Given the city's past, and that the missile was not armed, authorities were not necessarily surprised or alarmed by the discovery; school officials say they believe the missile was left over from an old show-and-tell program. But that doesn't mean missile have a benign history in Rialto, a city of nearly 100,000 people 60 miles east of Los Angeles. For missiles, armed and not, contain rocket fuel, and within rocket fuel is perchlorate.

The chemical allows combustion and has many uses, from space shuttle rocket boosters to road flares. But it also has been proved to inhibit the production of growth hormones in fetuses and young children. At high levels, it can cause thyroid cancer.

Officials have found perchlorate in 319 wells in California, but nowhere is harder hit than Rialto. The city relies solely on groundwater for its drinking supply, while other California communities have alternate sources. Five of Rialto's 15 wells have been closed because of perchlorate contamination.

The timing couldn't be worse. Drought is lowering water levels, and water is being drained from the system for the construction of a new freeway into town.