Data & Methodology — Stanislaus County

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

Contaminant Data — All Analytes

1713 well testing events on record, covering 27 analytes. Data spans 1934 to 2023.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. CA Avg
Chloride 1561 1934–2021 100%
33% of limit ~ typical
Sulfate 1352 1934–2021 99%
18% of limit ~ typical
Uranium 271 1994–2021 99%
22% of limit ↑ 306% above
Nitrate 1145 1947–2021 76%
36% of limit ↑ 52% above
Manganese 463 1950–2021 58%
8% of limit ↓ 97% below
PFOS municipal 241 2019–2025 9%
199% of limit ~ typical
Iron 419 1950–2021 39%
7% of limit ↓ 95% below
Arsenic 404 1966–2021 91%
40% of limit ~ typical
PFOA municipal 241 2019–2025 7%
110% of limit ↓ 34% below
1,2,3-Trichloropropane 46 2006–2021 4%
1250% of limit ~ typical
Radon 52 1994–2006 100%
9% of limit ↓ 90% below
Lead 351 1959–2021 43%
1% of limit ↓ 84% below
PFHxS municipal 241 2019–2025 10%
40% of limit ↓ 29% below
PFNA municipal 241 2019–2025 0%
15% of limit ↓ 72% below
Chromium (total) 44 2006–2021 86%
2% of limit ↓ 37% below
Boron 44 2006–2021 100%
5% of limit ↓ 70% below
Fluoride 578 1950–2021 90%
4% of limit ↓ 39% below
HFPO-DA (GenX) municipal 236 2020–2025 0%
— —
Nitrite 783 1984–2021 20%
2% of limit ↑ 62% above
Perchlorate 46 2006–2021 83%
7% of limit ~ typical
Tds 44 2006–2021 100%
— ↓ 24% below
Sodium 1582 1934–2021 100%
— ~ typical
Total Coliform 42 2002–2021 24%
— ~ typical
PFBS municipal 241 2019–2025 3%
— ↓ 23% below
pH 1546 1939–2021 100%
— ~ typical
E. coli 42 2002–2021 23%
— ~ typical
Hardness 1543 1934–2021 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 Stanislaus County

Of the 404 arsenic tests on record for Stanislaus County (1966–2021), 9% found none, and half were below 3.8 µg/L.

Of the 1561 chloride tests on record for Stanislaus County (1934–2021), 0% found none, and half were below 82 mg/L.

Of the 44 chromium (total) tests on record for Stanislaus County (2006–2021), 14% found none, and half were below 1.5 µg/L.

Of the 578 fluoride tests on record for Stanislaus County (1950–2021), 10% found none, and half were below 140 µg/L.

Of the 419 iron tests on record for Stanislaus County (1950–2021), 61% found none.

Of the 351 lead tests on record for Stanislaus County (1959–2021), 57% found none.

Of the 463 manganese tests on record for Stanislaus County (1950–2021), 42% found none, and half were below 80 ng/L.

Of the 1145 nitrate tests on record for Stanislaus County (1947–2021), 24% found none, and half were below 2.48 mg/L.

Of the 42 pfoa tests on record for Stanislaus County (2019–2021), 86% found none.

Of the 42 pfos tests on record for Stanislaus County (2019–2021), 88% found none.

Of the 1352 sulfate tests on record for Stanislaus County (1934–2021), 1% found none, and half were below 42.8 mg/L.

Of the 44 tds tests on record for Stanislaus County (2006–2021), 0% found none, and half were below 300 mg/L.

Of the 271 uranium tests on record for Stanislaus County (1994–2021), 1% found none, and half were below 6.33 µg/L.

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Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Chloride 1561 samples
  • Sulfate 1352 samples
  • Uranium 271 samples
  • Nitrate 1145 samples
  • Manganese 463 samples
  • PFOS 241 samples
  • Iron 419 samples
  • Arsenic 404 samples
  • PFOA 241 samples
  • 1,2,3-Trichloropropane 46 samples
  • Radon 52 samples
  • Lead 351 samples
  • PFHxS 241 samples
  • PFNA 241 samples
  • Chromium (total) 44 samples
  • Boron 44 samples
  • Fluoride 578 samples
  • HFPO-DA (GenX) 236 samples
  • Nitrite 783 samples
  • Perchlorate 46 samples
  • Tds 44 samples
  • Sodium 1582 samples
  • Total Coliform 42 samples
  • PFBS 241 samples
  • pH 1546 samples
  • E. coli 42 samples
  • Hardness 1543 samples

Public vs. Private Water in Stanislaus County

193 Active public water systems
513,089 Residents on public water
9% Households on private wells

Public water systems in Stanislaus 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 Stanislaus 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 Stanislaus County Prevalence CA Average Source Year
Arsenic Cancer prevalence 5.7% 6.2% 2020
Arsenic Kidney disease rate 3.1% 3.0% 2020
Lead Heart disease rate 6.3% 6.3% 2020

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