<p>Extreme disequilibrium in uranium isotope ratios (²³⁴U/²³⁸U &gt; 10 by activity) in groundwater is rare, as most groundwaters generally exhibit a moderate excess of ²³⁴U (²³⁴U/²³⁸U = 1–3). One hypothesis attributes ²³⁴U/²³⁸U &gt; 10 to past permafrost conditions. To test this hypothesis, groundwaters were investigated along the contact between the Baltic Shield and the East European Platform, in the Leningrad region and western Karelia, northwest Russia, where permafrost existed during the Wichlesian Ice Age. Groundwater ²³⁴U/²³⁸U ratios were found to reach 25.8. To link the extreme excess of ²³⁴U in groundwater to past permafrost conditions, the major ion, isotopic (δ¹⁸O, δ²H) composition and tritium (³H) abundance of groundwater were also determined. Groundwaters were found to contain four different components. The first component, with ²³⁴U/²³⁸U ~ 1, corresponded to fresh groundwater derived from modern recharge, containing ³H and stable isotope compositions reflecting modern precipitation. The second component, with ²³⁴U/²³⁸U ~ 1 corresponded to fresh groundwater with a recharge age of ~ 50 years or slightly older, as stable isotope compositions were close to modern precipitation, but ³H activity was below the limit of detection. The third component, with ²³⁴U/²³⁸U = 1–3, corresponded to postglacial groundwater recharge derived from the melting of the Scandinavian ice sheet, reflecting an extremely depleted stable isotope composition. The fourth component, with ²³⁴U/²³⁸U ≤ 25, corresponded to “revived” groundwater formed during the permafrost thawing in the Holocene. This classification of groundwater origin corroborates with regional paleogeographic reconstructions and supports the hypothesis that long-term permafrost conditions influence excess ²³⁴U in groundwater.</p>