SummaryNumber of people whose drinking water supply is potentially unhealthy. Whether or not water is unhealthy to drink will depend on the concentrations of specific contaminants as well as synergistic effects of multiple contaminants. General Information about this IndicatorWhat is it?: Californians expect access to clean drinking water. Contaminants may be present in drinking water due to contaminated source waters, incomplete purification, or contamination during movement of drinking water from source to tap.