Processes controlling 234 U and 238 U 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±0.014, was measured in Ca-HCO 3type 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 -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 -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.

Fracturing‐induced release of radiogenic 4He and 234U into groundwater during the last deglaciation: An alternative source to crustal helium fluxes in periglacial aquifers

“…[], and Méjean et al . []. The regional aquifer of the Becancour River watershed is mainly composed of fractured Ordovician carbonate‐shale deposits belonging to the St. Lawrence Platform [ Larocque et al ., ].…”

“…Méjean et al . [] measured the 234 U/ 238 U activity ratios (( 234 U/ 238 U) act hereafter) in the groundwater of the same aquifer as Vautour et al . [].…”

“…Méjean et al . [] found an inverse relationship between the ( 234 U/ 238 U) act and the 3 He/ 4 He ratios: freshwater, devoid of radiogenic helium, shows an ( 234 U/ 238 U) act of close to unity (i.e., at secular equilibrium); older groundwater is progressively enriched in radiogenic helium (i.e., the 3 He/ 4 He ratio decreases) and the ( 234 U/ 238 U) act increases to as much as 6. This relationship calls for a local enrichment mechanism for both 234 U and 4 He, within the aquifer rather than an external source of helium as suggested by Vautour et al .…”

“…Méjean et al . [] suggested that the opening of new fractures might provide additional surfaces from which 234 U migrates by α‐recoil and 4 He migrates by diffusion into water at high rates but did not quantify these processes.…”

“…Helium and uranium data from Méjean et al . [] and Vautour et al . [] are revisited and integrated in a coupled model of 234 U‐ 238 U fractionation and radiogenic 4 He release into groundwater by glaciation‐induced fracturing, using equations developed by Andrews et al .…”