When a therapy dog refused to drink at a San Diego grade school, it was the first clue that something was wrong with the water. Tests revealed why the pup turned up its nose — the presence of polyvinyl chloride, the polymer in PVC pipes that degrade over time. But further analysis found something else that had gone undetected by the dog, the teachers and students of the San Diego Cooperative Charter School, and the school district: elevated levels of lead.