It was 1997 when Californians began to worry in earnest about a chemical called perchlorate. For decades, the ingredient in jet fuel had been seeping from missile factories and rocket testing sites into groundwater across the state and, thanks to a new testing method, it was suddenly clear it had reached hundreds of drinking water wells. Soon, researchers discovered that the toxic chemical had reached Lake Mead, the picturesque reservoir that supplies water to 25 million people in the American Southwest and irrigates the fields that grow the lion’s share of the nation’s winter produce.