Regional assessment of composite groundwater parameters, illustrated by the Saguenay-Lac-Saint-Jean region, including methods and limitations

Julien Walter, Alain Rouleau, Romain Chesnaux

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

Session: Regional Aquifer Characterization III

ABSTRACT: Groundwater and aquifer systems have been mapped in the Saguenay-Lac-Saint-Jean (SLSJ) region, covering about 13 210 km2, as part of the Quebec Groundwater Knowledge Acquisition Program (Programme d™acquisition de connaissances sur les eaux souterraines, or PACES). The SLSJ area comprises a variety of aquifer types, composed of either fractured rocks or granular deposits (Figure 1). Figure 1: Conceptual diagram of the hydrogeological contexts of the Saguenay-Lac-Saint-Jean (SLSJ) region. Assessing regional hydrogeological processes over this large territory is a challenging task, particularly for those processes that are based on a number of input parameters. One example is the estimation of groundwater vulnerability, for which the PACES projects have adopted the DRASTIC approach. This approach uses a numerical ranking system that assigns relative weights to seven hydrogeological factors, which together yield the acronym DRASTIC (Figure 2). Each factor is assigned a rating from 1 to 10 based on a range of values, and the ratings are then multiplied by a relative weight ranging from 1 to 5. Regional vulnerability mapping requires values of the considered factors distributed over a region. This was done using GIS tools and a regional grid with a mesh size of 250m x 250m. The first step consisted of delimiting the regional aquifers on homogenized and simplified geological maps. The next steps required a series of calculations and interpolations in order to: 1) transform vector charts into raster charts with the desired resolution, and to 2) assign to each cell the correct rating for each one of the considered parameters. Aquifer recharge is one of the parameters with the highest weight in the DRASTIC vulnerability method, but also one of the most difficult to evaluate. Regional recharge has been estimated over the region at the required grid size, using a combination of daily weather data over the past 110 years and surface geological features. Regional assessment of composite groundwater parameters, illustrated by the Saguenay-Lac-Saint-Jean region, including methods and limitations

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