Data & Methodology — Humboldt County

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

Contaminant Data — All Analytes

238 well testing events on record, covering 27 analytes. Data spans 1950 to 2020.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. CA Avg
Manganese 117 1971–2020 95%
1258% of limit ↑ 306% above
Iron 171 1970–2020 91%
233% of limit ↑ 90% above
Radon 21 2009–2010 100%
88% of limit ~ typical
Sulfate 204 1954–2020 98%
4% of limit ↓ 78% below
Nitrate 7 2009–2020 43%
31% of limit ↑ 29% above
Lead 57 1994–2020 72%
5% of limit ↓ 45% below
Arsenic 47 2009–2020 94%
5% of limit ↓ 88% below
Chloride 7 2009–2020 100%
13% of limit ↓ 56% below
PFOS municipal 26 2020–2025 0%
— —
Fluoride 149 1954–2020 94%
2% of limit ↓ 64% below
HFPO-DA (GenX) municipal 23 2023–2025 0%
— —
Nitrite 47 2009–2020 19%
0% of limit ↓ 84% below
Perchlorate 4 2009 0%
— —
PFOA municipal 26 2020–2025 4%
32% of limit ↓ 80% below
Uranium 47 2009–2020 55%
0% of limit ↓ 97% below
Boron 7 2009–2020 100%
11% of limit ↓ 31% below
Chromium (total) 7 2009–2020 71%
1% of limit ↓ 66% below
1,2,3-Trichloropropane 4 2009 0%
— —
PFHxS municipal 26 2020–2025 4%
10% of limit ↓ 82% below
PFNA municipal 26 2020–2025 0%
— —
pH 7 2009–2020 100%
— ~ typical
Tds 7 2009–2020 100%
— ~ typical
Total Coliform 14 2009–2020 23%
— ~ typical
Hardness 222 1952–2020 100%
— ~ typical
E. coli 14 2009–2020 9%
— ~ typical
Sodium 218 1952–2020 100%
— ↓ 87% below
PFBS municipal 26 2020–2025 4%
— ↓ 77% below

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 Humboldt County

Of the 47 arsenic tests on record for Humboldt County (2009–2020), 6% found none, and half were below 0.37 µg/L.

Of the 149 fluoride tests on record for Humboldt County (1954–2020), 6% found none, and half were below 100 µg/L.

Of the 171 iron tests on record for Humboldt County (1970–2020), 9% found none, and half were below 470 µg/L.

Of the 57 lead tests on record for Humboldt County (1994–2020), 28% found none, and half were below 0.121 µg/L.

Of the 117 manganese tests on record for Humboldt County (1971–2020), 5% found none, and half were below 580 µg/L.

Of the 204 sulfate tests on record for Humboldt County (1954–2020), 2% found none, and half were below 11 mg/L.

Of the 47 uranium tests on record for Humboldt County (2009–2020), 45% found none, and half were below 5 ng/L.

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Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Manganese 117 samples
  • Iron 171 samples
  • Radon 21 samples
  • Sulfate 204 samples
  • Lead 57 samples
  • Arsenic 47 samples
  • PFOS 26 samples
  • Fluoride 149 samples
  • Nitrite 47 samples
  • PFOA 26 samples
  • Uranium 47 samples
  • PFHxS 26 samples
  • PFNA 26 samples
  • Hardness 222 samples
  • Sodium 218 samples
  • PFBS 26 samples

Limited data (<15 samples) — interpret with caution

  • Nitrate 7 samples
  • Chloride 7 samples
  • Perchlorate 4 samples
  • Boron 7 samples
  • Chromium (total) 7 samples
  • 1,2,3-Trichloropropane 4 samples
  • pH 7 samples
  • Tds 7 samples
  • Total Coliform 14 samples
  • E. coli 14 samples

Public vs. Private Water in Humboldt County

91 Active public water systems
133,864 Residents on public water
7% Households on private wells

Public water systems in Humboldt 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 Humboldt 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 Humboldt County Prevalence CA Average Source Year
Arsenic Cancer prevalence 6.1% 6.2% 2020
Arsenic Kidney disease rate 2.9% 3.0% 2020
Lead Heart disease rate 6.3% 6.3% 2020

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