Critics of running the Southwest Corridor light-rail through tunnels in Minneapolis are urging the mayor and City Council to delay a crucial vote on the project until more study can be done of the potential environmental impact on two nearby lakes.

Former Hennepin County Attorney Tom Johnson, who represents some residents along the future light-rail route through the Kenilworth corridor, wrote Mayor Betsy Hodges and City Council President Barb Johnson that approving plans without a comprehensive study would "endanger the environment."

The letter was sent this week as officials of Minneapolis and the Metropolitan Council, the agency overseeing the light-rail project, hold closed-door negotiations over the plans with a mediator.

Supporters of the $1.68 billion transit project, the most costly in the Twin Cities, are expected to seek federal funding for it this fall if Minneapolis and four other cities along the nearly 16-mile route approve the plans. The Met Council has set a July 14 deadline.

"Mayor Hodges and the City Council have long expressed concerns about environmental impacts in this corridor," John Stiles, the mayor's chief of staff, said Tuesday. "Beyond that, we are still in mediation with the Met Council and respect the confidentiality of the process."

Johnson noted that the last comprehensive environmental assessment was done in 2012, when plans called for the light-rail trains to run at ground level in the Kenilworth corridor. That idea was rejected by the Met Council in favor of running light rail in two tunnels, with the tracks emerging to cross a bridge over a channel between Lake of the Isles and Cedar Lake.

The potential environmental impact of the tunnels has been the subject of two separate studies by consultants hired by the Met Council. They concluded that the tunnels would have minimal impact on the lakes and channel.

But Johnson is calling for a more comprehensive environmental impact study before City Council action.

"The City Council does not have all the information it needs to make an informed decision," he wrote. He said city approval before such a comprehensive environmental review would violate state law, but the Met Council disagreed.

"Under federal and state law, the municipal consent process runs parallel to the environmental review process," Met Council spokeswoman Meredith Vadis said Tuesday. "The law does not preclude the submission of preliminary design plans to municipalities under the municipal consent process before completion of the environmental reviews."

Johnson has represented homeowners along the Kenilworth corridor who oppose the project and say they have raised money to mount a potential legal fight over the plan. He wrote to the city on behalf of the "Lakes and Parks Alliance of Minneapolis." Some of the homeowners previously favored a tunnel running through the entire corridor, including under the water channel. That more costly option was rejected by the Met Council.

The Minneapolis Park and Recreation Board wants the Met Council to conduct a detailed engineering feasibility study and cost comparison of tunneling under the channel.

The most recent review commissioned by the Met Council, from a Kansas City consultant last winter, analyzed how tunnels would affect groundwater flow and lake levels, and identified areas where more information should be gathered if the project moves into the engineering and environmental assessment phase.

The study was commissioned in part because Gov. Mark Dayton said last October that he wanted more questions answered about the light-rail line.

Pat Doyle • 612-673-4504