Noble gas, carbon and nitrogen in a St. Lawrence Lowlands bedrock aquifer (eastern Canada)

English Abstract:

Flow patterns and residence times are fundamental parameters for quantifying groundwater resources, yet they are difficult to estimate and are better defined with an integrated isotopic approach. Noble gases are excellent groundwater tracers because they are inert. In order to decipher groundwater circulation and sources in one of the several watersheds providing groundwater in the vicinity of the Montreal metropolitan area, He, Ar, Ne, N₂, CO₂ and CH₄ elemental and isotopic composition were analyzed. The Vaudreuil-Soulanges watershed (895 km²) has Cambrian sandstone and dolostone fractured aquifers confined by Quaternary clays. Fractured Cretaceous alkaline intrusions and Precambrian bedrock (Mont Rigaud) outcrop in the northwestern part of the basin, creating an important recharge area. Preliminary results from 16 wells show variable helium contents from 6.15.10^-8 to 1.32x10^-5 ccSTP/g. The He isotopic ratios (R), normalized to that of air (Ra = 1.386 x 10^-6), range from 0.18 to 2.33, indicating the occurrence of both radiogenic ⁴He and tritiogenic ³He excesses compared to the atmospheric composition. Tritium contents indicate pre-bomb recharge waters (³H<0.8TU) and modern water infiltrated during the last decades (³H = 10.8TU). The pre-bomb waters are Na-Cl type and are located in the most confined and chemically evolved water in the region. Helium components can be explained by the mixing of a freshwater tritium-rich component (from 12 to 65 TU) mixed with groundwater containing mainly crustal He, slightly contaminated by 2% of mantle helium. Interestingly, three groundwater samples suggest that they contain up to 16.7% of mantle He. N₂/Ar ratios range between 47 and 101, higher than the ASW (air saturated water) value of 37, indicating some N in excess of the atmospheric content. A relation between the CO₂/³He molar ratio and the ³He/⁴He ratio indicates that both CO₂ and He derive from a mixing between a mantle source (max 25%) and crustal sources with variable CO₂/³He ratios. Fossil mantle gases could be sourced by groundwater-rock interactions with the numerous intrusions buried under the Quaternary cover and related to the Cretaceous episode of the Monteregian Hills. Particularly, the carbonatite intrusion of Oka is 20 km away from the studied basin, where ³He/⁴He ratios up to 2.54 Ra have been measured.