Massive ships spilling sheets of oil across the sea might make for dramatic photo ops — yet the vast majority of oil pollution in North America comes not from leaking oil rigs or troubled tankers, but rather from thousands of small, diverse sources, most of them on the land, according to a new report by the U.S. National Academy of Sciences. These non-point pollution sources are estimated to equal an Exxon Valdez-scale spill — 10.9 million gallons — every eight months. In total, non-point sources, which range from oil-streaked streets to lawn mowers to personal watercraft, account for about 85 percent of the 29 million gallons of marine oil pollution in North America every year. Meanwhile, spills from oil transport vessels totaled less than 250,000 gallons in 1999, down from more than 6 million in 1990, largely thanks to substantial tightening of environmental regulations following the 1989 grounding of the Exxon Valdez. Non-point pollution sources are far harder to regulate and at least as damaging to the environment, because runoff often carries oil into sensitive bays and estuaries.