After reexamining the impact a Purple Line could have on springs in Chevy Chase and Rock Creek Park, the U.S. Fish and Wildlife Service has found that the light-rail line’s construction would not harm an endangered species living nearby.

The finding — outlined in a Jan. 7 letter to the Federal Transit Administration — could spark a lawsuit over Maryland’s plan to build a 16-mile Purple Line between Montgomery and Prince George’s counties.

John M. Fitzgerald, an environmental lawyer, said the nonprofit Center for Sustainable Economy and some Chevy Chase residents are preparing a “notice of intent to sue.” He said a lawsuit would argue that the federal government hasn’t done enough to protect two rare, tiny crustaceans: the Hay’s Spring amphipod, which has been listed as a federally protected endangered species since 1982, and the Kenk’s amphipod, which is a candidate for federal listing.

Fitzgerald said the agency’s finding — that the shrimplike creatures would be a safe distance from the project’s construction or rely on water unaffected by it — didn’t consider impacts from new development that would follow a rail line, including near downtown Bethesda and Chevy Chase Lake.

Adding people to those communities would increase the likelihood of sewer overflows into local streams, he said. Replacing trees cut for the rail line and new development with asphalt also would send more rainwater rushing into streams, he said. That could undercut the stream bank that supports the one spring where the Kenk’s amphipod lives, he said.

Map of the proposed Purple Line

“You have to look at the overall impacts,” Fitzgerald said. “You have to plan, not just hope, for the recovery of these species.”

Concerns about the little-known crustaceans surfaced in November, after Fitzgerald and other environmental activists said the Maryland Transit Administration had erred in leaving the two amphipods out of the Purple Line’s final environmental analysis.

Fitzgerald said the animals are significant because they survive only in particularly clean and cool water, signaling the quality of the groundwater springs that feed Rock Creek.

Some Purple Line advocates have questioned the focus on an endangered species as an attempt to block or delay the project. They say the localized impacts of a transitway’s construction would pale in comparison to its overall environmental benefits, such as curbing traffic growth and focusing new development around rail stations.

In the letter, the Fish and Wildlife Service said the closest Hay’s Spring amphipod habitat in Rock Creek Park is in the District, about 41 / 2 miles downstream of where Purple Line trains would cross Rock Creek on a bridge. That distance is too far to affect the amphipod, the agency said, and any changes in the flow or quality of water in Rock Creek wouldn’t harm it.

The Kenk’s amphipod has been found within a quarter-mile of the Purple Line alignment, in a spring south of Coquelin Run near Chevy Chase, the agency said. However, the spring is 40 feet above the stream and is not fed by the groundwater sources that would be near the line’s construction, the letter said.

The Maryland Transit Administration is awaiting federal approval of the final environmental study. After that approval, which officials say is expected any day, lawsuits could be filed within 150 days.

State officials have said they hope to begin building the $2.2 billion line in 2015 and open it in 2020 as a way to provide faster, more reliable east-west transit between Bethesda and New Carrollton. The project does not have full construction funding.

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