A federal judge in Maryland has rejected the third and final lawsuit brought by opponents of the state’s Purple Line project.

In a 21-page ruling issued Monday, U.S. District Judge James K. Bredar rejected the opponents’ arguments that the U.S. Army Corps of Engineers had improperly allowed construction crews to discharge dredge and fill into streams.

The plaintiffs, Bredar wrote, “have not proposed an obviously superior alternative that the Corps overlooked” to improve east-west transit in the Maryland suburbs while causing less damage to wetlands and waterways. The opponents, he said, also did not show that the agency’s decision to allow the work was “arbitrary, capricious, an abuse of discretion” or illegal under the Clean Water Act.

The Maryland Transit Administration (MTA), the judge wrote, “clearly demonstrated” that the Purple Line design now under construction was the “least environmentally damaging practicable alternative.”

The ruling resolves the final lawsuit against the 16-mile light-rail project, meaning that construction may proceed unimpeded between Bethesda in Montgomery County and New Carrollton in Prince George’s County. The Purple Line is scheduled to open in two phases, beginning in late 2022.

Veronica Battisti, a spokeswoman for the MTA, said the ruling allows the state “to fully focus on advancing a critical project that will provide essential transit connections and improve access to employment, housing, health care, education and recreation.”

The first lawsuit, filed in U.S. District Court in the District, delayed construction by almost a year before a federal appeals court rejected it in 2017.

A U.S. District Court judge in D.C. dismissed a second lawsuit last year.

In the third lawsuit, filed last year, the plaintiffs — the nonprofit Friends of the Capital Crescent Trail, Chevy Chase resident Leonard Scensny and former Chevy Chase resident John M. Fitzgerald — alleged that the Corps permit allowing dredging and other work in streams and wetlands is illegal under the Clean Water Act.

The agency, the plaintiffs said, should have required the MTA to prove that it would be “impracticable” to improve east-west transit in a way that would avoid harming streams and wetlands.

For example, they said, the state could have expanded and improved existing bus service, which wouldn’t have required widening bridges over streams to accommodate light-rail trains.

The Purple Line’s construction, they said, permanently affects a half-acre of wetlands and 5,108 linear feet of streams, including Rock Creek, Sligo Creek, Long Branch Creek, Northwest Branch Creek and Northeast Branch Creek.

Lawyers for the MTA, which intervened in the lawsuit on behalf of the Corps, said the state and Corps rejected options to build a rapid bus line because light-rail would be faster, have higher ridership and damage less wetlands.

A spokeswoman for the Corps’s Baltimore district previously said the state must mitigate the project’s impacts by restoring Paint Branch Stream in College Park and creating wetlands in the area of Ken-Gar Palisades Park in Kensington.