FRACTIONATION OF URANIUM ISOTOPES IN WATER FROM SAN MARCOS DAM IN CHIHUAHUA, MEXICO

The objective of this work was to assess the fractionation and distribution of isotopic uranium contained in suspended matter and dissolved fraction in water of the San Marcos Dam located in Chihuahua, Mexico Uranium was measured in three points along the San Marcos dam: water input, midpoint, and near to dam wall. This last point was also sampled in two depths. Every water sample was filtered to obtain the suspended matter with particle size of >20µm. Isotopic uranium was determined by liquid scintillation. The distribution coefficient k_d ranged between 1.7 ·10² to 1·10³ L g^-1. In dissolved fraction, high concentrations of ²³⁴U were found, where the activity ratio (AR, ²³⁸U/²³⁴U) ranged from 2.1 to 3.5. In average, uranium contents tend to be concentrated in suspended matter, with AR near to unit. Uranium concentration varies along the dam, being the highest ²³⁸U concentration at water input point. The results showed a possible lixiviation of uranium from geological substrate and, also, an important fractionation of uranium isotopes suggesting that those are non-homogeneously distributed along San Marcos dam.