Data & Methodology — Washington County

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

Contaminant Data — All Analytes

2089 well testing events on record, covering 22 analytes. Data spans 1957 to 2018.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. MN Avg
PFOA municipal 7098 1–2025 92%
425% of limit ~ typical
PFOS municipal 4453 2023–2025 97%
400% of limit ↑ 99% above
PFHxS municipal 2706 2023–2025 97%
70% of limit ~ typical
Manganese 843 1957–2025 34%
75% of limit ↓ 85% below
Iron 973 1957–2025 45%
31% of limit ↓ 96% below
Radon 9 1996–2014 100%
7% of limit ↓ 86% below
PFNA municipal 373 1–2025 47%
13% of limit ~ typical
Chloride 1101 1957–2025 98%
10% of limit ↑ 30% above
Nitrate 313 1961–2018 83%
15% of limit ↓ 38% below
Arsenic 644 1979–2025 18%
10% of limit ↓ 88% below
Lead 749 1972–2025 5%
1% of limit ↓ 97% below
Fluoride 414 1957–2025 57%
4% of limit ↓ 49% below
HFPO-DA (GenX) municipal 195 1–2025 7%
19% of limit ~ typical
Nitrite 144 1979–2018 12%
1% of limit ↓ 42% below
Sulfate 864 1957–2025 97%
4% of limit ↓ 77% below
Uranium 24 1979–2018 75%
2% of limit ↓ 39% below
E. coli 5 1999–2018 1%
— ~ typical
Hardness 95 1957–2018 100%
— ↓ 26% below
PFBS municipal 2032 2023–2025 96%
— ↓ 38% below
pH 245 1957–2018 100%
— ~ typical
Sodium 869 1961–2025 100%
— ↓ 77% below
Total Coliform 5 1999–2018 22%
— ~ 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 MN.

Compare your result with Washington County

Of the 644 arsenic tests on record for Washington County (1979–2025), 82% found none.

Of the 1101 chloride tests on record for Washington County (1957–2025), 2% found none, and half were below 24.4 mg/L.

Of the 414 fluoride tests on record for Washington County (1957–2025), 43% found none, and half were below 100 µg/L.

Of the 973 iron tests on record for Washington County (1957–2025), 55% found none.

Of the 749 lead tests on record for Washington County (1972–2025), 95% found none.

Of the 843 manganese tests on record for Washington County (1957–2025), 66% found none.

Of the 313 nitrate tests on record for Washington County (1961–2018), 17% found none, and half were below 813 µg/L.

Of the 7001 pfoa tests on record for Washington County (1–2025), 8% found none, and half were below 15 ng/L.

Of the 4356 pfos tests on record for Washington County (2024–2025), 1% found none, and half were below 16 ng/L.

Of the 864 sulfate tests on record for Washington County (1957–2025), 3% found none, and half were below 10.6 mg/L.

Of the 24 uranium tests on record for Washington County (1979–2018), 25% found none, and half were below 0.488 µg/L.

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Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • PFOA 7098 samples
  • PFOS 4453 samples
  • PFHxS 2706 samples
  • Manganese 843 samples
  • Iron 973 samples
  • PFNA 373 samples
  • Chloride 1101 samples
  • Nitrate 313 samples
  • Arsenic 644 samples
  • Lead 749 samples
  • Fluoride 414 samples
  • HFPO-DA (GenX) 195 samples
  • Nitrite 144 samples
  • Sulfate 864 samples
  • Uranium 24 samples
  • Hardness 95 samples
  • PFBS 2032 samples
  • pH 245 samples
  • Sodium 869 samples

Limited data (<15 samples) — interpret with caution

  • Radon 9 samples
  • E. coli 5 samples
  • Total Coliform 5 samples

Public vs. Private Water in Washington County

195 Active public water systems
260,077 Residents on public water
19% Households on private wells

Public water systems in Washington 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 Washington 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 Washington County Prevalence MN Average Source Year
Lead Heart disease rate 4.6% 6.6% 2020
Arsenic Cancer prevalence 6.0% 7.0% 2020
Arsenic Kidney disease rate 2.2% 2.9% 2020

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

Last updated: 2026-08-04

Full methodology →