United Rigging and Hauling Inc., which along with the Potomac Electric Power Co. has been part of a year-long state grand jury investigation into allegations of illegal disposal of PCBs in Beltsville, pleaded guilty yesterday to violating Maryland's water pollution laws and was fined $75,000.

Assistant Attorney General Jane Barrett said the fine, levied in Prince George's Circuit Court against the scrap metal firm, was the largest ever against a company for violating the state's water pollution laws.

As part of the plea agreement between the company and the Maryland attorney general's office, United Rigging and Hauling will not face separate state criminal charges related to the disposal of PCBs on its 15-acre site in Beltsville. The company also agreed to "cooperate and testify truthfully" in the grand jury investigation.

The plea leaves Pepco as the focus of what a federal judge last week called an "imminent" indictment by the state grand jury on charges of violating Maryland's hazardous-waste disposal laws. State officials have said the site has the worst PCB contamination ever discovered in the state.

"This is the end of the line in the legal proceedings for United Rigging and Hauling," said Arthur B. Pearlstein, an attorney for the company. "We've said all along that we didn't know anything about PCBs. We're really delighted about what happened."

United Rigging and Hauling pleaded guilty to three criminal charges of allowing oil and grease to seep from its grounds in the Beltsville Industrial Park on Ammendale Road into a nearby stream. Judge William H. McCoullough fined the company $25,000 for each offense.

State police and health employes found the pollution during checks conducted from May 1 through May 4 last year, according to court records. Some areas on the banks of the stream were so coated with oil that they appeared to be black in photographs, court records said.

The grand jury has been investigating since last May whether state hazardous-waste disposal laws were violated by United Rigging and Hauling, where more than 100 electrical transformers -- including 75 from Pepco -- were found to contain dangerous levels of PCBs.

PCBs, or polychlorinated biphenyls, are chemicals used for insulating equipment to prevent electrical fires. The manufacture of PCBs was banned in 1977 because the chemicals were found to cause cancer and birth defects in laboratory animals.

United Hauling and Rigging, a national company headquartered in Beltsville, has claimed that it was told by Pepco that any PCBs in the transformers had been flushed out before the company bought them as scrap metal.

Pepco has maintained that it has complied with federal regulations on the disposal of PCBs, but the grand jury investigation has focused on Maryland hazardous-waste disposal laws. Pepco sought unsuccessfully in federal court in Baltimore to get a ruling that federal laws regulating the disposal of PCBs preempt Maryland laws.

Last week, U.S. District Court Judge Walter E. Black Jr., in his ruling upholding the state's right to regulate the disposal of PCBs, said, "There is little question from the record . . . that an indictment of Pepco for alleged responsibility for or participation in the United Rigging contamination is imminent."

United Rigging and Hauling and Pepco signed an agreement last year with the U.S. Environmental Protection Agency and state officials to clean up the PCB contamination. The cleanup was finished in December.