DEVELOPMENT OF A 3D GEOMODEL FOR ESKERS : THE CASE OF THE SAINT-MATHIEU – BERRY ESKER, ABITIBI, QUÉBEC, CANADA

As part of a groundwater thematic study, a 3D geomodel (24 x 11 km) was developed for a continuous esker segment in the Abitibi area of Québec, Canada. This segment was chosen because it is representative of a glaciated environment where the ice retreated in contact with a large water body, here Glacial Lake Ojibway. The esker is water bearing, and the high groundwater quality and conflicting usage of the esker, such as sand and gravel exploitation, have raised concerns from the principal user, Amos, a municipality of 14 000 people. Our goal is to understand groundwater circulation patterns in the esker and surroundings to provide local authorities with a decision making tool for regulating activities on and near the esker. We have chosen to work with a 3D modeling and visualisation software called Gocad® of Earth Decision Sciences, in order to recreate a five-layers (bedrock, till, esker, Ojibway clay and nearshore sand) model of the surficial geology. We have integrated archival data (surface information from bedrock and surficial geology maps and borehole data from mining exploration wells and water wells), supplemented by our own ground penetrating radar surveys and soundings. The geomodel suggests that there is a control on water circulation from the underlying bedrock topography, but the primary groundwater circulation is a transverse, shallow flow from infiltration at the ridge crest to resurgence at the clay-esker contact. The secondary longitudinal flow is to the north from a bedrock high located underneath the southern tip of the esker. Water levels are up to 25 m below the esker surface with the piezometric surface in the Amos area being remarkably flat. The saturated thickness of sediments is in the order of 10-20 m. Irregular bedrock topography and confinement of the aquifer by Ojibway clays suggest there may be some slow-moving, old water trapped beneath dynamic circulation cells fed by modern precipitation. In the northern region of the aquifer, the flatness of the piezometric surface and the mobility of groundwater sub-basin boundaries suggest that small changes in the natural water regime by groundwater removal thru pumping might substantially change circulation patterns. We hope that the model provided will help local authorities better manage their groundwater resource.