Bacteria in St. Lawrence River after sewage dump nothing out of the ordinary, experts say

The raw sewage dump into the St. Lawrence River may have filled it with nasty bacteriabut 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 cole 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.

Atone 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 waterto 120,000.

Explore the numbersand 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 dumpare many times lower than the bacterial concentration in raw sewage, which is 4 million units per 100 mL.

Dornernotes that the high readings seen upstream, around Verdun and the Old Port, are similar to dailyreadings on the eastern end of the island, wheretreated wastewater is normally pumped out to the river.

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

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

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

Although the gap between before and after samples seems dramatic, it's important to note that the before numbers were probably taken indry weatherconditions, 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

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

"We need to establish toxicity," saidDaniel Cyr, professor of toxicology and biology at the Institutnational 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, alongmost 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 wasdumped directly into the river. The Nun's Island sampling station was right beside the sewage pipe.