Data & Methodology — San Diego County

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

Contaminant Data — All Analytes

3812 well testing events on record, covering 27 analytes. Data spans 1915 to 2024.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. CA Avg
Radon 80 2004–2017 100%
133% of limit ↑ 46% above
Manganese 1405 1948–2022 86%
61% of limit ↓ 80% below
Iron 2106 1942–2022 85%
17% of limit ↓ 86% below
Chloride 144 2008–2012 100%
46% of limit ↑ 55% above
Sulfate 144 2008–2012 100%
24% of limit ↑ 21% above
Uranium 166 2004–2021 99%
14% of limit ↑ 165% above
Arsenic 603 1964–2022 78%
18% of limit ↓ 54% below
Nitrate 2360 1939–2022 89%
8% of limit ↓ 66% below
PFOA municipal 163 2023–2025 4%
235% of limit ↑ 41% above
PFOS municipal 163 2023–2025 4%
134% of limit ↓ 36% below
Perchlorate 144 2008–2012 7%
95% of limit ↑ 1272% above
Boron 144 2008–2012 90%
10% of limit ↓ 37% below
PFHxS municipal 163 2023–2025 5%
77% of limit ↑ 38% above
Lead 319 1957–2022 34%
7% of limit ↓ 21% below
PFNA municipal 163 2023–2025 1%
93% of limit ↑ 75% above
Fluoride 2810 1951–2022 99%
10% of limit ↑ 44% above
Nitrite 574 1981–2022 25%
1% of limit ~ typical
HFPO-DA (GenX) municipal 163 2023–2025 0%
— —
Chromium (total) 144 2008–2012 4%
1% of limit ↓ 64% below
1,2,3-Trichloropropane 7 2011–2012 0%
— —
Radium 226 53 2008–2009 100%
3% of limit ↓ 85% below
Radium 228 53 2008–2009 100%
7% of limit ↓ 78% below
pH 144 2008–2012 100%
— ~ typical
Tds 144 2008–2012 100%
— ↑ 23% above
PFBS municipal 163 2023–2025 6%
— ↑ 34% above
Sodium 3299 1939–2022 100%
— ↑ 50% above
Hardness 3558 1939–2022 100%
— ↑ 34% above

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 San Diego County

Of the 603 arsenic tests on record for San Diego County (1964–2022), 22% found none, and half were below 1.16 µg/L.

Of the 144 chloride tests on record for San Diego County (2008–2012), 0% found none, and half were below 116 mg/L.

Of the 144 chromium (total) tests on record for San Diego County (2008–2012), 96% found none.

Of the 2810 fluoride tests on record for San Diego County (1951–2022), 1% found none, and half were below 400 µg/L.

Of the 2106 iron tests on record for San Diego County (1942–2022), 15% found none, and half were below 30 µg/L.

Of the 319 lead tests on record for San Diego County (1957–2022), 66% found none.

Of the 1405 manganese tests on record for San Diego County (1948–2022), 14% found none, and half were below 20 µg/L.

Of the 2360 nitrate tests on record for San Diego County (1939–2022), 11% found none, and half were below 606 µg/L.

Of the 144 sulfate tests on record for San Diego County (2008–2012), 0% found none, and half were below 61.2 mg/L.

Of the 144 tds tests on record for San Diego County (2008–2012), 0% found none, and half were below 487 mg/L.

Of the 166 uranium tests on record for San Diego County (2004–2021), 1% found none, and half were below 3.92 µg/L.

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Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Radon 80 samples
  • Manganese 1405 samples
  • Iron 2106 samples
  • Chloride 144 samples
  • Sulfate 144 samples
  • Uranium 166 samples
  • Arsenic 603 samples
  • Nitrate 2360 samples
  • Perchlorate 144 samples
  • Boron 144 samples
  • Lead 319 samples
  • Fluoride 2810 samples
  • Nitrite 574 samples
  • Chromium (total) 144 samples
  • Radium 226 53 samples
  • Radium 228 53 samples
  • pH 144 samples
  • Tds 144 samples
  • Sodium 3299 samples
  • Hardness 3558 samples

Limited data (<15 samples) — interpret with caution

  • 1,2,3-Trichloropropane 7 samples

No private-well PFAS data for San Diego County

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

197 Active public water systems
3,715,027 Residents on public water
2% Households on private wells

Public water systems in San Diego 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 San Diego 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 San Diego County Prevalence CA Average Source Year
Arsenic Cancer prevalence 5.4% 6.2% 2020
Arsenic Kidney disease rate 2.5% 3.0% 2020
Lead Heart disease rate 5.0% 6.3% 2020

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