Quantitative assessment of regional rock aquifers, south-western Quebec, Canada

The population of the St. Lawrence Lowlands platform in south-western Quebec, Canada, is heavily dependent on groundwater. The present study summarizes the quantity, quality and sustainability estimations of the groundwater resources found mainly in sedimentary rock aquifers. Results show that the regional groundwater flow in the considered domain of 1,500 km(2) is 97.7 Mm(3)/y with: 86.6% infiltration from precipitation, 9.6% inflow from neighboring aquifers, and 3.8% induced recharge. The regional sustainability is defined with simulated drawdowns from uniform withdrawal scenarios. The current withdrawal rate of 18 Mm(3)/y results in median drawdown of 0.6 m, compared to pre-development conditions. This drawdown is situated well in the sustainable range, an indication that regional aquifers are not overexploited. Hypothetical pumping rate of 24 Mm(3)/y resulting in an average drawdown of 2.2 m is estimated as sustainable limit. Increasing exploitation from 24 to 51 Mm(3)/y would need tight control and planning. Pumping rates beyond 51 Mm(3)/y are judged not sustainable as regional drawdowns become high, similar to 8 m. The water levels in recharge areas are the most sensitive to groundwater extraction. Combining drawdown maps, groundwater quality zones, and aquifer vulnerability to surface contamination delineated the areas most suitable for future groundwater developments.