The spread of the chemical pollutant PCB to human and animal food in 17 states was spotted "just by chance" and could have gone unnoticed under current government procedures, a Department of Agriculture official said yesterday.

Assistant Secretary Carol Tucker Foreman told a House hearing that bureaucratic routine and slipups accounted for the six-week delay between the taking of a food sample that proved to be PCB-loaded and the day that the Food and Drug Administration was notified of the contamination. But "even under the best of circumstances" it would have taken three to four weeks, she said.

The FDA is still tracking down shipments nationwide of cake mixes, waffle mixes and other products made with eggs that might have contained PCB from a Montana animal feed plant, according to acting FDA commissioner Sherwin Gardner. He said he was "reasonably certain" that none had escaped the FDA net but would not know for sure until all the reports are in and certified in another 30 days.

PCB is polychlorinated biphenyl, a possibly cancer-causing chemical that has been linked to skin and liver disease, bone problems and other ailments. Banned in 1977, it previously was used as a lubricant and high-temperature coolant and continues to exist in older electrical transformers and industrial machinery.

It was such a transformer, apparently damaged by some kind of farm machinery or loading vehicle as it sat in a machine storage shed, that leaked about 200 gallons of oil containing PCB at the Pierce Packing Co. of Billings, Mont., according to the FDA.

The oil went into a drainage system that was routinely cleansed of fats for inclusion in animal feed. Foreman told the hearing that 399,000 pounds of affected poultry and 16,000 pounds of fresh pork have been located and held off the market. The FDA and USDA have also tracked the chemical to 73,000 pounds of egg products, about 52,000 pounds of which have been withheld. The rest is being traced.

The Agriculture Department monitoring system spot checks animal carcasses and is designed to catch trends over a year-long period, Foreman said. "It was just by chance that we picked up this single incident relatively quickly," she added. "It is also possible that a single incident of this size could go entirely undetected."

She said there are 120 million head of livestock and 3.5 billion poultry marketed each year. "The cost of testing all of them for [dangerous chemical] residues would be about $100 billion," she continued. Further, she said there are some dangerous chemicals like the cancer-causing nitrofuranes, for which no residue-detecting test exists.

The PCB in this incident was found first in a chicken taken routinely from a processing plant in Provo, Utah, on July 6, Foreman related. But the USDA veterinarian who would have mailed the sample to be tested was on vacation and his replacement did not know a sample was waiting, so the mailing was delayed for a week.

On Aug. 3, the USDA inspection office in Alameda, Calif., was told the sample contained PCB, but waited until receiving written confirmation Aug. 8 before notifying the Utah office. That office traced the hen to the Pierce plant by the 10th but did not notify the FDA until Aug. 16, six weeks from the day the sample was taken.

"We should have far more sensitivity," Foreman said. "There should have been earlier and more complelling notification." She said corrective measures are being taken at the Department of Agriculture, but warned, "We have to realize that as long as we use chemicals in agriculture. . . accidents will occur."

Idaho Gov. John V. Evans objected in his testimony to "lapses . . .slipshod procedurs. . .inexcusable delays" in government agency performance in the case. He recommended improved communication, technical and laboratory facilities and called for some sort of indemnification program for citizens damaged by future pollution problems.

"We've got to eliminate PCBs completely," he said. 0ut the subcommittee chairman, Rep. Bob Eckhardt (D-Tex.), said that would be "a massive and extremely difficult, costly thing." An Environmental Defense Fund witness, Jacqueline Warren, told the hearing there are still 275 million gallons of PCB oils in transformers in use.

'Rep. Andrew Maguire (D-N.J.) asked whether it would be possible to ban PCB at least in the nation's 125,000 food processing plants. Foreman and Gardner said they thought that would be "feasible."