The report by the Manitoba Energy Justice Coalition said a rupture on the proposed Energy East pipeline would seep into any number of waterways which feed into Winnipeg's water supply.
The pipeline transporting oil from Alberta and Saskatchewan to refineries and port terminals on the East Coast would partly run underneath an aqueduct carrying Winnipeg's drinking water from Shoal Lake near the Ontario boundary.
Dennis LeNeveu, a retired biophysicist and author of the report, said a 40-year old repurposed natural gas line would be used across Manitoba. Such pipelines can get corroded and have ruptured four times in Manitoba in the last 20 years, he said.
The entire length of Winnipeg's 100-year-old aqueduct would be in danger of contamination from the pipeline, which would run parallel to it, LeNeveu said.
"Small, continuous, undetected leaks will occur and seep unseen into the ground causing ground and surface water contamination," he said following the release of the report Monday. "One spill, one leak — it doesn't have to be a big leak — almost anywhere along that line can be carried over our aqueduct."
There would also be "a significant risk of rupture and explosion" from a nearby natural gas line in Manitoba, LeNeveu said. Such an explosion could "easily be as large or larger" than the train derailment and explosion that killed 47 people in Lac Megantic, Que., almost two years ago, the report said.
"The smoke plume from such an explosion and fire could necessitate the immediate evacuation of the entire population of Winnipeg should it occur nearby."
Calgary-based TransCanada Corp. (TSX:TRP), the company behind the $12-billion pipeline, said it would be safe. Spokesman Tim Duboyce said the company already does a thorough inspection of the existing line with technology that can detect erosion as small as a pencil tip.
Such defects are immediately repaired, he said. Energy East would be monitored around the clock and would be shut down the minute any leak were detected.
"We're proceeding with the preparation of this project with safety at top of mind," Duboyce said.
TransCanada has never had an oil pipeline leak because of a problem with the "integrity" of the line, he said.
Critics say even a small risk of contaminating Manitoba's water is too great.
"There is absolutely no replacement for water in sustaining life," said Vicki Burns, director of the Save Lake Winnipeg Project. "On the other hand, we know there are new technologies that actually will allow us to meet our energy needs without relying on the problems of fossil fuels."
Alex Paterson with the energy justice coalition called on the provincial government to oppose the proposal, even though it is federally regulated. Paterson said the province still controls building permits and conducts its own environmental assessment.
"The reality is, if they wanted to protect the water, the only sure way to protect our water is not have this pipeline go through."
Conservation Minister Tom Nevakshonoff declined to be interviewed. Spokesman Al Foster said in an emailed statement the department was studying the report and it would be taken into consideration during National Energy Board hearings on the project.