If you get your water from a private well, nobody is testing it but you. The federal Safe Drinking Water Act covers public water systems, not private household wells, so testing is up to the owner. CDC and EPA both recommend testing a private well at least once a year for total coliform bacteria, nitrate, total dissolved solids and pH, using a state-certified lab, plus extra tests after floods, repairs, or any change in taste, color or smell.
That’s the short answer. The rest of this guide explains why each test matters, when to test more often, how to find a certified lab, and what shock chlorination is if your well tests positive for bacteria.
Why well owners are on their own
According to EPA, around 15 percent of the US population, more than 43 million people, rely on private wells for drinking water. That is a lot of people outside the system that TapWaterSafety.org grades.
Public water systems have to sample on a schedule, meet enforceable limits, notify customers about violations, and publish a yearly water quality report. (Our methodology page explains how we use those public records to grade 1,000 large systems.) A private well has none of that. EPA states plainly that private wells are not regulated by the federal government under the Safe Drinking Water Act, so the owner is responsible for making sure the water is safe.
The upside is control. You decide what to test for and how often, and many fixes are within reach of a homeowner and a licensed well contractor.
The four tests to run every year
Both CDC and EPA list the same core annual panel:
Total coliform bacteria. Coliforms are a family of bacteria that are mostly harmless themselves, but their presence signals that surface water, soil or waste may be getting into the well. If total coliform is positive, labs typically also check for E. coli, which indicates fecal contamination.
Nitrate. Nitrate comes from fertilizer, septic systems and animal waste, and it is a particular risk for babies under six months. If you use well water to mix formula, ask your pediatrician. See our full guide to nitrate in tap water.
Total dissolved solids (TDS). A measure of everything dissolved in the water. It isn’t a health test on its own, but a change from year to year can be an early sign that something has shifted.
pH. Water that is too acidic can corrode plumbing and pull metals such as lead and copper into your water.
EPA adds that you should test more often if a pregnant or nursing person, small children or older adults live in the home.
Tests to add based on where you live
The annual panel is a baseline, not a complete picture. Both agencies say to ask your state or local health department which contaminants are common in your area’s groundwater. Depending on geology and land use, that can include:
- Arsenic, which occurs naturally in rock in many regions
- Radium and uranium, also natural
- Lead, mostly from plumbing and well components rather than the aquifer
- Volatile organic compounds, mercury, and pesticides or herbicides, which CDC lists as other tests to consider
EPA gives two examples of matching tests to land use. Near intensive agriculture, test for nitrate, nitrite, pesticides and coliform bacteria. Near oil and gas drilling, test for chloride, sodium, barium and strontium.
When to test again, outside the yearly schedule
CDC and EPA both say to test right away if:
- You hear about water problems or groundwater contamination in your area
- Flooding, land disturbance, new construction or industrial activity happens near the well, or there is a waste disposal site nearby
- You repair or replace any part of the well system
- The water’s taste, color or smell changes
CDC adds two family events: when someone in the home becomes pregnant, and when a child comes to live with you. Minnesota’s health department specifically recommends testing for nitrate before bringing a newborn home.
How to find a certified lab
Use a lab certified for drinking water testing. CDC says your local health department or county government can give you a list of certified labs nearby, and EPA points to your state’s certified laboratory listings. Some health departments test for common contaminants themselves, and EPA notes that some offer free private well testing.
A few practical tips:
- Get the bottles from the lab. Bacteria samples in particular need sterile bottles and must reach the lab quickly.
- Follow the sampling instructions exactly. Where you sample, whether you remove the aerator, and how long you run the tap all affect the result.
- Keep your results. Year-over-year records are how you spot a slow change, like nitrate creeping upward.
Home test strips can be useful for a quick check between lab tests, but they are not a substitute for a certified lab result. Our guide on how to test your tap water at home covers the trade-offs.
If your well tests positive for bacteria
A positive total coliform result means the well needs attention. Until the problem is fixed and a follow-up test is clean, use bottled water or boil water for drinking, cooking and brushing teeth (a rolling boil for 1 minute, or 3 minutes above 6,500 feet, per CDC).
The usual next step is shock chlorination: putting a strong dose of chlorine bleach into the well and plumbing to kill bacteria, then flushing it out. It is also commonly done after flooding and after repairs. Here is how the process works according to CDC and the Minnesota Department of Health, which publishes one of the most detailed state guides.
The basic steps
- Bypass treatment equipment. Chlorine can damage water softeners and other devices. Minnesota says to bypass them and remove filters first.
- Turn off power to the pump before opening the well.
- Mix the bleach. Use plain, unscented household liquid bleach with no added detergents, and not pool chemicals. The amount depends on the well’s water depth and casing diameter. Minnesota’s table ranges from about 2 cups to 10 cups; CDC has its own tables for drilled and dug wells.
- Add it to the well through a funnel, keeping bleach off wires and metal parts.
- Circulate it. Run a garden hose from an outside faucet back into the well, then open each indoor faucet until you smell chlorine.
- Let it sit. This is where guidance differs. Minnesota says a minimum of two hours, preferably six hours or overnight. CDC’s post-emergency guidance says at least 12 hours.
- Flush it out until the chlorine smell is gone. CDC says to drain it onto an area without plants, such as a driveway, and not into streams, ponds or other open water. Follow your state’s rules on where to discharge.
- Retest. Test for total coliform only after all the chlorine is gone. CDC says to wait 7 to 10 days after disinfection, then retest again in 2 to 4 weeks and in 3 to 4 months. Minnesota recommends retesting 2 to 4 weeks after disinfection.
Safety notes
Wear goggles or a face shield and work in a ventilated area. CDC warns never to enter a well pit, because gases can build up. CDC also recommends having a well or pump contractor do the disinfection because of these safety concerns.
If bacteria keep coming back
Shock chlorination is a one-time disinfection, not a permanent fix. If coliform returns, something is letting contamination in: a cracked casing, a missing or damaged cap, poor grading around the wellhead, or a nearby septic problem. Both CDC and Minnesota say to repeat disinfection once, and if bacteria persist, contact your local health department or a licensed well contractor. The well may need repair, cleaning, or continuous treatment.
What about filters?
A filter is a response to a specific test result, not a replacement for testing. Reverse osmosis can reduce contaminants such as nitrate, arsenic and radium, but an ordinary carbon pitcher won’t fix a bacteria problem, and no filter should be your answer to a well that keeps testing positive for E. coli. Test first, then match any treatment to what you found, and look for an NSF/ANSI certification that covers that specific contaminant.
Bottom line
Test every year for coliform bacteria, nitrate, TDS and pH. Add local contaminants your health department recommends. Retest after floods, repairs, pregnancies and any change you can see, smell or taste. Use a certified lab, keep your records, and treat a positive bacteria result as a reason to find the source, not just to disinfect.
If you’re on city water instead, you can look up your utility’s EPA record and grade, or read how groundwater and surface water systems compare.