Uranium content in clayey strata of the basin-Sacramento Chihuahua, Chih. Mexico.

Abstract

It is proposed that the natural contamination due to uranium in the aquifer at Northwest of Chihuahua basin may be deposited in argillaceous bodies in the subsoil, forming part of the alluvium deposits. For the study, samples were taken from layers of the aquifer, provided as fragments extracted while drilling of well perforation in the Chihuahua-Sacramento valley. The petrographic classification of these samples was performed, and they were studied in the gamma-ray spectrometer based on a NaI(Tl) detector. The specific activities for isotopes 238U, 232Th and 40K were determined through the application of the Standard Stripping code, performing the decomposition of the sample spectral components by means of standards. The code calculates the specific activities of mentioned isotopes and propagates the errors and their uncertainties. Correlations between the specific activities of isotopes and the clay presence have been observed. It is suggested that the clays play a reducing role, contributing to the immobilization of uranium in form of the U+4 ion. 238U decays to 234Th and quickly to 234U, which by recoil nucleus effect changes the valence to U+6, that is soluble. This way, the groundwater has activity ratio 234U/238U >1. The clay existence in layers of the Chihuahua-Sacramento basin, together with the abundance of uranium in the sources of recharge of the aquifer, explains the uranium contents in groundwater of the zone.