Dashwood-Mapleguard

Hydrostratigraphic Unit

Project
Nanaimo Lowlands

Datasets

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Dashwood-Mapleguard unit description

Update:2016-02-03
The Dashwood-Mapleguard hydrogeological unit includes a part of the Dashwood Sediments and the Mapleguard geological unit. Dashwood Sediments overlie the Mapleguard unit. They are composed of cobbles and gravel (Armstrong and Clague, 1977; Fyles, 1963; Hicock and Armstrong, 1983). The gravel is a mixture of plutonic rocks derived from the Coast Mountains and volcanic and sedimentary rocks from the east coast of Vancouver Island. The geochemical signature of Dashwood has an elevated concentration of metals compared to Quadra Sand (Knight et al., 2015). Dashwood Drift was deposited during the penultimate glaciation early Wisconsinan, as deduced from the presence of marine shells which date beyond the range of radiocarbon in overlying sediments (Clague, 1980; Hicock, 1980; Hicock and Armstrong, 1983; Ryder and Clague, 1989). Mapleguard Sediments unconformably overlie bedrock and are the oldest Pleistocene deposits present in the Nanaimo Lowland. They are composed of bedded sand, silt, clay and minor gravel up to 10 m thick along the sea cliffs; however Fyles (1963) reported more than 20 m in some boreholes. They are considered to be outwash deposited at the onset of the penultimate glaciation. According to Fyles (1963), Mapleguard proglacial outwash deposits could be coarser than the Quadra Sand and are expected to have good aquifer potential; however, the vertical and spatial extent of this unit is not very well known.
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%).
Area141.00 km2
Lithology
DescriptionThe unit is composed of cobbles and gravel overlying bedded sand, silt, clay and minor gravel.
Source
Aquifer
Aquifers

Unit properties

Surficial aquifer media
Typical value:porous
Description The unit is composed of cobbles and gravel overlying bedded sand, silt, clay and minor gravel.
Confinement
Typical value:confined
Groundwater storage
Description Specific storage: 1.5E-4 (1/m)
Hydrogeological unit thickness
Description mean thickness 19 m and median thickness 14 m
Surficial unit hydraulic conductivity
Range: [2e-06 to 0.0046] 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).
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 of the unit: 0 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:medium
Description According to Fyles, the Mapleguard's proglacial deposits may be coarser than the Quadra sands. They probably have a better aquifer potential. However, the vertical and spatial extent of the unit is poorly constrained due to lack of drilling to adequate depths. The Mapleguard sediment granulometry shows an upward-fining sequence, suggesting a transition to an aquitard unit on the surface of the unit.
Source Surficial Geology of Horne Lake and Parksville map Areas, Vancouver Island, British Columbia
Aquifer vulnerability
Typical value:low
Threats
Description saltwater intrusion caused by pumping