Numerical modelling of groundwater flow and contaminant transport to Point Pelee marsh, Ontario, Canada

A numerical model was developed for simulating groundwater–wetland interactions and contaminant transport. The model calculates transient hydraulic head and a transient free surface in a two‐dimensional, heterogeneous domain, with variable and transient boundary conditions (infiltration, evapotranspiration, surface water), and water and contaminant fluxes across the aquifer–wetland interface. Contaminant transport is also simulated, with contaminant sources located at the free surface, wetland, or within the saturated domain. The model was applied to assess groundwater–wetland interactions and the transport of septic‐system‐derived contaminants at Point Pelee, Ontario, Canada. The model successfully simulated the field observations of groundwater flow and contaminant plumes. Where the barrier bar is narrow, the seasonal reversal in the direction of groundwater flow is caused by differences in the elevation of the water surface of Lake Erie and that of the marsh. This, in turn, induces the contaminants to oscillate between movement towards the lake during the winter and towards the marsh during the summer. Hence, contaminant plumes are bimodal in shape. Where the barrier bar becomes wider, the lake and the marsh have less effect, and hence contaminants move in one direction, along the principal direction of groundwater flow towards the marsh.