Physically-based assessment of intrinsic groundwater resource vulnerability in an urban catchment

T. Graf, R. Therrien, J-M. Lemieux, J. Molson

Dans les comptes rendus d’articles de la conférence: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Groundwater Management II

ABSTRACT: Several methods exist to assess intrinsic groundwater (re)source vulnerability for the purpose of sustainable groundwater management and protection. Unfortunately many are highly subjective and are poorly defensible scientifically. Recent studies suggest that a physically-based approach could be better suited to provide a general, conceptual and operational basis for groundwater vulnerability assessment. A novel method for physically-based assessment of intrinsic aquifer vulnerability is currently under development and testing to explore the potential of an integrated modelling approach, combining groundwater travel time probability and future scenario modelling in conjunction with the fully integrated HydroGeoSphere model. To determine the intrinsic groundwater resource vulnerability, a fully coupled 2D surface water and 3D variably-saturated groundwater flow model in conjunction with a 3D geological model (GoCAD) has been developed for a case study of the Rivière Saint-Charles urban catchment (Québec/Canada). The model has been calibrated under transient conditions for the hydrogeological, variably-saturated subsurface system, coupled with the overland flow zone by taking into account monthly recharge variation and evapotranspiration. Computations has been optimised by using a parallelised version of HydroGeoSphere, linked with the PEST (Parameter Estimation) model. To better determine the intrinsic groundwater vulnerability, two independent approaches are considered and subsequently combined in a simple, holistic multi-criteria-decision analyse. To date, the first approach has been applied and tested, relying on numerical modelling of the groundwater travel time probability (TTP). Multiple model outcomes will be generated to take into account uncertainty due to the model input data for a spatial illustration of the TTP from the model recharge area (land surface) to the calibrated water table. The results give a first look at the simulated flow patterns, allowing to draw a preliminary picture of the regional and local vulnerability. The next step of the overall vulnerability assessment consists of modelling future vulnerability scenario(s), applying realistic changes to the model by using PEST with SENSAN for subsequent sensitivity analysis. This comprehensive, physically-based vulnerability assessment is less subjective and will provide reproducible results with respect to the associated uncertainty derived from the non-uniform quality and quantity of the available data. KEY WORDS: Groundwater vulnerability, HydroGeoSphere, PEST, 3D Coupled modelling, Sensitivity analysis, Travel Time Probability, DPSIR

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