ABSTRACT: Evaluation of groundwater vulnerability to contamination in the Laurentian Piedmont using the drastic method.

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As part of an hydrogeological project in the Laurentian Piedmont the groundwater vulnerability to contamination was assessed using the DRASTIC method. Groundwater vulnerability to contamination is one of many tools that may be used to enhance the protection and the conservation of the groundwater resource. Groundwater vulnerability maps are constructed to assist land planners in their decision making process concerning the localization of activities presenting potential contamination threats to groundwaters. These maps also serve to raise the public awarness about the existence of a precious groundwater resource as well as potential conflicting usages. Hyrdrogeological maps were done in a region of approximately 1600 square kilometers encompassing the southern part of the Portneuf MRC. This region is charactarized by the presence of two important sand and gravel deltaic deposits located along the Sainte-Anne and Jacques-Cartier Rivers. These surficial granular deposits constitute the most productive and most exploited aquifers of the region. Also present, is a fairly thick marine silt and clay deposit that constitute the main regional aquitard. People living on the clay plain are constrained to use bedrock aquifers as a drinking source of water. Although bedrock aquifers provide sufficient quantities of water for individual needs they are less productive and have a poorer water quality than the deltaic sands and gravels. The DRASTIC method was used to assess the vulnerability of these unconfined granular and fractured aquifers to contamination. DRASTIC is a regional and empirical vulnerability assessment method combining the concept of hydrogeological units with a system of numerical ranking and weighting. The parameters that are ranked and evaluated are: D; depth to water table, R; recharge, A; aquifer media, S; soil types, T; topography, I; the vadose zone, C; hydraulic conductivity. The evaluation of these seven parameters required that numerous sources of data be surveyed. These data were collected through different Ministries, public and private agencies and integrated into a database. The delineation of the different zones for each parameter and the assignation of classes and values were performed using MapInfo. IDRISI was used for the integration of all seven parameters into the final DRASTIC map. In the Portneuf region, thirteen hydrogeologic settings were identified. The final results showed that the sand and gravel aquifers are the most vulnerable areas to contamination. The DRASTIC method provided a rational mean to evaluate aquifers vulnerabuility to contamination. The overlay of the DRASTIC synthesis map on the landuse map provides an excellent tool for land use planners.

Bibliography of Canadian Geomorphology