Abstract

REYES-CORTES, Manuel et al. Orign and distribution of the natural radioactivity in the northern part of the Chihuahua basin, Mexico. Rev. mex. cienc. geol [online]. 2012, vol.29, n.3, pp.659-675. ISSN 2007-2902.

The Chihuahua valley is a north-south oriented basin 8 km wide and 32 km long, forming part of the Chihuahua hydrologic basin. This basin supplies most of the water for the Chihuahua City. Water enriched in organic matter from the San Marcos dam in the upper part of the basin has up to 75% of the uranium content incorporated as colloidal or suspended particles. Water andfish from the San Marcos dam and from some deep wells in the northwestern region of Chihuahua valley have relatively high uranium concentrations. A lithostratigraphic study from several drill cuttings, logging and gamma spectrometry data defines the presence of sandy clay lenses and beds bearing carbonaceous material. These horizons show uranium anomalies in the northwestern part of the Chihuahua valley with values up to 612 Bq/kg or approximately 50 ppm. The detected radiometric anomalies in the water from some deep wells may correspond to the presence of fine particles of carbonaceous material, formed by some paleo-soils or paleo-sediments from flooding plains. It is suggested that these clay horizons are uranyl ion collectors. The uranyl may suffer a reduction process by the presence of organic material. Depending on pH and Eh conditions, groundwater can oxidize and re-mobilize the uranyl ions that can be detected in some deep wells for agricultural water supply.

Keywords : Uranium; radioactive contamination; hydrogeochemistry; Chihuahua; Mexico.