One knotty issue is the presence in the water supply of residues from prescription drugs, personal care products like sunscreen or shampoo, or other substances that scientists call “emerging contaminants.” The E.P.A. says researchers have found these compounds in water almost everywhere they have looked. They attribute this to improved sensors as well as greater prevalence.
Many of these substances seem impervious to water treatment regimes. But are they a problem for people? So far, that question remains unresolved.