppb and of thorium ranged from 0.33 to 0.39 ppb. Geochemical modeling indicated that uranium was mobilized through the transport of the uranyl ions (UO₂²⁺) and their complexes, especially carbonates, although other elements such as Fe, Mg, HCO₃, and Ca could also be involved in the mobility and transportation of uranium. The obtained results suggest that the mobilization of thorium is very low, almost negligible in some cases, so this element did not present a quantifiable hydrogeochemical pattern. In spite of the low concentrations of U and Th, the existence of high radon concentrations in several samples indicates that additional radioactive species could be naturally present in the hydrogeological environment.