That humble staple of the fast-food industry - the french fry - is more dangerous than it looks. A recent study by the non-profit Columbia Basin Institute found that fry-makers in the Columbia River Basin waste cheap water and poison residential wells. The 100-page report, Value Added and Subtracted, says fry-makers use only half of each potato and spread more than 3 billion pounds of potato sludge over fields in Washington and Oregon each year. The waste breaks down and releases nitrates, which are at unsafe levels in 30 percent of residential wells along the Columbia, the report says. In addition, the industry leaves its mainly Mexican-American workforce seasonally unemployed while it receives federally subsidized water and state development tax credits. The report calls for companies like McDonald's to take the lead in planning local programs to reduce pollution and ensure employee benefits. It also suggests increasing the price of water from dams to encourage conservation instead of subsidizing the multimillion-dollar fry industry. To receive the report, or a 10-page summary, write the Columbia Basin Institute, Box 3795, Portland, OR 97208 (503/222-6541).