In a Dec. 17 letter, the Wild Equity Institute claims the federal agency failed to perform its required duty of granting or denying the group's petition against PG&E's Gateway Generating Station.
The lack of response puts an endangered butterfly species found only in the Antioch waterfront area in danger, said Laura Horton, Wild Equity's staff attorney.
"We're trying to get them to actually respond to the petition," Horton said. "We're trying to protect this endangered butterfly and get these agencies to do what is right."
The notice of intent gives 60 days for the agency to remedy the situation, or the group will file a lawsuit in U.S District Court of Northern California, she said.
Wild Equity argues that the permit does not address the authorization needed from the Fish and Wildlife Service for "incidental take" of species affected by pollution from the Gateway plant.
The Bay Area Air Quality Management District approved the proposed permit, issuing it in late October.
An EPA spokesman said this week the agency has not received the notice, or any legal filings, and could not comment on it. He added the agency will review them when the documents are available.
This week's action is the latest in a series of legal notices over the past two years against the PG&E plant.
The group has also targeted four other power plants -- all located within a mile of each other on the Antioch-Oakley border.
Wild Equity officials say the emissions jeopardize the Lange's metalmark butterfly, which is found only at the Antioch Dunes National Wildlife Refuge, by helping invasive weeds grow and smothering its food supply.
The recent legal issue is a little different from past notices but related to the same butterfly protection campaign, Horton said.
PG&E officials did not response directly to the notice but reiterated this week that Gateway, which started operating in early 2009, is state-of-the-art and emits extremely low levels of nitrogen oxides while being in full compliance with limitations set by its clean air permits.
It also has a voluntary Safe Harbor Agreement for 12 acres of dune habitat on its property for the butterfly and plant species, has created a vegetation management plan and holds twice-a-year weed control days.
The Antioch shoreline refuge is the only place in the world where the bright reddish-orange Lange's metalmark is found. Their numbers have dwindled from more than 25,000 early in the 20th century to 2,500 in the late 1990s and as low as 32 in 2011. There were 78 tallied this fall.
Contact Paul Burgarino at 925-779-7164. Follow him at Twitter.com/paulburgarino.