Data & Methodology — Monterey County

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

Contaminant Data — All Analytes

493 well testing events on record, covering 30 analytes. Data spans 1951 to 2023.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. CA Avg
Radon 76 2005–2013 100%
128% of limit ↑ 41% above
Hexavalent Chromium 13 2011 62%
85% of limit ~ typical
Sulfate 416 1971–2023 100%
31% of limit ↑ 53% above
Manganese 219 1998–2023 63%
14% of limit ↓ 95% below
Nitrate 221 1973–2023 86%
21% of limit ~ typical
Chloride 416 1971–2023 100%
32% of limit ~ typical
Perchlorate 142 2011–2023 80%
30% of limit ↑ 331% above
Arsenic 409 1971–2023 88%
16% of limit ↓ 59% below
Iron 222 1971–2023 58%
11% of limit ↓ 91% below
Boron 142 2011–2023 66%
10% of limit ↓ 39% below
Nitrite 220 1998–2023 20%
4% of limit ↑ 241% above
PFOS municipal 165 2022–2025 2%
122% of limit ↓ 41% below
Uranium 214 2005–2023 63%
10% of limit ↑ 83% above
PFOA municipal 165 2022–2025 1%
174% of limit ~ typical
Lead 195 2005–2023 25%
9% of limit ~ typical
Fluoride 415 1971–2023 88%
8% of limit ~ typical
PFHxS municipal 165 2022–2025 2%
62% of limit ~ typical
PFNA municipal 165 2022–2025 0%
— —
Radium 228 78 2011 28%
31% of limit ~ typical
Chromium (total) 142 2011–2023 70%
4% of limit ↑ 37% above
1,2,3-Trichloropropane 143 2005–2023 1%
40% of limit ↓ 97% below
Radium 226 78 2011 10%
25% of limit ↑ 27% above
HFPO-DA (GenX) municipal 165 2022–2025 0%
— —
Tds 142 2011–2023 100%
— ~ typical
Sodium 415 1971–2023 100%
— ~ typical
Hardness 416 1971–2023 100%
— ~ typical
Total Coliform 8 2005–2023 13%
— ~ typical
PFBS municipal 165 2022–2025 2%
— ↑ 36% above
pH 418 1971–2023 100%
— ~ typical
E. coli 8 2005–2023 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 Monterey County

Of the 409 arsenic tests on record for Monterey County (1971–2023), 12% found none, and half were below 1.1 µg/L.

Of the 416 chloride tests on record for Monterey County (1971–2023), 0% found none, and half were below 80.8 mg/L.

Of the 142 chromium (total) tests on record for Monterey County (2011–2023), 30% found none, and half were below 1.3 µg/L.

Of the 415 fluoride tests on record for Monterey County (1971–2023), 12% found none, and half were below 280 µg/L.

Of the 222 iron tests on record for Monterey County (1971–2023), 42% found none, and half were below 10.3 µg/L.

Of the 195 lead tests on record for Monterey County (2005–2023), 75% found none.

Of the 219 manganese tests on record for Monterey County (1998–2023), 37% found none, and half were below 1.53 µg/L.

Of the 221 nitrate tests on record for Monterey County (1973–2023), 14% found none, and half were below 1.64 mg/L.

Of the 416 sulfate tests on record for Monterey County (1971–2023), 0% found none, and half were below 76.7 mg/L.

Of the 142 tds tests on record for Monterey County (2011–2023), 0% found none, and half were below 435 mg/L.

Of the 214 uranium tests on record for Monterey County (2005–2023), 37% found none, and half were below 1.08 µg/L.

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Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Radon 76 samples
  • Sulfate 416 samples
  • Manganese 219 samples
  • Nitrate 221 samples
  • Chloride 416 samples
  • Perchlorate 142 samples
  • Arsenic 409 samples
  • Iron 222 samples
  • Boron 142 samples
  • Nitrite 220 samples
  • PFOS 165 samples
  • Uranium 214 samples
  • PFOA 165 samples
  • Lead 195 samples
  • Fluoride 415 samples
  • PFHxS 165 samples
  • PFNA 165 samples
  • Radium 228 78 samples
  • Chromium (total) 142 samples
  • 1,2,3-Trichloropropane 143 samples
  • Radium 226 78 samples
  • HFPO-DA (GenX) 165 samples
  • Tds 142 samples
  • Sodium 415 samples
  • Hardness 416 samples
  • PFBS 165 samples
  • pH 418 samples

Limited data (<15 samples) — interpret with caution

  • Hexavalent Chromium 13 samples
  • Total Coliform 8 samples
  • E. coli 8 samples

Public vs. Private Water in Monterey County

312 Active public water systems
476,792 Residents on public water
11% Households on private wells

Public water systems in Monterey 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 Monterey 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 Monterey County Prevalence CA Average Source Year
Arsenic Cancer prevalence 5.2% 6.2% 2020
Arsenic Kidney disease rate 2.9% 3.0% 2020
Lead Heart disease rate 5.8% 6.3% 2020

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