TRANSITIONING A REGIONAL-SCALE GEOLOGICAL MODEL TO A WORKABLE 3-DIMENSIONAL HYDROSTRATIGRAPHIC MODEL FOR LARGE-SCALE INTEGRATED GROUNDWATER – SURFACE WATER MODELLING

Fully-integrated 3D groundwater – surface water (GW-SW) modelling is being progressively applied towards larger and larger simulation domains (i.e. >100,000 km²), with the goal of providing water resources insights for decision making. Such hydrologic modelling applications are addressing previously unanswerable questions related to the role of groundwater, and groundwater – surface water interactions, on regional hydrologic characteristics. In order to construct large-scale fully-integrated models that are usefull and defensible for decision making applications, key hydrostratigraphic characteristics need to be incorporated in order to provide a meaningful representation of the groundwater flow system. Constructing the hydrostratigraphic models that form the subsurface component of the fully-integrated GW-SW model is not without challenges. For example, geologic detail is needed for the full extent of the hydrologic domain and must be amenable to a 3D structural representation. Also, the geologic detail must form a suitable baseline for hydrostratigraphic model construction, such that regionally important aquifer and aquitard units can be resolved. Spatial resolution is a further consideration, in that large-scale hydrologic models are typically constrained by vertical and horizontal spatial resolution limits in order to keep simulation run times within reasonable limits. Accordingly, it is important to consider the end use objectives for an integrated hydrologic model when constructing the underlying hydrostratigraphic model. In this presentation we review the process of constructing a regional scale hydrostratigraphic model for Southern Ontario, Canada, for use in fully-integrated GW-SW modelling with HydroGeoSphere (HGS). A coarse spatial-resolution version of the HGS model carries a 17 layer hydrostratigraphic sequence through surficial geology and Phanerozoic sedimentary bedrock deposits to the top of Precambrian crystaline bedrock, while a fine resolution HGS model version carries the same surficial geology sequence as the first, but truncates the Phanerozoic sequence at a locally present fresh water – high TDS water interface, such that there are only 12 hydrostratigraphic layers. Hydraulic property parameterization strategy for the hydrostratigraphic model will be discussed, along with the influence of hydrostatigraphic model resolution on hydrologic model simulation results.