Data & Methodology — Fresno County

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

Contaminant Data — All Analytes

2584 well testing events on record, covering 25 analytes. Data spans 1937 to 2023.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. CA Avg
Sulfate 2078 1937–2024 100%
212% of limit ↑ 948% above
Radon 117 1993–2014 100%
15% of limit ↓ 84% below
Nitrate 1438 1951–2023 94%
57% of limit ↑ 140% above
Chloride 2081 1937–2024 100%
44% of limit ↑ 47% above
1,2,3-Trichloropropane 60 2006–2024 27%
1410% of limit ~ typical
Uranium 374 1993–2024 97%
28% of limit ↑ 418% above
Manganese 1119 1979–2024 75%
40% of limit ↓ 87% below
Boron 60 2006–2024 98%
4% of limit ↓ 75% below
PFOS municipal 575 2023–2025 10%
168% of limit ~ typical
PFOA municipal 575 2023–2025 9%
145% of limit ~ typical
Iron 1145 1958–2024 71%
13% of limit ↓ 89% below
Arsenic 875 1979–2024 86%
20% of limit ↓ 49% below
Nitrite 995 1984–2024 28%
4% of limit ↑ 225% above
PFHxS municipal 575 2023–2025 5%
55% of limit ~ typical
Lead 624 1979–2024 34%
5% of limit ↓ 42% below
Fluoride 1160 1951–2024 86%
5% of limit ↓ 28% below
HFPO-DA (GenX) municipal 569 2023–2025 0%
— —
Perchlorate 56 2010–2024 55%
19% of limit ↑ 167% above
PFNA municipal 575 2023–2025 1%
56% of limit ~ typical
Chromium (total) 60 2006–2024 63%
2% of limit ↓ 44% below
Tds 60 2006–2024 100%
— ↑ 36% above
Hardness 2080 1937–2023 100%
— ↑ 61% above
PFBS municipal 575 2023–2025 5%
— ~ typical
Sodium 2098 1937–2024 100%
— ↑ 234% above
pH 2430 1937–2024 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 Fresno County

Of the 875 arsenic tests on record for Fresno County (1979–2024), 14% found none, and half were below 2 µg/L.

Of the 2081 chloride tests on record for Fresno County (1937–2024), 0% found none, and half were below 110 mg/L.

Of the 60 chromium (total) tests on record for Fresno County (2006–2024), 37% found none, and half were below 0.46 µg/L.

Of the 1160 fluoride tests on record for Fresno County (1951–2024), 14% found none, and half were below 180 µg/L.

Of the 1145 iron tests on record for Fresno County (1958–2024), 29% found none, and half were below 30 µg/L.

Of the 624 lead tests on record for Fresno County (1979–2024), 66% found none.

Of the 1119 manganese tests on record for Fresno County (1979–2024), 25% found none, and half were below 10 µg/L.

Of the 1438 nitrate tests on record for Fresno County (1951–2023), 6% found none, and half were below 4.36 mg/L.

Of the 2078 sulfate tests on record for Fresno County (1937–2024), 0% found none, and half were below 530 mg/L.

Of the 60 tds tests on record for Fresno County (2006–2024), 0% found none, and half were below 540 mg/L.

Of the 374 uranium tests on record for Fresno County (1993–2024), 3% found none, and half were below 7.16 µg/L.

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Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Sulfate 2078 samples
  • Radon 117 samples
  • Nitrate 1438 samples
  • Chloride 2081 samples
  • 1,2,3-Trichloropropane 60 samples
  • Uranium 374 samples
  • Manganese 1119 samples
  • Boron 60 samples
  • PFOS 575 samples
  • PFOA 575 samples
  • Iron 1145 samples
  • Arsenic 875 samples
  • Nitrite 995 samples
  • PFHxS 575 samples
  • Lead 624 samples
  • Fluoride 1160 samples
  • Perchlorate 56 samples
  • PFNA 575 samples
  • Chromium (total) 60 samples
  • Tds 60 samples
  • Hardness 2080 samples
  • PFBS 575 samples
  • Sodium 2098 samples
  • pH 2430 samples

Public vs. Private Water in Fresno County

338 Active public water systems
1,053,467 Residents on public water
10% Households on private wells

Public water systems in Fresno 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 Fresno 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 Fresno County Prevalence CA Average Source Year
Arsenic Cancer prevalence 5.1% 6.2% 2020
Arsenic Kidney disease rate 3.2% 3.0% 2020
Lead Heart disease rate 6.4% 6.3% 2020

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