Data & Methodology — Los Angeles County

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

Contaminant Data — All Analytes

3667 well testing events on record, covering 27 analytes. Data spans 1908 to 2024.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. CA Avg
Manganese 1980 1952–2023 81%
35% of limit ↓ 89% below
Radon 138 2000–2018 100%
8% of limit ↓ 91% below
Arsenic 1111 1964–2023 90%
27% of limit ↓ 31% below
PFOS municipal 1047 2021–2025 17%
395% of limit ↑ 90% above
PFOA municipal 1047 2021–2025 14%
190% of limit ~ typical
Sulfate 2714 1908–2023 99%
21% of limit ~ typical
Chloride 2841 1908–2023 100%
10% of limit ↓ 67% below
Lead 568 1968–2023 21%
9% of limit ~ typical
Nitrite 1447 1968–2023 28%
2% of limit ↑ 38% above
Iron 2165 1908–2023 69%
5% of limit ↓ 96% below
Nitrate 2565 1908–2023 83%
11% of limit ↓ 55% below
Uranium 331 2000–2023 92%
11% of limit ↑ 103% above
PFHxS municipal 1042 2021–2025 13%
48% of limit ~ typical
PFNA municipal 1047 2021–2025 0%
2580% of limit ↑ 4768% above
Fluoride 2537 1947–2023 99%
10% of limit ↑ 44% above
Boron 5 2006–2021 100%
3% of limit ↓ 84% below
Chromium (total) 5 2006–2021 20%
0% of limit ↓ 83% below
1,2,3-Trichloropropane 8 2006–2021 0%
— —
HFPO-DA (GenX) municipal 1047 2021–2025 0%
— —
Perchlorate 8 2006–2021 25%
8% of limit ~ typical
Hardness 2911 1908–2023 100%
— ↓ 29% below
Tds 5 2006–2021 100%
— ~ typical
Sodium 2842 1944–2023 100%
— ↓ 32% below
pH 3228 1901–2023 100%
— ~ typical
PFBS municipal 1047 2021–2025 11%
— ↑ 23% above
Total Coliform 2 2005–2022 31%
— ~ typical
E. coli 2 2005–2022 9%
— ~ 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 Los Angeles County

Of the 1111 arsenic tests on record for Los Angeles County (1964–2023), 10% found none, and half were below 2 µg/L.

Of the 2841 chloride tests on record for Los Angeles County (1908–2023), 0% found none, and half were below 24.8 mg/L.

Of the 2537 fluoride tests on record for Los Angeles County (1947–2023), 1% found none, and half were below 400 µg/L.

Of the 2165 iron tests on record for Los Angeles County (1908–2023), 31% found none, and half were below 5.8 µg/L.

Of the 568 lead tests on record for Los Angeles County (1968–2023), 79% found none.

Of the 1980 manganese tests on record for Los Angeles County (1952–2023), 19% found none, and half were below 4 µg/L.

Of the 2565 nitrate tests on record for Los Angeles County (1908–2023), 17% found none, and half were below 727 µg/L.

Of the 2714 sulfate tests on record for Los Angeles County (1908–2023), 1% found none, and half were below 52 mg/L.

Of the 331 uranium tests on record for Los Angeles County (2000–2023), 8% found none, and half were below 2.94 µg/L.

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Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Manganese 1980 samples
  • Radon 138 samples
  • Arsenic 1111 samples
  • PFOS 1047 samples
  • PFOA 1047 samples
  • Sulfate 2714 samples
  • Chloride 2841 samples
  • Lead 568 samples
  • Nitrite 1447 samples
  • Iron 2165 samples
  • Nitrate 2565 samples
  • Uranium 331 samples
  • PFHxS 1042 samples
  • PFNA 1047 samples
  • Fluoride 2537 samples
  • HFPO-DA (GenX) 1047 samples
  • Hardness 2911 samples
  • Sodium 2842 samples
  • pH 3228 samples
  • PFBS 1047 samples

Limited data (<15 samples) — interpret with caution

  • Boron 5 samples
  • Chromium (total) 5 samples
  • 1,2,3-Trichloropropane 8 samples
  • Perchlorate 8 samples
  • Tds 5 samples
  • Total Coliform 2 samples
  • E. coli 2 samples

Public vs. Private Water in Los Angeles County

277 Active public water systems
10,112,558 Residents on public water
0% Households on private wells

Public water systems in Los Angeles 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 Los Angeles 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 Los Angeles County Prevalence CA Average Source Year
Arsenic Cancer prevalence 4.9% 6.2% 2020
Arsenic Kidney disease rate 2.8% 3.0% 2020
Lead Heart disease rate 5.6% 6.3% 2020

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