Numerical modelling in support of a conceptual model for groundwater flow and geochemical evolution in the southern Outaouais Region, Quebec, Canada

A two-dimensional vertical-section numerical model for groundwater flow and transport using age, tritium and chloride was used to help validate a conceptual model of geochemical evolution within a representative regional-scale hydrogeological system in the Outaouais Region, Quebec, Canada. The flow system includes up to 30 m of Quaternary sediments and marine clays overlying fractured silicate rock of the Canadian Shield. Calibration of the regional flow model using observed piezometric levels and tritium concentrations showed that shallow groundwater flow is dominated by local flow systems limited to 30–40 m depth, 1–5 km long, and with groundwater residence times of 10–50 years. Intermediate systems, on the order of 5–15 km long, are less extensive than initially thought and are characterised by maximum depths of about 100 m and residence times of 200–6000 years. A model-calibrated hydraulic conductivity of 8 × 10−5 m.s−1 was required in the upper 50 m of the fractured bedrock. The active flow zone was inferred to extend to depths of about 100–150 m, with any deeper regional flow essentially negligible. Differences between tritium-based ages and simulated mean residence times were attributed to mixing of groundwater in open boreholes. Concentrations of 4He could be explained by diffusive transport from deeper and older groundwater, exacerbated by sampling. With new insight from the numerical modelling, the conceptual flow model has been updated to now include only a weak component of regional flow combined with significant local- and intermediate-scale flow systems connected to the upper fractured bedrock. The simulated flow system is also consistent with the geochemical evolution of the region, which is dominated by young Ca-HCO3-type waters in the unconfined aquifer and by older Cl− signatures from the remnant Champlain Sea seawater