Data & Methodology — Mendocino County

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

Contaminant Data — All Analytes

250 well testing events on record, covering 27 analytes. Data spans 1951 to 2025.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. CA Avg
Radon 30 1993–2012 100%
103% of limit ~ typical
Iron 161 1969–2020 75%
97% of limit ↓ 21% below
Manganese 143 1969–2020 80%
16% of limit ↓ 95% below
Boron 13 2009–2020 100%
8% of limit ↓ 49% below
Arsenic 90 1982–2020 70%
5% of limit ↓ 86% below
PFOS municipal 52 2019–2025 10%
112% of limit ↓ 46% below
Lead 88 1982–2020 33%
2% of limit ↓ 83% below
PFOA municipal 52 2019–2025 2%
172% of limit ~ typical
Chloride 183 1960–2020 100%
3% of limit ↓ 89% below
Perchlorate 12 2009–2019 25%
4% of limit ↓ 37% below
HFPO-DA (GenX) municipal 46 2023–2025 0%
— —
Uranium 46 2009–2020 63%
0% of limit ↓ 97% below
Nitrite 44 1981–2020 25%
0% of limit ↓ 84% below
Sulfate 176 1960–2020 84%
4% of limit ↓ 78% below
PFNA municipal 52 2019–2025 0%
— —
Chromium (total) 13 2009–2020 8%
4% of limit ↑ 37% above
1,2,3-Trichloropropane 12 2009–2019 0%
— —
Fluoride 166 1960–2020 93%
2% of limit ↓ 64% below
Nitrate 63 1960–2020 87%
7% of limit ↓ 69% below
PFHxS municipal 49 2019–2025 6%
36% of limit ↓ 36% below
Tds 13 2009–2020 100%
— ↓ 49% below
Total Coliform 1 2009–2020 32%
— ~ typical
pH 213 1960–2020 100%
— ~ typical
Sodium 180 1960–2020 100%
— ↓ 79% below
E. coli 1 2009–2020 9%
— ~ typical
Hardness 183 1960–2020 99%
— ↓ 45% below
PFBS municipal 49 2019–2025 10%
— ~ 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 Mendocino County

Of the 90 arsenic tests on record for Mendocino County (1982–2020), 30% found none, and half were below 0.165 µg/L.

Of the 183 chloride tests on record for Mendocino County (1960–2020), 0% found none, and half were below 8.13 mg/L.

Of the 166 fluoride tests on record for Mendocino County (1960–2020), 7% found none, and half were below 100 µg/L.

Of the 161 iron tests on record for Mendocino County (1969–2020), 25% found none, and half were below 84 µg/L.

Of the 88 lead tests on record for Mendocino County (1982–2020), 67% found none.

Of the 143 manganese tests on record for Mendocino County (1969–2020), 20% found none, and half were below 1.6 µg/L.

Of the 63 nitrate tests on record for Mendocino County (1960–2020), 13% found none, and half were below 467 µg/L.

Of the 176 sulfate tests on record for Mendocino County (1960–2020), 16% found none, and half were below 8.22 mg/L.

Of the 46 uranium tests on record for Mendocino County (2009–2020), 37% found none, and half were below 11 ng/L.

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Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Radon 30 samples
  • Iron 161 samples
  • Manganese 143 samples
  • Arsenic 90 samples
  • PFOS 52 samples
  • Lead 88 samples
  • PFOA 52 samples
  • Chloride 183 samples
  • Uranium 46 samples
  • Nitrite 44 samples
  • Sulfate 176 samples
  • PFNA 52 samples
  • Fluoride 166 samples
  • Nitrate 63 samples
  • PFHxS 49 samples
  • pH 213 samples
  • Sodium 180 samples
  • Hardness 183 samples
  • PFBS 49 samples

Limited data (<15 samples) — interpret with caution

  • Boron 13 samples
  • Perchlorate 12 samples
  • Chromium (total) 13 samples
  • 1,2,3-Trichloropropane 12 samples
  • Tds 13 samples
  • Total Coliform 1 sample
  • E. coli 1 sample

Public vs. Private Water in Mendocino County

147 Active public water systems
83,529 Residents on public water
28% Households on private wells

Public water systems in Mendocino 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 Mendocino 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 Mendocino County Prevalence CA Average Source Year
Arsenic Cancer prevalence 8.1% 6.2% 2020
Arsenic Kidney disease rate 3.8% 3.0% 2020
Lead Heart disease rate 6.2% 6.3% 2020

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