The Washington firefighters' union officially declared its contract negotiations with the city deadlocked yesterday, throwing the issue into mediation and then possibly into binding arbitration.

A letter asserting that the talks between Local 36 of the AFL-CIO International Association of Firefighters and the city negotiators have reached an "impasse" was delivered yesterday to Mayor Marion Barry by William E. Mould, the union president.

The letter represents the first time negotiations have broken down since the collective bargaining provisions of the city's Merit Personnel Act were invoked earlier this year.

Mould told the mayor that after seven months of negotiations with city representative Donald Weinberg, only seven of 48 issues have been resolved, some of them of limited scope. He also contended that Weinberg was pleading with the union to accept a wage increase of under 5 percent just before the mayor granted a 5 percent rise, plus a 2 percent bonus, to nonunion city employes.

Union vice president Ken Cox said the law provides for a mediator now to attempt to resolve the dispute. If he does not do so within 30 days, a three-member arbitration panel would hold hearings and recommend a binding solution.

There was no immediate comment from city officials.