Water can also become contaminated after it leaves the treatment plant but before it reaches your faucet, says Shane Snyder, Ph.D., co-director of the Arizona Laboratory for Emerging Contaminants and professor of chemical and environmental engineering at the University of Arizona. "Water may sit in a tank, sometimes for over a week, mixing with the chlorine used for disinfection — which may result in elevated levels of disinfection by-products," he says. The consequence: chemicals that might be harmful.
Source: Good House Keeping