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

Noble gases, in particular He-3/He-4 (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 He-3/He-4 ratios range from 0.224 +/- 0.012 to 1.849 +/- 0.036Ra, where Ra is the atmospheric He-3/He-4 ratio (1.386 x 10(-6)). These results suggest the occurrence of He-3 produced by decay of tritium and terrigenic He-4 produced by decay of U and Th. Calculated H-3/He-3 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 He-4 (He-4(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 He-4(terr) ranges from 3.4 x 10(-9) to 2.2 x 10(-6) cm(3)STP g(-1) and shows a clear gradient with depth, suggesting addition of a He-4(terr) flux entering the bottom of the eskers. Modeled He-4(terr) fluxes range from 2.0 x 10(-8) cm(3)STP cm(-2) yr(-1) at the Harricana moraine to 6.6 x 10(-7) cm(3)STP 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/He-4 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. (C) 2015 Elsevier B.V. All rights reserved.