Data & Methodology — Sonoma County

Full contaminant data, sample history, and sourcing for Sonoma County. For readers who want to go beyond the summary.

Contaminant Data — All Analytes

426 well testing events on record, covering 25 analytes. Data spans 1951 to 2022.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. CA Avg
Radon 60 2004–2012 100%
118% of limit ↑ 30% above
Manganese 218 1981–2022 89%
59% of limit ↓ 81% below
Iron 361 1971–2022 70%
13% of limit ↓ 89% below
Arsenic 249 1976–2022 83%
15% of limit ↓ 62% below
Boron 67 2004–2022 97%
5% of limit ↓ 68% below
PFOS municipal 115 2022–2025 3%
112% of limit ↓ 46% below
PFOA municipal 115 2022–2025 4%
100% of limit ↓ 40% below
Fluoride 358 1971–2022 92%
3% of limit ↓ 57% below
Chloride 366 1950–2022 100%
3% of limit ↓ 90% below
Nitrate 218 1978–2022 59%
4% of limit ↓ 85% below
Sulfate 361 1971–2022 99%
6% of limit ↓ 70% below
Nitrite 217 2002–2022 18%
0% of limit ↓ 76% below
HFPO-DA (GenX) municipal 114 2022–2025 0%
— —
Perchlorate 69 2004–2022 19%
3% of limit ↓ 49% below
Uranium 185 2003–2022 75%
0% of limit ↓ 96% below
PFNA municipal 115 2022–2025 0%
— —
Chromium (total) 69 2004–2022 43%
1% of limit ↓ 59% below
1,2,3-Trichloropropane 69 2004–2022 0%
— —
Lead 211 1976–2022 38%
2% of limit ↓ 84% below
PFHxS municipal 115 2022–2025 7%
40% of limit ↓ 29% below
Tds 68 2004–2022 100%
— ↓ 22% below
Sodium 365 1971–2022 100%
— ↓ 83% below
Hardness 366 1950–2022 100%
— ↓ 46% below
PFBS municipal 115 2022–2025 6%
— ~ typical
pH 372 1971–2022 100%
— ~ typical

Distribution shows the share of samples in each concentration band relative to the EPA Maximum Contaminant Level (MCL): Low = below half the MCL, Moderate = between half and the MCL, High = above the MCL. Analytes without an MCL (e.g. sodium, pH) show — in the limit columns. State average is based on county median values across CA.

Compare your result with Sonoma County

Of the 249 arsenic tests on record for Sonoma County (1976–2022), 17% found none, and half were below 0.95 µg/L.

Of the 366 chloride tests on record for Sonoma County (1950–2022), 0% found none, and half were below 7.5 mg/L.

Of the 69 chromium (total) tests on record for Sonoma County (2004–2022), 57% found none.

Of the 358 fluoride tests on record for Sonoma County (1971–2022), 8% found none, and half were below 100 µg/L.

Of the 361 iron tests on record for Sonoma County (1971–2022), 30% found none, and half were below 20 µg/L.

Of the 211 lead tests on record for Sonoma County (1976–2022), 62% found none.

Of the 218 manganese tests on record for Sonoma County (1981–2022), 11% found none, and half were below 14.4 µg/L.

Of the 218 nitrate tests on record for Sonoma County (1978–2022), 41% found none, and half were below 66.5 µg/L.

Of the 361 sulfate tests on record for Sonoma County (1971–2022), 1% found none, and half were below 15.4 mg/L.

Of the 68 tds tests on record for Sonoma County (2004–2022), 0% found none, and half were below 312 mg/L.

Of the 185 uranium tests on record for Sonoma County (2003–2022), 25% found none, and half were below 40 ng/L.

Have a number from your lab? Enter it to see where it falls. Nothing is sent anywhere.

Want the full picture for your well? Check all your results.

Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Radon 60 samples
  • Manganese 218 samples
  • Iron 361 samples
  • Arsenic 249 samples
  • Boron 67 samples
  • PFOS 115 samples
  • PFOA 115 samples
  • Fluoride 358 samples
  • Chloride 366 samples
  • Nitrate 218 samples
  • Sulfate 361 samples
  • Nitrite 217 samples
  • HFPO-DA (GenX) 114 samples
  • Perchlorate 69 samples
  • Uranium 185 samples
  • PFNA 115 samples
  • Chromium (total) 69 samples
  • 1,2,3-Trichloropropane 69 samples
  • Lead 211 samples
  • PFHxS 115 samples
  • Tds 68 samples
  • Sodium 365 samples
  • Hardness 366 samples
  • PFBS 115 samples
  • pH 372 samples

Public vs. Private Water in Sonoma County

445 Active public water systems
481,821 Residents on public water
15% Households on private wells

Public water systems in Sonoma County are regulated by the EPA and must test and report contaminant levels. Private well owners are responsible for their own testing — there is no routine monitoring of private wells by any government agency.

CDC Health Outcome Correlations

Where contaminants detected in Sonoma County have established associations with specific health outcomes, we cross-reference CDC PLACES county-level prevalence data. This is a contextual signal, not a causal claim.

Contaminant Associated Condition Sonoma County Prevalence CA Average Source Year
Arsenic Cancer prevalence 5.8% 6.2% 2020
Arsenic Kidney disease rate 2.5% 3.0% 2020

Source: CDC PLACES county-level estimates. Raw data: Download Sonoma County CDC PLACES data →

Data Sources

This report aggregates data from the following public databases:

Methodology

Raw records are downloaded from the Water Quality Portal and normalized to µg/L (ppb). Records are deduplicated by sample ID and date, and certified outliers are excluded. Analyte names are mapped to EPA canonical forms. Detection rates, distribution bands, and MCL comparisons are computed from the normalized dataset.

Distribution bands use the EPA Maximum Contaminant Level as the threshold: concentrations below 50% of the MCL are classed as Low, between 50% and 100% as Moderate, and above 100% as High. For analytes without an MCL (sodium, hardness, pH), distribution is not computed.

State comparison uses the median of county median values across all counties in CA with at least one sample for that analyte.

Last updated: 2026-09-10

Full methodology →