The Environmental Protection Agency says that more than a quarter of the nations? one million underground gasoline and oil tanks leak. And since Valdez ran aground, smaller oil barges have been responsible for two of the three worst U.S. spills. The General Accounting Office says an average of 16,000 small oil spills seep into waterways each year half of them during loading or unloading operations. In more recent years, the GAO estimates that 46 million gallons have spilled per year, more than four times the Valdez spill.