Contribution of natural uranium isotopes to the study of groundwater flow

Abstract

Uranium is a lithophile element that is preferentially concentrated in acid rather than basic/ultrabasic igneous rocks. Its average crustal abundance corresponds to 2.5 μg/g, composed of two primary isotopes, ²³⁸U (~99.3%) and ²³⁵U (~0.7%). Despite the technological importance of ²³⁵U, as it is the fissile isotope that is the basis of nuclear energy production, it appears that its contribution as a natural source of radioactivity is small, since the specific activity of ²³⁸U is about 20 times greater than ²³⁵U. Isotopes ²³⁸U and ²³⁵U are progenitors of radioactive decay series, the greater number of descendants corresponding to ²³⁸U, many of which possessing a long half-life, such as ²³⁴U formed in the ²³⁸U decay series from the following sequence: ²³⁸U (4.49 Ga, α) → ²³⁴Th (24.1 days, β-) → ²³⁴Pa (1.18 min, β-) → ²³⁴U (248 ka, α) →... Both ²³⁸U and ²³⁴U are emitters of alpha particles and, in groundwater, the assessment of the extent of radioactive imbalance between these uranium isotopes has allowed the development of numerous studies of hydrogeological interest since the 1960’s. The main mechanisms that explain such disequilibrium are the ²³⁴U selective leaching relatively to ²³⁸U from the crystalline lattice of minerals and alpha recoil that introduces ²³⁴Th into the liquid phase, which forms ²³⁴Pa by beta decay, radionuclide that produces ²³⁴U, also by beta decay, causing ²³⁴U-enrichment in groundwaters, i.e., ²³⁴U/²³⁸U activity ratios greater than unity. These ratios, together with the dissolved uranium concentration, have been extensively utilized in hydrological applications, such as those described in this paper.