The Nankan River in Kumamoto Prefecture was hit by a 5,000-liter oil spill Friday after a nearby food processing plant stocked up on more oil than usual before a gas surcharge was reinstated earlier in the week. Iwamoto Co., based in the town of Nankan, had increased its stores of heavy oil to stock up before higher gasoline and other surcharges were reinstated Thursday by the government after the ruling bloc-controlled Lower House rammed through a related bill. The oil is used as fuel for boilers. The plant has an 8,000-liter oil tank buried five to six meters underground that is usually filled with 5,000 liters, company officials said. After the prospect of reinstating gas surcharges became certain, however, the food processor increased the amount to 7,300 liters, they said. Despite being rated for 8,000 liters, the oil somehow leaked into the soil and then into the nearby Nankan River, local fire officials said, suggesting the tanks may have been improperly installed. The environmental damage has yet to be tallied. Before the gas surcharges were reinstated Thursday, motorists were seen swamping gas stations for cheap, last-minute fill-ups. The binge was so heavy that fire stations nationwide are on special alert for explosions and fires that may result from improper storage of gasoline at home, which is illegal.