Joan Mulhern, an environmental advocate and lobbyist for Earthjustice, a law firm that works on environmental advocacy. She was an authority on the Clean Water Act. (Courtesy of Earthjustice/COURTESY OF EARTHJUSTICE)

Joan Mulhern, a forceful advocate for the environment who lobbied Congress and often rallied public support to sway lawmakers to her cause, died Dec. 18 of liver disease at MedStar Georgetown University Hospital. She was 51.

Her death was confirmed by a sister, Marie Mulhern.

Ms. Mulhern had been the senior legislative counsel for Earthjustice, a nonprofit environmental law firm, since 1999. She fought repeated attempts by Congress to limit the scope of the Clean Water Act and battled coal companies and government officials over mountaintop-removal coal mining, in which mountains are blasted away to create strip mines.

In a regulatory world constantly affected by changing political winds, Ms. Mulhern found herself refighting battles that had seemingly been settled decades before. The Clean Water Act was signed into law in 1972, but it was continually modified by rule changes that restricted the law to fewer and fewer waterways.

In October 1999, a federal judge ruled that mountaintop-removal mining operations in West Virginia could no longer dump debris within 100 feet of streams. Within a month, powerful West Virginia Sen. Robert C. Byrd (D) attached an amendment to an appropriations bill to override the judicial ruling by making streams near mountaintop mines exempt from the Clean Water Act.

Byrd mocked environmentalists as “head-in-the-clouds individualists who peddle dreams of idyllic life among old-growth trees,” but Ms. Mulhern taught the veteran senator a lesson in real-world political gamesmanship.

She launched a counterattack that included grass-roots protests, letter-writing campaigns, op-ed essays and Capitol-corridor politicking that turned a spotlight on mountaintop removal, which had been mostly a regional issue. In the end, Byrd gave up the fight and withdrew his amendment.

“Joan did what Joan does best, and that was to loose the dogs of war,” Martin Hayden, Ms. Mulhern’s former boss at Earthjustice, said in an interview. “Joan was in­cred­ibly knowledgeable, extremely smart and politically skilled, and in my opinion was one of the most tenacious and effective advocates I’ve ever seen. She would never back down.”

In December 2008, after a series of regulatory changes during the administration of President George W. Bush allowed operators of mountaintop mines to dump rubble in Appalachian rivers and streams, Ms. Mulhern had seen enough.

“With two months left in power,” she told the New York Times, “the Bush administration is determined to cement its legacy as having the worst environmental record in history.”

She wasn’t any easier on environmental officials after President Obama took office. After regulatory officials blocked only six of 48 mountaintop-removal projects in 2009, Ms. Mulhern pronounced it a “big disappointment.”

“Until they can clearly tell us their policy is to stop the destruction of mountains and streams in Appalachia,” she told the Times, “we cannot support them.”

Joan Marie Mulhern was born Aug. 23, 1961, in Wellesley, Mass., and grew up in Sherborn, Mass. After graduating from the University of Vermont in 1985, she went to work for the Vermont Public Interest Research Group, an environmental and consumer advocacy organization, and eventually became its executive director.

She came to Washington in 1994 to attend Georgetown University’s law school, from which she graduated in 1997. Before joining Earthjustice, she worked for two years at Public Citizen, the consumer rights group founded by Ralph Nader.

Throughout her career, Ms. Mulhern worked closely with many journalists and Capitol Hill staff members. She was on the board of directors of Appalachian Mountain Advocates, a legal group opposed to mountaintop-removal mining, and presented seminars around the country for law students and environmental advocates. She spent a good deal of time in Appalachia and other regions affected by environmental concerns.

“The grass-roots people out there loved Joan,” Hayden said. “They looked at her as their bulwark.”

Survivors include two sisters and a brother.

In 2007, Ms. Mulhern was one of several “eco-heroes” featured in a photo essay in Vanity Fair magazine.

She was known for her wit and for creative ways of making her point on Capitol Hill. One year, instead of handing out an appointment calendar, she distributed a “disappointments calendar,” with pictures of 12 lawmakers with poor records on environmental protection.

At least one congressman featured in the calendar wanted to know how he could get extra copies.