Processes controlling 234U and 238U isotope fractionation and helium in the groundwater of the St. Lawrence Lowlands, Quebec: The potential role of natural rock fracturing

Abstract The goal of this study is to explain the origin of 234 U– 238 U fractionation in groundwater from sedimentary aquifers of the St. Lawrence Lowlands (Quebec, Canada), and its relationship with 3 He/ 4 He ratios, to gain insight regarding the evolution of groundwater in the region. ( 234 U/ 238 U) activity ratios, or ( 234 U/ 238 U) act , were measured in 23 groundwater samples from shallow Quaternary unconsolidated sediments and from the deeper fractured regional aquifer of the Becancour River watershed. The lowest ( 234 U/ 238 U) act , 1.14 ± 0.01, was measured in Ca–HCO 3 -type freshwater from the Quaternary Shallower Aquifer, where bulk dissolution of the carbonate allows U to migrate into water with little 234 U– 238 U isotopic fractionation. The ( 234 U/ 238 U) act increases to 6.07 ± 0.14 in Na–HCO 3 –Cl-type groundwater. Preferential migration of 234 U into water by α-recoil is the underlying process responsible for this isotopic fractionation. An inverse relationship between ( 234 U/ 238 U) act and 3 He/ 4 He ratios has been observed. This relationship reflects the mixing of newly recharged water, with ( 234 U/ 238 U) act close to the secular equilibrium and containing atmospheric/tritiogenic helium, and mildly-mineralized older water ( 14 C ages of 6.6 kyrs), with ( 234 U/ 238 U) act of ≥6.07 and large amounts of radiogenic 4 He, in excess of the steady-state amount produced in situ . The simultaneous fractionation of ( 234 U/ 238 U) act and the addition of excess 4 He could be locally controlled by stress-induced rock fracturing. This process increases the surface area of the aquifer matrix exposed to pore water, from which produced 4 He and 234 U can be released by α-recoil and diffusion. This process would also facilitate the release of radiogenic helium at rates greater than those supported by steady-state U–Th production in the rock. Consequently, sources internal to the aquifers could cause the radiogenic 4 He excesses measured in groundwater.