Groundwater age investigation of eskers in the Amos region, Quebec, Canada

•Aquifers in eskers are the only source of drinking water at high latitudes.•Ages and flow difficult to quantify because heterogeneity of the medium.•Eskers in Amos contain precious water resources but no age available.•Fast flow dominate esker's top aquifers, recharged in the last 30 years.•Flow is slow at the base of eskers with occurrence of fossil meltwater.

SummaryNoble gases, in particular 3He/4He (R) ratios, were measured together with tritium activity in groundwater from eskers and moraines of the Abitibi-Temiscamingue region of northwestern Quebec (eastern Canada). These high-latitude glaciofluvial landforms contain precious freshwater resources that need to be quantified. Here we provide estimates of residence time for groundwater in glaciofluvial sediments forming the Saint-Mathieu-Berry (SMB) and Barraute eskers, the Harricana moraine and in the underlying fractured bedrock aquifer. The 3He/4He ratios range from 0.224 ± 0.012 to 1.849 ± 0.036Ra, where Ra is the atmospheric 3He/4He ratio (1.386 × 10−6). These results suggest the occurrence of 3He produced by decay of tritium and terrigenic 4He produced by decay of U and Th. Calculated 3H/3He apparent ages of groundwater from the SMB esker and the Harricana moraine range from 6.6 ± 1.1 a to 32 ± 7.4 a. Terrigenic 4He (4Heterr) was found in the deeper wells of the SMB esker and in the wells tapping water from the deeper fractured aquifer located below the eskers and moraines and confined by postglacial clays. The amount of 4Heterr ranges from 3.4 × 10−9 to 2.2 × 10−6 cm3STP g−1 and shows a clear gradient with depth, suggesting addition of a 4Heterr flux entering the bottom of the eskers. Modeled 4Heterr fluxes range from 2.0 × 10−8 cm3STP cm−2 yr−1 at the Harricana moraine to 6.6 × 10−7 cm3STP cm−2 yr−1 in the southern section of the SMB esker. Calculated fluxes are highly variable and 5-165 times lower than the helium continental crustal flux, suggesting local helium sources, with helium being driven upward through preferential pathways such as local faults. Maximum U-Th/4He ages obtained for the groundwater in the fractured bedrock range from 1473 ± 300 a to 137 ± 28 ka, suggesting the occurrence of several generations of fossil meltwater trapped under the clay plain after the last two glaciations.