Fractured rock aquifer - Gulf Islands

Hydrogeological Context

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Version	Metadata
	86a97fce-424c-4fe0-9919-0a7a649ea3f4

Project
Gulf Islands Fractured Aquifers

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Fractured rock aquifer - Gulf Islands unit description

Update:2014-02-19

Physiography
The Gulf Islands include two aquifers: the granular and the fracture rock aquifer. The granular one consists of marine, fluvial and glacial material. It is generally thin (less than 18 m in thickness) or absent on the islands. Granular aquifers are found at local scale and aren't well known. Locally, perched aquifers may be observed. The regional fractured rock aquifer is widely exposed and consists primarily of interbedded sandstone-conglomerate and mudstone. The aquifer is consequently under unconfined conditions, showing moderate to very high vulnerability along the coast. The main threat is saline water intrusion, which is more present in the summer since the population increases during tourist season and the precipitations decreases. Surface water percolates in the topographic highs, flows down the rock fractures roughly following the surface topography. Water flows primarily from fractures as secondary permeability, reflecting the low primary porosity and permeability of the bedrock. All lithologies are highly cemented, dominantly by calcite. Sandstone-dominant unit with low fracture has low aquifer potential. Mudstone-dominant unit with high fracture density and elevated permeability shows high aquifer and good groundwater storage capacity. This aquifer represents the primary source of potable water for the islands. Groundwater is used for water supply for individual domestic use, small communities and for small mixed farming operations in the region. Groundwater quality is found to vary relative to the position. Recharge areas have fresh water, but deeper water is brackish. High fluoride concentrations are also observed.
Hydrogeological region	Cordillera
Hydrogeological context
Relief	Topography of the lowland exhibits moderate relief up to 600 m and is primarily controlled by northwesterly trending bedrock structures. The minimal elevations are near the sea level on the coast.
Area	478.00 km2
Lithology
Description	The unconsolidated deposits comprised of marine, fluvial and glacial material are generally thin (less than 18 m in thickness) or absent on the islands. Bedrock consequently is widely exposed and consists primarily of clastic rocks of the Nanaimo Group. This Group is made of a sequence of indurated marine and non-marine sedimentary rocks comprised of conglomerate, sandstone, shale and coal. The strata are gently folded with a uniform direction of dip towards the northeast into the Georgia Basin. Older rocks belonging to the Sicker Group, consisting of massive tuffs, volcanic breccias, limestone, argillite, quartzite and greenschist are found on Saltspring Island. Major longitudinal reverse faults trending northwesterly occur on Vancouver Island, parallel to the margins of the Georgia Basin and subsidiary transverse faults striking northeasterly. Other numerous and widely distributed structural lineaments may be observed in the region.
Source
Aquifer
Aquifers	List of aquifers fractured rock aquifer potential granular aquifers

Unit properties

Surficial aquifer media


Typical value:	porous
Description	Glacial sediments: sand and gravel layers (granular aquifer); punctual aquifers
Source	Water Budget Project: RDN Phase One (Gabriola, DeCourcy & Mudge Islands)

Bedrock aquifer media

Typical value:	fractured
Description	Fractured bedrock (inter-layered mudstone and sandstone) mudstone formations are more densely fractured than sandstone
Source	Water Budget Project: RDN Phase One (Gabriola, DeCourcy & Mudge Islands)

Confinement

Typical value:	unconfined
Description	The aquifer is generally unconfined, but depending on glacial deposits thicknesses, it may be semi-confined or confined in some places
Source	Water Budget Project: RDN Phase One (Gabriola, DeCourcy & Mudge Islands)

Groundwater storage

Description	The storativity coefficient was assumed to be 1E-4.
Source	Water Budget Project: RDN Phase One (Gabriola, DeCourcy & Mudge Islands)

Surficial sediment thickness
Range: [0 to 30] m
Description	The thickest deposits occur in lowlands between ridges where they may reach 30 m. Over a majority of the islands, surficial cover has been eroded down to, or nearly down to, bedrock.
Source	Water Budget Project: RDN Phase One (Gabriola, DeCourcy & Mudge Islands)

Hydrogeological unit thickness
Range: [ to 4000] m
Description	The sedimentary sequence is up to 4 km thick
Source	Water Budget Project: RDN Phase One (Gabriola, DeCourcy & Mudge Islands)

Well depth
45.72 m Range: [0.61 to 274.32] m

Groundwater depth
4.03 m Range: [0 to 80.77] m

Bedrock hydraulic conductivity
Range: [1e-07 to 3e-06] m/s
Description	Typical hydraulic conductivity were in range from 3E-6 to 1E-7 m/s
Source	Water Budget Project: RDN Phase One (Gabriola, DeCourcy & Mudge Islands)

Regional precipitation
Range: [811 to 1506] mm/y
Description	Mean total annual precipitation ranges from 811 mm at Victoria to 1506 mm at Courtenay.
Source	Ground Water Resources of British Columbia Chapter 9 -- Ground Water Resources of the Basins, Lowlands and Plains 9.1.3 GULF ISLANDS

Regional recharge
Range: [103 to 178] mm/y
Description	Recharge to groundwater was estimated as in range of 103 to 178 mm/year for Gabriola Island, however in other hydrogeological projects on the Gulf Islands, similar recharge values were determined. Unconsolidated deposits are anticipated to have a significant control on recharge.
Source	Water Budget Project: RDN Phase One (Gabriola, DeCourcy & Mudge Islands)

Regional discharge

Description	The aquifer drains very rapidly to ocean shores.
Source	Water Budget Project: RDN Phase One (Gabriola, DeCourcy & Mudge Islands)

Hydrogeological unit potential

Typical value:	medium - low
Description	Sandstone-dominant unit with low fracture has low aquifer potential, indicating a poorly productive aquifer that can be pumped, but only at low pumping rates. Mudstone-dominant unit with high fracture density and elevated permeability shows high aquifer potential, suggesting a typical ""moderately"" productive aquifer (good for residential wells but not high enough for large industrial pumping wells or city water supplies).
Source	Water Budget Project: RDN Phase One (Gabriola, DeCourcy & Mudge Islands)

Groundwater usage

Typical value:	agricultural - domestic
Description	Groundwater from bedrock wells is an important source of water supply for individual domestic use, small communities and for small mixed farming operations in the region.
Source	Ground Water Resources of British Columbia Chapter 9 -- Ground Water Resources of the Basins, Lowlands and Plains 9.1.3 GULF ISLANDS

Aquifer vulnerability

Typical value:	high local - medium regional - medium local - low local
Description	In higher elevation areas, the vulnerability is very low to low. It is moderate along major fault and fracture systems or in areas underlain by permeable materials such as glacial deposits (sand and gravel). Regions of high and very high susceptibility include coastal areas.
Source	Water Budget Project: RDN Phase One (Gabriola, DeCourcy & Mudge Islands)

Threats

Description	The main threat is saltwater intrusion. The more sensitive period is summer, due to the population growth in the tourist season.
Source	Water Budget Project: RDN Phase One (Gabriola, DeCourcy & Mudge Islands)

Groundwater Quality

Typical value:	brackish - fresh
Description	Natural ground water quality is found to vary relative to sampling position within evident ground water flow systems. Recharge areas are characterized by low mineralized (low specific conductance), calcium and sodium-bicarbonate type ground waters while deeper portions of flow systems and discharge areas are dominated by brackish sodium - choride type ground waters. High fluoride concentrations are also observed.
Source	Ground Water Resources of British Columbia Chapter 9 -- Ground Water Resources of the Basins, Lowlands and Plains 9.1.3 GULF ISLANDS