Under fields, past homes and across waterways, a pipeline has run through one of Canada's most populous corridors for nearly four decades, quietly pumping oil between southern Ontario and Montreal.

While it hasn't generated much national attention in the past, Line 9, which runs through part of Cambridge, is now being thrust into the spotlight as the company that operates it seeks approval to reverse its flow and increase its capacity.

On one side of the debate that will take place before the National Energy Board in Montreal and Toronto this month is Calgary-based Enbridge Inc. – which insists safety is its top priority – and on the other are residents and environmentalists who suggest the project will put a number of communities at risk.

Opponents to the Line 9 reversal, some of whom have staged protests and held sit-ins at pumping stations, worry that Enbridge plans to run a heavier, and what they claim is a more corrosive kind of oil, through the line that will stress the aging infrastructure and increase the chance of a leak.

But Enbridge – which is still cleaning up after a 2010 pipeline rupture in Michigan – says there's a lot of misinformation being circulated.

"It's been a hard thing to really get our story out in an independent way," Dave Lawson, Enbridge vice-president of major projects, told The Canadian Press.

"The environmental groups...it's difficult for them to target the oilsands but if they slow down the pipeline or stop the pipeline from moving that oil, that's shutting in the oilsands crude. We've seen a lot of that."

Enbridge says the 831-kilometre-long line is constantly monitored from an Edmonton control centre and can be shut down in up to 10 minutes if an unexplained reading comes in. A sudden loss of pressure means an automatic shutdown. The line is also patrolled on foot and by air.

If a leak occurs, a team can be on site in up to three hours, but the company is working to improve that by adding an emergency crew in Mississauga, Ont., to deal with problems in the Greater Toronto Area.

"If a leak happens we have to be able to respond to it, but the focus isn't on planning for a leak, it's really focusing on making sure we've done the operations and maintenance component to make sure this doesn't happen," said Lawson.

Line 9 originally shuttled oil from Sarnia, Ont., to Montreal but was reversed in the late 90s in response to market conditions, to pump imported crude westward. Enbridge is now proposing to flow oil back eastwards to service refineries in Ontario and Quebec.

It plans to move 300,000 barrels of crude oil per day through the line, up from the current 240,000 barrels, with no increase in pressure.

"The scope of our reversal is actually very, very small," said Lawson

A portion of the line has already received approval for reversal and has been sending oil from Sarnia to North Westover, Ont. –about 30 kilometres northwest of Hamilton –since August.

Enbridge stresses what will flow through the line will not be a raw oilsands product –although there will be a mix of light crude and processed bitumen.

"It really is dependent on what the refineries are set up for. Right now the refineries can take a small portion of heavy crude...they'd have to invest significant infrastructure to take greater heavy volume," said Lawson.

In defending the contents of Line 9, the company points to its own studies and a report from the U.S. National Academy of Sciences which found diluted bitumen is no more corrosive to pipelines than other crude. The American report was commissioned by the U.S. government after Enbridge's Michigan spill, which leaked 20,000 barrels of crude into the Kalamazoo River.

That spill has been repeatedly pointed to by Line 9 opponents, who say the same thing could happen in Ontario or Quebec when more oil is pumped through the pipe.

"This pipeline is nearly identical in design and age to the pipeline that spilled in Kalamazoo," said Adam Scott, climate and energy program manager at Environmental Defence, one among a consortium opposing Enbridge.

"This is a proposal that dramatically changes what the pipeline is for. Shipping tarsands diluted bitumen as is being proposed is considerably more risky...If there was a spill those chemicals would evaporate very quickly and cause an immediate health risk."

Some opponents suggest the Line 9 reversal is ultimately so Enbridge can transport oil to the Atlantic coast for export -- something the company denies, saying the project is currently about "keeping Canadian crude in Canada."

Scott contends the project is similar to a previous Enbridge bid called project Trailbreaker, which he said would've reversed Line 9 and a pipeline between Montreal and Portland.

"If Line 9 is reversed, there's huge pressure to reverse the Portland-Montreal pipeline," he said. "This project is ultimately a piece of the network that's designed to export oil to other countries...it doesn't affect the day-to-day lives of Canadians except to expose us to considerable risk."

But at least one observer suggests the reversal is a better alternative to an international player meeting the refinery demands Enbridge is responding to.

"This project wants to bring lower priced oil from western Canada to replace foreign oil," said Phil Walsh, a Ryerson professor with the university's Centre for Urban Energy.

"My biggest concern is that environmentalists are not considering that the market will seek out sources from other jurisdictions that don't have the same level of environmental standards that we have here in Canada."

Indeed, the degree of scrutiny Line 9 is being subjected to will ensure it's one of the safest pipelines in Canada, said Warren Mabee, director of the Queen's University Institute for Energy and Environmental policy.

"I worry about the environmental impacts no matter where these pipes run but...they're having to take a hard look at the infrastructure that they have and presumably that means that they will find faults and they will fix them."

Whatever the result of the upcoming NEB hearings, Mabee believes the conversation Line 9 has generated is worth having.

"This is good for Canada, that we're talking about this infrastructure, because it ties into this discussion about how we develop our energy future," he said. "Whether this is a positive or negative outcome for the company, I think it's a positive outcome for Canada."