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Conceptual model of regional groundwater flow based on hydrogeochemistry (Montérégie Est, Québec, Canada)

The groundwater geochemistry of the fractured rock aquifer system in the Montérégie Est region, southern Quebec, Canada, was studied as part of a regional groundwater resources assessment. The 9218 km² study area included three major watersheds that were divided into five hydrogeological contexts: Northern St. Lawrence Lowlands, Southern St. Lawrence Lowlands, Appalachian Uplands, Appalachian Piedmont and Monteregian Hills. A large part of this study area was invaded by the Champlain Sea from 13,000 to 11,000 years ago. Study objectives were to identify the mechanisms controlling groundwater composition and to support the understanding of the aquifer hydrodynamics. Groundwater from 206 wells drilled into the rock aquifer was sampled and analyzed for conventional parameters and isotopic analyses were also done on selected samples (δ2H, δ18O and 3H of water; δ13C and 14C of dissolved inorganic carbon). The interpretation of geochemical results was based on a multivariate statistical analysis, which led to the definition of eight water groups. The study allowed the delineation of a 2200-km² zone containing brackish groundwater of marine origin in the northwestern part of the study area. This zone is surrounded by sodic and alkaline groundwater originating from Na+-Ca2+ ionic exchange. Young groundwater and therefore recharge zones were only encountered in the southern part of the Lowlands, in the northern part of the Piedmont and in the Appalachian Uplands. In the southern part of Lowlands, recharge is presumed to be slow and water composition shows the influence of the former presence of the Champlain Sea. Relatively deep groundwater circulation was also inferred to occur from the Appalachian Uplands toward mixing zones mainly located to the west at the Appalachian frontal thrust faults and around the Monteregian Hills. The geochemical interpretation provided indications on regional recharge and discharge zones as well as groundwater flow, which could not have been determined otherwise.