Columbia First Bank, an Arlington-based savings and loan, said yesterday that it will increase its reserve against potential loan losses by nearly $8 million over the next year because of increasing uncertainty in the Washington real estate market.

Columbia First set aside an additional $2.1 million for troubled loans for its fiscal third quarter ended June 30 and will add $3.3 million more before Sept. 30 if needed. It plans to increase the fund by $2.4 million in 1991.

The increased loan-loss provision was a key factor in the 92 percent decline in fiscal third-quarter earnings that Columbia First reported yesterday. It said its profit totaled $44,000 (1 cent a share) for the quarter, compared with $533,000 (19 cents) in the same quarter a year ago.

The loan-loss provision offset higher interest income and reduced expenses. In addition, the 1989 third-quarter results included a $3.4 million gain from the sale of Federal Home Loan Mortgage Corp. stock.

The third-quarter loan-loss provision had less of an impact on nine-month earnings, which rose more than four-fold on the strength of higher interest income, lower expenses and a $2.4 million gain from the settlement of an insurance claim and litigation.

Columbia First earned $2.4 million (81 cents) in the first nine months, compared with $465,000 (17 cents) in the same period a year ago.

The thrift's nonperforming loans -- past-due loans or those on which it is not being paid interest -- increased to $18 million from $5.6 million over the last year, reflecting the area's real estate slowdown.

"Our problems don't appear to be of the depth of larger institutions in the area," said a spokesman for Columbia First. "But we are starting to see an increase in nonperforming loans." The troubled loans are largely secured by real estate or are unsecured loans to individuals in the development community, the spokesman said.

Columbia First stock closed at $5 yesterday, down 25 cents a share.