FLINT, MICH., OCT. 22 -- Warren E. Barge Jr., a key figure in a federal probe of District contracts, was convicted today on 16 counts of mail fraud for allegedly supplying water instead of chemicals to the General Motors Corp. and paying three GM officials at least $18,000 in bribes.

Assistant U.S. Attorney Robert W. Haviland said the evidence showed that GM paid Barge $277,000 in 1982 and 1983 to provide industrial chemicals for use in removing paint from spray-painting booths at the Chrysler truck-assembly plant in Flint, Mich. While Barge provided some legitimate chemicals, Haviland said, he "slipped in" barrels of water or weak solutions of water and chemicals, depriving GM of $129,714.

Haviland said the three GM employes who received bribes from Barge -- the plant engineer, the plant supervisor for maintenance, and a third employe -- were not charged because the government could not prove that they knew Barge's materials were faulty.

Barge, 43, of Burton, Mich., said after the jury returned its verdict that he had not received a fair trial and would appeal the conviction. "I did not defraud them," he said.

Barge was an unwitting partner in a 17-month FBI undercover operation that centered on the award of District contracts. Barge has acknowledging giving David E. Rivers, former director of the Department of Human Services, two pairs of expensive cowboy boots but said the boots had nothing to do with the award of $133,000 worth of contracts to a firm in which he and an undercover FBI agent were partners.

Federal authorities are investigating whether the boots were part of a bribe in return for the contracts. Rivers has denied any wrongdoing.

Haviland said that Barge's indictment on the charge of defrauding GM was not an attempt to pressure Barge to cooperate with the investigation of District contracts. But he suggested that the indictment may have been delayed so as not to jeopardize the FBI's undercover investigation in Washington, which ended May 22.

Haviland said the evidence showed that Barge gave one GM employe $6,000 and carpeting for his house. He said a second employe received two payments totaling $9,000 from Barge, in addition to other payments of smaller amounts. He said Barge gave a third GM employe $3,300 worth of savings bonds. In addition, Haviland said, Barge paid for 10 trips by two of the GM officials, including free vacations at Barge's Hilton Head, N.C., vacation house.

Haviland sought to show that Barge gave the gifts and payments to GM officials in an attempt to influence their decision to award him business and to ignore complaints about his product's quality.

Barge faces a maximum sentence of five years and a $5,000 fine on each count.