Apples at a Washington area Safeway store were found to contain residue of a possibly cancer-causing chemical, even though the chain had pledged not to market apples with the substance, a private lab disclosed yesterday.

Alar, the trade name for the chemical daminozide, was found on Red Delicious apples in one out of five stores sampled. Alar levels averaged 0.8 parts per million on the sample of 10 apples, well below the legal limit of 20 parts per million.

Slightly higher levels were found in four out of five Safeway stores in Sacramento and two out of five stores in Los Angeles. No Alar residue was found on apples sampled in the chain's San Francisco stores.

NutriClean, an Oakland-based company, coordinated the testing, but the chemical analyses were run by the National Food Laboratory, an independent lab in Dublin, Calif. An NBC television affiliate in Chicago, preparing a news story, had contracted with NutriClean to perform the tests.

Felicia del Campo, manager of public affairs at Safeway's corporate offices, said she didn't know whether the test results are "factual," but said, "We are nevertheless concerned."

Del Campo said the chain has written guarantees from all of its apple suppliers stating that they do not use Alar on apples sold to Safeway distributors. Since the disclosure of the findings, the chain has been reminding suppliers of the policy, she said. Approximately 90 percent of Safeway's apples are purchased from growers in the State of Washington.

In July 1986, the Oakland-based supermarket chain agreed to stop selling apples treated with the chemical, which is used to prevent premature fruit drop and to produce a redder, harder product.

Other major supermarket chains, including Giant Food, as well as several food manufacturers, agreed to stop purchasing apples from suppliers who used Alar. Safeway's apples sold in the Washington area are purchased from a variety of growers.

Consumer rights advocate Ralph Nader, who led the campaign urging companies to stop using Alar, said yesterday that Safeway had "betrayed the public trust."

The Environmental Protection Agency proposed banning Alar in 1985 after tests indicated that it caused cancer in laboratory animals. The agency backed down from its proposal after industry groups and an advisory panel of scientists argued that the tests were faulty. The chemical is under review, pending the submission of new test results from Uniroyal, the chemical's manufacturer.

Stanley Rhodes, president of NutriClean, would not disclose the Washington area Safeway whose apples were found to contain Alar residue.