Evolution of Uranium Isotopic Composition of the Groundwater in Jabal Sayid-Mahd Adhab Region, Western Saudi Arabia

Uranium isotopic composition of shallow groundwater in the Jabal Sayid-Mahd Adhab area of western Saudi Arabia was explored to evaluate geochemical changes resulting from water-rock interactions. The wide range of uranium concentrations (0.75–29.3 ppb) and 234U/238U activity ratios (1.11–3.11) reflects variable redox and dissolution conditions across the aquifer. Samples with high uranium concentrations but low activity ratios in-dicate recent uranium release from mineral phases, enhanced by the presence of fluoride ions that form stable soluble complexes with uranyl species. Conversely, higher isotopic ratios in low-uranium samples suggest longer water-rock interaction and preferential leaching of 234U by alpha-recoil processes. The positive correlation between uranium and salinity parameters further supports that uranium enrichment is linked to increased ionic strength and the abundance of complexing ligands. The activity ratio 234U/238U (AR) vs. 1/U relationship in the studied samples indicates that uranium behavior in the shallow aqui-fer is dominated by open-system leaching, with local binary mixing superimposed in few sites. The findings emphasize that uranium isotopic composition is a valuable tool for identifying localized groundwater mixing and evaluating the hydrogeochemical effects of nearby mineralized areas on the aquifer system.