Data & Methodology — Santa Clara County

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

Contaminant Data — All Analytes

296 well testing events on record, covering 27 analytes. Data spans 1952 to 2023.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. CA Avg
Manganese 141 1980–2022 78%
184% of limit ↓ 40% below
Chloride 167 1974–2022 100%
32% of limit ~ typical
Radon 21 2007–2011 100%
19% of limit ↓ 79% below
Sulfate 146 1975–2022 100%
25% of limit ↑ 24% above
Iron 140 1980–2022 63%
25% of limit ↓ 80% below
PFOA municipal 287 2022–2025 3%
138% of limit ~ typical
PFOS municipal 287 2022–2025 2%
115% of limit ↓ 45% below
Nitrate 123 1974–2022 89%
35% of limit ↑ 49% above
PFNA municipal 287 2022–2025 0%
— —
Fluoride 147 1974–2022 99%
4% of limit ↓ 39% below
HFPO-DA (GenX) municipal 287 2022–2025 0%
— —
Nitrite 117 1997–2022 32%
0% of limit ↓ 76% below
Arsenic 110 1980–2022 93%
6% of limit ↓ 84% below
Uranium 63 2007–2022 94%
1% of limit ↓ 74% below
Perchlorate 1 2011 100%
4% of limit ↓ 40% below
PFHxS municipal 286 2022–2025 8%
35% of limit ↓ 38% below
Lead 67 2001–2022 36%
7% of limit ↓ 22% below
Boron 1 2011 100%
4% of limit ↓ 74% below
Chromium (total) 1 2011 100%
0% of limit ↓ 97% below
1,2,3-Trichloropropane 1 2011 0%
— —
Tds 1 2011 100%
— ~ typical
pH 208 1974–2022 100%
— ~ typical
Hardness 146 1974–2022 100%
— ↑ 33% above
E. coli 15 2022 0%
— —
PFBS municipal 287 2022–2025 3%
— ~ typical
Sodium 146 1974–2022 100%
— ↓ 21% below
Total Coliform 15 2022 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 Santa Clara County

Of the 110 arsenic tests on record for Santa Clara County (1980–2022), 7% found none, and half were below 0.555 µg/L.

Of the 167 chloride tests on record for Santa Clara County (1974–2022), 0% found none, and half were below 78.8 mg/L.

Of the 147 fluoride tests on record for Santa Clara County (1974–2022), 1% found none, and half were below 170 µg/L.

Of the 140 iron tests on record for Santa Clara County (1980–2022), 37% found none, and half were below 21.2 µg/L.

Of the 67 lead tests on record for Santa Clara County (2001–2022), 64% found none.

Of the 141 manganese tests on record for Santa Clara County (1980–2022), 22% found none, and half were below 21.4 µg/L.

Of the 123 nitrate tests on record for Santa Clara County (1974–2022), 11% found none, and half were below 3.19 mg/L.

Of the 146 sulfate tests on record for Santa Clara County (1975–2022), 0% found none, and half were below 62.7 mg/L.

Of the 63 uranium tests on record for Santa Clara County (2007–2022), 6% found none, and half were below 0.353 µg/L.

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Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Manganese 141 samples
  • Chloride 167 samples
  • Radon 21 samples
  • Sulfate 146 samples
  • Iron 140 samples
  • PFOA 287 samples
  • PFOS 287 samples
  • Nitrate 123 samples
  • PFNA 287 samples
  • Fluoride 147 samples
  • HFPO-DA (GenX) 287 samples
  • Nitrite 117 samples
  • Arsenic 110 samples
  • Uranium 63 samples
  • PFHxS 286 samples
  • Lead 67 samples
  • pH 208 samples
  • Hardness 146 samples
  • E. coli 15 samples
  • PFBS 287 samples
  • Sodium 146 samples
  • Total Coliform 15 samples

Limited data (<15 samples) — interpret with caution

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

Public vs. Private Water in Santa Clara County

122 Active public water systems
2,097,472 Residents on public water
2% Households on private wells

Public water systems in Santa Clara 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 Santa Clara 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 Santa Clara County Prevalence CA Average Source Year
Nitrate Cancer prevalence 5.2% 6.2% 2020

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