Data & Methodology — Williamson County

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

Contaminant Data — All Analytes

1181 well testing events on record, covering 22 analytes. Data spans 1962 to 2025.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. TX Avg
Radon 3 2016–2017 100%
107% of limit ↑ 196% above
Iron 419 1938–2022 74%
57% of limit ↓ 63% below
Fluoride 1032 1938–2022 99%
50% of limit ↑ 109% above
Sulfate 60 1977–2022 100%
8% of limit ↓ 72% below
PFOS municipal 123 2023–2025 4%
125% of limit ~ typical
Nitrate 102 1972–2022 76%
17% of limit ↓ 25% below
PFOA municipal 123 2023–2025 3%
124% of limit ~ typical
Manganese 354 1938–2022 26%
8% of limit ↓ 88% below
Lead 137 1977–2022 9%
8% of limit ↓ 53% below
Chloride 60 1977–2022 100%
6% of limit ↓ 83% below
Arsenic 131 1977–2022 32%
4% of limit ↓ 88% below
HFPO-DA (GenX) municipal 123 2023–2025 0%
— —
Nitrite 54 1987–2022 19%
1% of limit ↓ 42% below
PFHxS municipal 123 2023–2025 15%
37% of limit ~ typical
PFNA municipal 123 2023–2025 1%
50% of limit ↓ 60% below
Uranium 57 2007–2022 54%
2% of limit ↓ 86% below
E. coli 3 2016–2017 33%
— ~ typical
Hardness 61 1977–2022 100%
— ↑ 44% above
PFBS municipal 123 2023–2025 37%
— ↑ 26% above
pH 134 1977–2022 100%
— ~ typical
Sodium 61 1977–2022 100%
— ↓ 91% below
Total Coliform 3 2016–2017 33%
— ~ 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 TX.

Compare your result with Williamson County

Of the 131 arsenic tests on record for Williamson County (1977–2022), 68% found none.

Of the 60 chloride tests on record for Williamson County (1977–2022), 0% found none, and half were below 15 mg/L.

Of the 1032 fluoride tests on record for Williamson County (1938–2022), 1% found none, and half were below 1.9 mg/L.

Of the 419 iron tests on record for Williamson County (1938–2022), 26% found none, and half were below 100 µg/L.

Of the 137 lead tests on record for Williamson County (1977–2022), 91% found none.

Of the 354 manganese tests on record for Williamson County (1938–2022), 74% found none.

Of the 102 nitrate tests on record for Williamson County (1972–2022), 24% found none, and half were below 1.3 mg/L.

Of the 60 sulfate tests on record for Williamson County (1977–2022), 0% found none, and half were below 19.6 mg/L.

Of the 57 uranium tests on record for Williamson County (2007–2022), 46% found none, and half were below 55 ng/L.

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Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Iron 419 samples
  • Fluoride 1032 samples
  • Sulfate 60 samples
  • Nitrate 102 samples
  • Manganese 354 samples
  • Lead 137 samples
  • Chloride 60 samples
  • Arsenic 131 samples
  • Nitrite 54 samples
  • Uranium 57 samples
  • Hardness 61 samples
  • pH 134 samples
  • Sodium 61 samples

Limited data (<15 samples) — interpret with caution

  • Radon 3 samples
  • E. coli 3 samples
  • Total Coliform 3 samples

No private-well PFAS data for Williamson County

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

51 Active public water systems
703,121 Residents on public water
2% Households on private wells

Public water systems in Williamson 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 Williamson 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 Williamson County Prevalence TX Average Source Year
Nitrate Cancer prevalence 5.6% 6.1% 2020
Lead Heart disease rate 5.0% 7.4% 2020

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

Last updated: 2026-08-04

Full methodology →