How often should you test well water for bacteria?

CDC says to test total coliform bacteria at least once every year at a state-certified laboratory. Test again after flooding, well repairs, or a change in taste, smell, or color. Penn State Extension also puts bacteria on a yearly list. A present coliform still goes to the local health department.

When should you test well water?

Once a year for CDC's floor: total coliform, nitrates, TDS, and pH. Test right away after flooding, well-system repairs, a change in taste, smell, or color, nearby land-use changes, a local well-water notice, pregnancy, or a child moving in. EPA prints the same immediate list. Buying a house with a well is another EPA-named reason to test.

Does well water need to be tested?

Yes. EPA's private-well pages and CDC's well-testing guidelines put testing on the owner. Public-system rules do not cover a privately owned well. Use a state-certified laboratory. Bacteria or nitrate still goes to the local or state health department.

Should I test well water when buying a house?

EPA's Home Water Testing fact sheet says to test if you are buying a home and want to assess the existing supply. The listed panel starts with coliform, nitrate, lead, radon, iron, hardness, pH, sulfate, TDS, and a corrosion index, plus extras that follow nearby land use. Use a state-certified lab. Bacteria or nitrate still goes to the health department.

What should I test well water for?

CDC's yearly floor is total coliform, nitrates, total dissolved solids, and pH. Ask the local health department what else is common where you live. EPA matches extras to nearby activity: agriculture, lead plumbing, gas stations, mining. A store strip cannot replace the yearly bacteria or nitrate record.

Guide · Updated October 19, 2026

How often should you test your well water?

Direct answer

CDC and EPA tell private-well owners to test at least once a year for total coliform, nitrates, TDS, and pH at a state-certified lab. Test again after flooding, well work, or a change in taste, smell, or color. Extra tests follow the house and the geology. Bacteria or nitrate still goes to the local health department.

What to test
What to test	Why	EPA MCL / SMCL or guidance
Total coliform	CDC’s annual floor for a private well. A present result is a pathway warning, not a shopping trip.	Present/absent — any detected coliform is a positive until a clean retest CDC well-testing guidelines
Nitrate as N	Same yearly panel as coliform. A result at or above the public-system line goes to the health department.	10 mg/L as N (EPA public-system MCL) EPA National Primary Drinking Water Regulations
pH	CDC annual floor. Low pH is a corrosion clue, not an iron-filter job.	6.5–8.5 SMCL (aesthetic guideline) EPA secondary standards
Total dissolved solids	CDC annual floor with coliform, nitrate, and pH.	500 mg/L SMCL (aesthetic guideline) EPA secondary standards

What to circle on the sheet

A teaching diagram, not a lab form. Circle these names on your real report. Bacteria or nitrate still goes to the health department.

Iron
Orange or rust stains. Secondary (aesthetic) number is 0.3 mg/L. Treatment class, not a health-department line.
Manganese
Brown to black stains. Secondary number is 0.05 mg/L. Often travels with iron.
H2S
Rotten-egg odor. The gas often leaves a mailed bottle. Trust the nose; ask for a field kit if you need a number.
Coliform
Presence or absence. Health department. This site will not turn that line into a filter shopping list.
Nitrate
Compared to 10 mg/L as N. Health department. Not a softener or iron-filter problem.

Private wells are not city water. CDC and EPA put testing on the owner. This page is a yearly routine, not medical advice.

Steps

Once a year, order bottles from a state-certified drinking-water lab. CDC's floor is total coliform bacteria, nitrates, total dissolved solids, and pH. EPA's Protect Your Home's Water page prints the same four.
Sample the untreated tap the way the lab printed on the bottle. Bacteria has a hold time. A Friday drop-off that sits over a weekend is often void.
Read the sheet. Circle total coliform, E. coli, nitrate, and nitrite first. Units and "ND" live on how to read a well water lab report.
If coliform is present, or nitrate is at or above 10 mg/L as N (EPA's public-system MCL), take the sheet to the local or state health department. See coliform bacteria in well water and nitrates in Wisconsin well water.
Add extras that follow the house and the geology: iron or manganese for stains, lead if the solder is old, arsenic or radon where the state names them, PFAS only as a certified-lab panel. Well water test kits can screen iron or hardness. They cannot replace the yearly bacteria or nitrate record.

When to test right away

Do not wait for the anniversary if any of these happen. CDC and EPA both print an extra-test list.

Flooding, land disturbance, or a nearby waste site. If floodwater reached the wellhead, do not start here. Start with Well flooded? What to test and when.
Well repairs, a new pump, or other well-system work. CDC: test after you replace or repair any part of the well system.
A change in taste, smell, or color. CDC and EPA both treat that change as a reason to test now.
Nearby septic problems, construction, or a new industrial neighbor. EPA: test when conditions near the well change, including new construction or industrial activity.
Pregnancy, nursing, or a child in the home. CDC: test if you or someone you live with becomes pregnant, or if a child starts living with you. EPA says you may want to test more often if small children or elderly adults live there, or if someone is pregnant or nursing. Wisconsin DNR also wants a nitrate test before the well will be used by someone who is or may become pregnant.
Buying a house with a well. EPA's Home Water Testing fact sheet: if you are buying a home and want to assess the existing supply, test coliform, nitrate, lead, radon, iron, hardness, pH, sulfate, TDS, and a corrosion index, plus extras that follow nearby land use.
A notice of well-water problems in your area. CDC and EPA both name that alert as a same-season test.

CDC also says to check the well itself every spring for mechanical problems. That is a wellhead look, not a substitute for the lab sheet.

State lab links

Use a laboratory certified for drinking water in your state. Start at certified labs by state, then the inventory that matches the house:

Some state pages print a longer cadence than CDC's four-item floor. Penn State Extension: bacteria every year; pH and TDS every three years; extras that follow land use every three years. Wisconsin DNR: bacteria once a year and when taste, color, or smell changes; nitrate once a year and before someone who is or may become pregnant uses the well; arsenic every five years for most wells, and every year in Outagamie, Winnebago, or Brown counties or if a prior sample found arsenic. Those are state recommendations. They do not cancel CDC's yearly bacteria and nitrate pair.

What to test

CDC: at least once every year, test total coliforms, nitrates, total dissolved solids, and pH, plus other germs and chemicals of concern where you live. Ask the local health or environmental department, or EPA's Drinking Water Hotline at 800-426-4791, what else is common in your groundwater.

EPA's Protect Your Home's Water chart matches tests to nearby activity: coliform after recurring gastrointestinal illness; nitrate, nitrite, pesticides, and coliform near intensive agriculture; lead and copper if plumbing may contain lead; VOCs near a dump, landfill, gas station, or dry cleaner. PFAS in private well water is a certified-lab panel, not a store strip.

USGS Circular 1332 is the national backdrop: contaminants in domestic wells are often invisible in the glass. A clear tap is not a result.

After the sheet comes back

CDC: if the result shows harmful germs or chemicals, consider bottled water or another safe source and contact the local health or environmental department. That department can help you read the sheet and may want more frequent tests. This site will not turn a number into a diagnosis.

Official certified-lab and health-department paths come first. Tap Score / SimpleLab is a class example of a mail-in certified-lab kit, not a ranked pick (Awin tracked URL). Well Diagnosis may earn a commission if you buy through an approved affiliate link on this page. That does not change the price you pay. See the disclosure. SoftPro is not on this page. Bacteria or nitrate still goes to the health department.

FAQ

How often should you test well water for bacteria?: CDC says to test total coliform bacteria at least once every year at a state-certified laboratory. Test again after flooding, well repairs, or a change in taste, smell, or color. Penn State Extension also puts bacteria on a yearly list. A present coliform still goes to the local health department.
When should you test well water?: Once a year for CDC's floor: total coliform, nitrates, TDS, and pH. Test right away after flooding, well-system repairs, a change in taste, smell, or color, nearby land-use changes, a local well-water notice, pregnancy, or a child moving in. EPA prints the same immediate list. Buying a house with a well is another EPA-named reason to test.
Does well water need to be tested?: Yes. EPA's private-well pages and CDC's well-testing guidelines put testing on the owner. Public-system rules do not cover a privately owned well. Use a state-certified laboratory. Bacteria or nitrate still goes to the local or state health department.
Should I test well water when buying a house?: EPA's Home Water Testing fact sheet says to test if you are buying a home and want to assess the existing supply. The listed panel starts with coliform, nitrate, lead, radon, iron, hardness, pH, sulfate, TDS, and a corrosion index, plus extras that follow nearby land use. Use a state-certified lab. Bacteria or nitrate still goes to the health department.
What should I test well water for?: CDC's yearly floor is total coliform, nitrates, total dissolved solids, and pH. Ask the local health department what else is common where you live. EPA matches extras to nearby activity: agriculture, lead plumbing, gas stations, mining. A store strip cannot replace the yearly bacteria or nitrate record.

State pages and the labs hub

Sources

1.CDC. Guidelines for Testing Well Water.
2.U.S. EPA, Private Wells. Protect Your Home’s Water.
3.U.S. EPA. Private Drinking Water Wells.
4.U.S. EPA. Fact Sheet: Home Water Testing.
5.U.S. EPA. National Primary Drinking Water Regulations.
6.U.S. Geological Survey. Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004 (Circular 1332).
7.Penn State Extension. Testing Your Drinking Water.
8.Wisconsin Department of Natural Resources. Test your private well water annually.