Data & Methodology — San Joaquin County

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

Contaminant Data — All Analytes

1238 well testing events on record, covering 27 analytes. Data spans 1943 to 2023.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. CA Avg
Manganese 415 1950–2023 50%
132% of limit ↓ 57% below
Arsenic 409 1970–2023 98%
40% of limit ~ typical
Chloride 1151 1943–2023 100%
15% of limit ↓ 51% below
Nitrate 752 1943–2020 89%
20% of limit ~ typical
Sulfate 772 1944–2023 96%
7% of limit ↓ 64% below
1,2,3-Trichloropropane 44 2004–2019 11%
180% of limit ↓ 87% below
Iron 432 1950–2023 43%
10% of limit ↓ 92% below
PFOS municipal 230 2019–2025 6%
135% of limit ↓ 35% below
PFOA municipal 230 2019–2025 7%
126% of limit ↓ 24% below
Boron 45 2004–2019 100%
4% of limit ↓ 79% below
Uranium 160 1993–2023 89%
5% of limit ~ typical
PFHxS municipal 230 2019–2025 10%
37% of limit ↓ 34% below
Nitrite 296 1978–2020 22%
2% of limit ↑ 30% above
PFNA municipal 230 2019–2025 0%
— —
Fluoride 634 1950–2023 90%
3% of limit ↓ 57% below
HFPO-DA (GenX) municipal 193 2023–2025 0%
— —
Perchlorate 40 2005–2019 72%
8% of limit ~ typical
Radon 28 1993–2005 93%
9% of limit ↓ 90% below
Lead 138 1970–2023 48%
4% of limit ↓ 55% below
Chromium (total) 44 2005–2019 80%
3% of limit ~ typical
pH 42 2011–2019 100%
— ~ typical
Tds 44 2005–2019 100%
— ↓ 36% below
Hardness 1005 1943–2023 100%
— ↓ 24% below
E. coli 37 2002–2019 2%
— ~ typical
Sodium 1004 1947–2023 100%
— ↓ 56% below
PFBS municipal 230 2019–2025 6%
— ~ typical
Total Coliform 37 2002–2019 6%
— ~ 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 San Joaquin County

Of the 409 arsenic tests on record for San Joaquin County (1970–2023), 2% found none, and half were below 4 µg/L.

Of the 1151 chloride tests on record for San Joaquin County (1943–2023), 0% found none, and half were below 37 mg/L.

Of the 44 chromium (total) tests on record for San Joaquin County (2005–2019), 20% found none, and half were below 2.6 µg/L.

Of the 634 fluoride tests on record for San Joaquin County (1950–2023), 10% found none, and half were below 110 µg/L.

Of the 432 iron tests on record for San Joaquin County (1950–2023), 57% found none.

Of the 138 lead tests on record for San Joaquin County (1970–2023), 52% found none.

Of the 415 manganese tests on record for San Joaquin County (1950–2023), 50% found none.

Of the 752 nitrate tests on record for San Joaquin County (1943–2020), 11% found none, and half were below 1.72 mg/L.

Of the 37 pfoa tests on record for San Joaquin County (2019–2020), 86% found none.

Of the 37 pfos tests on record for San Joaquin County (2019–2020), 95% found none.

Of the 772 sulfate tests on record for San Joaquin County (1944–2023), 4% found none, and half were below 16.1 mg/L.

Of the 44 tds tests on record for San Joaquin County (2005–2019), 0% found none, and half were below 256 mg/L.

Of the 160 uranium tests on record for San Joaquin County (1993–2023), 11% found none, and half were below 1.04 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 415 samples
  • Arsenic 409 samples
  • Chloride 1151 samples
  • Nitrate 752 samples
  • Sulfate 772 samples
  • 1,2,3-Trichloropropane 44 samples
  • Iron 432 samples
  • PFOS 230 samples
  • PFOA 230 samples
  • Boron 45 samples
  • Uranium 160 samples
  • PFHxS 230 samples
  • Nitrite 296 samples
  • PFNA 230 samples
  • Fluoride 634 samples
  • Perchlorate 40 samples
  • Radon 28 samples
  • Lead 138 samples
  • Chromium (total) 44 samples
  • pH 42 samples
  • Tds 44 samples
  • Hardness 1005 samples
  • E. coli 37 samples
  • Sodium 1004 samples
  • PFBS 230 samples
  • Total Coliform 37 samples

Public vs. Private Water in San Joaquin County

309 Active public water systems
776,769 Residents on public water
10% Households on private wells

Public water systems in San Joaquin 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 Joaquin 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 Joaquin County Prevalence CA Average Source Year
Arsenic Cancer prevalence 5.1% 6.2% 2020
Arsenic Kidney disease rate 2.9% 3.0% 2020

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