Data & Methodology — San Luis Obispo County

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

Contaminant Data — All Analytes

1008 well testing events on record, covering 27 analytes. Data spans 1941 to 2023.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. CA Avg
Radon 21 2005–2013 100%
197% of limit ↑ 116% above
Sulfate 802 1927–2023 100%
212% of limit ↑ 948% above
Manganese 472 1954–2023 53%
40% of limit ↓ 87% below
Boron 23 2008–2023 100%
16% of limit ~ typical
Nitrate 600 1927–2023 92%
20% of limit ~ typical
Iron 528 1942–2023 62%
8% of limit ↓ 93% below
Uranium 86 1980–2023 91%
19% of limit ↑ 245% above
PFOA municipal 128 2020–2025 9%
158% of limit ~ typical
PFOS municipal 128 2020–2025 9%
170% of limit ~ typical
Chloride 945 1927–2023 100%
18% of limit ↓ 39% below
Arsenic 87 1980–2023 93%
13% of limit ↓ 67% below
PFHxS municipal 127 2023–2025 17%
43% of limit ↓ 23% below
Lead 70 1954–2023 39%
3% of limit ↓ 71% below
PFNA municipal 128 2020–2025 1%
40% of limit ↓ 25% below
Chromium (total) 23 2008–2023 43%
1% of limit ↓ 59% below
1,2,3-Trichloropropane 23 2008–2023 0%
— —
Fluoride 730 1942–2023 99%
10% of limit ↑ 44% above
HFPO-DA (GenX) municipal 128 2020–2025 0%
— —
Nitrite 338 1991–2023 18%
2% of limit ↑ 70% above
Perchlorate 23 2008–2023 61%
9% of limit ↑ 36% above
Tds 23 2008–2023 100%
— ↑ 66% above
pH 23 2008–2023 100%
— ~ typical
Sodium 811 1927–2023 100%
— ~ typical
Total Coliform 2 2005–2023 0%
— —
PFBS municipal 127 2023–2025 17%
— ~ typical
E. coli 2 2005–2023 0%
— —
Hardness 879 1927–2023 100%
— ↑ 220% 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 Luis Obispo County

Of the 87 arsenic tests on record for San Luis Obispo County (1980–2023), 7% found none, and half were below 1.2 µg/L.

Of the 945 chloride tests on record for San Luis Obispo County (1927–2023), 0% found none, and half were below 46 mg/L.

Of the 23 chromium (total) tests on record for San Luis Obispo County (2008–2023), 57% found none.

Of the 730 fluoride tests on record for San Luis Obispo County (1942–2023), 1% found none, and half were below 400 µg/L.

Of the 528 iron tests on record for San Luis Obispo County (1942–2023), 38% found none, and half were below 8 µg/L.

Of the 70 lead tests on record for San Luis Obispo County (1954–2023), 61% found none.

Of the 472 manganese tests on record for San Luis Obispo County (1954–2023), 47% found none, and half were below 0.265 µg/L.

Of the 600 nitrate tests on record for San Luis Obispo County (1927–2023), 8% found none, and half were below 1.61 mg/L.

Of the 802 sulfate tests on record for San Luis Obispo County (1927–2023), 0% found none, and half were below 530 mg/L.

Of the 23 tds tests on record for San Luis Obispo County (2008–2023), 0% found none, and half were below 658 mg/L.

Of the 86 uranium tests on record for San Luis Obispo County (1980–2023), 9% found none, and half were below 4.66 µg/L.

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Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Radon 21 samples
  • Sulfate 802 samples
  • Manganese 472 samples
  • Boron 23 samples
  • Nitrate 600 samples
  • Iron 528 samples
  • Uranium 86 samples
  • PFOA 128 samples
  • PFOS 128 samples
  • Chloride 945 samples
  • Arsenic 87 samples
  • PFHxS 127 samples
  • Lead 70 samples
  • PFNA 128 samples
  • Chromium (total) 23 samples
  • 1,2,3-Trichloropropane 23 samples
  • Fluoride 730 samples
  • HFPO-DA (GenX) 128 samples
  • Nitrite 338 samples
  • Perchlorate 23 samples
  • Tds 23 samples
  • pH 23 samples
  • Sodium 811 samples
  • PFBS 127 samples
  • Hardness 879 samples

Limited data (<15 samples) — interpret with caution

  • Total Coliform 2 samples
  • E. coli 2 samples

Public vs. Private Water in San Luis Obispo County

171 Active public water systems
348,510 Residents on public water
15% Households on private wells

Public water systems in San Luis Obispo 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 Luis Obispo 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 Luis Obispo County Prevalence CA Average Source Year
Arsenic Cancer prevalence 7.3% 6.2% 2020
Arsenic Kidney disease rate 3.0% 3.0% 2020

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