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 characteriza- tion 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 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 frac- tured aquifers is believed to be the major conduit for groundwater flow.

The characterization of groundwater hydrogeochemistry of the regional county municipalities of Argenteuil, Mirabel, Deux-Montagnes, and Thérèse-de Blainville is underway as part of a regional hydrogeological mapping project of the fractured aquifers of southwestern Quebec. The goal of the hydrogeochemistry component is to eval- uate regional groundwater quality and identify geochemical processes controlling this quality. Three sampling campaigns were conducted, one in spring 1999, one in summer 1999, and another in summer 2000. Preliminary results of the measurement of physico-chemical parameters and the analysis of groundwater samples for inor-ganic and isotopic (stable isotoped 18 O) concentrations reveal significant variations in groundwater geochemistry. These variations seem to be controlled in part by geological units and by hydrogeological context. Moreover, pre- cipitation was analyzed for stable isotopes to establish the local meteoric waterline for the area. This will help understand the origin of groundwater.

A study of stable isotope (H, C, O) analysis of groundwater from the Laurentian piedmont aquifers is underway in the Portneuf region county municipality in order to characterize both granular and fractured aquifers in the region. Isotopic results indicate a short residence time for most groundwater samples. The influence of the limestone on the composition of groundwater from a sandy unit was identified, based on d13C-DIC and chemistry results. The obtained d18O-SMOW and d2H values suggest that groundwater recharge is provided mostly by the infiltration of snowmelt. The d18O-SMOW and d2H ratios show a linear correlation (r = 0.940, n = 36). The Portneuf groundwater line was calculated as d2H = 8.7 d18O + 22.7. The main particularity of the Portneuf region groundwater is the excess in deuterium (d =d2H - 8 d18O) which averages 14.8 o/oo. Such excess seems to be related to the specific climatic conditions of Eastern Canada which determine the oxygen and hydrogen isotopic composition of precipitation and groundwater of the Laurentian piedmont aquifers.