Fracturing‐induced release of radiogenic 4He and 234U into groundwater during the last deglaciation: An alternative source to crustal helium fluxes in periglacial aquifers. Issue 7 (13th July 2017)

• Record Type: Journal Article
• Authors: Méjean, Pauline; Pinti, Daniele L.; Ghaleb, Bassam; Larocque, Marie
• Is Part Of: Water resources research. Volume 53:Issue 7(2017)
• Page Start: 5677
• Page End: 5689
• Publication Date: 2017-07-13
• Subjects: 234U/238U activity ratio -- 4He excess -- groundwater age -- α‐recoil -- diffusion -- fracturing -- deglaciation; Hydrology -- Periodicals; 333.91
• DOI: 10.1002/2016WR020014
• Languages: English
• ISSNs: 0043-1397

Abstract

Abstract: External 4 He sources have been invoked to explain 4 He 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), 4 He concentrations of up to 4.48 × 10 −5 cm 3 STP g H 2 O − 1 were measured. Such concentrations are ∼1000 times higher than would be expected from in situ production. A concomitant increase in 4 He concentration and 234 U/ 238 U activity ratio is shown, suggesting a common release process in groundwater for 234 U and 4 He. This process has tentatively been identified as glaciation‐induced rock fracturing following the Laurentide Ice Sheet retreat. The resulting increase in exposed grain surface facilitates 234 U release by α‐recoil and that of radiogenic 4 He by diffusion. Using a model of helium diffusion from a spherical grain, it is shown that rock fracturing facilitated the release of accumulated 4 He at rates ranging from 4.2 × 10 −10 to 1.06 × 10 −8 cm 3 STP g H 2 O − 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 × 10 −13 cm 3 STP g r o c k − 1 yr −1 . Integration of 4 He release rates over time yields a radiogenic 4 He concentration of between 3.85 × 10 −6 and 7.12 × 10 −5 cm 3 STP g H 2 O − 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. Key Points: Crustal fluxes are often invoked to explain groundwater helium in excess of steady state local production The relationship between 234 U/ 238 U activity ratios and 4 He contents suggests a common process releasing 234 U and 4 He into groundwater locally Modeling 4 He release from comminuted grains after glaciation‐induced fracturing could explain local 4 He excesses in groundwater