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There's one thing that has to be made straight about the Big Pipe, the sewer project that has sparked a battle between Toronto and its northern neighbours in York Region.

There is no Big Pipe.

Rather, there is a grid that collects raw sewage from households and businesses in York Region and transports it to a sewage treatment plant, run jointly by York and Durham Regions, on the Lake Ontario shore in Pickering. The so-called Big Pipe is actually a broad plan to expand the system, and it's the environmental consequences of that plan that are at issue.

Digging sewer pipe may seem an improbable cause for hostilities, but the $700-million plan has Toronto's civic leaders worried enough to get involved.

On Thursday, Toronto City Council voted 34 to 3 to join environmental activists in fighting the project tooth and claw. The city will spend $100,000 to study the impact of construction on Toronto's watershed, and ask Ontario's Environment Minister to defer approving construction.

York Region chairman Bill Fisch said that Toronto's interference in the approvals process is unprecedented, and should not have happened on York's sewer project, which will take a decade to complete. "On this issue, there seemed to be something in the air that indicated that reason would not prevail," he said yesterday.

But Toronto politicians said the city's move will protect its interests. "It is not meddling," said Shelley Carroll, chair of the Toronto council's works committee. "It is advocacy for our own citizens."

The city is worried there was inadequate study of the system's impact on groundwater in the Rouge River Valley before the system was approved by the Ministry of the Environment, and that the losses could result in permanent damage to the river.

Gord Miller, Ontario's Environment Commissioner has said that, because of the scope of the project, the provincial Ministry of the Environment should have conducted a full-scale environmental assessment for the project, which would have meant public hearings.

Instead, it expedited a paper review for the parts of the project that it has approved so far.

York Region has been planning the expansion since 1997, and there are seven major projects in the current plan, including a 50-per-cent increase in the capacity of the Pickering sewage plant and six sewer pipe projects. Some have been built, some are being built and others are in the planning stages.

The two most controversial lines are the second phase of a line across 16th Avenue in Markham, which will be finished next spring; and a proposal to build another, starting next spring, along 19th Avenue east from Yonge Street and down Leslie Street to link up with the existing system.

Opponents are concerned that the 16th Avenue line is taking away groundwater from the Oak Ridges Moraine aquifer, which supplies the headwaters of the Rouge River.

David Donnelly, as the lawyer for Markham resident Jim Robb, is leading a court challenge to the York project.

He said an engineering study commissioned by the group Environmental Defence shows the sewer system "is robbing the ecosystem of an enormous amount of water."

He said that the report estimates that 100,000 cubic metres of groundwater a day flow out of the YDSS into the treatment plant in Pickering, a volume that leaves the regional ecosystem short of water.

And once the dewatering during construction is finished, "the infiltration [of ground water directly into the sewer pipe]is of much more serious concern to environmentalists. It really robs the ecosystem of the groundwater."

York Region's current sewer system was built in the 1970s and 1980s, and is now close to the limits of its capacity. That system will be strained further as York Region grows from its current population of 910,000 to 1.5 million in 2031. The region says the Big Pipe will service developments that have already been planned.

But the 19th Avenue line -- designed to relieve pressure on the existing system -- will run through the heart of the Oak Ridges Moraine, making it easy to service that sensitive area for development. Opponents worry about long-term development pressure on the Moraine.

Sewers potentially reduce the amount of groundwater in two ways. To keep the workers who build sewers from drowning, water will be pumped out while digging the tunnels for sewer pipes that can be two to three metres in diameter.

And after the sewer is installed, there may be leakage of water into the pipe in areas, such as passages through a deep aquifer, where the external water pressure is higher than the internal pressure in the sewer, but such leaks are usually detected through regular inspection.

York Region has acknowledged that the first phase of building the 16th Avenue sewer line, finished in 2003, had a negative impact on local watersheds.

But the region defends its current work as environmentally sensitive. For the second phase, which is about 40 per cent done, the region received authority from the province to pump out as much as 38,000 litres of groundwater a minute, said Debby Korolnek of York Region's works department.

But the volume pumped out never reached the limit, taking about 20,000 litres for most of the construction, she said. The heavy pumping for the 16th Avenue sewer will end in December, and will stop altogether when the line is finished in the spring, she added.

The broad disagreement on the pipe's impact could have major political consequences. Other important issues, such as transit and transportation, extend across regional boundaries, and if this turns out to be the opening battle in a wider war, it would be a major setback for regional co-operation.

"I hope it doesn't bring problems across the Greater Toronto Area," Mr. Fisch said.