资源环境科技发展态势分析平台

External He-4 sources have been invoked to explain He-4 concentrations in groundwater greater than those expected from in situ U and Th production. In a fractured aquifer of Ordovician age located in the St. Lawrence Lowlands (Quebec, Canada), He-4 concentrations of up to 4.48 x 10(-5) cm(3) STP g(H2O)(-1) were measured. Such concentrations are similar to 1000 times higher than would be expected from in situ production. A concomitant increase in He-4 concentration and U-234/U-238 activity ratio is shown, suggesting a common release process in groundwater for U-234 and He-4. This process has tentatively been identified as glaciationinduced rock fracturing following the Laurentide Ice Sheet retreat. The resulting increase in exposed grain surface facilitates U-234 release by alpha-recoil and that of radiogenic He-4 by diffusion. Using a model of helium diffusion from a spherical grain, it is shown that rock fracturing facilitated the release of accumulated He-4 at rates ranging from 4.2 x 10(-10) to 1.06 x 10(-8) cm(3) STP g(H2O)(-1) yr(-1). These release rates are between 1000 and 30,000 times higher than the local U and Th steady state production rate, of 3.5 +/- 1.4 x 10(-13) cm(3) STP g(rock)(-1) y(-1). Integration of He-4 release rates over time yields a radiogenic He-4 concentration of between 3.85 x 10(-6) and 7.12 x 10(-5) cm(3) STP g(H2O)(-1), in the range of concentrations measured in the St. Lawrence Lowlands fractured aquifers. Results support the occurrence of a local radiogenic helium source to explain the excesses measured in groundwater without requiring a significant external He crustal flux.