Spontaneous combustion was believed to be the cause of a small fire Sunday at Pine Tree Landfill in Hampden. Pine Tree employees called the fire department at 5:15 p.m. after noticing smoke on the east side of the landfill, where a fire had started underneath a capped section of waste, Hampden fire Lt. Frank Coombs said Sunday. A combination of the recent rain and decomposing waste was believed to have started the fire, he said. Fire crews ran hoses about 350 feet up the side of the landfill and smothered the fire with 5,000 gallons of water, Coombs said. After about an hour, firefighters left the scene and landfill employees covered the area with a layer of clay, he said.