3H/3He, 14C and (U–Th)/He groundwater ages in the St. Lawrence Lowlands, Quebec, Eastern Canada

• Groundwater ages investigated in a watershed of St. Lawrence Lowlands, Quebec
• 3He/3He and 14C ages suggest mixing of modern and fossil groundwater.
• Fossil groundwater rested since the last deglaciation.
• Excesses of radiogenic 4He in fossil water from local production in the crust

This study attempts to place constraints on groundwater residence times using helium isotopes and 14C of a regional groundwater flow system – the Becancour River watershed – located midway between Montreal and Quebec City (Quebec, Canada). This densely populated region is one of the main targets for shale gas exploitation in Eastern Canada. For this reason, this watershed has been the focus of a detailed aquifer study to gain a better understanding of groundwater resources, both in terms of availability and quality. In the current study, noble gases were sampled and analyzed in twenty-eight wells from a Quaternary granular aquifer and a regional bedrock aquifer of Ordovician age. Tritium (3H) and radiocarbon (A14C) activities were measured on selected wells. Helium isotopic ratios 3He/4He (R) normalized to that of the atmosphere (Ra = 1.386 × 10− 6) range from R/Ra = 0.039 ± 0.003 to 3.109 ± 0.065. The helium isotopic signatures point to the presence of three water bodies: 1) modern infiltration water with nearly atmospheric helium isotopic ratio and little post-bomb tritium recharging the shallower granular aquifer; 2) mid-50s tritium-rich water slightly mineralized; and 3) an older water component rich in terrigenic helium flowing in from the bedrock fractured aquifer. Uncorrected 14C ages range from 15 ka to modern. 14C is affected by several dead carbon reservoirs related to carbonate dissolution, cations exchange, oxidation of organic matter and methanogenesis. Adjusted 14C ages calculated with NETPATH range from 7 ka to modern. Older 14C ages correspond to the end of the regional re-organization of the hydrological system following deglaciation and isostatic rebound. Noble gas recharge paleotemperatures are 4–9 °C colder than the present temperatures, although no clear relation with ages has been found. The relationship between the helium isotopic ratios and 14C ages suggests that the regional bedrock aquifer is affected by mixing between these three water sources. Calculated (U–Th)/4He ages can be partially explained by in situ production of 4He in the aquifer rock and the addition of a radiogenic helium source external to the aquifer. Calculated minimum helium fluxes of 1.0 × 10− 8 to 1.8 × 10− 7 cm3 STP cm− 2 yr− 1 are tens to hundreds of times lower than the average continental crust flux of 3.3 × 10− 6 cm3 STP cm− 2 yr− 1, suggesting local sources, possibly from production of radiogenic helium in the shale gas formations underlying the studied aquifers. The occurrence of old groundwater in this aquifer system clearly limits the renewable resource and increases the risk of overexploitation in the case of increased use or in the case of pollution from different sources.