Other than a big spike on a rainy day, the concentration of bacteria throughout the raw sewage dump into the St. Lawrence River was as expected.

The City of Montreal released the results of the water quality tests performed throughout the four-day sewage dump.

While fecal bacterial levels were far above average concentrations, they resembled numbers seen during heavy rains, when sewers overflow into the river, a water quality expert in Montreal said.

"I'm not surprised by the numbers," said Sarah Dorner, an associate professor at the Polytechnique Montréal's Department of Civil, Geological and Mining Engineering.

See how the bacterial levels changed throughout the dump for 10 selected sampling stations in the chart below.


On a mobile device? See the chart here.

The big spike on Nov. 12 was registered by a station close to one of the pipes that spewed the raw sewage, near LaSalle Boulevard and Stephens Street in Verdun.

It likely detected some of the waste before it had a chance to dilute.

The reading that day was 2.7 million coliform units per 100 mL of river water. Raw sewage has around 4 million.

"It is clearly a site that is directly hydraulically connected to the overflow point," Dorner said.

The average concentration in the St. Lawrence around Montreal since 2008 is 193 coliform units per 100 mL, safe enough for human contact.

The spike was also likely caused by the heavy rains that day. When the sewer system is overloaded by rain, it dumps excess sewage into the river.

"You could get a re-suspension of sewer deposits that get flushed out and increase concentrations," Dorner said.