ISOTOPIC URANIUM IN GROUNDWATER USED IN AGRICULTURE IN CHIHUAHUA, MEXICO

Some uranium outcrops are located in the San Marcos environment at the northwest of Chihuahua city. Those outcrops might be an area of influence of natural radioisotopes pollution in water of the main aquifer that supplies that region. The objective of this study was to determine the specific activities of isotopic uranium in groundwater. Seventeen wells were sampled in the Chihuahua-Sacramento Valley and some of them near to the San Marcos area. Specific activity concentrations of ²³⁸U and ²³⁴U were determined by liquid scintillation, using a Perals spectrometer. In addition, physic-chemical parameters were also quantified in every sample. The results showed that pH values were slightly acid to neutral, whereas the amount of total dissolved solids ranged from 80 to 310 ppm. The specific activity of ²³⁸U and ²³⁴U in the water from wells sampled ranged from 0.003 to 6.75 (Bq L^-1), and from 0.017 to 27.030 (Bq L^-1), respectively. The uncertainty to ²³⁸U was less than 10%, while for ²³⁴U was less than 4%. The highest uranium concentrations were found in wells closest to the San Marcos area. Only 17.64% showed a ²³⁸U specific activity above the maximum permissible limit for alpha emitters in drinking water established by the Mexican Official Norm (0.56 BqL^-1), as well as the limit recommended by the EPA (0.37 BqL^-1).