The contaminant, listed as a probable carcinogen, appears to be coming from the Orange County Water District. The wells were closed after tests revealed the presence of 1,4-dioxane, often used as an industrial solvent.

It is not related to dioxin, which is known to be a far more toxic chemical. The wells were closed down largely to ease potential public concern. "We did this voluntarily," says Diana M. Leach, general manager of the Mesa Consolidated Water District in Costa Mesa. "We don't want to create a fear factor."

The levels of the chemical found in the wells were small, the highest just below 20 parts per billion. Trace amounts also were found in wells in Huntington Beach, although these were not shut down because the levels were lower than the state's recommended threshold of three parts per billion. By comparison, Orange County Water District officials say that a variety of food and drink contain far higher levels of 1,4-dioxane, including coffee, shrimp and chicken. Additives placed in such items can contain as much as 10,000 parts per billion of the chemical. Household products such as shampoo, cosmetics or baby lotion can contain tens to hundreds of thousands of parts per billion.