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

Abstract

External 4He sources have been invoked to explain 4He 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), 4He concentrations of up to 4.48 × 10−5 cm3 STP gnormalH2O−1 were measured. Such concentrations are ∼1000 times higher than would be expected from in situ production. A concomitant increase in 4He concentration and 234U/238U activity ratio is shown, suggesting a common release process in groundwater for 234U and 4He. 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 234U release by α‐recoil and that of radiogenic 4He by diffusion. Using a model of helium diffusion from a spherical grain, it is shown that rock fracturing facilitated the release of accumulated 4He at rates ranging from 4.2 × 10−10 to 1.06 × 10−8 cm3 STP gnormalH2O−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 cm3 STP grock−1 yr−1. Integration of 4He release rates over time yields a radiogenic 4He concentration of between 3.85 × 10−6 and 7.12 × 10−5 cm3 STP gnormalH2O−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.

Environmental isotopes and noble gas ages of the deep groundwater with coupled flow modelling in the Baltic artesian basin

“…These differences in δ 18 O of groundwater are due to changes in global paleoclimatic conditions and their impact on the groundwater formation during the Pleistocene and Holocene [6]. The δ 18 O values of groundwater in most aquifers in North Poland, Kaliningrad District of Russia, Lithuania and Latvia territory range from -7.7 to -13.9‰ [6,7]. However, in the Island of Gotland (Sweden Homocline) at the same depth groundwater has significantly higher δ 18 O values ranging from -5.7 to -6.1‰ [8].…”

“…In the active zone near surface, dominant δ 18 O values are from -8.2 up to -11.6‰ that are related to the modern meteoric water recharge and by features of lateral partitioning of the δ 18 O isotope ratio of precipitation across the region [5]. Zones with more positive δ 18 O values (from -8.2 to -10.5‰) are located mostly in the southwestern part of the Baltic Region. In Estonia, extremely depleted zones with δ 18 O values from -14 to -22.5‰ are observed reflecting palaeorecharge during the cold climate Pleistocene time.…”

“…Below the 1.8 km depth, stagnant chloride type groundwater with more than 100 g/L in TDS is formed. Thus, the three main zones according to δ 18 O values distribution in the abovementioned groundwater zones were distinguished in the BAB.…”

“…In Estonia, extremely depleted zones with δ 18 O values from -14 to -22.5‰ are observed reflecting palaeorecharge during the cold climate Pleistocene time. These differences in δ 18 O of groundwater are due to changes in global paleoclimatic conditions and their impact on the groundwater formation during the Pleistocene and Holocene [6]. The δ 18 O values of groundwater in most aquifers in North Poland, Kaliningrad District of Russia, Lithuania and Latvia territory range from -7.7 to -13.9‰ [6,7].…”

“…The δ 18 O values of groundwater in most aquifers in North Poland, Kaliningrad District of Russia, Lithuania and Latvia territory range from -7.7 to -13.9‰ [6,7]. However, in the Island of Gotland (Sweden Homocline) at the same depth groundwater has significantly higher δ 18 O values ranging from -5.7 to -6.1‰ [8].…”