Data & Methodology — Colusa County

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

Contaminant Data — All Analytes

614 well testing events on record, covering 25 analytes. Data spans 1956 to 2025.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. CA Avg
Manganese 268 1960–2021 71%
200% of limit ↓ 35% below
Iron 271 1960–2021 79%
27% of limit ↓ 78% below
Sulfate 385 1956–2021 99%
15% of limit ↓ 25% below
Chloride 382 1956–2021 100%
19% of limit ↓ 36% below
Arsenic 302 1960–2021 96%
10% of limit ↓ 74% below
Uranium 37 1997–2021 95%
9% of limit ↑ 59% above
Fluoride 278 1956–2021 99%
8% of limit ~ typical
Lead 89 1960–2017 49%
0% of limit ↓ 100% below
Nitrate 470 1956–2021 99%
2% of limit ↓ 91% below
PFOA municipal 14 2024 0%
— —
Boron 5 2006–2016 100%
36% of limit ↑ 119% above
Chromium (total) 5 2006–2016 60%
6% of limit ↑ 122% above
1,2,3-Trichloropropane 7 2006–2016 0%
— —
PFOS municipal 14 2024 0%
— —
Perchlorate 7 2006–2016 14%
23% of limit ↑ 237% above
PFNA municipal 14 2024 0%
— —
Radon 3 2006–2009 100%
7% of limit ↓ 92% below
Nitrite 63 1997–2021 52%
0% of limit ↓ 100% below
HFPO-DA (GenX) municipal 14 2024 0%
— —
PFHxS municipal 14 2024 0%
— —
Tds 5 2006–2016 100%
— ↑ 55% above
Sodium 397 1956–2021 100%
— ~ typical
Hardness 322 1956–2021 100%
— ~ typical
pH 524 1956–2021 100%
— ~ typical
PFBS municipal 14 2024 0%
— —

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 Colusa County

Of the 302 arsenic tests on record for Colusa County (1960–2021), 4% found none, and half were below 1 µg/L.

Of the 382 chloride tests on record for Colusa County (1956–2021), 0% found none, and half were below 48.2 mg/L.

Of the 278 fluoride tests on record for Colusa County (1956–2021), 1% found none, and half were below 300 µg/L.

Of the 271 iron tests on record for Colusa County (1960–2021), 21% found none, and half were below 30 µg/L.

Of the 89 lead tests on record for Colusa County (1960–2017), 51% found none.

Of the 268 manganese tests on record for Colusa County (1960–2021), 29% found none, and half were below 30 µg/L.

Of the 470 nitrate tests on record for Colusa County (1956–2021), 1% found none, and half were below 173 µg/L.

Of the 385 sulfate tests on record for Colusa County (1956–2021), 1% found none, and half were below 38 mg/L.

Of the 37 uranium tests on record for Colusa County (1997–2021), 5% found none, and half were below 2.6 µg/L.

Have a number from your lab? Enter it to see where it falls. Nothing is sent anywhere.

Want the full picture for your well? Check all your results.

Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Manganese 268 samples
  • Iron 271 samples
  • Sulfate 385 samples
  • Chloride 382 samples
  • Arsenic 302 samples
  • Uranium 37 samples
  • Fluoride 278 samples
  • Lead 89 samples
  • Nitrate 470 samples
  • Nitrite 63 samples
  • Sodium 397 samples
  • Hardness 322 samples
  • pH 524 samples

Limited data (<15 samples) — interpret with caution

  • Boron 5 samples
  • Chromium (total) 5 samples
  • 1,2,3-Trichloropropane 7 samples
  • Perchlorate 7 samples
  • Radon 3 samples
  • Tds 5 samples

No private-well PFAS data for Colusa County

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

29 Active public water systems
35,121 Residents on public water
16% Households on private wells

Public water systems in Colusa 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 Colusa 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 Colusa County Prevalence CA Average Source Year
Arsenic Kidney disease rate 3.1% 3.0% 2020
Lead Heart disease rate 7.0% 6.3% 2020
Arsenic Cancer prevalence 5.3% 6.2% 2020

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