Amid growing concern that a gasoline ingredient intended to make the air cleaner is now polluting water, a group of well owners from Garden City, New York sued the gasoline industry today, trying to force it to pay to clean contaminated water sources. The ingredient, methyl tertiary butyl ether, or M.T.B.E., has been widely used since 1990, when Congress ordered oil companies to use cleaner burning fuel in major metropolitan areas. The problem is that the additive, a possible carcinogen, can spread quickly through groundwater after gasoline leaks or spills.