Bacteria in Well Water: Testing, Shock Chlorination and UV

A lab report that says “coliform present” is unsettling, but it’s common and usually fixable. The order matters: find out why the bacteria got in before you buy anything.

Short answer: if your well tests positive for total coliform bacteria, drink boiled or bottled water, retest to confirm, and have the well checked for an opening that lets surface water or septic effluent in. Then shock-chlorinate the well and test again. Many one-time problems end there. If bacteria keep coming back and the cause can’t be fixed, you need continuous disinfection, most often a UV system certified to NSF/ANSI 55 Class A, with any pretreatment it needs. Of the 20 UV brands we looked up, only a handful have a Class A listing.

What does a positive bacteria test mean?

Most well tests look for three things, from broad to specific:

• Total coliform bacteria. A large group that occurs naturally in soil, on vegetation and in surface water, the Wisconsin DNR notes. Most don’t make people sick. They’re an indicator: the Minnesota Department of Health says their presence suggests disease-causing germs may be there too, and Penn State Extension says it shows “a contamination pathway exists” between a source such as surface water, a septic system or animal waste and your water.
• Fecal coliform bacteria. The subgroup that lives in the intestines of warm-blooded animals.
• E. coli. Penn State calls a positive E. coli result “much more serious than coliform bacteria alone because it indicates that human or animal waste is entering the water supply.”

The EPA says people who drink water with harmful microorganisms can get gastrointestinal illnesses and infections, and Penn State lists stomach upset, fever, cramps and diarrhea as the most common symptoms. Minnesota adds that infants, children, older adults and people with weakened immune systems are more likely to get sick.

How common is bacteria in well water?

Common enough that a yearly test is the standard advice. Three studies give a sense of scale, though they used different methods, regions and years, so the numbers don’t add up to one national rate:

• National: the US Geological Survey’s study of domestic wells sampled from 1991 to 2004 (Circular 1332) detected total coliform in 34% of about 400 wells tested for it, and E. coli in 7.9%.
• Wisconsin: health departments tested nearly 4,000 rural wells for a program serving low-income families from 2007 to 2010; coliform bacteria exceeded safe limits in 18% (Knobeloch and colleagues, 2013).
• Maryland: in 118 private wells, total coliforms turned up in 25.4% and E. coli in 3.4% (Murray and colleagues, 2018).

The EPA recommends testing your well every year for total coliform bacteria, and right away after flooding, after any repair to the well system, or when the water’s taste, color or smell changes. A coliform test is usually cheap: Penn State puts it at about $10 to $30. Our well water testing guide covers the rest of the yearly panel and how to find a state-certified lab.

What should you do first after a positive test?

1. Stop drinking the water untreated. The Wisconsin DNR says not to consume it unless you boil it at a rolling boil for at least 1 minute. Minnesota suggests boiled or bottled water, and warns that if your test also found nitrate or arsenic, boiling may not be the safest option, because it can concentrate them. Use bottled water in that case.
2. Retest. Wisconsin’s first step is to resample, to rule out a sampling mistake. Follow your lab’s instructions exactly.
3. Look for the way in. Wisconsin and Minnesota both point to a loose, broken or missing well cap, a cracked or corroded casing, and a septic system or other waste source too close to the well; Wisconsin adds a casing that’s too short or poorly grouted. Penn State adds that an E. coli result is a reason to check your septic system. Some defects can be repaired; Wisconsin notes that some can’t easily be, and the well may need replacing.
4. Call your local health department. It can help read the results, and many state well programs list licensed well contractors who can inspect and disinfect the well.

How do you shock chlorinate a well?

Shock chlorination means filling the well and your plumbing with a strong chlorine solution, letting it sit, then flushing it out. It kills the bacteria present in the system at that moment. State guides agree on the steps, but not on the numbers:

	Minnesota	Wisconsin	Texas A&M
Chlorine strength	50–200 ppm, never above 200	100–300 ppm	200 ppm
Contact time	At least 2 hours, preferably 6 or overnight	At least overnight, preferably 24 hours	At least 12, preferably 24 hours
Retest after flushing	2 to 4 weeks	About a week	—

Because the amount of bleach depends on your well’s depth and diameter, and the tables differ between states, use your own state’s instructions or hire a licensed well contractor. A few points from the guides:

• Use plain, unscented household bleach. Minnesota specifies 6 or 8.25% bleach and no pool products.
• Never mix chlorine with ammonia products, which releases a toxic gas, and don’t work in an enclosed space such as a well pit or pump house (Wisconsin).
• Mind the wiring. Wisconsin warns that you’ll be working with water and dangerous voltages, and suggests getting help from a licensed professional if you’re not familiar with electrical systems.
• Bypass water softeners, reverse osmosis units and appliances that can’t tolerate bleach, and disinfect them separately as their makers direct (Minnesota).
• Don’t drink the water, bathe or shower in it while it’s heavily chlorinated (Wisconsin).
• Flush it away from your septic system, landscaping and any stream or pond; strong chlorine harms all three.
• In arsenic areas, ask first. Wisconsin limits shock chlorination in its arsenic problem areas to 100 ppm for no more than 30 minutes, because stronger or longer treatment can release arsenic from the bedrock into the water.

Does shock chlorination fix the problem for good?

Sometimes. Penn State says it’s meant for “one-time contamination incidents,” such as bacteria introduced when a new submersible pump is installed, or surface runoff. If the cause is still there, the bacteria come back.

The evidence on how often it lasts is limited but sobering. In a Penn State study, researchers disinfected 16 coliform-positive wells and fitted them with sanitary caps; coliform was back in 7 of them within 30 to 60 days, and in all but two within a year (Swistock and Sharpe, 2005). The few successes had low coliform counts and no E. coli. Penn State Extension sums it up as success for one year in 15% of the wells, and the authors suggest the contamination often came from beyond the wellhead. It was a small study, but it’s a good reason not to stop at one clean test.

So test twice. Penn State recommends retesting 10 to 14 days after disinfection and again two to three months later; if bacteria return in either test, it says continuous disinfection is necessary. Texas A&M suggests considering it if bacteria show up a third time.

When do you need UV, and what does Class A mean?

Continuous disinfection treats every drop that enters the house. Penn State describes the main options:

• Continuous chlorination: a small feed pump meters chlorine into the water, enough to leave about 0.3 to 0.5 mg/L after roughly 30 minutes of contact time.
• UV light: no chemicals added and no residual. It only kills germs inside the unit.
• Ozone: Penn State says it’s generally not recommended because it costs much more than chlorination or UV.

UV is a popular choice for a home well, but it only works under the right conditions:

• Class A, not Class B. Under NSF/ANSI 55, Class A systems deliver at least 40 mJ/cm² at their alarm set point and are designed to disinfect water that may be contaminated; Class B systems deliver at least 16 mJ/cm² and are only meant to reduce harmless “nuisance” microbes, per the Ohio Department of Health. Ohio’s well code requires Class A for UV in a private water system, with an automatic shut-off or warning device, and says Class B units “shall not be used for continuous disinfection.”
• Clear water. Ohio’s fact sheet calls for a pre-filter when turbidity is 5 NTU or more, and treating iron and hardness first when iron is 0.3 ppm or more and hardness is 6 grains per gallon or more, because they coat the quartz sleeve and block the light. Each maker sets its own limits too.
• Not too many germs. Penn State doesn’t recommend UV where total coliform exceeds 1,000 colonies per 100 mL or fecal coliform exceeds 100. Fix the source first.
• Sized for peak flow, the most water your house uses at once (Ohio), at the flow rate listed for its Class A dose.
• Upkeep. Lamps lose strength and need replacing about once a year, even if they still glow (Penn State, Ohio). The Connecticut Department of Public Health says to disinfect the whole plumbing system with chlorine whenever the unit is serviced, that you may need to run water to waste after a power outage until the lamp is back to full strength, and to test monthly for the first six months.

Which UV systems are certified Class A?

Fewer than the marketing suggests. On October 12, 2026, NSF’s NSF/ANSI 55 listing had 125 Class A and 235 Class B models from 19 listed companies, several of which sell the same units under their own names. We searched the NSF, WQA and IAPMO listings for 20 UV brands sold to US homeowners. Of those 20, only VIQUA (its PRO series), Luminor, Culligan and Puretec, an Australian company, have Class A listings. Aquasana is listed for Class B only. The other 15 (Pelican, SpringWell, iSpring, Express Water, Wyckomar, Pure Water Products, Alfaa, Acuva, US Water Systems, Kinetico, Hague, Waterdrop, APEC, Aquasure and Big Blue/Bluonics) weren’t listed under NSF/ANSI 55 by any of the three. Outside that list, UV Pure (Hallett) and Watts (UV-COM, at WQA) also have Class A listings.

Whole-house Class A models from US-market brands, with the flow each is listed for at its Class A dose:

Brand	Listed Class A models (flow)	Certifier
VIQUA	PRO10 (10 gpm), PRO20 (20 gpm), PRO30 (30 gpm), and their “S” versions	NSF
Luminor	LB6-10XA (6.3 gpm), LB6-15XA (7.9 gpm), LBH6-15XA (5.4 gpm), LBH6-25XA (7.9 gpm), LBH6-40XA (18 gpm)	NSF
Culligan	CUV6-15XA, CUVH6-15XA, CUVH6-25XA, CUVH6-40XA	NSF
UV Pure	Hallett 500PN (16.5 gpm), Hallett 750PN (27.4 gpm)	NSF
Watts	UV-COM C196C (16.5 gpm), C222C (27.4 gpm)	WQA

The flow rating is where Class A costs you. Luminor’s LBH6-15XA and LBH6-15XB use the same lamp, but NSF lists the Class A version at 5.4 gpm and the Class B version at 13 gpm. A unit sold as “15 gpm” may only reach the Class A dose at a third of that.

Certified

Listed for this contaminant under the named standard by an accredited certifier (NSF, WQA, IAPMO, CSA or similar).

Lab-tested only

The maker publishes an independent lab report, but the product is not certified for this claim.

Maker claim only

No certification listing or independent lab report that we could find.

Luminor BLACKCOMB 6.1 A

Luminor’s BLACKCOMB 6.1 A is sold through dealers, with no price on the site. It has a UV monitor, audible lamp-out and lamp-change alarms, and a solenoid shut-off is an optional module. Luminor uses a “1” in the model number for the US plug (LB6-101A, LB6-151A); NSF lists the models with an X in that spot, which its footnote says stands for the type of power cord.

• Disinfection, Class A (6.3 and 7.9 gpm)CertifiedNSF/ANSI 55 · NSF Listing

VIQUA

VIQUA’s PRO series is listed Class A. Its VH200-V and VH410-V models, a common sight in well-water setups, are listed as Class B only. We couldn’t open VIQUA’s own website from our location to check current prices or claims.

• Disinfection, Class A (10, 20 and 30 gpm)CertifiedNSF/ANSI 55 · NSF Listing

• Nuisance microbes only, Class B (7.8 and 14 gpm)CertifiedNSF/ANSI 55 · NSF Listing

Aquasana Rhino Well Water with UV

The Rhino Well Water with UV ($1,749 on sale on October 12) is a whole-house carbon filter with Aquasana’s AQ-UV-SF UV unit, rated 6 gpm. Aquasana is upfront that the UV unit is certified to NSF/ANSI 55 Class B, and NSF’s listing agrees. The same page markets it for well water and says it sterilizes “99.99% of viruses and bacteria like E.coli”; Class B certification doesn’t cover that. The replacement lamp costs $125.99 and is due every 12 months. The page mentions no UV-intensity sensor or automatic shut-off.

• Nuisance microbes only, Class B (6 gpm)CertifiedNSF/ANSI 55 · NSF Listing
• 99.99% of viruses and bacteria like E. coliMaker claim only

SpringWell UV Water Purification System

SpringWell’s 15 gpm UV system (SPRW-UV-15gpm) was $1,100.50 on sale on October 12, with a replacement bulb at $118.65, due every 365 days. It says it “kills 99.9% of bacteria and viruses.” SpringWell isn’t listed under NSF/ANSI 55, so neither the class nor the flow at a disinfecting dose has been certified.

• Kills 99.9% of bacteria and virusesMaker claim only

iSpring UVF55FS

iSpring’s UVF55FS ($338.99 on October 12) is rated up to 12 gpm and switches on with flow; iSpring’s description mentions no UV-intensity sensor or automatic shut-off. It’s marketed for well water. The page also carries a generic “iSpring vs Other Brands” table, written for its reverse osmosis systems, with the row “NSF/ANSI 58 Certification — Yes (full system certified)”. NSF/ANSI 58 is the reverse osmosis standard and doesn’t cover a UV unit, and iSpring isn’t listed under NSF/ANSI 55.

• Tackles "various water concerns in well water"Maker claim only

How much does UV cost to run?

The lamp is the main running cost. Among the systems above with prices, a replacement lamp was $118.65 to $125.99 a year on October 12. Add a sediment pre-filter cartridge where one is needed, any pretreatment for iron or hardness, and the yearly coliform test that tells you it’s working. Installation is a plumbing job, and an electrician may be needed for the power supply.

The bottom line

• A positive coliform test is a signal, not a diagnosis. Use boiled or bottled water, retest, and find out how the bacteria got in.
• Fix the well, then shock it. A good cap, an intact casing and distance from the septic system come first. Shock chlorination clears one-time contamination; retest twice to be sure.
• If bacteria keep coming back, choose continuous disinfection. For UV, that means NSF/ANSI 55 Class A, sized for your peak flow at its Class A rating, with any pretreatment and a yearly lamp.
• Check the listing, not the box. Most UV brands sold to homeowners aren’t listed under NSF/ANSI 55 at all, and some well-water bundles use Class B units.

For what else to test in a private well, see our well water testing guide and the well water hub.