Assessing rock aquifer vulnerability using downward advective times from a 3D model of surficial geology: A case study from the St. Lawrence Lowlands, Canada

Abstract

An aquifer vulnerability assessment method which relates vulnerability directly to groundwater Downward Advective Time (DAT) from a 3D geologic model is tested over a 1400 km2 area in the St. Lawrence Lowlands in SW Québec. The goal was to assess the vulnerability of the regional rock aquifer which is overlain by discontinuous and unconsolidated Quaternary units. Hydrogeologic parameters such as groundwater recharge and porosity of each unit overlying the aquifer were integrated to a gOcad 3D stratigraphic grid. This grid was generated from a 3D geologic model made of interlocked surfaces representing the boundaries of Quaternary units and the underlying bedrock topography. Calculations were carried out using a deterministic approach and DAT estimates were obtained for 74% of the model area. Results are tentatively grouped into 6 DAT classes which are interpreted in terms of a relative vulnerability index. The spatial distribution of this index is in good agreement with the hydrogeological settings and available hydrogeochemical data. Results indicate that 40% of the evaluated area falls within the moderately high to very high vulnerability classes. Comparison between this map and a DRASTIC map of the same area reveals some significant discrepancies, particularly in areas of shallow depth to bedrock which are characterized by fast DAT through a thick unsaturated zone. In these areas DRASTIC produces low vulnerability scores while this assessment yields high vulnerability ranking. Overall, this assessment provides a sound basis for regional planning and decision making.