Water Stewardship Information Sources

Citation Barroso, S et al. 2013. Groundwater quality in the lower Cowichan River aquifer complex. BC Ministry of Environment.
Organization Ministry of Environment
URL http://www.env.gov.bc.ca/wsd/plan_protect_sustain/groundwater/library/groundwater_quality_in_the_lower_cowichan_river_aquifer_complex.pdf
Abstract/Description or Keywords Groundwater is an important resource in British Columbia. It has been estimated
that over 750,000 people in B.C. depend on groundwater as their source of potable
drinking water. In the Cowichan Valley region of southern Vancouver Island,
groundwater is an essential source of potable water for domestic purposes, including
municipal water supplies. It also provides water for industrial, commercial and
agricultural purposes and critical baseflow to streams during dry periods. In the lower
Cowichan Valley underlying the City of Duncan there are several very productive,
layered, sand and gravel aquifers that provide fresh water for the area. As the population
of the region increases, the demand on groundwater and these aquifers is expected to
The Lower Cowichan River layered aquifer system is composed of three sand and
gravel aquifers known as the Lower Cowichan River aquifers 186, 187 and 188 (also
commonly known as the Lower Cowichan Aquifers A, B, and C, referring to the upper,
middle and lower aquifers respectively). This layered aquifer complex is found along the
lower Cowichan River floodplain and consists of interbedded fluvial and glaciofluvial
sand and gravel deposits, variably separated by lower permeability layers of silt, clay, and
The two uppermost aquifers have been identified as being highly to moderately
vulnerable to potential contamination from surface activities and land-use overlying the
aquifer, based on factors including the depth to groundwater and the lithology of the
materials overlying the aquifer i.e. the presence or absence and relative thickness of
confining low permeability sediments such as clay or till.
The main source of groundwater recharge is likely infiltration of precipitation at
the land surface; in addition, the aquifer complex, in particular the upper and middle
aquifers, are considered hydraulically connected to the Cowichan River. The river is a
source of aquifer recharge and receives groundwater discharge within different reaches
and seasonally during the year. Because of this interconnection, water quality in the
aquifer has the potential to be impacted by surface water quality, while, equally,
production well pumping adjacent to the river may affect river base flow during dry periods. The latter concerns are to be addressed in separate studies currently being
completed in the Cowichan watershed.
In 2002, the B.C. Ministry of Environment initiated this study of the groundwater
quality within the Lower Cowichan River aquifer complex, focussing on wells
constructed in the uppermost aquifer layers (aquifers 186 and 187). The objectives of this
study were to establish a baseline of ambient groundwater chemistry which could be used
to evaluate future changes, and to assess seasonal variability and temporal trends in
groundwater quality.
Six large-capacity production wells, owned and operated by municipalities or fish
hatcheries, were sampled between 2002 and 2011. The majority of the study wells are
situated adjacent to the Cowichan River (200 m distance), with the exception of site 5
which is located closer to the Koksilah River, approximately 1.5 km south of the
Cowichan. Available water chemistry data for the same period from two provincial
observation wells constructed in these aquifers were also included in the evaluation.
Groundwater samples were analyzed for water quality parameters including pH,
conductivity, alkalinity, turbidity, major anions and cations, and total or dissolved metals.
Based on the sampling program, groundwater in the Lower Cowichan River (upper and
middle) aquifers was determined to be an immature, calcium-bicarbonate type
groundwater without evidence of saltwater intrusion. Compared to the Guidelines for
Canadian Drinking Water Quality, the concentrations of all measured chemical
parameters were below the maximum acceptable concentrations for all health-based
There were two exceedences for iron and one exceedence for manganese, both
considered naturally occurring aesthetic parameters that affect the taste and appearance of
the water. Measured concentrations of chloride at site 6 were below the drinking water
guidelines but elevated compared to concentrations at sites 1 to 5, and nitrate-nitrogen
concentrations were also higher at site 6 compared to the other production and
observation wells. These slightly higher concentrations of chloride and nitrate may be
indicative of surface land use impacts and onsite activities, or differences in the aquifer
properties at the location. The Lower Cowichan River aquifer system is a highly productive source of quality
drinking water for the City of Duncan, the Municipality of North Cowichan, and the
Cowichan Tribes. It is used extensively by agricultural and industrial operations in the
area, and provides baseflow to the Cowichan River during dry periods. The groundwater
quality is currently very good; however the aquifer is also highly susceptible to
anthropogenic impacts making aquifer protection a priority. Although well head
protection plans have been developed for essential municipal wells, priority should be
placed on aquifer protection, specifically land use decisions, hazard identification, and
risk management at both a wellhead and an aquifer level. It is recommended that all
levels of government continue to work together to share resources and information to
better understand and monitor this important aquifer system. Future studies could include
more widespread sampling of domestic and small to medium well users, spatially
distributed over the aquifer, in areas at higher risk to contamination due to aquifer
properties and the type of land use (e.g. industrial or agricultural use).
Information Type report
Regional Watershed Vancouver Island South
Sub-watershed if known Cowichan
Aquifer #
Project status complete
Contact Name
Contact Email