PHYSICAL HYDROGEOLOGICAL CHARACTERIZATION AND 3-D NUMERICAL MODELLING OF GROUNDWATER FLOW IN TILL AT A CENTRAL ALBERTA SITE

The physical hydrogeology of a till in central Alberta, Canada, was studied as part of a remediation research project at a decommissioned sour gas treatment site. Field investigations revealed seasonally dynamic groundwater responses in the upper 2 to 4 m of weathered, fractured till, but not in the underlying unweathered, sparsely fractured till. Piezometers completed in the unweathered till showed delayed and attenuated responses to infiltration relative to the piezometers completed in the overlying till. Precipitation (494 mm/y) and evapotranspiration (545-575 mm/y) dominated the water budget. Vertical hydraulic gradients (up to 2.5 m/m) indicated perched water table conditions. Horizontal flow rates were interpreted to be minimal due to low horizontal hydraulic gradients, low conductivity, and lack of geologic continuity. Soil moisture monitoring indicated the potential influence of dual porosity processes where the till was previously hydraulically fractured. Groundwater extraction tests showed only local, yet spatially erratic, head responses.

To confirm field interpretations, groundwater flow was simulated with FRAC3DVS, a finite-element simulator. Model calibration was achieved through modification of the timing and magnitude of the net surface boundary flux, and the magnitudes of hydraulic conductivity (K) and specific storage (S_s). Overall, the best calibration was in the hydraulically fractured till, perhaps at the expense of calibration in outlying areas, where data were sparse. Modelling results confirmed the field-based finding that groundwater extraction caused only local drawdown, reinforcing the dominant roles of dual porosity flow and storage. Model validation analyses showed erroneous responses to enhanced dewatering stresses in the hydraulically fractured till, indicating that the calibrated model had compensating errors. These errors were likely caused by the heterogeneous, dual porosity nature of the deposit, and corresponding variations in the magnitude and distribution of K and S_s.