Data & Methodology — Siskiyou County

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

Contaminant Data — All Analytes

144 well testing events on record, covering 25 analytes. Data spans 1939 to 2020.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. CA Avg
Radon 35 2010 100%
93% of limit ~ typical
Manganese 52 2005–2020 54%
13% of limit ↓ 96% below
Iron 63 1972–2020 38%
4% of limit ↓ 97% below
Chloride 98 1958–2020 100%
1% of limit ↓ 95% below
Arsenic 64 2003–2020 75%
4% of limit ↓ 90% below
Nitrite 52 2010–2020 4%
0% of limit ↓ 76% below
HFPO-DA (GenX) municipal 10 2024 0%
— —
Fluoride 73 1958–2020 90%
3% of limit ↓ 60% below
Perchlorate 2 2010 100%
2% of limit ↓ 67% below
PFOS municipal 10 2024 0%
— —
Sulfate 85 1958–2020 88%
3% of limit ↓ 86% below
Uranium 52 2010–2020 90%
0% of limit ↓ 94% below
PFOA municipal 10 2024 0%
— —
PFNA municipal 10 2024 0%
— —
Boron 2 2010 50%
1% of limit ↓ 93% below
Chromium (total) 2 2010 100%
2% of limit ~ typical
1,2,3-Trichloropropane 2 2010 0%
— —
Lead 34 2003–2020 53%
5% of limit ↓ 47% below
Nitrate 74 1958–2020 82%
3% of limit ↓ 89% below
PFHxS municipal 10 2024 0%
— —
Tds 2 2010 100%
— ↓ 63% below
Sodium 91 1958–2020 100%
— ↓ 88% below
Hardness 104 1958–2020 100%
— ↓ 45% below
PFBS municipal 10 2024 0%
— —
pH 123 1958–2020 100%
— ~ 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 Siskiyou County

Of the 64 arsenic tests on record for Siskiyou County (2003–2020), 25% found none, and half were below 0.28 µg/L.

Of the 98 chloride tests on record for Siskiyou County (1958–2020), 0% found none, and half were below 3.65 mg/L.

Of the 73 fluoride tests on record for Siskiyou County (1958–2020), 10% found none, and half were below 100 µg/L.

Of the 63 iron tests on record for Siskiyou County (1972–2020), 62% found none.

Of the 34 lead tests on record for Siskiyou County (2003–2020), 47% found none, and half were below 40 ng/L.

Of the 52 manganese tests on record for Siskiyou County (2005–2020), 46% found none, and half were below 0.18 µg/L.

Of the 74 nitrate tests on record for Siskiyou County (1958–2020), 18% found none, and half were below 164 µg/L.

Of the 85 sulfate tests on record for Siskiyou County (1958–2020), 12% found none, and half were below 5.96 mg/L.

Of the 52 uranium tests on record for Siskiyou County (2010–2020), 10% found none, and half were below 72 ng/L.

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Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Radon 35 samples
  • Manganese 52 samples
  • Iron 63 samples
  • Chloride 98 samples
  • Arsenic 64 samples
  • Nitrite 52 samples
  • Fluoride 73 samples
  • Sulfate 85 samples
  • Uranium 52 samples
  • Lead 34 samples
  • Nitrate 74 samples
  • Sodium 91 samples
  • Hardness 104 samples
  • pH 123 samples

Limited data (<15 samples) — interpret with caution

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

No private-well PFAS data for Siskiyou County

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

71 Active public water systems
41,189 Residents on public water
34% Households on private wells

Public water systems in Siskiyou 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 Siskiyou 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 Siskiyou County Prevalence CA Average Source Year
Arsenic Cancer prevalence 6.0% 6.2% 2020
Arsenic Kidney disease rate 2.9% 3.0% 2020

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