After The Times reported last week that nearly 300 drinking water wells and other water sources in California had been contaminated with toxic chemicals linked to cancer, readers wanted to know what they could do. For many, it was the first time they had read about this class of chemicals, called perfluoroalkyl and polyfluoroalkyl substances — or PFAS. We talked to industry experts, and the following are their best answers to some of the most often-asked questions we received.