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.