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. |
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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 |
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Comments | |
Project status | complete |
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