Numerical Simulation of Groundwater Flow in the Chateauguay River Aquifers

The Chateauguay River watershed extends over northeastern New York State (USA) and southwestern Quebec (Canada). Fractured sedimentary rocks of the St. Lawrence Platform host the regional aquifers. Quaternary sediments of variable thickness of up to 45 m overlie the bedrock. The geometric mean hydraulic conductivity of the bedrock aquifers obtained from 548 field measurements is 5.1 x 10(-5) m/s with a standard deviation of 0.7 of the logarithms. The modelled area extends from the foothills of the Adirondacks to the St. Lawrence River and covers 2,850 km(2). The numerical groundwater flow model was developed using the finite element simulator FEFLOW. The model has 13 layers with layer thicknesses ranging from 5 m for the top layer to 75 m for the bottom layer. The average thickness of the numerical model is 655 m, for a total volume of 1,868 km(3). The St. Lawrence River is considered as a specified head boundary; the base and other lateral limits are considered as no-flow boundaries, whereas a head and conductivity-dependent boundary is specified along major streams and wetlands. Spatial recharge rate is applied as a specified flux across the top of the model and was fixed during calibration to reduce model uncertainty. Groundwater withdrawal of 34 Mm(3)/yr is assigned using sinks for major wells and as a uniform negative flux across the top of the model to account for domestic and other diffuse uses. Calibration was carried out against 153 hydraulic head measurements, with horizontal hydraulic conductivity and vertical anisotropy used as calibration parameters. The regional groundwater flow amounts to 268 Mm(3)/yr: 12.7% is withdrawn for domestic purposes; aquifer contribution to streams and wetlands is 176 Mm(3)/yr, and 55 Mm(3)/yr is discharged to the St. Lawrence River. Groundwater flow appears to be controlled by the sub-horizontal bedding planes contributing to relatively high vertical anisotropy.