The developer of the Keystone oil pipeline has agreed to lay stronger pipe near a lake and aquifer, and drill a Sheyenne River crossing to save nearby trees and foilage, the state Public Service Commission says.

Members of the commission, who held a special meeting Wednesday to discuss the pipeline changes, said the heavier pipe would provide a stronger safeguard against leaks.

TransCanada Corp. agreed to install the pipe on short segments of the line, even though the company argued the move was unnecessary. State regulators also lack the authority to order the use of the heavier pipe, a company attorney said.

Opponents of the pipeline, including the Dakota Resource Council, a Dickinson-based environmental group, and some landowners along its suggested route now have 10 days to comment on the change if they choose.

The city of Fargo, which formally intervened in the case after public hearings on the pipeline's route had concluded, also may weigh in.

TransCanada is pushing for final PSC approval of the pipeline's route, to allow the company to begin construction this spring. The commission may finish its work on a siting order next month.

Commissioner Kevin Cramer said 2.3 miles of thicker pipe will be installed near the Fordville aquifer in Walsh and Grand Forks counties.

Another 1.4 miles of heavier pipe will be installed east of Lake Ashtabula, near Valley City.

"The reason we wanted a higher design standard … is because both of those places represent municipal drinking water supplies. In the case of Lake Ashtabula, it is also a recreational area," Cramer said. "So we just felt like those areas deserved a little higher level of protection."

The Keystone project is intended to transport oil from Alberta province in western Canada to locations in Illinois and Oklahoma. As planned, its North Dakota segment would stretch for 218 miles through Cavalier, Pembina, Walsh, Nelson, Steele, Barnes, Ransom and Sargent counties.

The pipeline's proposed route burrows beneath the Sheyenne River in Ransom County in southeastern North Dakota. TransCanada recently agreed to use horizontal drilling at the river crossing to spare nearby trees and shrubs.

Commissioner Tony Clark said that if TransCanada had not agreed to use the thicker pipe in specific locations, the commission could have altered the company's preferred route for the line.

The PSC does not have the power to specify types of pipe to be used in the pipeline, but it can direct where a pipeline is built, Clark said.

"We're not in the business of designing pipes, and we can't order specific designs with regard to pipes," Clark said. "But we do have the authority to site certain infrastructure in a given place or not. So, the answer could be, 'We're not designing your pipe for you. We're just telling you that given this set of factors, we're not comfortable with it here.'"