Data & Methodology — Cook County

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

Contaminant Data — All Analytes

25 well testing events on record, covering 14 analytes. Data spans 1967 to 2025.

Contaminant Samples Years Detection Rate Distribution LowModHigh vs. Limit vs. MN Avg
Iron 23 1995–2004 100%
110% of limit ↓ 85% below
Manganese 23 1995–2004 96%
72% of limit ↓ 85% below
Lead 8 1995–1996 50%
99% of limit ↑ 121% above
Chloride 23 1995–2004 87%
0% of limit ↓ 97% below
Arsenic 10 1995–1999 50%
5% of limit ↓ 94% below
Fluoride 23 1995–2004 17%
23% of limit ↑ 230% above
Nitrate 23 1995–2004 9%
6% of limit ↓ 77% below
Nitrite 15 1999–2004 33%
0% of limit ↓ 59% below
PFNA 2 2017 0%
— —
PFOA 2 2017 0%
— —
Sulfate 23 1995–2004 83%
1% of limit ↓ 97% below
Sodium 23 1995–2004 100%
— ↓ 94% below
pH 15 1999–2004 100%
— ↓ 22% below
Hardness 15 1999–2004 93%
— ↓ 80% 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 MN.

Compare your result with Cook County

Of the 23 chloride tests on record for Cook County (1995–2004), 13% found none, and half were below 480 µg/L.

Of the 23 fluoride tests on record for Cook County (1995–2004), 83% found none.

Of the 23 iron tests on record for Cook County (1995–2004), 0% found none, and half were below 329 µg/L.

Of the 23 manganese tests on record for Cook County (1995–2004), 4% found none, and half were below 30.6 µg/L.

Of the 23 nitrate tests on record for Cook County (1995–2004), 91% found none.

Of the 23 sulfate tests on record for Cook County (1995–2004), 17% found none, and half were below 1.24 mg/L.

Have a number from your lab? Enter it to see where it falls. Nothing is sent anywhere.

Want the full picture for your well? Check all your results.

Data Coverage & Gaps

Well-sampled analytes (15+ samples)

  • Iron 23 samples
  • Manganese 23 samples
  • Chloride 23 samples
  • Fluoride 23 samples
  • Nitrate 23 samples
  • Nitrite 15 samples
  • Sulfate 23 samples
  • Sodium 23 samples
  • pH 15 samples
  • Hardness 15 samples

Limited data (<15 samples) — interpret with caution

  • Lead 8 samples
  • Arsenic 10 samples
  • PFNA 2 samples
  • PFOA 2 samples

Public vs. Private Water in Cook County

101 Active public water systems
8,374 Residents on public water
74% Households on private wells

Public water systems in Cook 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 Cook 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 Cook County Prevalence MN Average Source Year
Arsenic Cancer prevalence 6.1% 7.0% 2020
Arsenic Kidney disease rate 3.5% 2.9% 2020
Lead Heart disease rate 8.3% 6.6% 2020

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