In any instance in which a landfill has a useful life of less than one and one-half (1½) years, the Director of the Division of Environmental Quality may authorize any city utilizing that landfill to transport solid waste outside the boundaries of the regional solid waste management district. Provided, however, in no instance shall that authority be extended after a landfill with a useful life in excess of one and one-half (1½) years becomes available within the district for accepting the solid waste of the city.