Regional-Scale Distribution of Helium Isotopes in Aquifers: How Informative Are They as Groundwater Tracers and Chronometers?

• Published in: Water
• Main Authors: Daniele Luigi Pinti, Marie Larocque, Pauline Méjean, Marion Saby, Mario Alberto Hernández-Hernández, Sylvain Gagné, Emilie Roulleau, Yuji Sano, Maria Clara Castro, Takuya Matsumoto, Viorel Horoi
• Format: Article in Journal/Newspaper
• Language: English
• Published: MDPI AG 2022
• Subjects: recharge; groundwater age; helium isotopes; 14 C; noble gas paleotemperatures; Hydraulic engineering; TC1-978; Water supply for domestic and industrial purposes; TD201-500; Canada; Ice Sheet
• Online Access: https://doi.org/10.3390/w14121940; https://doaj.org/article/6cf84ddd63684a7c89decbdae2a7c6ef

This study presents an almost entirely unpublished dataset of 121 samples of groundwater analyzed for helium concentration and its isotopic ratio ( 3 He/ 4 He) in two adjacent watersheds of the St. Lawrence Lowlands, in a region with intensive agricultural activities in the southern Québec Province, Eastern Canada. Most of the samples were collected in the regional bedrock fractured aquifer hosted in mid-Ordovician siliciclastic shales, on a total surface of 7500 km 2 . Even with this low-density sampling, and in a heterogeneous and fractured aquifer, the helium isotopes bring precious information on the recharge conditions and on chemical evolution of water. The helium spatial interpolation does not show a clear isotopic gradient through the basin. However, it shows progressive enrichment of radiogenic 4 He in the confined part of the aquifer. The atmospheric and/or tritiogenic-rich helium occurs at the recharge in the Appalachians and in the middle of the plain, where impermeable cover is limited, and local infiltration of meteoric freshwater reaches the bedrock aquifer. The relation between the total dissolved solids (TDS) and 3 He/ 4 He ratios remains elusive. However, on discriminating the samples with the dominant chemistry of water, a clear trend is observed with 3 He/ 4 He ratio, suggesting that radiogenic 4 He accumulates together with dissolved solids and with increasing time (indicated by progressively older 14 C ages). Finally, the noble gas temperatures (NGTs) obtained from concentrations of the other noble gases (Ne, Ar, Kr and Xe) brings constraints on the earlier recharge conditions during the Holocene. Particularly, the NGTs showed that the studied aquifers were continuously replenished, even under ice-sheet cover in the last 10,000 years.