As spring rains pelted Toronto last month, the City dumped 1.3 million cubic metres of partially treated sewage into Lake Ontario — the equivalent of 521 Olympic sized pools.

But these “bypasses” don’t mean you should give up your water sports this summer.

As a basic rule, people should start thinking about water quality like the weather — “it’s something that changes every day,” said Krystyn Tully, the vice-president of the Lake Ontario Waterkeeper.

Exercise caution for 48 hours after it’s rained, but “if it hasn’t rained for several days, then for the most part we don’t think people should be afraid of the lake at all.”

Frank Quarisa, the director of wastewater treatment with Toronto Water, said it’s been a wet year and the “bypasses are driven by weather conditions.”

In 2016, a “very, very dry year,” there were only four sewage bypasses at the city’s Ashbridges Bay Treatment Plant, Quarisa said. Since January this year there have been five, putting the plant on track for an average year with about 12 bypasses, he said.

But not all sewage bypasses are alike.

In May, Ashbridges had two sewage bypasses, one that lasted 10 ½ hours and released 405,789 cubic metres of partially treated sewage and stormwater into the lake and one that lasted five hours and released 192,317 cubic metres.

Together the bypasses accounted for two per cent of the plant’s total flow. Over at the Humber treatment plant another bypass lasted 32.5 hours and released 512,834 cubic metres of partially treated wastewater.

When heavy rains force the city to send some partially treated sewage and stormwater into the lake to protect its wastewater treatment plants, the sewage receives primary treatment, which removes solids, goes through phosphorous removal, and is disinfected with chlorine in an effort to kill off bacteria.

Some of the wastewater, though, doesn’t receive secondary treatment, a biological treatment, which helps remove more contaminants and accounts for about 30 per cent of the total treatment process, Quarisa said.

For Tully, the primary concern during heavy rainfall isn’t sewage bypasses, it’s for the “creeks and rivers and other places along the waterfront where sewage isn’t making it to the treatment plant at all.”

The city can’t say how much untreated sewage and stormwater made its way into the lake in May from combined sewer overflows, but Tully said we can be sure some of it ended up in Toronto waterways.

“In an old system like the City of Toronto, if you have so much rain that the wastewater treatment plants are bypassing, you know that further upstream there’s raw sewage that is also going into the water,” said Tully.

Water quality is less of a concern at Toronto’s beaches where it is monitored daily and most are located in areas that are less affected by sewage bypasses and combined sewer overflows than areas like the inner harbour.

“The bigger concern is for people who are surfing, paddling, sailing, using the water in places other than those official beach locations,” Tully said.

They tend to use areas — like the inner harbour — that are more affected by sewage pollution but monitored less.

“The concern for people coming and trying to use these spaces is that they can’t get access to the data that they need to effectively protect their health,” she said.

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Instead they may avoid the lake altogether out of caution even though there are many days in the summer when the water is safe to enjoy.

Moving forward the Lake Ontario Waterkeeper wants the city to share water quality information more proactively, so people know when there are water quality alerts in place and when they’ve ended.

Tully also wants the city to start monitoring water quality in the inner harbour, where people frequently kayak and sail during the summer months.

Little kids learning to sail or kayakers practising their rolls end up in the water just like swimmers on the beach, she said.

In 2016 the city estimated 2.35 million cubic metres, or 942 Olympic sized swimming pools, of raw sewage and stormwater overflowed into Toronto waterways from combined sewer overflows. But it doesn’t monitor combined sewer overflows, offering only annual estimates based on modeling in reporting to Environment and Climate Change Canada.

In a statement the city said it would be costly and time consuming to monitor combined sewage overflows, with 2013 estimates predicting upfront costs of $7 million and $6.3 in annual operating and maintenance costs.

“Given the number of potential discharge points that we have and where they’re located it’s just economically prohibitive to tell us what we already know which is when it rains a (combined sewer overflow) has a fairly high likelihood of happening,” Quarisa said.

By not investing in better monitoring, Tully said the city is simply passing the costs of sewage pollution on to the Toronto residents, visitors, and businesses.

Toronto “should be taking notes” from the City of Kingston, about 260 kilometres east along the shores of Lake Ontario, which launched real time public notifications of sewage overflows last month.

Regular water monitoring would give them the information they need to avoid waterborne illnesses, anything from a rash to a stomach ache or a more serious virus, and allow them to comfortably enjoy the lake.

For now, the Waterkeeper conducts water monitoring in the inner harbour and recreational water users can check their online swim guide before taking a dip.