Water Quality Percentile: The percentile of a given area’s average annual concentration of 13 common water contaminants across drinking water system boundaries in LA County
For more information on the California Office of Environmental Health Hazard Assessment’s methodology, visit the CalEnviroScreen homepage.
California Office of Environmental Health Hazard Assessment – CalEnviroScreen 2.0 (2014), CalEnviroScreen 3.0 (2017), CalEnviroScreen 4.0 (2021)
Data collected from 2009-2011 and published in 2014
Data collected from 2012-2014 and published in 2017
Data collected from 2017-2019 and published in 2021
Why are these variables important to measure?
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.
Faust, John et al. “Update to the California Communities Environmental Health Screening Tool: CalEnviroScreen 3.0.” Office of Environmental Health Hazard Assessment & California Environmental Protection Agency, January 2017. Link
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