St. Mary Lake Water Quality2018-07-10T00:04:53+00:00


St. Mary Lake is Salt Spring’s largest single water source holding some 14.7 million cubic meters of water (at 40.0m elevation). Considered a critical island resource, St. Mary is designated a multi-use lake providing drinking water, fishing and recreational use for island residents and visitors alike.

A shared resource, the North Salt Spring 
Waterworks District (NSSWD) alone withdraws 
nearly ~85 million imperial gallons (2013) or ~385 million litres annually, while the Fernwood Highland Water District, resorts and many lakeside residents also withdraw water and enjoy recreational activities on and around the lake.

Weather Station at St. Mary Lake

Decades of Concern

Since as far back as the 1970s, the water quality of St. Mary has been a concern to NSSWD, the Ministry of the Environment, island residents and many others. A 1975 water quality report described the level of 
algal growth and hypolimnetic oxygen depletion as symptoms of a “eutrophic” lake.

“Cultural Eutrophication” is a term used by scientists to describe the negative impacts on lake water quality due to human activities such as land development, recreational activities, road maintenance, logging and 
farming. These activities can lead to increased nutrients levels and excessive algal growth in lakes. At one time or another, all these activities have been carried out within the 
St. Mary watershed and some continue today.

However, natural conditions such as extreme rainfall events, watershed topography and types of ground cover, water temperature, water chemistry, lake morphometry (form), wind and airborne deposits all affect water quality to some degree as well. In addition, with a surface area of 182 hectares and a 
watershed drainage area of only 525 hectares, depending on the amount of rainfall in a given year St. Mary has a very low flushing rate. This means that any nutrients deposited in the lake often stay in the lake for a long time, further impacting water quality. Our longer, hotter summers, due to changing climate patterns, also impact water quality by promoting algal growth. These conditions may particularly favour the “algal toxin” producing “cyanobacteria” which have been dominant over the past number of years.

All these elements combined have created a challenging situation for a water purveyor such as NSSWD. A more sophisticated water treatment plant is the only way to consistently meet BCs’ water quality standards and regulations.

DAF Meets the Challenge

Without a treatment process such as Dissolved Air Flotation (DAF) that can remove both suspended and dissolved organic material, the finished water will develop “Disinfection By-Products” when combined with the chlorine added for disinfection. The Canadian Guidelines for Drinking Water Quality set the standards for disinfection by-products such as Trihalomethanes and Haloacetic acids. 
At times, during heavy algal blooms, levels of these disinfection by-products in the District’s St. Mary system have exceeded those standards. As chlorine must be used to ensure the water is free of harmful bacteria, the only way to reduce levels of disinfection by-products is to remove the organic matter from the water.

The “Taste and Odour” associated with drinking water from St. Mary Lake is also due to the high levels of algae and organics in the water and the chlorine required for adequate disinfection. The DAF water treatment process removes not only organics but also algal cells. Once these substances have been removed, less chlorine will be required for disinfection and the taste and odour of the finished water will be significantly improved.

Managing the Lake and the Watershed

Many agencies are involved in the management of the lake and watershed. The District, the Capital Regional District, the Ministry of Environment, the Ministry of Forests, Lands and Natural Resource Operations, the Islands Trust, the Salt Spring Island Watershed Protection Authority (SSIWPA), the Salt Spring Water Council, the Salt Spring Water Preservation Society and many concerned residents have worked and continue to work to improve water quality in St. Mary and other island lakes.

As a water supplier, the District does not have the authority to protect the lake and its watershed. However, the District does have the responsibility to supply potable water and the water treatment plant will enable the District to meet that responsibility now and in the future regardless of the state of St. Mary Lake.

In 2009 NSSWD installed an updated aeration system in an attempt to prevent or at least reduce algal blooms. A ban on gas motors on the lake has been in effect for over 20 years. A watershed management plan has been developed but implementation is challenging due to the multi-jurisdictional nature of watersheds.

While efforts continue, solutions are a long way off at this time and improvements in the lake will not eliminate the requirement for a new water treatment plant for the district’s ratepayers. To ensure all public water supplies meet the legislated requirements of the Drinking Water Protection Act, Island Health developed performance targets and standards for all drinking water systems that use surface water as a source. As a result, a new water treatment plant would be required for the St. Mary Lake system regardless of improvements in the condition of the lake. By law, all surface water sources need adequate treatment and disinfection even if the lake is healthy.

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