Water Stewardship Information Sources

Citation Pommen Water Quality Consulting. 2004. Water quality assessment of the Koksilah River at Highway 1 Bridge (1971-2003). Prepared for BC Ministry of Environment.
Organization Ministry of Environment
URL http://www.env.gov.bc.ca/wat/wq/quality/koksilah_riv/koksilah.pdf
Abstract/Description or Keywords The Koksilah River watershed is located in the southwest corner of British Columbia on
the southeast coast of Vancouver Island. The headwaters are located in the Vancouver
Island Mountains, and the river flows east and north to join the Cowichan River one
kilometre upstream from the Cowichan Bay estuary. The water quality sampling station
on the Koksilah River at Highway 1 is located two kilometres upstream from the
Cowichan River. The river provides habitat for sport and commercial fish species, and is
used for recreation and domestic, irrigation, and industrial water supplies. The main
human activities in the Koksilah River watershed are forestry, agriculture, and residential
and commercial development. This assessment is based on up to 33 years of water quality
data during 1971-2003. The water quality trends identified below have not yet been
confirmed by statistical analysis.
Conclusions
• The water was soft with respect to hardness, but had a low sensitivity to acids.
• The water quality objectives for ammonia, lead, and zinc were attained.
• Water quality indicators of fecal contamination were often exceeded, but the
objectives may be too stringent for the water uses in the river. The objectives
appear unrealistically low because the turbidity in the river would require partial
treatment (e.g., filtration) plus disinfection before drinking, thereby allowing a
higher objective for the water.
• Periphyton algal growth exceeded the recreation and aquatic life guidelines in
1990, but not in 1988. A 1998 survey found large quantities of algal growth
throughout the lower river wherever the river was not heavily shaded. There was
ample nitrogen and phosphorus to support algal growth.
• Colour exceeded the aesthetic drinking water guideline during winter high flow.
• Total copper exceeded the objectives during 1998-2003, although dissolved
copper met the objectives.
• Iron occasionally exceeded the guideline for drinking water aesthetics and aquatic
life, especially during winter high flows.
• Water quality objectives for dissolved oxygen were often not attained.
• Water temperatures were suitable for cold-water fish species, but the aesthetic
drinking water guideline was exceeded in the summer, when the water was warm
enough for swimming.
• Turbidity often exceeded the drinking water guideline for health, and partial
treatment plus disinfection is needed before drinking water use.
• There was an increasing (deteriorating) trend in manganese during summer low
flow due to very low flows in 2002-03. The aesthetic drinking water guideline
was exceeded during the late summer in 1998 and 2002-3.
• Other water quality indicators had apparent increasing and decreasing trends, but
they were attributed to decreasing minimum detectable limits over time or
increased sampling frequency during 2000-03. Recommendations
• Reconsider the objectives for indicators of fecal contamination.
• Update the zinc objectives in accordance with the new guidelines for zinc.
• Continue biweekly monitoring and reassess the data for trends in five years, when
ten years of consistent, regular data are available.
• Reduce the minimum detection limit for cadmium to at least ten times below the
lowest water quality guideline when the technology becomes available.
Information Type report
Regional Watershed Vancouver Island South
Sub-watershed if known Koksilah
Aquifer #
Comments
Project status complete
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