Data & Methodology — Napa County

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

Contaminant Data — All Analytes

113 well testing events on record, covering 25 analytes. Data spans 1952 to 2023.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. CA Avg
Manganese 82 2001–2022 94%
184% of limit ↓ 41% below
Radon 25 2004–2012 100%
170% of limit ↑ 86% above
Iron 84 2001–2022 80%
14% of limit ↓ 89% below
Arsenic 84 2001–2022 93%
50% of limit ↑ 28% above
Fluoride 84 2001–2022 99%
6% of limit ~ typical
Boron 24 2004–2022 100%
11% of limit ↓ 33% below
Chloride 86 1949–2022 100%
6% of limit ↓ 79% below
PFOS municipal 32 2019–2025 3%
305% of limit ↑ 46% above
PFOA municipal 32 2019–2025 3%
160% of limit ~ typical
Lead 57 2004–2022 54%
2% of limit ↓ 74% below
Nitrate 76 2001–2022 46%
5% of limit ↓ 80% below
Uranium 69 2004–2022 74%
0% of limit ↓ 91% below
Chromium (total) 24 2004–2022 33%
1% of limit ↓ 75% below
1,2,3-Trichloropropane 27 2004–2022 0%
— —
PFHxS municipal 30 2019–2025 0%
— —
PFNA municipal 32 2019–2025 0%
— —
HFPO-DA (GenX) municipal 31 2022–2025 0%
— —
Nitrite 76 2001–2022 26%
0% of limit ↓ 68% below
Perchlorate 27 2004–2022 33%
4% of limit ↓ 40% below
Sulfate 84 2001–2022 100%
6% of limit ↓ 71% below
Tds 24 2004–2022 100%
— ~ typical
Hardness 86 1949–2022 100%
— ↓ 60% below
Sodium 84 2001–2022 100%
— ↓ 44% below
pH 90 2001–2022 100%
— ~ typical
PFBS municipal 32 2019–2025 0%
— —

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 Napa County

Of the 84 arsenic tests on record for Napa County (2001–2022), 7% found none, and half were below 4.5 µg/L.

Of the 86 chloride tests on record for Napa County (1949–2022), 0% found none, and half were below 15.8 mg/L.

Of the 24 chromium (total) tests on record for Napa County (2004–2022), 67% found none.

Of the 84 fluoride tests on record for Napa County (2001–2022), 1% found none, and half were below 240 µg/L.

Of the 84 iron tests on record for Napa County (2001–2022), 20% found none, and half were below 19.8 µg/L.

Of the 57 lead tests on record for Napa County (2004–2022), 46% found none, and half were below 78 ng/L.

Of the 82 manganese tests on record for Napa County (2001–2022), 6% found none, and half were below 85.1 µg/L.

Of the 76 nitrate tests on record for Napa County (2001–2022), 54% found none.

Of the 84 sulfate tests on record for Napa County (2001–2022), 0% found none, and half were below 14.7 mg/L.

Of the 24 tds tests on record for Napa County (2004–2022), 0% found none, and half were below 326 mg/L.

Of the 69 uranium tests on record for Napa County (2004–2022), 26% found none, and half were below 65 ng/L.

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Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Manganese 82 samples
  • Radon 25 samples
  • Iron 84 samples
  • Arsenic 84 samples
  • Fluoride 84 samples
  • Boron 24 samples
  • Chloride 86 samples
  • PFOS 32 samples
  • PFOA 32 samples
  • Lead 57 samples
  • Nitrate 76 samples
  • Uranium 69 samples
  • Chromium (total) 24 samples
  • 1,2,3-Trichloropropane 27 samples
  • PFHxS 30 samples
  • PFNA 32 samples
  • HFPO-DA (GenX) 31 samples
  • Nitrite 76 samples
  • Perchlorate 27 samples
  • Sulfate 84 samples
  • Tds 24 samples
  • Hardness 86 samples
  • Sodium 84 samples
  • pH 90 samples
  • PFBS 32 samples

Public vs. Private Water in Napa County

188 Active public water systems
161,950 Residents on public water
12% Households on private wells

Public water systems in Napa 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 Napa 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 Napa County Prevalence CA Average Source Year
Arsenic Cancer prevalence 7.1% 6.2% 2020
Arsenic Kidney disease rate 3.0% 3.0% 2020
Lead Heart disease rate 6.5% 6.3% 2020

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