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

Noble 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±1.1a to 32±7.4a. Terrigenic ⁴He (⁴He_terr) 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 ⁴He_terr ranges from 3.4×10^-9 to 2.2×10^-6cm³STPg^-1 and shows a clear gradient with depth, suggesting addition of a ⁴He_terr flux entering the bottom of the eskers. Modeled ⁴He_terr fluxes range from 2.0×10^-8cm³STPcm^-2yr^-1 at the Harricana moraine to 6.6×10^-7cm³STPcm^-2yr^-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±300a to 137±28ka, suggesting the occurrence of several generations of fossil meltwater trapped under the clay plain after the last two glaciations.