Data & Methodology — Orange County

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

Contaminant Data — All Analytes

2030 well testing events on record, covering 21 analytes. Data spans 1937 to 2020.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. CA Avg
Manganese 174 1980–2020 67%
17% of limit ↓ 95% below
Radon 63 1999–2011 100%
8% of limit ↓ 91% below
PFOS municipal 436 2023–2025 10%
242% of limit ~ typical
Uranium 122 1999–2020 93%
23% of limit ↑ 319% above
Iron 175 1980–2020 43%
17% of limit ↓ 86% below
PFOA municipal 436 2023–2025 7%
186% of limit ~ typical
Arsenic 131 1999–2020 92%
17% of limit ↓ 56% below
Nitrate 1664 1937–2020 98%
2% of limit ↓ 90% below
PFHxS municipal 436 2023–2025 11%
52% of limit ~ typical
Nitrite 151 1999–2020 20%
0% of limit ↓ 74% below
Fluoride 1490 1951–2020 100%
10% of limit ↑ 44% above
Chromium (total) 1 2006 0%
— —
1,2,3-Trichloropropane 1 2006 0%
— —
HFPO-DA (GenX) municipal 436 2023–2025 0%
— —
Lead 116 1999–2020 62%
3% of limit ↓ 68% below
Perchlorate 1 2006 0%
— —
PFNA municipal 436 2023–2025 0%
— —
pH 1 2006 100%
— ~ typical
PFBS municipal 436 2023–2025 2%
— ~ typical
Hardness 1870 1937–2020 100%
— ~ typical
Sodium 1799 1937–2020 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 Orange County

Of the 131 arsenic tests on record for Orange County (1999–2020), 8% found none, and half were below 1.7 µg/L.

Of the 1490 fluoride tests on record for Orange County (1951–2020), 0% found none, and half were below 400 µg/L.

Of the 175 iron tests on record for Orange County (1980–2020), 57% found none.

Of the 116 lead tests on record for Orange County (1999–2020), 38% found none, and half were below 0.23 µg/L.

Of the 174 manganese tests on record for Orange County (1980–2020), 33% found none, and half were below 1.31 µg/L.

Of the 1664 nitrate tests on record for Orange County (1937–2020), 2% found none, and half were below 203 µg/L.

Of the 122 uranium tests on record for Orange County (1999–2020), 7% found none, and half were below 5.75 µg/L.

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Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Manganese 174 samples
  • Radon 63 samples
  • Uranium 122 samples
  • Iron 175 samples
  • Arsenic 131 samples
  • Nitrate 1664 samples
  • Nitrite 151 samples
  • Fluoride 1490 samples
  • Lead 116 samples
  • Hardness 1870 samples
  • Sodium 1799 samples

Limited data (<15 samples) — interpret with caution

  • Chromium (total) 1 sample
  • 1,2,3-Trichloropropane 1 sample
  • Perchlorate 1 sample
  • pH 1 sample

No private-well PFAS data for Orange County

We have no private well sampling data for PFAS compounds (PFOA, PFOS, PFNA, and related chemicals) in Orange County. PFAS has been detected in local public water systems (UCMR 5 data) — indicated by the "municipal" badge in the table above — but this does not directly indicate private well contamination. PFAS testing for private wells requires a dedicated lab panel (~$300–$500). If you are near a military base, airport, or industrial site, consider testing proactively. Learn more about PFAS →

Public vs. Private Water in Orange County

46 Active public water systems
3,201,893 Residents on public water
1% Households on private wells

Public water systems in Orange 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 Orange 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 Orange County Prevalence CA Average Source Year
Arsenic Cancer prevalence 5.2% 6.2% 2020
Arsenic Kidney disease rate 2.5% 3.0% 2020

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