A major gas pipeline company accused of dumping cancer-causing polychlorinated biphenyls (PCBs) in 89 earthen pits along a 10,000-mile transmission route has agreed to pay a $15 million fine plus cleanup costs expected to reach $400 million, the Environmental Protection Agency said yesterday.

The agreement by Texas Eastern Transmission Corp. represents the highest penalty and cleanup settlement negotiated by the EPA, officials said.

According to the EPA, Texas Eastern disposed of PCB-laden liquids in 14 states from Texas to New Jersey, leaving soil contaminated at levels as high as 240 times the agency's safety guideline. Deposits of the banned chemical in mostly rural areas were found within 200 feet of residential drinking wells.

"The contamination was extensive," said Frederick F. Stiehl, an associate enforcement counsel for the EPA, explaining the record fine. "This is no mom-and-pop operation. They were fully capable of understanding what the law requires."

Texas Eastern spokesman Fred Wichlep said the agreement by the Houston-based company, one of the nation's largest pipeline distributors of natural gas and which last year had $1.3 billion in profits, does not represent an admission of wrongdoing. It was intended, he said, to "expedite a settlement and avoid the uncertainties of litigation."

The dumping "was not intentional," said Wichlep. "It was a problem created in the natural course of doing business" in which PCBs were used by Texas Eastern and other pipelines as a fire retardant.

But company executives have also admitted that the company discharged PCBs into earthen pits long after it pledged to stop.

Yesterday's announcement of an "agreement in principle" with Texas Eastern, to become a consent decree in federal court, completes the first phase of a wider EPA investigation into PCB dumping by the nation's 13 interstate pipeline companies. Stiehl said two or three other firms have released PCBs into the environment, but none as extensively as Texas Eastern.

The EPA banned disposal of PCBs in 1978 after tests on laboratory animals turned up evidence of tumors and reproductive disorders.

Texas Eastern began using PCBs in the 1950s as a lubricant to reduce the risk of fire in pipeline equipment. When liquid built up in the transmission system and blocked the flow of gas, it was purged from the lines by pressure from compressor stations. The residue, containing PCBs, was released into unlined ponds, some as deep as 17 feet and as wide as two acres, Stiehl said.

According to Wichlep, the company stopped using PCBs in 1977. But chemical residues remained in compressor crankcases and continued to taint pipeline liquids.

Texas Eastern and other companies whose pipelines were found to be contaminated by PCBs agreed in May 1981 to remove the chemical and dispose of it in landfills licensed to handle hazardous waste. Fifteen months later, after discovering high levels of PCBs in Texas Eastern's equipment, the EPA agreed to waive fines of $160,000 in return for a company pledge to properly dispose of the chemical.

A Texas Eastern executive testified to Congress last March that the company continued to discharge pipeline liquids, including PCBs, into the earthen pits until 1984.

There were 89 disposal pits along the pipeline route, which originates in the gas fields of Texas and Louisiana and fans out in a northeasterly direction across Arkansas, Mississippi, Alabama, Tennessee, Kentucky, Illinois, Indiana, Ohio, Missouri, Maryland, Pennsylvania and New Jersey.

An EPA list of 78 dump sites shows generally low levels of PCB in soil, stream sediment and runoff deposits. But several areas registered levels exceeding the EPA's cleanup standard of 25 parts per million, including Grantville, Pa., where soil concentrations reached 5,900 ppm.

Most of the contaminated sites are in remote areas far from homes, and they pose no immediate public health risks, the EPA said. In Lambertville, N.J., the EPA found 74 ppm in the soil within 200 feet of a residential drinking-water well.