Town’s sewage discharge part of fish-kill investigation on Cowichan River
Discharge from the Town of Lake Cowichan’s sewage-treatment facility is being investigated as a potential factor in a fish die-off in the Cowichan River last summer.
Kevin Pellett, south coast stock assessment biologist with Fisheries and Oceans Canada (DFO), told sixmountains.ca that “algal growth appeared to increase” downstream of the town’s sewage outfall pipe.
Hot sunny weather, low water flows, and the presence of nutrients such as nitrogen and phosphorus can lead to excess algal growth, DFO says.
In this case, low dissolved oxygen and high pH, potentially caused by decay of algae, created stressful conditions and put fish at greater risk of disease.
Lake Cowichan operates an open-lagoon sewage treatment facility on Hudgrove Road that employs aerators and both chlorination and de-chlorination systems before discharging to the upper Cowichan River.
Pellett said it’s important to be “cautious to look at all potential nutrient sources without jumping to conclusions.” Water data collection is continuing, with a follow-up report expected in the coming months.
The Ministry of Environment says the Lake Cowichan sewage treatment facility is authorized to discharge an average of 2,200 cubic meters per day and the maximum authorized rate of discharge is 4,500 cubic meters per day.
Discharge is treated to secondary-treatment standards and must not exceed a biochemical oxygen demand concentration of 45 milligrams per litre and a total suspended solids concentration of 60 milligrams per litre.
Total chlorine residual concentration in the discharge is limited to a maximum of 0.05 milligrams per litre.
Provincial compliance and enforcement files show that Lake Cowichan was fined $700 under the Environmental Management Act on July 14, 2023, for non-compliance with reporting/sampling requirements for effluent toxicity. The fine related to 22 contravention days from 2020 to 2022.
Pellett, who addressed the Cowichan Watershed Board this week, noted that this summer’s low river flows — 4.5 cubic metres per second — have occurred in the past during droughts without similar die-offs. “It’s tough. Why is this year different? It’s caught all of us off guard.”
About 50 samples have been submitted to fish-health experts for testing. Human consumption of fish from the river is not considered a health concern.
Pellett believes steelhead and rainbow trout (and potentially brown trout) “took this on the chin the hardest” given their presence in the main stem of the river in summer.
Cowichan Tribes Chief Lydia Hwitsum stressed the importance of studying the cumulative impacts throughout the entire river system.
Pellett said he expects a more rigorous monitoring program in future during low-flow periods, from Lake Cowichan to Cowichan Bay. “It’s very concerning, quite high in our priority list right now.”
DFO first received a report of a fish kill at Horseshoe Bend, downstream of Skutz Falls, on July 10. Over the next few days, more fish were confirmed dead as far upstream as 70.2 Mile Trestle.
On July 19, a survey of the upper river from Cowichan Lake to Skutz Falls found more mortalities as well as “significant algal growth” downstream of the Lake Cowichan sewage outfall.
Jas Sandhu, superintendent of public works and engineering services for Lake Cowichan, said that currently there are three lagoons in operation with a fourth awaiting funding so it can be connected to the rest of the system.
“People are moving away from them to new and better systems,” he said. “It’s definitely some of the older technology, but there’s a number of them around.”
(Photos of Lake Cowichan sewage-treatment facility. DFO map of upper Cowichan River).
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— Larry Pynn, Sept. 28, 2023