Drinking Water Contaminants

The drinking water contaminant index is calculated based on the concentration of 13 different types of contaminants commonly found in drinking water. Each census tract is given a percentile score for each type of water contaminant, ranking it compared to other census tracts. A census tract’s percentile scores for each contaminant are then added together to give a total drinking water contaminant index for each census tract.

Over 95% of Californians receive their drinking water from public sources, but water quality can vary greatly across geographic areas. Water can become contaminated with chemicals and bacteria through both natural causes like rocks and soil or human causes like factories, sewage, and farm runoffs. The California Office of Environmental Health Hazard Assessment (OEHHA) tests drinking water for 13 contaminants and 2 types of water quality violations. Prevalent contaminants in California groundwater include nitrate, perchlorate, and arsenic, which can cause birth defects and lung and bladder cancer. Although drinking water contaminants can have adverse health effects, California water systems generally have high rates of compliance with accepted standards.

OEHHA measures drinking water contaminants on a census tract level, and census tracts sometimes contain multiple public drinking water systems and groundwater resources. As a result, the drinking water quality variable may not reflect the water that an individual within a census tract is drinking, but rather the average water quality across the census tract.