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A Water Quality Assessment of the Cowichan and Koksilah Rivers and Cowichan Bay

Abstract

A recent British Columbia Environment Water Quality Status Report for the Cowichan and Koksilah river systems said the water quality is generally considered good with only a few objectives not being met. The objective for fecal coliforms (drinking water) was not regularly met in the Cowichan and Koksilah rivers during the summer. The dissolved oxygen objective was also not regularly attained in the lower reaches of the rivers and chlorophyll-a (a measure of algal growth) has exceeded its objective in the lower portion of the Cowichan River. The report recommended more monitoring to determine sources of coliforms and nutrients.

In 1998 and 1999, British Columbia Environment conducted a project to further assess the water quality in Cowichan River, Koksilah River, and Cowichan Bay. The purpose of the project was to identify sources of coliforms and nutrients with emphasis on non-point sources. Additional substances associated with urban or industrial runoff were also sampled. The short-term goal was to provide information to citizens, agencies, and regulators to assist in protecting water resources. The long-term goal is improvement of water quality to meet drinking water quality objectives in the rivers and shellfish water quality objectives in Cowichan Bay.

This report summarizes the results of this project. Initially a survey of the two rivers was completed to identify appropriate sampling locations. Sampling sites were located on each river and on several tributary streams. In addition, a number of sampling sites were also associated with ditches and stormwater drains located in urban and industrialized areas. Samples were collected and sent to labs where they were analysed for fecal coliforms, nutrients, total and dissolved metals and, in some cases, toxic substances. Environment Canada, through the Georgia Basin Environmental Initiative, assisted in the coordination of the sampling program and paid for a portion of the analytical costs.

The assessment revealed that although the Cowichan River frequently exceeded desired levels of fecal coliform bacteria, neither of the two discharges of sewage to the river were significant contributors to this pollution. Other non-point sources appear to have been responsible. The Koksilah River regularly contained excessive fecal coliform bacteria, again attributed to non-point sources. The report concluded that the discharge from the sewage treatment plant, serving the village of Cowichan Bay, is a primary contributor to fecal coliform bacteria pollution in the bay.

Excessive nutrient levels in the two rivers result in significant algae growth. This is a concern due to the potential for a negative impact on the natural benthic community and oxygen depletion during die-off, which could affect fish in both rivers.

Levels of metals and other toxics, such as oil and grease and hydrocarbons, in the Cowichan and Koksilah rivers are of little concern, though there were comparatively high concentrations of these contaminants measured in various stormwater conduits in urban and industrial areas. Levels of these substances were found to be typical for urban and commercial areas. On small streams, their impact would be significant, but loadings were small compared to dilution in the two rivers.

Full Report available by request at georgiabasin@ec.gc.ca

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