Data & Methodology — Fairfax County

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

Contaminant Data — All Analytes

1089 well testing events on record, covering 18 analytes. Data spans 1945 to 2026.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. VA Avg
Manganese 37 1976–2015 86%
97% of limit ↓ 22% below
Iron 56 1976–2015 80%
27% of limit ↓ 77% below
PFOA municipal 44 2023–2025 20%
152% of limit ~ typical
PFOS municipal 44 2023–2025 14%
139% of limit ~ typical
Sulfate 56 1976–2015 96%
3% of limit ↓ 54% below
Nitrate 54 1976–2015 98%
6% of limit ↑ 53% above
Uranium 31 1994–2015 61%
0% of limit ↓ 50% below
Radon 7 1994–1998 100%
13% of limit ~ typical
PFNA municipal 44 2023–2025 0%
— —
PFHxS municipal 44 2023–2025 0%
— —
Nitrite 54 1976–2015 35%
1% of limit ~ typical
Lead 26 1998–2015 27%
2% of limit ↑ 32% above
HFPO-DA (GenX) municipal 44 2023–2025 0%
— —
Fluoride 56 1976–2015 73%
2% of limit ↓ 57% below
Sodium 56 1976–2015 100%
— ↓ 26% below
pH 41 1979–2015 100%
— ~ typical
PFBS municipal 44 2023–2025 30%
— ~ typical
Hardness 56 1976–2015 100%
— ↓ 21% below

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 VA.

Compare your result with Fairfax County

Of the 26 arsenic tests on record for Fairfax County (1998–2015), 50% found none, and half were below 25 ng/L.

Of the 56 chloride tests on record for Fairfax County (1976–2015), 0% found none, and half were below 9.96 mg/L.

Of the 56 fluoride tests on record for Fairfax County (1976–2015), 27% found none, and half were below 70 µg/L.

Of the 56 iron tests on record for Fairfax County (1976–2015), 20% found none, and half were below 50 µg/L.

Of the 26 lead tests on record for Fairfax County (1998–2015), 73% found none.

Of the 37 manganese tests on record for Fairfax County (1976–2015), 14% found none, and half were below 38 µg/L.

Of the 54 nitrate tests on record for Fairfax County (1976–2015), 2% found none, and half were below 543 µg/L.

Of the 56 sulfate tests on record for Fairfax County (1976–2015), 4% found none, and half were below 5.7 mg/L.

Of the 31 uranium tests on record for Fairfax County (1994–2015), 39% found none, and half were below 52 ng/L.

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Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Manganese 37 samples
  • Iron 56 samples
  • Sulfate 56 samples
  • Nitrate 54 samples
  • Uranium 31 samples
  • Nitrite 54 samples
  • Lead 26 samples
  • Fluoride 56 samples
  • Sodium 56 samples
  • pH 41 samples
  • Hardness 56 samples

Limited data (<15 samples) — interpret with caution

  • Radon 7 samples

No private-well PFAS data for Fairfax County

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

44 Active public water systems
1,317,489 Residents on public water

Public water systems in Fairfax 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 Fairfax 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 Fairfax County Prevalence VA Average Source Year
Lead Heart disease rate 4.2% 6.7% 2020
Nitrate Cancer prevalence 5.5% 6.7% 2020

Source: CDC PLACES county-level estimates. Raw data: Download Fairfax 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 VA with at least one sample for that analyte.

Last updated: 2026-08-04

Full methodology →