Water Stewardship Information Sources
| Citation | Barroso, S et al. 2013. Groundwater quality in the lower Cowichan River aquifer complex. BC Ministry of Environment. |
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| 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 increase. 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 till. 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 parameters. 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 |
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| Project status | complete |
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