Lake Ontario May Suffer Fallout From Monday's Floods
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Lake Ontario May Suffer Fallout From Monday’s Floods

What effect do storm surges have on Lake Ontario's ecology?

The mouth of the Humber River, littered with debris in the aftermath of Monday’s storm.

If you were a fish, you might be a little worried about exactly what Monday’s floods washed into Lake Ontario. After all, the surge far overwhelmed the capacity of our wastewater treatment plants, and it’s been acknowledged that some raw sewage was released, though the exact amount is unknown. But Ontario is a big lake, and for decades those who dwell on its shores have relied on this vast volume and some natural filtration systems to reduce pollutants to levels that, while certainly not ideal, are at least low enough that it’s safe to go swimming, most of the time. So is the danger posed by the storm only temporary, or will it have a lasting effect on the stability of the lake’s ecosystem? The answer, it turns out, is up to us.

Water scientists and civil engineers talk about “100 year events,” a level of storm intensity that, based on past statistics, we can expect to see only about once a century. Monday’s storm certainly qualifies, but according to Gail Krantzberg, a professor of civil engineering at McMaster University, such events are becoming steadily more common. “The statistical norm for the 100 year event has been broken year after year for the past few years,” she says. Typically, sewer systems and wastewater treatment plants are designed based on the maximum intensity you’d expect during a much smaller window, only about two to three years. For larger events, the flow can be too great, and some of the extra water—in many areas a combination of rainwater and sanitary water from households—goes into overflow channels that empty directly into the lake.

This is obviously not an ideal system, but it may not be as bad as it seems. Treatment plants rely on microorganisms that are naturally present in the water to break down organic matter and devour potential disease-causing bacteria like E. coli. The same process happens in the lake itself, albeit more slowly. Lake Ontario has been dealing with bird and animal waste this way for millennia. So while it might be wise to stay away from the beaches for the next few days (indeed, many are still marked as unsafe), eventually the levels will drop back down to normal.

So, all clear? Not so fast, says Krantzberg. “When that storm surge came through, it would have scoured the highways and parking lots,” she says. “All of those paved surfaces were full of things like heavy metals, hydrocarbons from car exhaust, corrosive materials, oil, and grease.” Oil and water don’t mix, so these contaminants tend to stick to particles of dirt rather than stay dissolved in the water. In the lake, this dirt becomes sediments, prime habitat for invertebrates and young fish. “They may be actually picking up more contamination in the short-term period, and that can move those contaminants up into the food chain,” says Krantzberg.

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Of course, this process can take a long time, and a single storm may not count for much over the course of decades. But Krantzberg, who spent years as co-ordinator of the Ontario Ministry of the Environment’s remedial action plans for cleaning up pollution hot spots in the Great Lakes, says that if indeed such storms are becoming more frequent, they could have a cumulative effect. “We actually studied what climate change would do to the clean-up of these areas of concern,” she says. “Anything that has to do with severe storms, warmer water temperatures or dropping lake levels can have an effect on what we’re trying to do to restore these historically degraded areas.”

What about the fact that Lake Ontario is huge? Even a billion litres of polluted water would amount to less than one millionth of its total volume: surely it could dilute contaminants to a manageable level? Unfortunately, it’s not that simple. “The Great Lakes are meltwaters from glaciers. They’re like a giant swimming pool with a very little tap, which is rainwater,” says Krantzberg. “The replenishment of the Great Lakes is one per cent per year.” Civil engineers use a concept called residence time to measure how long, on average, a drop of water stays in a given vessel. For Lake Ontario, the figure is roughly 20 years, so pollutants dumped in by a storm have the potential to stick around for a long time. Even worse, the mixing in the lake isn’t uniform. “A severe storm like what we saw can be focused almost like a funnel, right down into an isolated area,” says Krantzberg, noting that while Etobicoke and Mississauga saw over 120 millimetres of rainfall, Hamilton received only two millimetres. “That is not going to get dissipated and diluted right out into the open waters of the lake; it’s going to stay fairly close to a shoreline for quite a while.”

Fortunately, there are solutions. Civil engineers have expressed hope that the storm will serve as a wake-up call, reminding taxpayers that we need to replace outdated infrastructure, including combined sewers. We can also create systems like detention tanks, which can hold excess stormwater underground and release it to the treatment plant during off-peak hours. Another important strategy is that of “soft engineering,” which aims to mimic natural water filtration systems. For example, bioswales are grassy ditches beside roads that hold water and filter it through soil, where contaminants are often rendered with less bioavailability than in sediments. Parking lots made of porous material would act like sponges instead of funnels, further reducing the load on the treatment plants. Whatever the method used, time is of the essence. “You can argue all you want about carbon emissions and so on, but the fact is, we are not going to stop seeing this, we’re going to see more of it,” says Krantzberg. “We need to adapt the way that we use the land to protect the water.”