Vashon-Capilano

Hydrostratigraphic Unit

Project
Nanaimo Lowlands

Datasets

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Vashon-Capilano unit description

Update:2016-02-02
The Vashon-Capilano hydrogeological unit overlies the Quadra Sand with thicknesses generally up to 30 m, except at observed locations where depressions left by deeply eroded Quadra Sand were filled by approximately 60 m of sediment (Fyles, 1963). It is composed of glaciofluvial deposits composed of gravel, sand and lenses of till forming the eskers, kame terraces, ice-contact fans, and deltas. The Vashon Drift geological unit is split across two hydrostratigraphic units; Vashon coarse Sediments (of the Vashon-Capilano) and Vashon Till. This unit is considered as an aquifer; however, it generally has poor aquifer potential due to its spatial discontinuity. Deposition is inferred to have been from glaciofluvial streams that initially would have flowed along the margins of the retreating Cordilleran and local glaciers, with some deltas or terraces building into short-lived ice-dammed lakes. As the Cordilleran ice occupying the Georgia Depression thinned, the sea inundated coastal lowland to an elevation of at least 150 m. The geochemical signature of Vashon Drift has a number of complicated trends over the unit thickness related to various element groupings (Knight, 2015).
Physiography
Hydrogeological regionCordillera
Hydrogeological context
ReliefThe regional topography ranges from sea level at the coast, up to 1819 m (Mount Arrowsmith). The coastal plain terrain is sub-horizontal with slopes lower than 6% except along incised rivers valleys (up to 20%).
Area306.00 km2
Lithology
DescriptionThe Vashon-Capilano hydrogeological unit consists of glaciofluvial deposits composed of gravel, sand and lenses of till forming the eskers, kame terraces, ice-contact fans, and deltas deposited by meltwater streams during early deglaciation.
Source
Aquifer
Aquifers

Unit properties

Surficial aquifer media
Typical value:porous
Description Glaciofluvial sand and gravel with lenses of tills
Confinement
Typical value:confined - unconfined
Description The Vashon-Capilano aquifer is confined where the Capilano glaciomarine Sediments are present; elsewhere they are unconfined.
Groundwater storage
Description Specific storage: 1.5E-4 (1/m)
Surficial sediment thickness
Hydrogeological unit thickness
Description mean thickness 8 m; median thickness 5 m
Surficial unit hydraulic conductivity
Range: [9e-06 to 0.0008] m/s
Source Compendium of Re-evaluated Pumping Tests in the Regional District of Nanaimo, British Columbia
Regional precipitation
Description The total precipitation is typically up to 5000mm in the mountains (Mount Arrowsmith) (Waterline, 2013).
Source Regional District of Nanaimo Phase One Water Budget Project
Regional recharge
Description For the lowland area, recharge calibrated values range from 112 to 396 mm/yr. Recharge for the unit: 500 mm/yr
Regional discharge
Description The regional analysis of the water budget reveals that 71% of the groundwater is discharging to surface water bodies, whereas the remaining 29% discharges along the coast of the Strait of Georgia.
Hydrogeological unit potential
Typical value:bas local
Description It has a poor aquifer potential, due to its discontinuity in nature.
Aquifer vulnerability