Data & Methodology — Yolo County

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

Contaminant Data — All Analytes

405 well testing events on record, covering 25 analytes. Data spans 1930 to 2020.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. CA Avg
Boron 2 2005 100%
108% of limit ↑ 551% above
Manganese 184 1965–2021 33%
60% of limit ↓ 81% below
Chloride 372 1940–2021 100%
17% of limit ↓ 44% below
Arsenic 156 1970–2021 97%
31% of limit ↓ 21% below
Nitrate 176 1955–2021 98%
18% of limit ↓ 23% below
Iron 190 1951–2021 35%
7% of limit ↓ 95% below
Sulfate 295 1940–2021 96%
13% of limit ↓ 35% below
Fluoride 252 1940–2021 100%
5% of limit ↓ 28% below
PFOA municipal 70 2023–2025 0%
— —
PFHxS municipal 70 2023–2025 0%
— —
Lead 72 1970–2020 14%
1% of limit ↓ 85% below
Radon 8 2005 100%
7% of limit ↓ 92% below
PFNA municipal 70 2023–2025 1%
42% of limit ↓ 21% below
Uranium 30 1997–2021 93%
4% of limit ↓ 32% below
PFOS municipal 70 2023–2025 0%
— —
Chromium (total) 2 2005 100%
24% of limit ↑ 787% above
1,2,3-Trichloropropane 5 2005 0%
— —
HFPO-DA (GenX) municipal 70 2023–2025 0%
— —
Nitrite 33 1981–2021 12%
2% of limit ↑ 62% above
Perchlorate 2 2005 0%
— —
Tds 2 2005 100%
— ↑ 53% above
Sodium 371 1940–2021 100%
— ↓ 23% below
PFBS municipal 70 2023–2025 0%
— —
pH 372 1951–2021 100%
— ~ typical
Hardness 371 1940–2021 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 Yolo County

Of the 156 arsenic tests on record for Yolo County (1970–2021), 3% found none, and half were below 3 µg/L.

Of the 372 chloride tests on record for Yolo County (1940–2021), 0% found none, and half were below 42 mg/L.

Of the 252 fluoride tests on record for Yolo County (1940–2021), 0% found none, and half were below 200 µg/L.

Of the 190 iron tests on record for Yolo County (1951–2021), 65% found none.

Of the 72 lead tests on record for Yolo County (1970–2020), 86% found none.

Of the 184 manganese tests on record for Yolo County (1965–2021), 67% found none.

Of the 176 nitrate tests on record for Yolo County (1955–2021), 2% found none, and half were below 1.73 mg/L.

Of the 295 sulfate tests on record for Yolo County (1940–2021), 4% found none, and half were below 29 mg/L.

Of the 30 uranium tests on record for Yolo County (1997–2021), 7% found none, and half were below 1 µg/L.

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Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Manganese 184 samples
  • Chloride 372 samples
  • Arsenic 156 samples
  • Nitrate 176 samples
  • Iron 190 samples
  • Sulfate 295 samples
  • Fluoride 252 samples
  • Lead 72 samples
  • Uranium 30 samples
  • Nitrite 33 samples
  • Sodium 371 samples
  • pH 372 samples
  • Hardness 371 samples

Limited data (<15 samples) — interpret with caution

  • Boron 2 samples
  • Radon 8 samples
  • Chromium (total) 2 samples
  • 1,2,3-Trichloropropane 5 samples
  • Perchlorate 2 samples
  • Tds 2 samples

No private-well PFAS data for Yolo County

We have no private well sampling data for PFAS compounds (PFOA, PFOS, PFNA, and related chemicals) in Yolo 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 Yolo County

89 Active public water systems
259,964 Residents on public water
10% Households on private wells

Public water systems in Yolo 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 Yolo 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 Yolo County Prevalence CA Average Source Year
Arsenic Cancer prevalence 5.5% 6.2% 2020
Arsenic Kidney disease rate 2.7% 3.0% 2020

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