Water Quality
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About Water Quality Viewer Data

Data displayed on the Drinking Water Quality Viewer are based on the Water Quality Results Sampling Results dataset and Public Water System Inventory Dataset. Water quality data are linked to approximate geographic locations of community water systems from the Water System Inventory dataset by matching the Public Water System Identification number found in both datasets. Data for these datasets are derived from the State Water Resources Control Board Water Quality Monitoring (WQM) Database, also known as the Water Quality Inventory (WQI), and Safe Drinking Water Information System (SDWIS) database.

Public Water System Inventory Dataset

The California Public Water System Inventory dataset is derived from SDWIS and contains information about all community water systems (CWS) in California. The data set contains one record for each year and CWS that was active for all or part of the reporting period. It includes additional information about how many retail connections a CWS has, how many people are served by each CWS, and the approximate location of each CWS. A CWS is a water system that serves its population year round and is defined as serving a population of 25 people or more and/or at least 15 retail connections. 

California Drinking Water Quality Sampling Results Dataset

The California Drinking Water Quality Sampling Results dataset links information from SDWIS and WQM database to estimate quarterly and annual measures of eight analytes: arsenic, nitrate, uranium, radium, TCE, PCE, atrazine and DEHP. 

Methods for Calculating Drinking Water Quality Measures

Each community water system (CWS) has multiple sampling results for each analyte at different time points and at different locations within the CWS (called sampling stations) each year. All sampling results are averaged by analyte, by year, and by sampling station within each CWS. Then, average concentrations by sampling station are averaged across all sampling stations within each CWS. This results in one average concentration value for each CWS, in each year, for each analyte. Maximum values for all analytes are derived by taking the highest sampling result in each year for each CWS.

Methods for Determining CWS Location

The California Public Water System Inventory Information (WQI) dataset contains information about all community water systems (CWS) in California. The data set contains one record for each year and CWS that was active for all or part of the reporting period. It includes additional information about how many retail connections a CWS has, how many people are served by each CWS, and the approximate location of each CWS.

Coordinates for all water system locations displayed on the map and in the Public Water System Inventory dataset are compiled from multiple available data sources. Data sources are prioritized in order of increasing precision, using coordinates that are likely to be less precise only if coordinates of water systems from higher priority data sources are not available. Ordered below are source data sets containing geographic locations for individual water systems, from highest to lowest spatial precision.  

  1. Service area polygon centroids: Tracking California collects and maintains geographic boundaries that approximate the retail service area of community water systems through our Water Boundary Tool. If a water system has a service area polygon in our Water Boundary Tool data set, the geometric center of the polygon will be used to approximate the location of the water system. Over 80% of public water system locations were approximated from centroids of verified service area polygons.
  2. Geocoded water system address: Contact addresses for each CWS are available in the Inventory data set and the Safe Drinking Water data set. Approximate locations can be derived and verified through geocoding services.
  3. Other: Though unlikely, in the absence of locations from the above data sources, other methods such as principal city served, zip code centroids or county centroids may be used to approximate the location of a CWS.

The locations are provided and intended for visualization purposes, such that the approximate location of the CWS can be described on a small-scale (city, county, state, or regional) map. These locations are not designed to be used in linking to health data; they are only intended for diagrammatic purposes.