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

This study presents an almost entirely unpublished dataset of 121 samples groundwater analyzed for helium concentration and its isotopic ratio (3He/4He) in two adjacent watersheds the St. Lawrence Lowlands, a region with intensive agricultural activities southern Québec Province, Eastern Canada. Most were collected regional bedrock fractured aquifer hosted mid-Ordovician siliciclastic shales, on total surface 7500 km2. Even this low-density sampling, heterogeneous aquifer, isotopes bring precious information recharge conditions chemical evolution water. The spatial interpolation does not show clear gradient through basin. However, it shows progressive enrichment radiogenic 4He confined part aquifer. atmospheric and/or tritiogenic-rich occurs at Appalachians middle plain, where impermeable cover is limited, local infiltration meteoric freshwater reaches relation between dissolved solids (TDS) 3He/4He ratios remains elusive. discriminating dominant chemistry water, trend observed ratio, suggesting that accumulates together increasing time (indicated by progressively older 14C ages). Finally, noble gas temperatures (NGTs) obtained from concentrations other gases (Ne, Ar, Kr Xe) brings constraints earlier during Holocene. Particularly, NGTs showed studied aquifers continuously replenished, even under ice-sheet last 10,000 years.