Three dimensional groundwater flow model of the Nanoose-Deep Bay area, Nanaimo Lowland, British Columbia

Abstract

The Nanaimo Lowland Groundwater Study is a collaboration between the British Columbia (BC) Ministry of Environment (MoE), Ministry of Forests, Lands, and Natural Resource Operations (MFLNRO), the Regional District of Nanaimo (RDN), and the Geological Survey of Canada (GSC). This Open File reports on the three-dimensional (3D) groundwater flow model (3DGFM) for the Nanaimo Lowland aquifer system developed to support improved sustainable groundwater management of the Nanaimo Lowland. The study area represents a coastal strip running from Nanoose to Deep Bay in the eastern part of Vancouver Island and covers approximately 580 km2. The groundwater flow model was based on the 3D hydrostratigraphic model (3DHSM) developed by Benoit et al. (2015). The hydrostratigraphic units were converted into a numerical grid and representative hydraulic properties were assigned based to each unit on statistical analysis of available values. The groundwater resurgences present along the steep slopes of the main riverbanks and at the shoreline were simulated with seepage boundary conditions allowing for a vertical hydraulic link between the main Quadra Sand aquifer and surface water bodies. Hydraulic conductivity and groundwater recharge rate were further calibrated through numerical inversion using static hydraulic heads recorded in existing wells and estimated baseflow values from streamflow records. Simulation results that two distinct groundwater flow systems: (1) the Quadra aquifer system with groundwater divides closely matching watershed boundaries containing relatively fresh water; and (2) the extensive bedrock aquifer system with older water and a flow patterns generally directed toward the sea coast. Simulations also reveal discharge to rivers and streams mainly from the Quadra Sand aquifer and the Capilano sediments, often locally present along the main river channels, with low contribution from the bedrock. Exception is the Englishmen River where the Quadra Sand is inexistent and the river is in direct hydraulic contact with the bedrock aquifer. The Quadra Sand aquifer is naturally protected from sea intrusion due to the higher altitude with respect to sea level, whereas the bedrock aquifer appears vulnerable to encroachment especially under heavy pumping conditions close to the shoreline. This 3DGFM provides then a meaningful framework for the understanding of the regional hydrogeology of Nanoose-Deep Bay aquifer system that would contributes to support sustainable management of groundwater resources.