Crews are mopping up about 31,000 gallons of heavy fuel that spilled into the Arthur Kill Monday.The spill happened as the oil was being transferred from a barge to the Chevron plant in Perth Amboy. About 16,000 gallons of oil and water have already been collected, but cleanup is expected to continue into Wednesday morning. Chevron has taken responsibility for the spill and is investigating the cause.This isn't the first oil spill in the Arthur Kill and environmentalists say they fear it won?t be the last. They say even thought the area is already polluted, it's filled with migratory ducks, birds and fish and they want to make sure no more harm is done.