Unocal Admits Spillage Into Ocean; Guilty Plea Expected : Pollution: Leakage over 15 years of 8.5 million gallons of oily fluid off Central Coast is now state’s largest.

Unocal Corp. disclosed Friday that it has leaked as much as 8.5 million gallons of petroleum thinner at its oil field near San Luis Obispo in what has quietly become the largest oil spill in California history.

Over the past 15 years, frequent leaks and pipeline breaks at the Guadalupe oil field have contaminated the ocean and ground water with a clear, diesel-like fluid called diluent--at times covering beaches along the Central California coast with an oily sheen.

Unocal, which has been accused of attempting to cover up the spills, is expected to plead guilty next week to as many as three misdemeanor counts of failing to report the leaks and to pay a fine of as much as $1.5 million, said an attorney who is familiar with the case. The plea would settle a 36-count criminal complaint that local authorities filed against the oil giant and six current and former employees.

“Yes, we have problems there,” said Unocal spokeswoman Janet McClintock. “Yes, we take full responsibility for it. We’re going to clean it up--the criminal case has no bearing on this.”

With most of the petroleum thinner underground or washed out to sea, authorities have had difficulty determining the size of the spill. Since the leaks became public in 1990, the company has acknowledged that recovery wells have pumped up about 650,000 gallons of the hazardous fluid.

But on Friday, Unocal announced that three experts hired by the company estimate that 4.6 million to 8.5 million gallons of diluent have leaked into the ground beneath the Guadalupe field. The estimates do not include the fluid that went into the ocean--an amount that may never be known.

Even the smallest estimate of the Guadalupe leak is larger than the infamous Santa Barbara oil spill of 1969, when an offshore oil well blew out and spilled 4.2 million gallons into the ocean and onto beaches.

The estimate of 8.5 million gallons would rank Guadalupe as the fourth-largest spill in the United States. By comparison, the Exxon Valdez spilled 11 million gallons of oil into the waters off Alaska when it crashed onto rocks off Prince William Sound in 1989.

Residents of the Central California coast had long complained that some sort of oily fluid from the Guadalupe field was contaminating beaches and harming dozens of sea lions, seals and other marine animals.

But it was not until 1990 that Unocal first reported the spills and took corrective action, including the installation of a bentonite wall on the beach to block the flow of diluent and wells to pump it out.

Subsequently, the state’s recently created Office of Oil Spill Prevention and Response began investigating the leaks and staged two raids on Unocal offices, hauling away more than 80 boxes of records.

Steve Sawyer, a lawyer for the agency, said the investigation indicated that there had been 190 leaks of diluent at the field, many of which went unreported.

However, the government’s case was weakened when San Luis Obispo County Dist. Atty. Barry La Barbera, who was handling the case, filed the criminal charges against Unocal more than a year after receiving an initial complaint of unreported spills in July, 1992. A judge found that the district attorney missed the deadline for the statute of limitations by two days--a finding likely to send the case into years of procedural litigation.

As a result, government agencies and the oil giant have tentatively agreed to the plea bargain, which would result in the dismissal of 33 counts against Unocal and the employees. Once the criminal case is resolved, however, the Office of Oil Spill Prevention and Response plans to bring a civil suit against Unocal that could result in additional fines beyond those in the criminal case.

Apart from any legal action, Unocal is faced with a massive cleanup job at its aging oil field 20 miles south of San Luis Obispo.

At least 18 plumes of the petroleum fluid have been located underground and in some places the diluent sits 5 feet thick on top of ground water.

In January, diluent was discovered surfacing on the beach beyond the retaining wall that was built to prevent further ocean spills.

Now, the company is proposing an emergency action to install a wall farther inland and excavate the entire beach area--all the way down to the water table--where diluent has been surfacing.

Unocal officials said this would mean removing 47,000 cubic yards of sand from a 1,000-foot length of beach--roughly the equivalent of hauling away three football fields six feet deep.

“We are in an emergency mode to find a solution to the problem,” McClintock said. “We want to excavate the beach all the way down to the ocean.”