Private well water: what to test for, county by county
About 37.2 million people in the lower 48 states use a private well (USGS 2010 estimate). Nobody publishes results for a private well, so a lab test is how you find out what is in yours. Pick your state to see your county’s modelled arsenic and nitrate.
Pick your state
- Alabama536,400 well users67 counties
- AlaskaTesting guidance (no USGS model)
- Arizona242,000 well users15 counties
- Arkansas470,700 well users75 counties
- California1.4 million well users58 counties
- Colorado340,700 well users64 counties
- Connecticut672,800 well users8 counties
- Delaware141,300 well users3 counties
- District of Columbia580 well users1 county
- Florida1.9 million well users67 counties
- Georgia1.4 million well users159 counties
- HawaiiTesting guidance (no USGS model)
- Idaho315,200 well users44 counties
- Illinois1.1 million well users102 counties
- Indiana1.4 million well users92 counties
- Iowa501,800 well users99 counties
- Kansas242,100 well users105 counties
- Kentucky532,700 well users120 counties
- Louisiana461,100 well users64 counties
- Maine510,200 well users16 counties
- Maryland726,100 well users24 counties
- Massachusetts430,400 well users14 counties
- Michigan2.4 million well users83 counties
- Minnesota1.1 million well users87 counties
- Mississippi365,900 well users82 counties
- Missouri870,600 well users115 counties
- Montana282,300 well users56 counties
- Nebraska232,500 well users93 counties
- Nevada111,600 well users17 counties
- New Hampshire396,100 well users10 counties
- New Jersey766,100 well users21 counties
- New Mexico269,400 well users33 counties
- New York1.9 million well users62 counties
- North Carolina2.2 million well users100 counties
- North Dakota96,900 well users53 counties
- Ohio1.8 million well users88 counties
- Oklahoma470,000 well users77 counties
- Oregon521,600 well users36 counties
- Pennsylvania2.2 million well users67 counties
- Rhode Island98,800 well users5 counties
- South Carolina877,400 well users46 counties
- South Dakota122,200 well users66 counties
- Tennessee698,000 well users95 counties
- Texas1.7 million well users254 counties
- Utah79,200 well users29 counties
- Vermont206,200 well users14 counties
- Virginia1.4 million well users133 counties
- Washington678,900 well users39 counties
- West Virginia430,800 well users55 counties
- Wisconsin1.5 million well users72 counties
- Wyoming112,600 well users23 counties
Bars show estimated well users (USGS 2010), relative to the state with the most. Hatched: testing guidance only, no USGS county model.
“Test your well water at least once every year for total coliform bacteria, nitrates, total dissolved solids, and pH level.”
CDC- Every yearTotal coliform bacteria
- Every yearNitrate
- Every yearTotal dissolved solids
- Every yearpH
Arsenic occurs naturally in groundwater in many areas. CDC suggests asking your local health department whether to test for it.
- After flooding or land disturbance near the well
- After the well is repaired or a part is replaced
- When the water’s taste, colour or smell changes
- If someone in the home is pregnant or nursing
- When an infant or child joins the household
Sources: EPA: Private Drinking Water Wells · EPA: Protect Your Home’s Water · CDC: Guidelines for Testing Well Water
Estimates for an area, not results for a well
The county figures come from two USGS models of groundwater used by private wells. The CDC publishes both, for every census tract, in its Well Water Index, with the USGS estimate of how many people use wells there. PurityMap adds the tracts up to counties and weights each tract by its well users, so empty land doesn’t count the same as the places where wells are. A county’s arsenic figure is the share of its well users the model places above the limit. These are area estimates; they are never a result for any single well.
- Arsenic model
Estimates the chance that arsenic is above 10 µg/L.
- Nitrate model
Estimates the typical nitrate level at well depth.
- Area, not your well
These figures describe the area, not your well. Only a lab test shows what’s in your water. CDC says the Well Water Index figures “do not describe risk to individual wells.”
- Model limits
The arsenic model is better at spotting low-arsenic areas than at confirming high ones. Well-user counts are 2010 USGS estimates; tract values are averages over each census tract.
Screening tells you if something is worth a closer look
A lab result is the number you can act on.
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Lab-certified results
ETR Labs Basic Water Quality Test Kit
ETR Laboratories
50+ substances, including lead, copper, arsenic, nitrate, hardness and coliform bacteria
- Analyzed by Environmental Testing and Research Laboratories (ETR Labs), Leominster, MA · ISO/IEC 17025:2017, ANAB certificate AT-2638
- Turnaround: 2–4 business days after the lab receives it
What it can't tell you: It does not test PFAS, and not every result on the panel falls under the lab's accredited scope.
Screening: quick check, not lab-grade
Watersafe Drinking Water Test Kit
Silver Lake Research
Lead, bacteria, nitrate, copper, iron, chlorine, hardness and pH
- Turnaround: Minutes; the bacteria test can take up to 48 hours
What it can't tell you: Each test gives a yes/no or a rough range, not a lab number, and it cannot detect PFAS or arsenic.
A mail-in kit’s lab is accredited (ISO/IEC 17025) but may not be certified by your state. For a home sale, a loan, or a health-department follow-up, use a state-certified lab.
Private well water
- CDC Environmental Public Health Tracking: Well Water Index (2024)
- Lombard 2021: arsenic in private wells, conterminous US (USGS data release)
- Ransom et al. 2021: nitrate in groundwater used for drinking supply (USGS data release)
- Johnson & Belitz 2019: domestic well locations and populations served, 2000 and 2010 (USGS data release)
- EPA: Private Drinking Water Wells
- EPA: Protect Your Home’s Water
- CDC: Guidelines for Testing Well Water
- State Certification Programs Certify Laboratories to Conduct Drinking Water Analyses, US EPA, August 2025
Data last updated 2026-10-08. PurityMap weights each census tract by its well users to give county figures. See something wrong? Submit a correction.
Are private wells covered by the Safe Drinking Water Act?
No. The quality and safety of drinking water from private domestic wells are not regulated by the Federal Government under the Safe Drinking Water Act nor by most state governments and laws. (EPA)
What should I test my well for?
Test your private well annually for total coliform bacteria, nitrates, total dissolved solids, and pH levels. (EPA) CDC also suggests asking your local health department about arsenic and other local concerns.
What do the county figures mean?
They are USGS model estimates for the area, published through the CDC Well Water Index and weighted by where well users live. They show how common high arsenic or nitrate is likely to be across a county, not what is in any single well.