Testing deserts: in 9 states, millions of well users have no certified lab in their county

Private-well owners are on their own for water testing. EPA says private well owners "are responsible for delivering safe drinking water to their households" and advises a test every year for total coliform bacteria, nitrates, total dissolved solids and pH. That test has to be done somewhere. We combined the official state lab certification lists on this site with USGS county data to ask a simple question: how many people who rely on a private well live in a county with no state-certified drinking-water lab inside it?

Why distance matters for one test in particular

Most well tests can travel. A sample for metals or nitrate can usually be mailed. Bacteria are different, because the sample has to reach the lab while any bacteria in it are still alive. The rules and advice differ:

So a lab in the next county, or a courier that runs on the right days, is often what makes a valid bacteria test possible. A county with no lab is not a county where testing is impossible. It is a county where the owner has to plan around the clock. The full sampling advice is in our guide on how to test well water.

The numbers, state by state

Vermont has the highest share: 70% of its 243,796 private-well users live in a county with no listed lab, and only 3 of its 14 counties have one. Minnesota follows at 60%, then Ohio at 54% and Virginia at 40%. At the other end, Washington (12%), Wisconsin (15%), Pennsylvania (18%) have a listed lab in the counties where most of their well users live.

In raw numbers the picture shifts toward the populous well states:

Ohio has about 1,003,575 private-well users in counties without a listed lab, Minnesota about 711,425 and Virginia about 628,573.

Where the arsenic estimate overlaps

USGS modeled the number of private-well users whose water is likely to exceed EPA's arsenic limit of 10 µg/L, county by county, on a 2010 population base. Vermont's health department puts it plainly: "You cannot see, smell or taste arsenic. Testing is the only way to know if arsenic is in your drinking water." Comparing that estimate with lab locations:

In Ohio, about 120,687 of the estimated 189,191 arsenic-affected well users live in counties with no listed lab, and in Minnesota about 57,804 of 80,354. USGS presents this kind of estimate as useful "for broad screening of the population at risk", so these are planning numbers, not counts of tested wells. Arsenic samples are not limited by the bacteria clock, so a mail-in test is a practical option; see arsenic in well water.

What this means if you live in one of these counties

• Look up your county: counties with at least 1,000 private-well users have a page with their well numbers and any listed labs located there, linked from your state page. The finder shows every lab on your state's list.
• For bacteria, call a lab first and ask about its kit, its holding time and which days it receives samples. Plan collection so the sample arrives in time.
• For arsenic, nitrate, metals, uranium and most other tests, ask whether the lab accepts samples by mail.
• Check whether your state or county runs a testing program: our state pages list the ones we verified.
• Use the which-tests checklist to decide what to order, and the finder to see every certified lab on your state's list, with the tests each one is certified for.

Method and limits

Private-well users are the USGS 2015 county estimate of people on domestic self-supplied water (Dieter and others, 2018). Arsenic figures are the USGS county estimates of domestic-well users with predicted arsenic above 10 µg/L (Ayotte, Medalie and Qi, 2017), on a 2010 population base. Labs are the laboratories on the official state certification lists shown on this site, placed in a county by the address on the list: the ZIP code, using the Census Bureau's 2020 ZCTA-to-county file; where a ZIP spans several counties, the county that contains the lab's town (Census 2020 place and county-subdivision files); where a list gives no ZIP (Maine's), the lab's town. A lab counts for the county it is located in, whichever state's list it appears on.

Limits we know about:

• Lists differ in scope. Some state lists cover only certain tests (Wisconsin's list on this site is its coliform-bacteria labs). New York is left out of the comparison because our New York list covers only labs in its PFAS rebate program. Pennsylvania's list comes from reports printed in June 2024.
• A county line is a rough measure of distance. A lab just across the line may be closer than one in the same county.
• Certified does not mean it takes homeowner samples. Some listed labs serve utilities only. The finder shows the homeowner flag where a state publishes one.
• Different years and methods. The well population (2015) and the arsenic estimate (2010 base) come from different USGS products, so they can disagree for some counties.
• States we have not added yet are not counted. This study covers 9 states. We will update it as lists are added.

The aggregate tables behind this page (by state and by county, counts only, no lab contact details) are not published as downloads. Journalists and researchers can request the data here: say who you are, where the work will appear and which data you need, and we will reply.

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