<p>This study investigates natural radioelements K (⁴⁰K), U (eU = ²²⁶Ra), Th (eTh = ²²⁸Th), and U-isotopes ²³⁸U and ²³⁴U in rocks of the Irati Formation (Passa Dois Group), Paraná Sedimentary Basin, Brazil, aiming to understand the consequences of water–rock interaction processes occurring there. Gamma-ray and alpha-spectrometry were applied to the analysis of rock and groundwater samples. The results confirm that lithology exerts a primary control, with shales showing higher U, Th, and K concentrations than limestones due to clay minerals’ adsorption capacity. Moreover, ²³⁴U/²³⁸U activity ratios in rocks were slightly above unity, suggesting recent ²³⁴U gain via water–rock interactions. In groundwater, however, ratios reached values as high as 27.9, indicating intense isotopic fractionation driven by α-recoil and preferential ²³⁴U leaching. Spatial variability in these ratios- from near-equilibrium values in oxidizing environments to extreme fractionation in confined aquifer zones reflects differences in groundwater residence time, redox conditions, and hydrologic confinement. Overall, this work demonstrates that the Passa Dois aquiclude is a heterogeneous hydrogeochemical system, where localized zones of high isotopic fractionation are governed by lithological variability and groundwater flow dynamics, providing a radiometric framework for understanding uranium mobility in sedimentary basins.</p>