Joan Mulhern, an aggressive advocate for Clean Water Act enforcement, died yesterday after a long illness at Georgetown University Medical Center. She was 51.

As senior legislative counsel at Earthjustice, Mulhern was known for her battles for tougher protections for wetlands and for her opposition to mountaintop-removal coal mining.

"For the Earthjustice family, it's a huge hole. And for the movement, it's a huge hole," Earthjustice Vice President Martin Hayden said this afternoon. "She brought determination and skill that I don't think we've seen the likes of."

Hayden recalled one of Mulhern's first fights after joining Earthjustice in 1999. After Sen. Robert Byrd (D-W.Va.) pushed for a spending rider to block legal action against a coal mining project in his home state, the Clinton administration was reluctant to fight the powerful appropriator -- until Mulhern got involved.

"And Joan did what Joan did best, and that was loosen the dogs of war, in a way only Joan could do," Hayden recalled in an interview. "By the middle of the next week, the Clinton administration was opposing the rider. And this is when Joan was just getting started."

Mulhern also served on the board of directors of Appalachian Mountain Advocates, a group of attorneys who fight mountaintop-removal permits and pursue pollution-related lawsuits. She was extremely well-known in the Appalachian coal fields.

"She was a forceful and tenacious advocate who worked tirelessly to stop mountaintop-removal mining and defend and strengthen the Clean Water Act," said the Sierra Club's environmental quality director, Ed Hopkins. "We will miss Joan's commitment to achieving environmental justice for communities harmed by pollution."

Mulhern graduated from the University of Vermont and had a law degree from Georgetown University. Before Earthjustice, she worked for the Vermont Public Interest Research Group and at Public Citizen, where she fought the tobacco industry.

Coby Dolan, legislative director and general counsel for Rep. Debbie Wasserman Schultz (D-Fla.), called Mulhern a "mentor, friend, legend." When undecided about what action to take on any issue, he said, he asks, "What would Joan do?"

"So much of what I know about [Capitol Hill] I learned from her. So much of what I know about protecting the environment I learned from her," Dolan said, adding that Mulhern taught him to "never back down from a fight but to do it with grace and kindness."

Mary Frances Repko, an aide to House Democratic Whip Steny Hoyer, said, "All of us who have worked on the Hill with Joan have been privileged to do so and will miss her profoundly.

"Through five congressional bosses, and both before and during her time at Earthjustice, I have sought Joan's counsel and been inspired by her deep commitment to ensuring that Americans have clean and safe water," Repko said.

"Joan was a tireless advocate for clean water and brought an unmatched passion and commitment to everything she did," said Bettina Poirier, Senate Environment and Public Works Committee staff director and chief counsel. "She was one of the best, she will be missed."

Mulhern got national notice in 2007 when Vanity Fair featured her in a photo essay it called "Eco Heroes." She was appeared alongside luminaries like Robert Kennedy Jr. and former U.S. EPA chief Carol Browner.

"In Congress when money and power calls the shots, Joan came in with a strong sense of right and wrong," Earthjustice campaign manager Liz Judge said. "She took the plight of the communities at the foot of [mountaintop-removal mine] sites deeply to heart."

Hayden said he couldn't recall any effort to weaken the Clean Water Act that slipped past Mulhern.

"I don't think anything made it all the way to the goal line that Joan put herself in the way of," he said.

"The bottom line on Joan," he added, "is there was no more dedicated and tenacious advocate for the underdog. Period."

Mulhern, who grew up in Sherborn, Mass., and was a lifelong Red Sox fan, is survived by two sisters and a brother.