The raw sewage dump into the St. Lawrence River may have filled it with nasty bacteria but it was nothing the river hasn't seen before.

The City of Montreal released data from water quality tests made before and after the sewage began to flow.

Though the numbers may look scary, it's similar to what comes out of the water treatment plant daily and during heavy rains when the sewers overflow into the river, a water quality expert said.

"What was expected to happen is what actually happened," said Sarah Dorner, an associate professor at Montreal's École Polytechnique.

"The numbers one day into the sewage dump are like the numbers we see when there are sewage overflows."

The City of Montreal provided before and after test results for 29 sampling stations between Verdun and the eastern tip of the island.

At one station, located just beside one of the underwater sewer pipes in Verdun, the reading jumped from 7 fecal coliform units per 100 millilitres of river water to 120,000.

Explore the numbers and compare them to historical values in the map below.

On a mobile device? Tap here to see the map.

The highest readings after the sewage dump are many times lower than the bacterial concentration in raw sewage, which is 4 million units per 100 mL.

Dorner notes that the high readings seen upstream, around Verdun and the Old Port, are similar to daily readings on the eastern end of the island, where treated wastewater is normally pumped out to the river.

That water, although treated, is not disinfected, and still has a relatively high coliform concentration of one million.

"That's being discharged every day, 24 hours a day," she said.

She said the introduction of disinfection by ozonation to treatment plants in the future will help reduce those concentrations.

Key bacteria concentrations Water quality in Montreal is measured, in part, by the number of colony-forming units of fecal coliform bacteria in 100 millilitres of water. 4 million: Raw sewage 1 million: Treated wastewater that's discharged into the St. Lawrence River daily Around 50,000: Parts of the St. Lawrence during heavy rains (sewage overflow) 1,000 or more: Avoid all recreational activities Between 200 and 1,000: Avoid direct contact with water 200 and below: Safe for human contact



Although the gap between before and after samples seems dramatic, it's important to note that the before numbers were probably taken in dry weather conditions, when there is no sewage overflow.

Dorner also stressed that coliform bacteria, like E. coli, don't survive very long in river water.

"E. coli naturally live in the guts of warm-blooded animals. When in the environment, they become deactivated. Also, there are other organisms in the water that will consume these bacteria," Dorner said.

"If you take sample of the river today, it would go down to what you saw before the sewage dump."

Other contaminants worrisome

Fecal bacteria is only one measure of water quality. Another expert interviewed by CBC said that other sewage contaminants, like hormones and Polychlorinated Biphenyls (PCBs) can persist in the environment and cause long term damage.

"We need to establish toxicity," said Daniel Cyr, professor of toxicology and biology at the Institut national de recherche scientifique.

"And I think once we know if it's accumulating in any areas then we can do much more detailed studies to try and see what kind of chemicals they are and then to see if we can predict some of the effects that might be occurring to the fish."

Overall quality improving over time

Historical water quality data going back to 2003 from the City of Montreal shows that, along most of the Montreal shoreline, the average coliform concentration has been safe enough for human contact: under 200 units per 100 mL.

Overall, bacterial concentrations have been generally decreasing except for a few spikes. The chart below plots the averages for each year.

The spike in 2007 was caused by a sewage problem in Nuns' Island, when raw sewage from 48 households was dumped directly into the river. The Nun's Island sampling station was right beside the sewage pipe.