Groundwater Flow and Age Tracer Modeling in the Montérégie Est Bedrock Aquifer using MODFLOW, MODPATH, and MT3D-USGS

Abstract

Groundwater residence times and flow pathways are difficult to constrain in regional fractured bedrock aquifers because groundwater movement is controlled by heterogeneous fracture networks, variable recharge conditions, and mixing between local and regional flow systems. This study developed a numerical coupled groundwater flow, particle-tracking, and age-tracer transport model for the 9,218 km² Montérégie Est fractured bedrock aquifer system in southern Québec, Canada. The model was constructed using MODFLOW-NWT to simulate steady-state regional groundwater flow, and hydraulic head, MODPATH-7 to evaluate advective flow pathways, and MT3D-USGS to simulate the infiltration, transport and decay of tritium (³H) and radiocarbon (¹⁴C) from major recharge zones in the region.

The model was developed and calibrated using hydrogeological and geochemical datasets from the 2010 PACES field campaign and was subsequently independently evaluated using groundwater samples collected during a 2025 field campaign. The 2025 sampling targeted wells located near model-derived regional flowpaths and included analyses of ³H, ¹⁴C, dissolved inorganic carbon, δ²H, δ¹⁸O, and major ions. Surface-water samples were also collected from the region's major watersheds to assess potential river influence.

Simulated hydraulic heads reproduced the regional groundwater gradient well, with strong agreement between measured and modelled heads. The model also captured the main spatial patterns in groundwater age-tracer distributions. ³H simulations reproduced the overall measured–predicted relationship across the 2010 and 2025 datasets, although local deviations indicate unresolved variability in recharge, mixing, and flowpath geometry. ¹⁴C simulations showed strong agreement with measured values from 2025, supporting the model's ability to reproduce first-order regional residence-time patterns, while also highlighting the presence of water-rock interactions. Stable isotope results suggest broadly similar meteoric recharge sources between the 2010 and 2025 sampling periods, while temporal changes in ³H and ¹⁴C were site-specific, suggesting no overall temporal shift in recharge conditions. Decreases in ³H at several resampled wells are consistent with radioactive decay and reduced modern recharge signals, whereas anomalous increases in ¹⁴C at selected wells likely reflect localized surface-water influence or enhanced recent recharge.

The results from the ground-truthed and validated model presented here support the main recharge-to-discharge structure previously proposed for Montérégie Est while highlighting remaining uncertainties associated with local recharge, brackish groundwater zones, and vertical borehole mixing.