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

To support improved sustainable groundwater management of the Nanaimo Lowland, British Columbia (BC), a three-dimensional (3D) hydrostratigraphic model was developed for the Nanaimo Lowland aquifer system. The study area is a coastal strip running from Nanoose to Deep Bay on eastern
Vancouver Island (~580 km2). The 3D model was developed using existing driller's logs, published cross-sections and new data which include rotosonic coring, borehole geophysics logs, seismic reflection profiles and surficial geology mapping. The detailed surficial geology consists of 31 different
units which, for modelling, are grouped into 9 major hydrostratigraphic units (from youngest to oldest): Capilano-Salish (sand aquifer), Capilano glaciomarine (silty clay aquitard), Vashon-Capilano (coarse sand and gravel aquifer), Vashon (till aquitard), Quadra (sand aquifer), Cowichan-Dashwood
(compact silt aquitard), Dashwood-Mapleguard (sand aquifer), sedimentary bedrock and metamorphic bedrock (aquifer near surface with potential decreasing with depth). This succession of Late Pleistocene to Holocene sediments is up to 140 m thick and is present over most of the study area, thinning to
the southwest with rising topography and bedrock outcrops. Capilano-Salish and Vashon-Capilano deposits are shallow and contain unconfined aquifers with relatively high vulnerability to surface contamination and low groundwater resource potential due to their limited thicknesses. Quadra Sand is the
most exploited aquifer unit. It underlies the ubiquitous and low permeability Vashon till and it overlies Cowichan-Dashwood aquitard or bedrock. Quadra sediment has an average thickness of 19 m and it is generally above sea and river levels. This would minimize the potential of seawater intrusion
and direct surface water extraction, respectively, in the event of intensive groundwater extraction in Quadra unit. The Quadra is an important aquifer, however only about one-third of its thickness is saturated likely due to the overlying Vashon till aquitard that limits groundwater replenishment.
Dashwood-Mapleguard sand is a lower confined aquifer unit that extends below sea level. It may have hydrogeological properties and resource potential similar to the Quadra sand, but it is present only in areas where the bedrock elevation is low, generally closer to the coastline. It may be more
vulnerable to sea water intrusion, and remains poorly understood. The relatively low permeability regional bedrock aquifers of the Nanaimo Group are generally exploited only where the Quadra aquifer is not present. Their hydrogeological potential is poorly understood due to lack of subsurface data.
This hydrostratigraphic model will be used to support groundwater flow modelling.

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