Regional hydrogeological mapping project of the St. Lawrence Lowlands of southwestern Quebec: hydrogeological characterization work 1999-2000

The GSC is carrying out a regional hydrogeological mapping project of fractured rock aquifers in the St. Lawrence Lowlands of southwestern Quebec. The regional aquifer consists of Paleozoic fractured sedimentary rocks. This aquifer is mostly confined with clay and silt sediments at lower altitudes whereas over topographic heights it becomes a water-table aquifer. This paper presents the fieldwork and results of the hydrogeological characterization based on 361 constant-head injection tests, 4 large-scale pumping tests, 4 pulse tests, and 3 step pumping tests, as well as slug tests performed in 23 boreholes and 10 observation wells. The first results indicate that 2001-D9 M. Nastev et al. 1 CURRENT RESEARCH RECHERCHES EN COURS 2001 Article Article Titles Titres Purchase Information Information pour acheter 1 INRS Georessources, Quebec Geoscience Centre 880 chemin Sainte-Foy, C.P. 7500, Sainte-Foy (Quebec) G1V 4C7 2 Laval University Geological Department, Pavillon Adrien-Pouliot, Sainte-Foy, Quebec, G1K 7P4 carbonate and sandstone rocks are the most exploitable regional aquifers in the region. The average transmissivity shows a weak decreasing trend with depth. When present, the deep sandy gravel layer overlying the top of fractured aquifers is believed to be the major conduit for groundwater flow. INTRODUCTION T Geological Survey of Canada is carrying out a regional hydrogeological mapping project of fractured rock aquifers in the St. Lawrence Lowlands of southwestern Quebec. The main scientific collaborators of the project are INRS-Georessources, Laval University, Environment Canada, and Queen’s CURRENT RESEARCH RECHERCHES EN COURS 2001 Article Article Titles Titres Purchase Information Information pour acheter 2001-D9 M. Nastev et al. 2 Resume La CGC poursuit actuellement des travaux de cartographie hydrogeologique regionale des aquiferes fractures dans les basses terres du Saint-Laurent du sud-ouest du Quebec. L’aquifere regional est situe dans des roches sedimentaires fracturees du Paleozoique. Cet aquifere, largement captif, est contenu en grande partie dans de l’argile et du silt a de basses altitudes, et devient un aquifere libre au-dessus des hauteurs topographiques. Le present article resume les travaux de terrain et les resultats de la caracterisation hydrogeologique fondee sur 361 essais a charge constante, 4 essais de pompage a grande echelle, 4 essais a impulsion, 3 essais a paliers, et plusieurs essais de permeabilite realises dans 23 forages et 10 puits d’observation. Les resultats preliminaires indiquent que les roches carbonatees et les gres representent la portion la plus productive du systeme aquifere de la region. La transmissivite moyenne de ces roches tend a diminuer faiblement avec la profondeur. Aux endroits ou elle est presente, la couche profonde de gravier sableux qui recouvre les roches paleozoiques pourrait constituer le conduit principal de l’ecoulement souterrain. University. The study area encompasses four regional county municipalities — Argenteuil, Deux-Montagnes, Mirabel, and Therese-de-Blainville. The project activities consist of collection of existing data, update of the bedrock and Quaternary geological maps, field hydrogeological tests and measurements, hydrogeochemical characterization, surface and borehole geophysics, data compilation, groundwater-flow modelling, and reporting (thematic maps, technical reports). The project specific objectives are aquifer delineation and characterization, mapping methodology development, as well as characterization and protection of the groundwater resource. The overall objectives of the GSC project include developing and testing field hydrogeological, geophysical, and geochemical techniques for determination of the hydraulic properties of fractured rock aquifers, designing tools for data management, providing technical protocols and guides for regional hydrogeological mapping of fractured bedrock aquifers, and finally, increasing public awareness of the importance for groundwater protection. A particular attention of the St. Lawrence Lowlands mapping project is paid to the study of spatial and vertical variation in hydraulic properties of bedrock aquifers using the results of conventional pumping, slug, and constant-head injection tests. The applied methodology of hydrogeological characterization described in this report was initiated by the need to develop a complete model of the groundwater dynamics in the St. Lawrence Lowlands heterogeneous aquifers. In effect, in contrast to unconsolidated porous media generally characterized by a relatively small range in hydraulic properties, the properties of fractured rock aquifers may vary considerably. Identifying and characterizing the spatial heterogeneity of rock environments in a regional study, such as the St. Lawrence Lowlands project, and ensuring adequate borehole coverage to capture the variations of these properties is difficult. Often, available hydrogeological data are sparse for the surface as well as the volume of rock under consideration. Thus the St. Lawrence Lowlands project is based on a multidisciplinary approach including hydraulic testing and geophysical and geochemical surveys, which, in conjunction with stratigraphic, lithological, and structural mapping, reduce the uncertainty of 2001-D9 M. Nastev et al. 3 CURRENT RESEARCH RECHERCHES EN COURS 2001 Article Article Titles Titres Purchase Information Information pour acheter