A Prince George's County grand jury is investigating whether Maryland hazardous waste handling laws were violated at a Beltsville industrial park where state officials have found the highest levels ever of potentially cancer-causing PCB contamination in Maryland, according to government sources.

About a half-dozen employes of United Rigging and Hauling in Beltsville have testified before the grand jury in the last three weeks, according to DeeWitt C. Sperau, executive vice president of the company, which bought used electrical transformers from Pepco, the federal Department of Agriculture and other companies to reclaim for scrap metals, such as copper.

PCB, or polychlorinated biphenyl, is a man-made chemical widely used as a coolant in electrical equipment before its manufacture was banned in 1977 because it was found to cause cancer, birth defects and liver damage in laboratory animals.

Alarm over pollution at the company was triggered last spring when a Prince George's County health inspector noticed oil pouring out of a storm drain into a creek at the company and sent a sample to a state health department laboratory for testing. The test results showing high levels of PCB prompted state officials in May to obtain a search warrant and enter the site at 6701 Ammendale Rd.

There they found about 750 junked electrical transformers on an asphalt strip in front of the company, according to John Koontz, chief of enforcement for the state Waste Management Administration.

An additional 50 transformers with levels as high as 750,000 parts per million of PCB were found stored in the back yard of the company, Koontz said. Soil test results revealed extensive contamination in the back area with levels as high as 128,000 parts of PCB to every million parts of dirt, officials said.

The U.S. Environmental Protection Agency considers PCB concentrations higher than 500 parts per million dangerous whether they are found in equipment, soil or water, according to Robert Caron, EPA's on-site coordinator.

Nancy Moses, spokeswoman for Pepco, said EPA tests linked 730 of the estimated 800 PCB-laden transformers found at the site to Pepco. Most of those transformers contained less than 50 parts per million of PCB, but about 75 contained hazardous levels, Moses said, including one with 8,460 parts per million of PCBs. No Pepco employe has testified before the grand jury, Moses said, but she declined to say if any company officials had been subpoenaed to testify.

Because of threats to the environment and public health posed by PCB levels found at United, the EPA began a $250,000 emergency cleanup at the site in May.

Removal of the 50 transformers stored there by Electrical Equipment Corporation of Lorton and the grossly contaminated soil is scheduled for completion by the end of next week, Caron said.

Earlier this month, Pepco and United signed a consent order with EPA and the Maryland Department of Health and Mental Hygiene in which the two companies agreed to spend $1.5 million to $3 million to clean up the site and safely dispose of the remaining 750 transformers.

The Prince George's grand jury must decide whether criminal negligence or wrongdoing on the part of any of the companies led to the contamination.

"I will confirm for you that the office is conducting a criminal investigation of the matter," said Maryland Attorney General Stephen Sachs, whose Hazardous Waste Strike Force has successfully prosecuted 36 companies since 1981 and is handling this case. "But the law doesn't permit me to confirm any more than that."

Federal and state hazardous-waste laws contain criminal sanctions against companies that generate, transport or store hazardous waste if the wastes are improperly handled at any point from their inception, or cradle, to their disposal or grave. In Maryland, criminal penalties of up to five years in prison and $500,000 in fines are included for each violation.

Moses, the Pepco spokeswoman, said the company does not believe that transformers it sold to United contained the high levels of PCBs revealed by the government testing. While agreeing to assume responsibility for the cleanup at the site, Pepco is conducting its own investigation to see whether the contaminated transformers were sold to United by government agencies or companies other than Pepco, Moses said.

Burton Davis, president of Electrical Equipment Corp. would not comment yesterday on the transformers that his company stored at United. But Caron, the EPA official, said that EPA had acted to remove those transformers because Electrical Equipment Corp. is financially unable to pay for their disposal.

"We're cooperating fully with Pepco and everyone involved in this to perform the cleanup," said Sperau, the United vice president.

Sperau said his company was unaware that the transformers they bought for metal recycling from Pepco, the Department of Agriculture and two other companies contained PCBs.