Dating and tracing groundwater resources in central Québec with noble gases, 14C and water chemistry

Canada water supply is derived mainly from lakes and rivers while groundwater accounts for less than 30% of the demand. Climatic and anthropogenic stress is endangering this natural resource. This study is part of a Quebec-funded program to characterize groundwater resources from major basins to ensure adequate water quality and management. The Bécancour River catchment area, 200 km NE of Montréal, is studied for this purpose. The region extends from the Appalachian Mts. down to the St. Lawrence River. Water chemistry shows the occurrence of Ca,Na-HCO₃, Ca-HCO₃, Na-HCO₃ and NaCl waters, mainly recharged in the Appalachians and flowing to the St. Lawrence River. Aquifers are also recharged locally through Quaternary sandy deposits throughout the plain. Main aquifers are found both in unconfined and confined Quaternary sands intercalated with clays, and also locally in fractured Ordovician carbonates. To identify groundwater flow paths and to estimate water residence times, a multi-isotopic study involving noble gases, ¹⁴C and U, Th isotopes was initiated. Noble gas preliminary results were obtained on two transects, one along the downward flow path and the second perpendicular to the first, along a deeper sinclinorium with NaCl waters (TDS =170-705 mg/L). The ³He/⁴He ratios versus ⁴He/²⁰Ne clearly points to mixing between three water bodies. The first has been recharged very recently displaying a near-atmospheric ratio of 0.79 Ra (Ra=atmospheric ³He/⁴He ratio of 1.386 x 10^-6) and a ⁴He/²⁰Ne of 0.2 close to the Air Saturated Water value, i.e. the ratio of atmospheric He and Ne dissolved in groundwater. The second water body is slightly older and shows the occurrence of tritiogenic ³He (³He_tri) excesses up to 1.07x10^-13 ccSTP/g. These waters are found along the transect parallel to the main flow path. Using a local tritium decay curve and measured ³He_tri, an age of 19-20 years is suggested. The third water body contains large amounts of radiogenic ⁴He produced by U and Th decay in aquifer rocks up to 4.48x10^-5 ccSTP/g. These waters are found close to the Appalachian recharge and in the transversal sinclinorium. They are from relatively deep wells (50 m depth), likely tapping the Ordovician carbonate basement. A simple in situ U-Th-⁴He age model gives ages of 2.0 to 6.6 Ma, which might suggest either: (1) the occurrence of isolated pockets of old water in the fractured basement or (2) mixing with deeper fossil brines containing large amounts of radiogenic ⁴He occurring in the Bécancour area. ¹⁴C measurements, in progress, should be helpful in calibrating the He age model and to test whether a ⁴He source external to aquifers might exist. Preliminary calculated paleotemperatures using atmospheric noble gases suggest values varying between 0° to 7 °C. The higher temperature represents actual recharge conditions during spring. The lower temperatures might correspond to Holocene waters recharged during the last deglaciation or contribution from recent glacial meltwater.