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TitleHydrostratigraphic and Groundwater Flow Models of a Complex Unconsolidated Aquifer System, Nanaimo Lowlands, British-Columbia, Canada
AuthorBenoit, N; Paradis, D; Bednarski, J; Russel, H
SourceIAH-CNC Conference 2015, abstract volume; .
Alt SeriesEarth Sciences Sector, Contribution Series 20140418
MeetingIAH-CNC Conference 2015; Waterloo; CA; 2015
ProgramAquifer Assessment & support to mapping, Groundwater Geoscience
AbstractTo support sustainable groundwater management, tridimensional (3D) hydrostratigraphic and groundwater flow models were developed for an unconsolidated aquifer system in the Nanaimo Lowlands, British-Columbia (Canada). The study area is a coastal strip running from Nanoose Bay to Deep Bay on eastern Vancouver Island (~580 km2). A 3D hydrostratigraphic model was developed using existing well logs and published cross-sections as well as new data from roto-sonic coring, borehole geophysics, seismic reflection surveys and surficial geology mapping. The detailed surficial geology consists of 31 different units, which are grouped into 8 major hydrostratigraphic units: 5 aquifers and 3 aquitards. These are (from the surface down): Capilano-Salish (sand), Capilano glaciomarine (silty clay), Vashon-Capilano (sand and gravel), Vashon (till), Quadra (sand), Dashwood-Cowichan (compact silt), Mapleguard (sand), and siltstone to sandstone bedrock. This succession of Late Pleistocene to Holocene sediments is up to 140 m thick and is presents over most of the study area, thinning to the southwest with rising topography and bedrock outcrops. Capilano-Salish and Vashon-Capilano units are shallow aquifers with relatively high vulnerability to surface contamination and low groundwater potential due to their limited thickness. The Quadra sand is the most exploited aquifer unit. It underlies the ubiquitous low permeability Vashon till and overlies Dashwood-Cowichan aquitard or the bedrock. The Quadra has a thickness of up to 50 m in place and it is predominantly above sea level, which minimizes issues of seawater intrusion in the event of intensive groundwater extraction in this aquifer. And, only about one-third of its thickness is saturated likely due to: (1) the covering by the Vashon aquitard that limits groundwater replenishment; and (2) its deep incision by modern rivers that substantially drain its flanks. Based on few available descriptions, Mapleguard sand is a confined aquifer unit that extends below sea level and may have groundwater potential similar to the Quadra sand, despite both its magnitude and spatial extent away from the coastline remain incompletely understood. The relatively low permeability sedimentary rock aquifer is also extensively exploited, but only in areas where the Quadra aquifer is not present. Results of 3D groundwater flow modelling based on the hydrostratigraphic model show that baseflow to major rivers is provided mostly by groundwater seepage from heavily incised Quadra sand. Different regional groundwater flow patterns are also observed reflecting stratigraphic controls; flow within the Quadra aquifer generally follows surface water drainage basin with discharge to rivers, whereas flow in underlying Mapleguard and bedrock aquifers is directed towards the Strait of Georgia without significant hydraulic connection with rivers. Finally, hydrogeological insights gained through this study shows that the complexity of the study aquifer system can make groundwater management challenging and that each system component should be carefully understood to ensure sustainable management.
Summary(Plain Language Summary, not published)
This paper focuses on the development of mathematical models of the regional hydrostratigraphy and groundwater flow for Nanaimo (BC). Nanaimo regional aquifers represent one of the key aquifers considered by the ESS Groundwater Geosciences Program. The paper summarises the current hydrogeological knowledge and allows for visual representation of the main concepts and hydrogeological parameters for the study area.