How often to test well water, and 9 events that call for an extra test

Test a private well at least once a year for total coliform bacteria and nitrate; CDC and EPA add total dissolved solids (TDS) and pH. The slower-changing tests (arsenic, lead, uranium, manganese and the rest of a mineral panel) come back every two to ten years, or just once, depending on your state. On top of that, nine events call for an extra test whatever the calendar says.

EPA's private wells page says water from private domestic wells is "not regulated by the Federal Government under the Safe Drinking Water Act nor by most state governments and laws." Federal rules cover public water systems: at least 15 service connections, or an average of at least 25 people for at least 60 days a year. A household well sits below that line, so the schedule is yours to keep.

How often should you test your well water?

Every year, for at least bacteria and nitrate. CDC's guidelines for testing well water say: "Test your well water at least once each year for total coliform bacteria, nitrates, total dissolved solids, and pH levels." EPA's Protect your home's water page names the same four. Of the 26 state schedules in our table below, 23 name a yearly bacteria test (Alabama, Connecticut and Mississippi give a yearly test without naming bacteria) and 17 add nitrate.

The rest varies. Maine wants a long list every three to five years, Massachusetts a similar list every ten, Minnesota arsenic, lead and manganese "at least once."

Why yearly for some tests and years apart for others? Bacteria and nitrate can arrive from the surface: EPA's septic systems page says wastewater "may contaminate your drinking water through an unsecured well cap or cracks in the well casing." Arsenic, uranium and radium are tied to the ground: Connecticut's well statute lets a local health director require those tests where there are reasonable grounds to suspect them, such as known local geology. That, in our reading, is why most states test them less often.

Virginia's Household Water Quality Program analyzed 3,760 samples in 2024, and its annual report says "Annual well testing is recommended," then reports that 37% of participants said they had never tested their water before and 31% had tested only once.

The federal baseline: what CDC and EPA say

CDC's page is dated July 1, 2024; EPA's was last updated February 26, 2026.

The which tests do I need checklist turns EPA's conditions table into tick boxes.

Your state's schedule: 26 states, each in its own agency's words

Each row is paraphrased from the agency in the last column and fact-checked against its page. Your county health department may add tests.

Three notes. Connecticut's row predates its 2022 law: under CGS § 19a-37, every private well built since October 1, 2022 must be lab-tested before household use (see the new-well section). In Colorado, CSU Extension's fact sheet says coliform every one to two years, though its checklist says once a year; follow CDPHE's yearly line. And we haven't yet verified a state schedule for Kentucky, Michigan, Ohio, Pennsylvania or Virginia; there, follow the CDC and EPA yearly panel.

Test more often when a result was close, or when you treat

• Nevada's extension sheet: test more often if levels are close to the standards.
• Washington DOH: if nitrate is 5 mg/L or higher, you may want to retest in six months.
• Wisconsin DNR: arsenic goes from every five years to every year once it has been found.
• Oregon Health Authority: retest if arsenic is above 8 ppb; with treatment, keep testing every year at the faucet you drink from and every three years at the wellhead.
• MDH: if you treat your water, test the water after it goes through treatment.

For the numbers, see how to read a well water test report or use the results checker.

Nine events that call for an extra test

1. After a flood, heavy rain or snowmelt

EPA says floods and other disasters "can impact the safety of your drinking water by allowing contaminants to enter your private well system." Its first steps: stay away from the flooded pump to avoid electric shock, and don't drink or wash with the water. MDH's flooded-well page says the water "cannot be regarded as safe for drinking or food preparation until the well and plumbing system have been flushed and disinfected, and a water test shows that it is safe."

Bacteria is the first test. South Dakota DANR asks for more: after heavy rain, snowmelt or flooding near your well, submit a sample for at least bacteria and nitrate. Missouri and Colorado also list flooding.

One clean result isn't the end. The Oregon Health Authority's emergencies and wells page warns: "Your well may not be a safe source of water for many months after a flood." Its advice: "Repeated testing is strongly recommended." See our well maintenance guide for flood preparation, the shock chlorination guide for disinfection, and the sediment page for muddy water after rain.

2. After a wildfire

Flames don't have to reach the casing. CDC's guidance on private wells after a wildfire, written for health departments, names three routes: lost power or water pressure, damaged well equipment, piping and wiring, and damage near the home, including septic systems. Where a well lost pressure, residents should not drink the water until it has been tested, and CDC says to "Test water for bacterial contamination at a minimum."

Burned plastic adds tests. Where plastic piping is damaged and there is fire damage in the area, CDC calls for VOC and SVOC (volatile and semivolatile organic compound) testing at a certified lab. For any post-fire sampling, it suggests broad methods "such as EPA methods 502.2 or 524.2," says to "Consider heavy metals testing (e.g., lead, copper, zinc, and iron)," and to "Allow stagnation time before sampling." On plastic piping it adds: "Consider SVOC or VOC testing as damage might not be detectable through the senses."

Oregon's version for homeowners: "We recommend testing for arsenic, nitrate, bacteria, lead, and, depending on damage assessment results, benzene, toluene, ethylbenzene, and xylenes (BTEX)," and testing matters "Even if the fire did not directly damage your well." CDC says "Do NOT handle or touch electrical wiring." See the VOC explainer; VOC-certified labs are in the lab finder and on state pages such as California, Oregon, Colorado and Washington.

3. Pregnancy, a new baby or mixing formula

Pregnancy is one of the most-cited triggers. CDC lists "You or someone you live with becomes pregnant" and "A child starts living with you"; EPA says you may want to test more often with someone pregnant or nursing, or small children, in the house. MDH's Safe Drinking Water for Your Baby page explains why: high nitrate "can affect how blood carries oxygen and can cause methemoglobinemia (also known as blue baby syndrome)," bottle-fed infants under six months are at highest risk, babies "drink more water for their size than older children and adults," and "Some contaminants can pass from mother to baby during pregnancy."

Two agencies give timing. Wisconsin DNR says to test nitrate "before the well will be used by a woman who is or may become pregnant." Oklahoma DEQ says it is preferable to test coliform and nitrate/nitrite early in a pregnancy, and recommends testing before bringing an infant home. MDH's heading reads "Test your well water before or during pregnancy," and its list is wider than the yearly pair: coliform and nitrate every year, plus lead, manganese and arsenic at least once.

Can you use well water for formula?

Only once it's tested, and the call belongs to you and your baby's doctor. Illinois EPA says to always test nitrate before giving well water to an infant. MDH says water can be harmful to a baby if nitrate is above 10 mg/L (as nitrogen) or manganese above 100 µg/L. Those are Minnesota health lines; federal limits such as EPA's nitrate MCL are set for public water systems, not private wells. MDH also says not to boil water to deal with nitrate, because boiling increases the concentration. On fluoride, MDH says to ask your baby's doctor or dentist whether to test (see our fluoride page). Background: the nitrate explainer; labs: nitrate labs.

4. Near farms, livestock or a septic system

Oregon State University's Well Water Program groups these: it suggests testing nitrate once a year if you live near fertilized fields, septic systems or livestock, or have had nitrate before.

Fertilizer, manure and fields

EPA's table pairs "Nearby areas of intensive agriculture" with "Nitrate, nitrite, pesticides, coliform bacteria." Pesticides (and, on the CDC/EPA panel, nitrite) are what a yearly panel leaves out, so ask your lab or health department which pesticides are used locally. CSU Extension lists feed yards nearby as a reason to test bacteria more often. Nitrate labs by state: nitrate testing in Iowa, Minnesota, California, Oregon.

A septic system nearby, or a septic failure

EPA says a well "is at greater risk of becoming contaminated if it is in the path of groundwater flow" from a septic system, and "Contamination is less likely the farther apart a well is from a septic system." The Oregon Health Authority says you may need to test more often if the septic system recently malfunctioned, and CSU Extension says the same for a failed system. Test coliform and nitrate; the coliform bacteria page explains a positive.

5. Near oil and gas drilling

EPA's table lists "Gas drilling operations nearby" with "Chloride, sodium, barium, strontium," none of which is on a yearly panel. Wyoming DEQ says uranium, radium, selenium and methane are also common in Wyoming groundwater, and owners in areas of possible oil and gas development should test for more. Our reading, not an agency rule: test before drilling starts, since only a before-result shows your baseline. Ask your state health department or oil and gas regulator which extra parameters to add.

6. A new well

Some states set the first test by law or rule. Connecticut's CGS § 19a-37 requires every private well built since October 1, 2022 to be lab-tested for coliform, nitrate, nitrite, sodium, chloride, iron, manganese, hardness, turbidity, pH, sulfate, color, odor, arsenic and uranium (plus first-draw lead if it serves an existing building), and the well may not be used for household purposes until the local health authority finds the results meet state limits. South Dakota DANR says every new domestic well must be tested for bacteria and several selected chemicals under the Centennial Environmental Protection Act of 1989. Vermont's rules require a set of tests for new or deepened wells and springs.

Elsewhere it's advice. Illinois DPH recommends testing all new wells, including nitrate; Iowa DNR and Oklahoma DEQ name a new well as a reason to test. Colorado's CDPHE lists a broader panel, including PFAS and a VOC screen, for a new well or one with no known testing.

7. After repairs, pump replacement or shock chlorination

CDC and EPA both list replacing or repairing any part of the well system. Iowa DNR says to test after any work on the well, pump or water system; Missouri DHSS, any time the well is serviced; Oklahoma DEQ names work on the pipes, pressure tank, pump or casing. Order a bacteria test once the system has been disinfected and flushed; MassDEP says to test "only after disinfection and substantial flushing of the water system." Disinfection after well work, and who should do it, is in our maintenance guide.

After shock chlorination

A shock needs two tests. MDH's well disinfection page says to sample once the bleach is flushed out, to confirm a negative total coliform result before drinking or cooking. Then: "Total coliform may regrow in the water system. For this reason, it is important to retest your water between two to four weeks after disinfection." Other agencies' retest timing is in the shock chlorination guide.

8. Buying or selling a home

Oregon is the clearest state rule. Under its Domestic Well Testing Act (ORS 448.271), the seller must have the water tested for arsenic, nitrate and total coliform bacteria by an accredited lab, and the state's rules say the tests can't be waived even if the buyer agrees. Connecticut requires that a buyer or tenant be given the health department's well-testing material, which recommends testing. Florida, Louisiana and Tennessee have no sale requirement; Florida DOH still recommends coliform, nitrate and lead tests.

Lenders fill the gap. Oklahoma DEQ says many lenders require a test before closing, most often total coliform/E. coli, lead and nitrate/nitrite; some want the test no more than 30 days before closing, and a positive coliform result can take several weeks to fix and retest, so confirm the list with your lender early. Loan-program rules are in our home sale test guide for FHA, VA and USDA loans; the physical check is in the well inspection guide.

9. A change in taste, smell or color

CDC, EPA, Missouri, Iowa and Colorado all list it. Wisconsin DNR ties it to bacteria: "once a year and when you notice a change in taste, color or smell." Wyoming DEQ says to test right away. EPA's table pairs an objectionable taste or smell with hydrogen sulfide, corrosion and metals, and stained fixtures or laundry with iron, copper and manganese. The smell, taste and stain guide matches symptoms to causes, and the sulfur smell guide covers the rotten-egg fix.

Unexplained illness, or a neighbor's bad result

EPA pairs "Recurring gastro-intestinal illness" with coliform bacteria, and Wyoming DEQ says to test right away if someone in the household has an unexplained illness. (See a doctor too; a water test diagnoses no one.) Colorado's CDPHE says to test if contaminants turn up in a neighbor's well, and EPA says to test immediately if there are known groundwater problems in your area. If a result is positive for E. coli, the E. coli page sets out what to do first.

Two worked examples

An Iowa household expecting a baby. Bacteria and nitrate were last tested 14 months ago, arsenic and manganese two years ago. Pregnancy is event 3: test bacteria and nitrate now. Iowa DNR's three-year interval puts arsenic and manganese due next year.

A Massachusetts bedrock well after a pump replacement. The full MassDEP list was run in 2021, so the ten-year panel is due by 2031. The pump work is event 7: after the system has been disinfected and flushed, order a coliform test, and keep the yearly coliform and nitrate date.

Next: run the which tests do I need checklist, read the well water testing guide for sampling and costs, and find a state-certified lab with the lab finder, or check free well water testing programs first.

Questions people ask