TROIS RIVIERES, Quebec (AP) _ Environmental officials worked Wednesday to contain a spill of about 1,768 gallons of oil laced with toxic PCBs upstream from the city's supply of drinking water.

''We still don't know how serious it is for the drinking water in Trois Rivieres,'' said Yvon Gelinas, spokesman for the provincial Environment Department.

He said the oil leaked into the St. Maurice River on Tuesday while it was being transfered at the Consolidated-Bathurst Inc. pulp and paper plant. Trois Rivieres, a city of 53,000 people, is midway between Montreal and the city of Quebec.

''We're trying to recover as much oil as we can from the river and put together an evaluation of the impact on the environment,'' Gelinas said.

PCBs, or polychlorinated biphenyls, are used mainly as a coolant in electrical transformers. Their production has been banned in Canada.

In August, a fire at a warehouse storing PCB-laced oil forced the evacuation of 3,300 people from a Montreal suburb.