waterbycounty
Compiled federal source snapshot

California Water Quality

Drinking water data for all 58 counties.

Avg SDWIS Compliance Index

44.8

Most Common Five-Year SDWIS Compliance Band

F

Counties with Data

58

of 58 total

County water atlas

California water signals by county

A state-level 2.5D view across drinking-water compliance, watershed impairment, monitoring density, and streamflow snapshot context. Pin any county, switch layers, then use the lens controls to compare index ranges, recorded violation rates, or impaired watersheds without leaving the page.

Counties

58

Avg SDWIS index

44.8

Watersheds

58

ATTAINS counties

Monitoring

58

44 fresh USGS snapshots

State atlas layers combine EPA SDWIS health-based violations, EPA ATTAINS 303(d) impairment assessments, EPA Water Quality Portal monitoring sites, and representative USGS NWIS streamflow gauges. Streamflow values appear only for fresh pipeline snapshots, not as a continuously updated stream. County pages include the observation time and year.

Prepared by ByCounty Editorial TeamPublished by WaterByCounty

Multi-source coverage in California

Beyond Drinking Water

EPA SDWIS

58/ 58

counties with drinking-water compliance data

2,560 health violations statewide (5yr)

EPA ATTAINS

70.4%

avg impaired across 58 counties

1,815 of 2,638 assessed bodies impaired

EPA WQP

4,062

monitoring sites across 58 counties

1,199,491 total readings (5yr window)

USGS NWIS

44

counties with a fresh pipeline snapshot

3 above38 below

State atlas notes

What stands out in California

County water quality is not one number. The strongest read comes from comparing drinking-water compliance against watershed impairment, monitoring density, and streamflow context. Use these signals as a starting point, then open any county profile for source-level detail.

Compliance spread

San Francisco County has the highest value in the published state SDWIS Compliance Index table at 86.2/100, while Madera County has the lowest at 7.0/100. That is a 79.2 point gap inside one state.

Zero health violations

1

3+ health violations

51

Watershed pressure

The atlas impairment layer points to counties where assessed water bodies are most likely to miss state quality standards. Assessment density varies, so compare the percentage with the number of assessed bodies on the county page.

Imperial County100.0%Kings County100.0%Sutter County100.0%

Monitoring hubs

San Diego County266 sitesRiverside County247 sitesSan Bernardino County230 sites

Lowest flow reads

Kern County0%Orange County0%San Benito County0%

Highest fresh streamflow pipeline snapshots: Shasta County (137%, observation 2026), Sacramento County (136%, observation 2026), Mono County (124%, observation 2026). These are source-data observations, not household tap-water conclusions.

All California Counties

CountySDWIS Compliance Index
San Francisco County86.2
Alameda County72.3
Orange County72.3
Placer County71.2
San Mateo County70.7
Sacramento County69.3
Marin County68.8
Los Angeles County68.5
Santa Clara County67.4
Riverside County66.5
Santa Barbara County64.1
Ventura County64.1
El Dorado County62.9
Yuba County60.7
Nevada County60.1
Sutter County58.6
San Diego County58.3
Glenn County57.6
Solano County57.3
Fresno County55.2
San Bernardino County53.6
Butte County52.9
Calaveras County49.5
Santa Cruz County49.3
Humboldt County48.1
Lassen County48.1
Imperial County47.6
Contra Costa County47.1
Sonoma County46.4
Napa County45.2
Yolo County44.8
San Joaquin County44.6
Kings County43.1
Amador County42.8
Merced County42.1
Stanislaus County42.0
Shasta County41.5
Mendocino County40.5
San Benito County40.5
Tuolumne County35.8
Lake County34.4
Del Norte County33.2
Mariposa County31.6
Kern County30.3
Monterey County28.9
Mono County25.2
San Luis Obispo County23.8
Tulare County23.6
Plumas County22.3
Sierra County21.1
Modoc County19.9
Siskiyou County19.7
Colusa County17.3
Alpine County13.1
Tehama County11.1
Trinity County10.4
Inyo County9.1
Madera County7.0

Frequently Asked Questions

Which California county has the highest SDWIS Compliance Index?
San Francisco County has the highest SDWIS Compliance Index in the published California cohort at 86.2/100. This WaterByCounty relative comparison is not an EPA score and does not establish current conditions for a utility or address.
What does the Five-Year SDWIS Compliance Band mean?
The band is derived only from each county aggregation's recorded health-violation rate per 100,000 people served for the stated five-year source window. It is separate from the relative SDWIS Compliance Index and is not a current determination for a specific utility or address.
What does EPA ATTAINS report across California?
Across 58 published California county records with EPA ATTAINS data, 1,815 of 2,638 assessed water bodies are listed as impaired. This is a watershed assessment and does not describe finished household tap water.
Which USGS streamflow records are shown across California?
WaterByCounty has 44 fresh USGS pipeline snapshots in the published California cohort. Each county page shows its observation timestamp and year. A pipeline snapshot is not a continuously updated reading or a tap-water safety measure.
Where can California residents find current official information?
Review the applicable utility's Consumer Confidence Report, active notices, and the state drinking-water agency. WaterByCounty's county-level historical aggregations do not establish the current status of a specific utility, address, private well, or household plumbing system.
What does Water Quality Portal monitoring volume show in California?
The published county records contain 1,199,491 results from 4,062 monitoring locations in the site's five-year aggregation. Monitoring volume does not by itself indicate better or worse water quality.

Data Sources

Drinking-water compliance data from the EPA Safe Drinking Water Information System (SDWIS) via the ECHO enforcement database. The WaterByCounty SDWIS Compliance Index reflects the recorded rate and is not an EPA score.

Disclaimer: This data is informational only. It is not health, legal, or professional advice. For concerns about your specific water supply, contact your local water utility.

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