Some 250,000 gallons of oil were spilled Tuesday into the waters off Rio de Janeiro, Brazil, the worst ecological disaster to hit the region in 25 years. The oil, which poured out of a ruptured pipeline running from a nearby refinery operated by the state-owned oil company Petrobras, had spread across a 24-square-mile area by yesterday afternoon, turning miles of shoreline black and contaminating a protected mangrove swamp. Environmental officials blamed Petrobras for poor maintenance of its old pipelines. The spill could have a deleterious impact on cranes, crabs, mussels, oysters, and several species of small fish.