Bacterial contamination is one of the most common problems in private wells, according to Penn State Extension, and a positive coliform test doesn't by itself mean the water has made anyone sick. It means something from the surface has found a way into the well or plumbing, and germs that do cause illness could follow. Stop drinking the water until the problem is fixed and a retest is clean. If the report also says E. coli, it's more serious, because that bacterium comes from feces.
Most guides stop at "shock the well." Penn State's follow-up data suggest one shock chlorination usually doesn't fix it for good, so this page also covers how to read your results and find the cause.
Your well just tested positive? Until a retest is clean, use bottled water, or water boiled for one minute (3 minutes above 6,500 feet, per CDC), for drinking, cooking, brushing teeth, rinsing produce and bathing babies. Throw out ice made with the well water. If your report also shows high nitrate or arsenic, don't boil: the Minnesota Department of Health warns boiling can concentrate them. Use bottled water instead.
What is coliform in water?
Coliform bacteria are a large group of bacteria that live in soil, on plants and in the feces of people and warm-blooded animals. Most don't cause illness. In a water test they are an indicator: their presence suggests surface contamination has reached the water and that disease-causing germs could be present.
That is how Penn State Extension, the Washington State Department of Health and MDH all describe them. Washington adds the practical reason labs use them: "Testing drinking water for all possible pathogens is complex, time-consuming, and expensive. It is relatively easy and inexpensive to test for coliform bacteria." The table below shows how the three bacteria lines on a report fit together.
Total coliform, fecal coliform and E. coli: three nested tests
The three names on a report are groups inside groups. Each smaller one points more specifically to sewage or manure.
| Result line | What the group is | Where it comes from | What a positive tells you |
|---|---|---|---|
| Total coliform | A large group of related bacteria | Soil, vegetation, surface water, and also feces | A pathway from the surface exists. The source is probably environmental, but pathogens could use the same route. |
| Fecal coliform | A subgroup of total coliforms | Mostly the intestines and feces of people and warm-blooded animals | Washington's health department: it "often indicates recent fecal contamination." |
| E. coli | A subgroup of fecal coliforms | The USGS: "almost always found only in feces" | Human or animal waste is reaching the water. Penn State: "much more serious than coliform bacteria alone." |
If your report shows E. coli, read E. coli in well water: what to do today for the household checklist, who is at higher risk, and how to find the fecal source.
Sources: Washington State Department of Health, Penn State Extension, Coliform Bacteria (updated September 8, 2025), USGS SIR 2008-5227 (linked below).
Why labs test for an indicator
The Minnesota Department of Health (MDH) explains why total coliforms work as a stand-in: they are "everywhere on the surface of the ground, but do not usually occur past a few feet into the soil." When they show up in a water test, they "can indicate that surface contamination has gotten into the water."
Penn State adds that coliforms "usually persist in water longer than most disease-causing organisms," so their absence is taken as a sign of safe water. EPA's drinking water regulations table calls total coliforms "Not a health threat in itself" and sets the health goal (MCLG) at zero. That rule covers public water systems only. Nobody enforces it at a private well, but the state health departments cited here treat any detection as a problem to fix.
About E. coli: the strain behind outbreaks in the news is usually O157:H7. Washington's health department says an "E. coli present" result does not mean that strain is there, "and in fact, it is probably not present." It does mean recent fecal contamination, which can carry other germs.
What the germs behind a positive can do
EPA says pathogens in human and animal waste "can cause diarrhea, cramps, nausea, headaches, or other symptoms" and "may pose a special health risk for infants, young children, and people with severely compromised immune systems." If anyone has symptoms you think could come from the water, see a doctor and mention the well.
How common is coliform in U.S. domestic wells?
The largest national look at untreated domestic-well water is the USGS study of wells sampled from 1991 to 2004 (USGS Scientific Investigations Report 2008-5227). In the wells tested for bacteria:
- Total coliform was detected in 33.5 percent of 397 wells.
- E. coli was detected in 7.9 percent of 378 wells.
- In targeted agricultural studies in two Appalachian carbonate-rock aquifers, the rates were 74.4 percent for total coliform and 47.3 percent for E. coli.
Earlier surveys compiled in the same report are similar: roughly a third to just over two-fifths of domestic wells tested positive for total coliform.
Show the numbers
| Item | Value | Note |
|---|---|---|
| USGS NAWQA aquifer studies, 1991–2004 | 33.5 % of sampled wells | 397 wells in 15 principal aquifers |
| USGS agricultural land-use studies | 74.4 % of sampled wells | 86 wells, Appalachian carbonate-rock aquifers |
| EPA National Statistical Assessment of Rural Water Conditions | 42.1 % of sampled wells | Rural households, random stratified (EPA 1984) |
| CDC survey of Midwest domestic wells | 41.3 % of sampled wells | Randomly selected wells in nine states (CDC 1998) |
Any detection counts. The USGS agricultural studies were in two Appalachian carbonate-rock aquifers. USGS wells: untreated water, one sample per well, not a statistically representative sample of all U.S. wells. EPA and CDC surveys used random sampling of rural or regional wells; methods differ, so compare with care. Source: USGS Scientific Investigations Report 2008-5227, Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991-2004 (DeSimone, 2009), pp. 53-55, 64 and Appendix 1, accessed 2026-11-01.
Show the numbers
| Item | Value | Note |
|---|---|---|
| USGS NAWQA aquifer studies, 1991–2004 (E. coli) | 7.9 % of sampled wells | 378 wells |
| USGS agricultural land-use studies (E. coli) | 47.3 % of sampled wells | 55 wells, Appalachian carbonate-rock aquifers |
| EPA National Statistical Assessment (fecal coliform) | 19.8 % of sampled wells | Rural households (EPA 1984) |
| CDC survey of Midwest domestic wells (E. coli) | 11.1 % of sampled wells | Randomly selected wells (CDC 1998) |
E. coli in the USGS and CDC surveys; the EPA survey measured fecal coliform, a broader group that includes E. coli. USGS wells: untreated water, one sample per well, not a statistically representative sample of all U.S. wells. EPA and CDC surveys used random sampling of rural or regional wells; methods differ, so compare with care. Source: USGS Scientific Investigations Report 2008-5227, Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991-2004 (DeSimone, 2009), pp. 53-55 and 64, accessed 2026-11-01.
Where the positives turned up hints at why. The USGS found detections "more frequent in samples from the carbonate- and crystalline-rock aquifers than in those from other aquifers," and total coliforms in more than half of sampled wells in several aquifers, including the Central Valley basin-fill, North Atlantic coastal plain, Valley and Ridge sandstone and Piedmont crystalline-rock aquifers. Wells with detections drew water closer to the surface; those with total coliform were older, with more farmland nearby.
State data look similar. Penn State reports that a survey of 450 Pennsylvania wells found coliform in about 35 percent and E. coli in about 15 percent. A USGS study of 78 Pennsylvania wells near farmland found total coliform in 62 percent, and in about 75 percent where the bedrock was carbonate (USGS WRIR 01-4206).
Read these as context, not as your odds: the USGS says its wells are "not a statistically representative sample of all domestic wells." Still, in fractured or carbonate rock, a shallow or old well, or near farmland, the yearly bacteria test matters most.
How bacteria get into a well
Groundwater usually starts out clean. MDH says "Most of Minnesota's groundwater is free of disease-causing microorganisms" because "The upper layers of soil and rock filter the water as it soaks into the ground." So after a positive, the question is how something got around that filter. Penn State's sanitary well construction article (updated June 15, 2026) names the two usual routes: surface water flowing down the outside of the casing, and a loose or missing cap.
The top of the well
- The cap. A standard cap is held on by side bolts and leaves a small air gap that Penn State says "can allow for insects, small mammals, or surface water to enter the well." A sanitary (vermin-proof) cap has a rubber gasket and a screened vent. Penn State describes a 2002 USGS study of more than 100 Pennsylvania wells in which nearly 50 percent had obvious insect infestations under the cap.
- The casing. MDH says to look for cracks, holes, corrosion and soil settling, and warns that bumping the well with a mower, truck or snowplow "can crack the well casing, connections, or the grout seal around the well."
- Buried casings and pits. EPA says not to cut the casing off below the land surface. MDH says a well pit "may serve as a catch basin for contaminated surface water" and says the casing should be extended above grade and the pit filled with clean earth.
The gap around the casing
A drilled well has a narrow annular space between the casing and the surrounding rock or soil. Penn State explains that grout (usually neat cement or bentonite clay) is pumped into it from the bottom up to seal it. Without a seal, surface water can run down the outside of the pipe.
In that same 2002 study, as Penn State describes it, ungrouted wells were "three times more likely to be contaminated with E. coli bacteria compared to grouted wells," yet coliforms were still common in grouted wells, and the authors theorized they entered at the cap. Separately, the 2001 USGS report on 78 Pennsylvania wells found total coliforms "just as likely to be found in sanitary wells" as in nonsanitary ones. Grout and a tight cap block different routes. Grouting an existing well is generally not possible, Penn State says, so for an older well the practical protection is ground that slopes away from the casing.
Rain, snowmelt and flooding
EPA's private well contaminants page (updated July 6, 2026) says "Water run-off from rainfall or snow-melt can contaminate private wells by washing microorganisms into the well system or seeping underground." After a flood, EPA's Protect your home's water page says to contact your local or state health department for advice on inspecting and testing the well, and to use a contractor who services drinking water wells if possible.
Septic systems, animals and setback distances
EPA also lists "effluent from septic leach fields" as a way microorganisms reach wells. Setback distances are set by each state's well code. Under Minnesota Rules 4725.4450, a well must be at least:
| Contamination source (Minnesota rule) | Minimum distance | "Sensitive" well |
|---|---|---|
| Septic tank or watertight sewage holding tank | 50 feet | 50 feet |
| Absorption area of a soil dispersal system (drainfield), or a privy | 50 feet | 100 feet |
| Cesspool, seepage pit, leaching pit or dry well | 75 feet | 150 feet |
| Animal feedlot, or pasture feeding or watering area, holding more than one animal unit | 50 feet | 100 feet |
| Stream, river, pond, stormwater pond or lake (ordinary high water level) | 35 feet | 35 feet |
A "sensitive" well, in Minnesota's definition, has less than 50 feet of watertight casing and the casing doesn't pass through confining layers at least 10 feet thick in total. Other states differ, so check your state's well code. Our state pages (10 states so far) list certified labs from each state's own list.
Wells at higher risk
MDH singles out wells built before 1974 (when the Minnesota Well Code was developed), dug wells "with leaking walls and rotten boards for covers," wells at the bottom of old frost pits, wells with casing corroded through, and wells too close to sewers, septic systems or feedlots.
Season and heavy rain change the result
Penn State retested 38 wells that had been coliform-positive in a year of near-normal rainfall, this time in a cold, dry spell a year later. Fewer than half still had coliforms, and most of those had lower counts. Its advice: "the highest number of bacteria will be found by testing your well shortly after several weeks of rainy weather, while the fewest bacteria will be found when testing during dry, cold conditions in the winter."
So to find a pathway problem, sample after a wet stretch. And if you're judging a well you might buy, one clean result from a dry January is weaker evidence than it looks. For bottle, tap and shipping details, see the guide to testing well water.
Reading the report: presence/absence or a count
Penn State describes three lab methods. Presence/absence is a color-change test where "Present" only means at least one bacterium in 100 mL; it's cheaper but says less about severity. Membrane filtration counts colonies grown from 100 mL, reported per 100 mL (often CFU/100 mL). Most probable number (MPN) estimates the count statistically from several tubes. If you only got Present/Absent, Penn State suggests asking for a count on the follow-up sample, since it says more about severity, which helps in choosing a fix (see the thresholds below).
| Your report shows | What it means | Next step |
|---|---|---|
| Total coliform Absent, E. coli Absent | No coliforms in the 100 mL sample, if it reached the lab in time | Retest yearly (MDH), and after flooding, well work or a change in the water |
| "Absent" with an "H" qualifier (Montana State University Well Educated reports) | The sample arrived after the 30-hour ideal hold time; MSU says results "may have limited reliability," and past 48 hours are "likely not reliable." With any lab, a slow sample weakens an "Absent" | Consider retesting (MSU); deliver the sample faster |
| Total coliform Present, E. coli Absent | A pathway exists, probably from soil or surface water. MSU: a late sample can't create a false positive | Switch drinking water, confirm, inspect, disinfect, retest (below) |
| Low total coliform count, e.g. 2 per 100 mL; no E. coli | A small problem. Penn State's best results for a cap-plus-chlorination fix were in wells under 3 colonies per 100 mL | Good candidate for a sanitary cap and shock chlorination |
| E. coli or fecal coliform Present | Human or animal waste is reaching the water | Same sequence, more urgently. Penn State: check the septic system; remove or divert animal waste near the well |
| TNTC ("too numerous to count") | Penn State: "generally higher than 200 colonies per 100 mL" | Resubmit for an accurate count |
| "Confluent" growth | Non-coliform bacteria overgrew the plate, so coliforms couldn't be identified | Resubmit a sample |
| Total coliform over 1,000 per 100 mL, or fecal coliform over 100 | Heavy contamination. Penn State does not recommend UV units at these levels | Fix the source; get a contractor's assessment first |
Sources: Penn State Extension (Coliform Bacteria; Sanitary Water Well Construction); Montana State University Extension, Retesting for Bacteria. Penn State also warns that a non-sterile container "may cause bacteria to be introduced," one reason to retest. Our results checker lines your numbers up against the benchmarks.
Coliform positive, E. coli negative
This is a common result: "Total coliform: Present," "E. coli: Absent." Washington's health department explains it: "If only total coliform bacteria are detected in drinking water, the source is probably environmental. Fecal contamination is not likely. However, if environmental contamination can enter the system, there may also be a way for pathogens to enter the system. Therefore, it is important to find the source and resolve the problem." Wisconsin DHS puts the risk the same way: "When total coliform bacteria are found, the well is at risk for more serious forms of contamination."
| Question | What the sources say |
|---|---|
| Does it mean the water will make us sick? | Not by itself. Penn State: the presence of coliform "does not guarantee that drinking the water will cause an illness. Rather, their presence indicates that a contamination pathway exists." |
| Can we keep drinking it? | Not until it's fixed. MDH's steps for any coliform positive start with boiled or bottled water "for cooking and drinking until you address the issue," and Wisconsin DHS says to "take action if bacteria are present." |
| Was E. coli really tested? | Check the report. Washington: when total coliform is present, the sample "will also be tested for either fecal coliform or E. coli, depending on the lab testing method." If neither line appears, ask the lab, and ask for a count on the retest (Penn State). |
| Is it worse than E. coli positive? | No. Penn State calls a positive E. coli result "much more serious than coliform bacteria alone"; that case has its own page, E. coli in well water. |
What to do is the sequence in the next section: switch drinking water, retest to confirm, look for the way in (cap, casing, seals, electrical conduit and drainage, per MDH), disinfect, and retest. Penn State says to retest "within 10 to 14 days and again several months later," and that if those follow-up tests are negative, "it is likely that a one-time contamination event occurred that has been successfully treated." A low count is the best case for a simple fix: Penn State's best results for a sanitary cap plus shock chlorination were in wells under 3 colonies per 100 mL. If total coliform keeps coming back, even with no E. coli, read recurring positives below.
What to do after a positive result: the sequence
We compared the steps from two state health departments, MDH and the Wisconsin Department of Health Services (revised April 17, 2026), with Penn State Extension's. The order is the same, with one difference: Wisconsin and Penn State retest before disinfecting, while MDH disinfects first and then retests.
- Switch your drinking water now. MDH: use boiled or bottled water, or another safe source, "for cooking and drinking until you address the issue." Wisconsin DHS adds rinsing produce, brushing teeth and bathing babies, and says to throw out ice made with the water. CDC: "bring water to a full rolling boil for 1 minute. At elevations above 6,500 feet, boil for 3 minutes." If nitrate or arsenic was also found, MDH warns boiling "may not be the safest option." See the nitrate explainer.
- Confirm the result. Wisconsin DHS says to retest to verify. Penn State: make sure a certified lab ran the test and you followed its sampling instructions with a sterile bottle, and ask for an E. coli test and a count. The lab finder filtered for bacteria lists certified labs in your state.
- Inspect the well. Check the cap, casing, seals, electrical conduit and drainage (MDH), and look for septic problems or animal waste nearby if E. coli was found (Penn State). MDH says it is important to fix defects "so that surface water, insects, vermin, or other contaminants cannot enter the well."
- Disinfect the well and plumbing. MDH: hire a licensed well contractor or follow its procedure. Wisconsin DHS and Penn State recommend a licensed or well professional.
- Retest after the chlorine is gone. MDH: once the well "no longer tests positive for coliform bacteria," you can use the water again without boiling.
- Retest later, and act if it comes back. Wisconsin DHS: if disinfection didn't work, consider replacing the well or installing a certified treatment device. Penn State: if bacteria return, continuous disinfection "will be necessary."
Retest timing differs by agency. Follow your own state's advice when it gives one:
| Source | First retest | Follow-up |
|---|---|---|
| Minnesota Department of Health | After the chlorine is flushed, before drinking | Two to four weeks after disinfection, because "Total coliform may regrow in the water system" |
| Penn State Extension | About 10 to 14 days after disinfection | Two to three months later if the first retest is clean |
| Wisconsin Department of Health Services | Before disinfecting, to verify | "Test again to make sure the disinfection worked" |
Shock chlorination: how it works and what can go wrong
Shock chlorination, in Penn State's words, means "introducing a high concentration of chlorine to the water for a short time," through the well and every pipe in the house, then flushing it out. It kills the bacteria in the system. It doesn't seal the crack or replace the cap that let them in.
The figures below are from MDH's well disinfection procedure. Read the full procedure, or your state's, before doing it yourself.
- Strength: 50 to 200 parts per million (ppm) chlorine in the recirculating water. MDH: "Do not use a bleach solution greater than 200 ppm as it will reduce the disinfection effectiveness."
- Bleach: unopened, unscented household bleach with no additives, made within six months, labeled 6 or 8.25 percent sodium hypochlorite. Not pool or hot tub chemicals, which "typically contain algicides and fungicides."
- Amount: MDH says to fill a clean 5-gallon pail about three-fourths full with water from the system, add the bleach, then pour it into the well through a funnel. The bleach amount depends on the depth of water in the well and casing diameter. For a 4-inch well MDH lists 2 cups for 10 or 50 feet of water, 3 cups for 100 feet and 4 cups for 300 feet, covering the well plus an average home's plumbing. Flooded, large-diameter or larger systems may need more; test strips confirm 50 ppm.
- Distribution: recirculate from a hose back into the well for about 30 minutes after you first smell bleach, then run every faucet, hot and cold, until a strip shows at least 50 ppm, including toilets, outdoor taps and outbuildings.
- Contact time: "a minimum of two hours, preferably six hours, or overnight," with nobody using the water.
- Flushing: can take "30 minutes to 24 hours or more." Keep it away from the septic system, landscaping and water bodies.
- Equipment: bypass softeners, reverse osmosis units and filters; Penn State notes some "can be damaged or exhausted by high chlorine concentrations."
Safety first. From MDH's procedure: water and electricity together "can be deadly," so turn off the pump breaker (use the lockout hasp if the box has one) and verify with a voltmeter before opening the well. During recirculation the pump is switched back on with the well open, and MDH warns: "The wires in the well casing are now energized." Keep hands, tools and bleach away from the wiring. If you aren't confident around electricity, hire a licensed contractor. Wear goggles or a face shield and rubber gloves. Never mix bleach with other chemicals; they can form harmful gases. "Do not leave bleach or the bleach solution unattended." Ventilate the area and keep children and animals away. Gases can build up in well pits and create a lack of oxygen, and MDH's Protecting Your Well page says only a properly trained licensed well contractor or pump installer should enter a pit.
Why shocking a well often doesn't last
Penn State's numbers are the reality check. In its 2005 study, 16 coliform-positive wells were chlorinated and fitted with a sanitary cap. A month later, 44 percent were coliform-free; after one year, 19 percent were. (Penn State's shock chlorination article gives the one-year figure as 15 percent.) Either way, most of those wells were positive again within a year. The fix worked best where counts were low: under 3 colonies per 100 mL in one Penn State summary, under 10 colonies and no E. coli in the other.
MDH agrees that "It is not unusual to disinfect a water system multiple times." Treat shock chlorination as a test as much as a fix: if the bacteria come back, the cause is still there.
Recurring positives point to a structural problem
When positives return after shocking, the pathway is the issue, not the chlorine. Penn State notes that some positives come from a "one-time contamination incident," such as a new pump or an unusually heavy rainfall, and those are what shock chlorination is for.
MDH lists the likely causes when "all disinfection attempts fail": small animals or debris in the well, cracks in the upper casing, or soil and rock "not adequately filtering the water." Patterns in your results can help narrow it down before you call a licensed well contractor:
| Pattern in your results | Cause a source links to it | Where to look |
|---|---|---|
| Positive right after pump work or a repair | Bacteria introduced during the work (Penn State) | The case shock chlorination is meant for; retest on schedule |
| Positive after weeks of rain, clean in a dry winter | Surface water reaching the well during recharge (Penn State, EPA) | Grading, casing height, cap, the annular space |
| Low counts, no E. coli, standard bolt-on cap | Insects or small animals entering at the cap (Penn State) | Fit a sanitary cap; remove nests or insects from the casing |
| E. coli or fecal coliform, more than once | Human or animal waste reaching the water (Penn State, Washington DOH) | Septic tank and drainfield, animal areas, setback distances |
| Positive again within weeks of a clean retest | Regrowth in the system, or debris or animals in the well (MDH) | Repeat disinfection; have the well cleaned or inspected |
| Dug well, pit well, or casing ending below ground | Construction MDH and EPA flag as vulnerable | Extend the casing above grade, fill the pit, or replace the well |
One cause can't be fixed at the wellhead. Penn State says wells drilled into aquifers contaminated by animal waste, septic systems or surface water "can contain coliform bacteria regardless of well-construction practices." There, continuous treatment or a different water source may be the realistic answer.
Long-term options: repairs first, then continuous disinfection
Repairs come first because they remove the cause. Penn State lists extending a buried casing above ground, sloping the ground away from it and fitting a sanitary cap, which it prices at $50 to $75 against $20 to $30 for a standard cap. For a new or deepened well, ask for a grout seal; Penn State says grouting typically adds $500 to $1,000. MDH lists a new well among the options when disinfection fails; Wisconsin DHS says to consider replacing the well or installing a certified treatment device.
If bacteria keep returning, Penn State says you need a system "that continuously treats all of the water entering the home." Two technologies do this:
| Technology | How it works | Limits and upkeep |
|---|---|---|
| Continuous chlorination | A feed pump injects sodium hypochlorite solution (or dry calcium hypochlorite) ahead of a storage tank. Penn State: satisfy the water's "chlorine demand" and leave about 0.3 to 0.5 mg/L residual, with about 30 minutes of contact time. | Pressure tanks are usually too small for that contact time, so a holding tank or coiled pipe may be needed. Iron, hydrogen sulfide and organic matter use up chlorine. Needs a sediment filter, regular checks and refills. The residual keeps protecting the pipes. |
| Ultraviolet (UV) light | Water flows past a UV lamp inside a quartz sleeve. No chemical is added. | Water must be clear. Penn State: not recommended above 1,000 total coliforms or 100 fecal coliforms per 100 mL. Lamp replacement about once a year. No residual: it only treats water inside the unit. |
For UV, the certification class matters. Ohio's private water system rule, Ohio Administrative Code 3701-28-15 (effective January 1, 2020), says a UV system used as the primary means of disinfection "shall meet all of the requirements of NSF 55-2017 for class A," must have "an automatic shut-off device or warning device," and that systems "that meet only NSF 55-2017 class B shall not be used for continuous disinfection or otherwise installed on a private water systems." The doses behind the two classes come from Ohio's older 2011 fact sheet, written for the previous version of the rule: NSF/ANSI Standard 55 Class A systems deliver at least 40 mJ/cm² at the alarm set point, with warnings or automatic shutoff below it, while Class B units run at 16 mJ/cm² and "are not intended for the disinfection of 'micro biologically unsafe water.'" NSF has issued newer editions of the standard since (most recently 2026), so check the class on the certification listing rather than a dose figure.
Penn State doesn't recommend boiling, ozone or iodine for continuous disinfection: boiling is short-term only, ozone costs more than chlorination or UV, and EPA recommends iodine only for short-term or emergency use. If your well also has nitrate, the nitrate explainer covers the treatments that address it.
Treatment doesn't end the testing. Wisconsin DHS says to test treated water after a device is installed, and warns that devices that aren't maintained "lose their effectiveness over time and can even make water quality worse."
Next step
If you haven't tested for bacteria in the past year, start there. CDC and EPA put total coliform in the yearly panel with nitrate, total dissolved solids and pH. Find a certified lab with the bacteria lab finder, see which other tests fit your situation with Which tests do I need?, and keep every report.
This page is general information, not medical advice. Federal limits quoted here apply to public water systems, not private wells. For decisions about an infant, a pregnancy or someone who is ill, ask a doctor or your health department.
Questions people ask
Is it safe to shower with coliform in well water?
What does total coliform positive, E. coli negative mean?
What should I do if my well tests positive for E. coli?
How long should I boil well water with coliform bacteria?
How long after shock chlorination should I retest my well?
Why does my well keep testing positive after shock chlorination?
How common is coliform bacteria in well water?
- USGS Scientific Investigations Report 2008-5227, Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004 (DeSimone, 2009; revised November 2009) (accessed 2026-11-01)
- USGS Water-Resources Investigations Report 01-4206, Relation between selected well-construction characteristics and occurrence of bacteria in private household-supply wells, south-central and southeastern Pennsylvania (Zimmerman, Zimmerman and Lindsey, 2001) (accessed 2026-11-01)
- U.S. EPA, National Primary Drinking Water Regulations (total coliforms MCLG zero; fecal coliform and E. coli health note) (updated August 31, 2026) (accessed 2026-11-01)
- U.S. EPA, Potential Well Water Contaminants and Their Impacts (updated July 6, 2026) (accessed 2026-11-01)
- U.S. EPA, Protect Your Home's Water (accessed 2026-11-01)
- CDC, Drinking Water Advisories: An Overview (March 4, 2024) (accessed 2026-11-01)
- CDC, Guidelines for Testing Well Water (July 1, 2024) (accessed 2026-11-01)
- Minnesota Department of Health, Bacterial Safety of Well Water (updated October 19, 2026) (accessed 2026-11-01)
- Minnesota Department of Health, Well Disinfection (updated October 18, 2024) (accessed 2026-11-01)
- Minnesota Department of Health, Protecting Your Well (updated October 18, 2024) (accessed 2026-11-01)
- Minnesota Rules 4725.4450, Water-Supply Well Distances from Contamination (accessed 2026-11-01)
- Minnesota Rules 4725.0100, Definitions (subp. 43a, sensitive water-supply well) (accessed 2026-11-01)
- Wisconsin Department of Health Services, Drinking Water: Bacteria (revised April 17, 2026) (accessed 2026-11-01)
- Wisconsin Department of Health Services, Treatment Devices for Private Well Contaminants, P-03494 (11/2024) (accessed 2026-11-01)
- Washington State Department of Health, Coliform Bacteria in Drinking Water (accessed 2026-11-01)
- Penn State Extension, Coliform Bacteria (updated September 8, 2025) (accessed 2026-11-01)
- Penn State Extension, Shock Chlorination of Wells and Springs (updated May 29, 2025) (accessed 2026-11-01)
- Penn State Extension, Sanitary Water Well Construction (updated June 15, 2026) (accessed 2026-11-01)
- Montana State University Extension Water Quality, Retesting for Bacteria (Well Educated program) (accessed 2026-11-01)
- Ohio Administrative Code 3701-28-15, Continuous disinfection, continuous filtration, cyst reduction filtration and point of entry water treatment (effective January 1, 2020). Official page did not respond; read from the Internet Archive capture of May 9, 2026 (accessed 2026-11-03)
- Ohio Department of Health, Bureau of Environmental Health, Ultraviolet Light for Continuous Disinfection (fact sheet revised July 2011; copy hosted by Greene County Public Health; predates the current 2020 rule) (accessed 2026-11-01)
Find coliform bacteria testing labs by state
Labs each state lists as certified for coliform bacteria, with the ones that take homeowner samples first.
Got a bad result?
- Ask the lab or your health department whether to disinfect the well and how to re-sample afterwards.
- Confirm the result: re-test with a state-certified lab (find one in the finder on this site) and ask how to collect the sample.
- Call your state or county health department and ask what is known about this contaminant in your area and what it recommends.