Well water in McHenry County, Illinois
1in10
Testing recommended
tested well samples in McHenry County exceeded the EPA limit for arsenic (10 ug/L), based on public data from USGS Water Quality Portal.
The strongest signal is PFOA: 18% of 11 local samples were above the limit, too few samples to carry the headline but first on the test list below.
3,356 test results across 15 contaminants from at least 174 wells, 1922 to 2021; chloride has the most at 398. Data refreshed September 15, 2026.
Compiled by Nick at Well Water Nerd from public records. How these pages are built
An estimated 65,640 people in McHenry County get their drinking water from a private well, per USGS 2015 county water use. That is about 21% of residents.
Separately, the USGS national arsenic model estimates that 4% of the 65,640 people on private wells in McHenry County draw water above 10 ug/L (the model's range is 4% to 5%), from geology and well depth rather than local tests.
What individual samples found
Each row summarizes every public sample for that contaminant inside McHenry County. Above limit means above the EPA maximum contaminant level; where EPA has no enforceable limit, the row shows a health guideline or nuisance level instead.
| Contaminant | Above limit | Samples | Detected | Median | Highest | Limit |
|---|---|---|---|---|---|---|
| PFOA 2020 to 2021 | 11 | 64% | 2.2 | 14 | 4 ng/L EPA limit | |
| Arsenic 1975 to 2021 | 271 | 55% | 0.15 | 70.3 | 10 ug/L EPA limit | |
| PFOS 2020 to 2021 | 11 | 18% | 0 | 16 | 4 ng/L EPA limit | |
| Nitrate 1979 to 2021 | 373 | 38% | 0 | 12.5 | 10 mg/L EPA limit | |
| Fluoride 1945 to 2021 | 394 | 98% | 0.37 | 3.7 | 4 mg/L EPA limit | |
| Copper 1972 to 2021 | 278 | 39% | 0 | 0.067 | 1.3 mg/L EPA limit | |
| Lead 1972 to 2021 | 274 | 14% | 0 | 0.431 | 15 ug/L EPA limit | |
| Uranium 2000 to 2021 | 124 | 71% | 0.056 | 8.04 | 30 ug/L EPA limit | |
| Radium (226 + 228) 2000 to 2017 | 34 | 53% | 0 | 4.7 | 5 pCi/L EPA limit | |
| Chloride 1922 to 2021 | no limit | 398 | 95% | 11.9 | 830 | 250 mg/L nuisance level |
| Iron 1922 to 2021 | no limit | 396 | 91% | 755 | 9,870 | 300 ug/L nuisance level |
| Manganese 1922 to 2021 | no limit | 394 | 90% | 24.4 | 740 | 300 ug/L health guideline |
| Sodium 1922 to 2021 | no limit | 381 | 100% | 15 | 365 | 20 mg/L health guideline |
| Radon 2000 to 2011 | no limit | 16 | 100% | 205 | 460 | 2,000 pCi/L health guideline |
Sampled fewer than 10 times, too few to summarize: E. coli (1).
Over time
Share of samples above the limit by period. A cell needs ten samples to show; a comparison needs twenty in each period. Later periods sample fewer wells, and often the ones already known to have a problem.
| Contaminant | before 2000 | 2000s | 2010s | 2020s |
|---|---|---|---|---|
| Fluoride | 0%215 samples | 0%22 samples | 0%105 samples | 0%52 samples |
| Nitrate | <1%201 samples | 5%21 samples | 0%99 samples | 0%52 samples |
| Copper | 0%154 samples | 0%22 samples | 0%50 samples | 0%52 samples |
| Lead | 0%150 samples | 0%22 samples | 0%50 samples | 0%52 samples |
| Arsenic | 3%147 samples | 0%22 samples | 22%50 samples | 21%52 samples |
| Uranium | n/a | 0%22 samples | 0%50 samples | 0%52 samples |
Fluoride, nitrate, copper, lead, arsenic, uranium have held within a few points between the two most recent periods with enough samples.
Closer than the county
See the sampled wells near your address
The same records, drawn around one point: which wells within a few miles were sampled and what they found, how deep the wells next to you were drilled, and any leaking tank or cleanup site within a mile.
Check my addressWhat this means
The contaminants most often above their limit here are pfoa (18% of 11 samples), arsenic (10% of 271 samples), and pfos (9% of 11 samples). A well next door can differ sharply from yours because bedrock fractures, well depth and plumbing all matter, so these rates describe the area, not your tap.
Chloride, Iron, and Manganese are widespread in local groundwater. They are mainly a staining and taste problem, but high manganese also carries a health advisory for infants.
What to test for in McHenry County
- 1PFOA18% of 11 local samples were above the limit.
- 2Arsenic10% of 271 local samples were above the limit.
- 3PFOS9% of 11 local samples were above the limit.
- 4Nitrate<1% of 373 local samples were above the limit.
- 5Coliform bacteriaRecommended for every private well, every year.
- 6RadonMedian local reading is 205 pCi/L.
- 7FluorideDetected in 98% of local samples.
Test your McHenry County well for PFOA, arsenic and PFOS
Certified lab analysis of your own tap water, bottles and prepaid return shipping included. The Essential well panel covers arsenic, uranium, nitrate, lead, copper, manganese, iron, sodium and bacteria; radon and PFAS are add-ons.
Order the Tap Score well water test kit, from $209Already tested? Upload the report
Just want bacteria checked first? Tap Score coliform and E. coli test, $99. Coliform and E. coli are the tests every well needs every year.
Want a quick look before paying for a lab? Home well water test strips on Amazon check hardness, iron, pH and nitrate in minutes. They are a screen, not a result: arsenic, bacteria, uranium and PFAS need a certified lab.
Affiliate links: the merchant pays us a commission and the price is the same for you. It does not change what the data above says. As an Amazon Associate we earn from qualifying purchases.
Where to get a well tested in Illinois: certified labs, state programs and prices.
Already have results back? Get your lab report read against the McHenry County numbers, $29. Every line explained, ranked by what matters, with the fix for each.
The rock under McHenry County
Not a principal aquifer
The USGS map places this area outside any principal aquifer, which usually means wells here draw from fractured bedrock or thin local deposits rather than a large regional aquifer. Bedrock wells of that kind are where arsenic, radon and uranium turn up, and yields vary from well to well.
Classification from the USGS Principal Aquifers of the United States (national map, 2003 edition, GIS release 2015), read at the county centroid. A single well can sit in a different unit, especially near a boundary.
Official help in Illinois
Two places to start that are not selling anything: the state program that answers private well questions, and its list of labs certified to run drinking water tests. A certified lab is the difference between a result you can act on and a number.
- Private Water & Closed Loop Wells, IDPH
- Accredited Labs, Illinois EPA
Treatment that fits the local profile
PFOA
Granular activated carbon whole-house; Point-of-use reverse osmosis; Anion exchange
Arsenic
Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
PFOS
Granular activated carbon whole-house; Point-of-use reverse osmosis; Anion exchange
Nitrate
Point-of-use reverse osmosis; Anion exchange; Distillation
Chloride
Point-of-use reverse osmosis; Distillation
Systems we would look at first
Some links below are affiliate links. If you buy through one, the merchant pays us a commission at no extra cost to you. It does not change which products we list or what the data says.
For PFOA
Under-sink reverse osmosis system
under-sink or countertop, about $300
Look for NSF/ANSI 58 certification listing the contaminant you need removed.
For PFOA
Whole-house PFAS carbon system
whole-house, at the point of entry, about $3,000
Granular activated carbon certified to NSF/ANSI 53 for PFOA and PFOS; replace media on schedule.
For PFOA
Aquasana SmartFlow reverse osmosis
under-sink or countertop, about $225
Certified to NSF/ANSI 58 and 53 to reduce arsenic (96%, pentavalent), fluoride (90%), lead (99%), nitrate, nitrite and PFOA/PFOS at the kitchen tap, with a remineralizer.
For arsenic
Whole-house arsenic and uranium filter
whole-house, at the point of entry, about $2,500
Adsorptive iron-based media; sized to your well yield.
For PFOS
Aquasana Claryum 3-Stage under-sink filter
under-sink or countertop, about $175
Certified to NSF/ANSI 53 to reduce 99% of lead and 99.7% of PFOA and PFOS at the tap. Not a fix for arsenic, nitrate or bacteria.
For PFOS
Aquasana HealthShield whole-house filter
whole-house, at the point of entry, about $2,099
Whole-house carbon system the maker certifies to reduce PFAS (PFOA and PFOS) and VOCs at every tap; a UV option adds bacteria protection.
Free, no obligation
Get on the list for a free in-home water test in McHenry County
No water treatment company serves McHenry County through Well Water Nerd yet. We are signing licensed companies on county by county: they test at your tap for free, walk you through the numbers, and quote treatment only if something needs fixing. Leave your details and you are first in line. Nothing is shared until a company joins for McHenry County, and you get an email naming it first.
Common questions
Is there arsenic in well water in McHenry County?
Arsenic was detected in 55% of 271 public well samples in McHenry County, and 10% were above the EPA limit of 10 ug/L. Arsenic has no taste or smell; a lab test is the only way to know your level.
How high is radon in well water in McHenry County?
The median radon level across 16 public samples in McHenry County was 205 pCi/L, with a high of 460 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in McHenry County?
Test for bacteria and nitrate every year. Test once for arsenic, uranium, radon, fluoride, manganese and PFAS, then again every three to five years or after any change in taste, color, or plumbing.
Counties near McHenry County
Closest first. Groundwater does not stop at a line on a map, so the next county over is a fair second opinion.
Want the numbers themselves? Download the Illinois dataset as CSV, every county and town, with the sources named.
Sources
- USGS Water Quality Portal: 3,356 samples in McHenry County. Data from the Water Quality Portal, a service of USGS, EPA and USDA; groundwater samples collected by USGS and cooperators. Retrieved September 3, 2026.
- USGS national arsenic model (Ayotte 2017): estimated share of 65,640 people on private wells in McHenry County above 10 ug/L of arsenic, from geology, well depth and land use. County estimates of domestic well users with arsenic above 10 ug/L from the USGS national logistic regression model (Ayotte, Medalie and Qi, 2017), with private well population from the USGS 2015 county water-use compilation (Dieter and others, 2018). Retrieved September 15, 2026.
Non-detects count as samples and are treated as zero when computing medians. Exactly at the limit counts as not above it. A sample belongs to the town containing the well's reported coordinates.