Sonoma County, California has about 485,375 residents (Census 2024 estimate) and 129 community water systems in EPA records. The largest local system is Santa Rosa, City of, serving about 178,452 people from groundwater.
Public-record figures, not a test of your tap. Not 100% accurate or guaranteed. Confirm with a water test, your utility or a qualified pro before you buy or act.
Santa Rosa, City of Estimate: untreated local water, not your tap · Low confidence
6.8 gpg (117 mg/L), moderately hard
Sonoma County: median of 1,341 results from 194 monitoring sites (streams and wells). Utilities may blend or treat water, so your tap can differ.
Source: Hard Water Map county data, samples through 2022
County estimates: Hard Water Map (hardwatermap.com), CC BY 4.0, derived from the USGS/EPA Water Quality Portal; file generated 2026-07-22. License
Public-record figures, not a test of your tap. Not 100% accurate or guaranteed. Confirm with a water test, your utility or a qualified pro before you buy or act.
For a 3-person home using 57 gallons per person per day and regenerating weekly, you'd remove about 8,140 grains between regenerations. Suggested size: 24,000 grain. This uses a low-confidence county estimate, so confirm with a test first.
Estimate only. Not 100% accurate or guaranteed. Confirm with a water test, your utility or a qualified pro before you buy or act.
The 6 biggest of 129 community water systems on EPA records for this county. Service areas don't follow county lines; your water bill names your provider.
CA4910009 · serves about 178,452 people · Groundwater
Lead 90th percentile: 1.3 ppb (period starting 2023-01-01) Older result. Action level: 15 ppb (drops to 10 ppb on Nov 1, 2027).
CA4910006 · serves about 61,427 people · Groundwater
Lead 90th percentile: <DL (below the lab detection/reporting limit; reported as 0) (period starting 2021-01-01) Older result. Action level: 15 ppb (drops to 10 ppb on Nov 1, 2027).
CA4910014 · serves about 43,821 people · Groundwater
Lead 90th percentile: <DL (below the lab detection/reporting limit; reported as 0) (period starting 2023-01-01) Older result. Action level: 15 ppb (drops to 10 ppb on Nov 1, 2027).
CA4910017 · serves about 25,561 people · Groundwater
Lead 90th percentile: <DL (below the lab detection/reporting limit; reported as 0) (period starting 2023-01-01) Older result. Action level: 15 ppb (drops to 10 ppb on Nov 1, 2027).
CA4910013 · serves about 23,140 people · Groundwater
Lead 90th percentile: <DL (below the lab detection/reporting limit; reported as 0) (period starting 2024-01-01). Action level: 15 ppb (drops to 10 ppb on Nov 1, 2027).
CA4910005 · serves about 10,972 people · Surface water
Lead 90th percentile: <DL (below the lab detection/reporting limit; reported as 0) (period starting 2021-01-01) Older result. Action level: 15 ppb (drops to 10 ppb on Nov 1, 2027).
Public-record figures, not a test of your tap. Not 100% accurate or guaranteed. Confirm with a water test, your utility or a qualified pro before you buy or act.
A PFAS detection is not a violation. Federal limits (4 ppt for PFOA and PFOS) apply to running annual averages, with compliance required by 2029. In May 2026 EPA proposed extending the PFOA/PFOS deadline to 2031 and removing other PFAS limits; not final as of 2026-10-21.
These are public records for whole systems. Your home's pipes, fixtures and any private well can change what you drink. A certified lab test or a hardness test strip gives you your own numbers. Then use our PFAS and lead filter selector to see which certifications to look for.
None of the 6 largest systems here have a health-based violation in EPA records since October 2021. Santa Rosa, City of's most recent lead 90th percentile was 1.3 ppb (2023), below the 15 ppb action level. Public records can't guarantee what comes out of your tap, especially with older plumbing, so a home test is the surest check.
There's no published utility figure in our data, but untreated local stream and well samples in Sonoma County have a median of about 6.8 grains per gallon (117 mg/L, moderately hard). That's a low-confidence estimate of source water, not your tap, so test before you buy equipment. For a 3-person home at 6.8 gpg, our softener calculator suggests a 24,000 grain unit as a starting point.
3 of the 6 sampled systems listed here detected at least one PFAS in EPA's UCMR 5 testing, and 3 results were above the level of a federal limit in at least one sample. A detection isn't a violation: federal PFAS limits apply to running annual averages starting in 2029.
Most homes are served by Santa Rosa, City of or one of the other systems listed on this page, but service areas don't follow county lines. Your water bill names your provider, and its annual Consumer Confidence Report has the full test results.
Data as of October 21, 2026. Figures are copied from public records and combined with our own calculations; they can be out of date or wrong. Tapped Out isn't affiliated with any utility or the EPA. Written and reviewed by the Tapped Out team. How we calculate · All sources.