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A positive total coliform or E. coli result. Stop drinking the water, shock the well, retest, and fix the cause before you rely on a UV unit.
A positive total coliform or E. coli result is the one well water problem you cannot see, smell or taste. The one-sentence answer: stop drinking the water until it tests clean, inspect and shock chlorinate the well, retest, and only if bacteria keep coming back install a UV filter for well water that is certified to NSF/ANSI 55 Class A, with the pre-treatment it needs to work. A UV unit bought before the well is fixed treats a symptom while the entry point stays open.
Coliform bacteria live on the ground surface and in the gut of warm-blooded animals. The Minnesota Department of Health explains that they are everywhere on the surface but do not usually occur past a few feet into the soil, which is why their presence in a well means something at the surface has found a path in. The usual paths: a cracked, buried or loose well cap; a casing that ends below grade or has corroded through; a failed grout seal; a septic drainfield, barnyard or manure pile within reach; flooding over the wellhead; or a pump or plumbing repair done without disinfecting afterward.
Penn State Extension cites a 2006 survey that found coliform in about 35 percent of Pennsylvania private wells and E. coli in about 15 percent; shallow dug and bored wells and old wells with short casings are the most exposed.
Total coliform is an indicator group. Most of its organisms are harmless, but Penn State describes a positive as a sign that a contamination pathway exists and that disease-causing bacteria could enter the same way. The EPA's Revised Total Coliform Rule treats it the same way for public systems: a positive is not a health violation on its own but a trigger for repeat samples and an assessment that finds and fixes the sanitary defect.
E. coli is a fecal coliform. The CDC says a positive fecal coliform or E. coli result indicates fecal contamination has entered the well, and Penn State calls it much more serious than coliform alone. Under the EPA rule, E. coli has a maximum contaminant level goal of zero.
High nitrate alongside bacteria supports a septic or livestock source. Slime, odor or black staining point to iron and sulfur bacteria, a nuisance rather than a health risk.
Yes. Minnesota DOH lists the illnesses carried by fecally contaminated water as diarrheas, dysenteries, salmonellosis, hepatitis and giardiasis, with infants, children, the elderly and the immunocompromised at highest risk. The EPA and the CDC both tell private well owners to test for total coliform at least once a year, and to retest after flooding, after work on the well, when a child joins the household or when the water changes.
If E. coli is present, do not drink the water; the CDC's interim advice is a rolling boil for one minute, or bottled water, until the well tests clean. Minnesota DOH adds: if the water also has high nitrate or arsenic, do not boil it, since boiling concentrates them.
Our testing guide covers finding a certified lab.
Most bacteria problems end here, and it is contractor work rather than a product. Replace a damaged cap with a sanitary vermin-proof cap, extend a short casing above grade, regrade so water runs away from the well, and fix the septic component if that is the source. Then disinfect.
Virginia Cooperative Extension's shock chlorination procedure targets about 200 ppm of chlorine in the well and plumbing; the bleach volume depends on casing diameter and water depth, so read its dosing table first. Bypass any softener, carbon filter or UV unit, since Penn State warns high chlorine can damage or exhaust them; pour the measured bleach, mixed with water, into the well and recirculate it with a hose until every faucet smells of chlorine; let it stand 12 to 24 hours; then flush it out through outside faucets onto ground where it will not harm plants, reach a stream or load the septic system. The CDC's disinfection guidance says to wait at least 7 to 10 days after disinfection and then retest; Penn State suggests retesting 10 to 14 days later and again two to three months on.
Penn State reports that shock chlorination permanently solved the problem in only about 15 percent of wells with coliform, and worked best on wells with fewer than 10 colonies per 100 mL and no E. coli. If bacteria return, Virginia Tech and Penn State say the same thing: a continuous source is present and continuous disinfection is required.
A UV lamp inside a quartz sleeve in a stainless chamber. The light damages the genetic material of bacteria, viruses and protozoa so they cannot reproduce. Nothing is added or removed: there is no residual, and it only works on water clear enough for the light to pass through.
Class A, not Class B. NSF/ANSI 55 is the standard the NSF applies to UV systems, and it has two classes. The WQA's UV fact sheet lays them out: Class A systems deliver a minimum dose of 40 mJ/cm2, are designed for water of unknown quality and may claim to disinfect water contaminated with bacteria, viruses, Cryptosporidium or Giardia, and must include a UV sensor and alarm. Class B systems deliver 16 mJ/cm2 and are for supplemental treatment of water already deemed safe by a health agency. Ohio's Department of Health UV fact sheet says Class B and unrated units are not intended for microbiologically unsafe water and calls anything but Class A for primary disinfection of a private well hazardous. For a well that has tested positive, buy Class A, sized so its rated flow meets your peak flow.
Pre-treatment is not optional. Ohio DOH recommends pre-filtration when turbidity reaches 5 NTU or suspended solids exceed 10 ppm, and says to treat the water before UV when dissolved iron is 0.3 ppm or higher or hardness is 6 gpg or higher, because both coat the quartz sleeve and block the light. In practice:
Penn State adds a ceiling: UV is not recommended where total coliform exceeds 1,000 colonies per 100 mL or fecal coliform exceeds 100 per 100 mL; a well that dirty needs chlorination. Maintenance is a new lamp every year (WQA gives about 10,000 hours of useful life; Ohio DOH says replace annually before the alarm trips), sleeve cleaning on a schedule set by your hardness, and a new sleeve every 3 to 5 years. UV goes last, after every filter and the softener.
A feed pump doses chlorine into a retention tank ahead of a carbon filter that removes the residual before the house. Penn State describes maintaining 0.3 to 0.5 mg/L of residual chlorine with about 30 minutes of contact time. It is the answer for wells too contaminated for UV, for springs, for wells that cannot be sealed and for water that also has iron or sulfur bacteria to oxidize. Dealer-installed, with chlorine to refill and carbon to replace.
Aquasana · Whole-house filter · $1,749 (sale) at Aquasana as of Oct 2026
Carbon and KDF whole-house tank with an NSF/ANSI 55 Class B certified UV stage, 500,000-gallon media life and no backwash. No iron or sulfur claim; 7 GPM rated flow and pre-filter changes every 2 months.
iSpring · UV disinfection · $339 at iSpring as of Oct 2026
55 W UV sterilizer rated 12 GPM with a flow-sensing ballast and 9,000-hour lamp for $338.99. No NSF/ANSI 55 certification or published dose; 1-year warranty; needs pre-treated water.
Pentair · UV disinfection · Check current price
10 GPM UV certified to NSF/ANSI 55 Class A with a dose sensor that shuts off flow below 75%, 10-year chamber warranty. Price not shown on the page; professional install recommended; needs 5-micron pre-filtration.
SpringWell · UV disinfection · $1,101 (sale) at SpringWell as of Oct 2026
15 GPM UV-C reactor with a lamp-life countdown and alerts, 10-year reactor warranty per the manual, and about $200 a year in lamp and sleeve. No NSF/ANSI 55 certification or published dose; needs pre-treated water.
Choose a Class A unit whose flow rating covers your peak demand, then build the pre-treatment from your lab results: a sediment cartridge always, an iron filter above 0.3 mg/L, a softener above about 6 to 7 gpg. Combination systems that package sediment, carbon and UV together suit low-iron wells. The sizing tool gives you the flow number to match.
Total coliform is an indicator group. Most of its organisms are harmless, but a positive means a pathway from the ground surface into the well exists and disease-causing bacteria could use it. The EPA's Revised Total Coliform Rule treats a positive the same way for public systems: not a violation on its own, but a trigger to find and fix the defect. For a private well, inspect the cap, casing and grading, shock chlorinate and retest. Penn State cites a survey that found coliform in about 35 percent of Pennsylvania private wells.
Stop drinking the water, or bring it to a rolling boil for one minute (three minutes above 6,500 ft), until a follow-up test taken at least a week after disinfection comes back clean; a second test a few months later confirms the fix held. E. coli has an EPA maximum contaminant level goal of zero. If the water also has high nitrate or arsenic, the Minnesota Department of Health says not to boil it, since boiling concentrates them; use bottled water instead.
Not usually on its own. Penn State reports that shock chlorination permanently solved the problem in only about 15 percent of wells with coliform, and worked best where counts were under 10 colonies per 100 mL with no E. coli. The CDC says to wait at least 7 to 10 days after disinfecting before retesting. If bacteria come back, a continuous source is present and the well needs repair or continuous disinfection such as UV or chlorination.
Class A systems deliver a minimum dose of 40 mJ/cm2, are designed for water of unknown quality, may claim to disinfect water contaminated with bacteria, viruses, Cryptosporidium or Giardia, and must include a UV sensor and alarm. Class B systems deliver 16 mJ/cm2 and are only for supplemental treatment of water a health agency already considers safe. Ohio's health department calls anything but Class A for primary disinfection of a private well hazardous.
UV only works on water clear enough for the light to pass through. Put a 5 micron sediment filter immediately ahead of the chamber on every installation, an iron filter if iron is at or above 0.3 mg/L, and a softener if hardness is above about 6 gpg, since iron and scale coat the quartz sleeve. Test UV transmittance on the raw water, because tannin-colored water blocks the light. UV goes last, after every filter and the softener, with a new lamp every year.
The EPA and CDC both recommend testing a private well at least once a year for total coliform bacteria, along with nitrate, total dissolved solids and pH, and again after flooding, any work on the well, a change in the water or when a child joins the household. Use a state-certified lab, a sterile bottle, and deliver the sample cold within the lab's holding time, usually 24 to 30 hours.
Figures and quotations above come from these pages as read on 2026-10-05. These are reference links, not affiliate links.
