Grid-based Hydrostratigraphic 3D Modelling of the Quaternary Sequence in the Chateauguay River Watershed, Quebec

Groundwater recharge, groundwater-surface water interaction, and protection and management of the groundwater resource are strongly constrained by the geological nature of the substratum. A grid-oriented technique has been developed in order to build a 3D stratigraphic model of the Quaternary sediments overlying a regional fractured rock aquifer. The technique is based on the integration of the surficial sediments map and borehole logs with the use of GIS and grid-calculator software Vertical Mapper. The applied methodology focused on estimating the thickness of the stratigraphic units rather than the elevation of the contacts. First, a regular grid was generated over the study area with 30 x 30 m cells. The bulk thickness of the Quaternary sequence for each grid cell was computed as the difference between the terrain digital elevation model and the rock surface krigged over more than 5000 drillers' logs available. Two computation methods for estimating the discrete thicknesses are discussed and evaluated: the absolute method, in which the thickness of a given unit is computed as an independent value, and the relative method, in which the thickness is computed as a fraction of the bulk thickness. The simplified Quaternary stratigraphy consists, from top to bottom, of organics (peat), alluvium, lacustrine, aeolian, coarse marine, marine clay and fine silt, fine sandy and silty glacio-fluvial, coarse sandy and gravelly glacio-fluvial, and glacial (silty clayey till) sediments. Their spatial distribution respects filly the surficial sediments map and contacts. At locations with missing stratigraphic data, the borehole logs database was improved with the addition of control points representing the anticipated variation of the successive layers. With the absolute method, derivation of the thicknesses of the layered strata in zones of irregular bedding proved to be difficult. The relative computation method gives more consistent results for various stratigraphic settings and allows rapid, internally consistent estimation of the overburden stratification.