The aquifer system in the Lower Laurentians region of southwestern Quebec is hosted primarily by Cambro-Ordovician fractured sedimentary rocks that are part of the geological province known as the St. Lawrence Platform. The most transmissive unit of the aquifer is found at the top of the rock sequence and consists of highly fractured rocks and a layer of glaciofluvial Quaternary sediments. The floor of the aquifer is located 100 m below the surface, where the rocks become less permeable, and the aquifer is capped by Quaternary deposits of till and clay. In terms of quality as it relates to human health, an evaluation of exceedances of government criteria indicates that groundwater quality throughout most of the study area is good. At the regional level, groundwater is relatively uncontaminated by human activities. The study area was subdivided into seven sectors, which were classified on the basis of relative groundwater quality. The sectors with the highest groundwater quality are Saint-Hermas, Riviere du Nord, and a sector made up of the three subsectors of Lachute/Saint-Janvier, Sainte- Monique/Saint-Eustache, and Cote Saint-Vincent. The areas most vulnerable to contamination from the surface occupy approximately 35% of the study area. The remaining 65% is naturally protected by impermeable till and clay layers. The rate of anthropogenic use of groundwater during the study period was 14.8 x 106 m3/a, equivalent to a water depth of 11 mm/a throughout the area. The primary uses of groundwater are human consumption (42%), quarry operations (37%) and, to a lesser extent, agricultural consumption (17%). Golf course irrigation (~1%) and commercial water bottling (3%) account for a small percentage of current use. The annual rate of total extraction for human activities is considered to be sustainable because the resulting drawdown does not exceed the average natural fluctuation in piezometric levels. In some areas, annual extraction could be sustainably increased to as much as 26 x 106 m3/a.