Bacteria in Well Water: Shock Chlorination vs UV Disinfection

A positive coliform result is the one well water number that does not wait. Iron stains, taste complaints, the rotten egg smell, all of that keeps until spring. Bacteria does not. The standard first response is well water shock chlorination: one slug of chlorine strong enough to disinfect the casing, the pump, the tank and the house plumbing. It is a corrective action, not a treatment system. If the well keeps testing positive afterward, something is still getting in, and that is where UV earns its price.

What total coliform and E. coli actually tell you

The test is not hunting for the organism that would make you sick. It looks for a signal. Coliform bacteria live in soil, on vegetation and in the guts of animals, most of them harmless. Labs culture them because they grow cheaply, and because whatever let coliform in could let something worse in. A positive is evidence of an opening. A cracked casing. A rotted cap gasket. Surface water down the annular space after a three-day rain.

E. coli is the serious one. It lives in the guts of warm-blooded animals, not in leaf litter, so a positive means fecal material reached your water. Stop drinking it. Boil one minute at a rolling boil if you need water now, and call your county health department the same day. Many retest free.

The EPA Total Coliform Rule sets the limit for public systems at zero, and private wells sit outside the Safe Drinking Water Act, so nobody enforces that on your property. Zero is still the target. There is no reassuring low reading.

Find the way in before you kill anything

Chlorine does not fix a hole. People shock the same well three times in a season and act surprised at the fourth positive, because nobody took the cap off and looked. Ten honest minutes at the wellhead first.

• Is the casing at least 12 inches above grade, with the ground sloping away from it?
• Is the cap vermin-proof, the gasket intact, the vent screened? Loose caps and chewed gaskets are the most common way in. Look for cracks, rust-through, or a pitless adapter weeping at the seal.
• How far to the nearest drainfield or livestock pen? Most codes want 50 to 100 feet, and a failing leach field reinfects a well indefinitely. If the tank is overdue, check the pumping interval for your tank size.
• Did the positive follow flooding or a pump job? Anyone who opens a well introduces bacteria, and that hit usually clears with one disinfection.

Found a defect? Fix it first, because shocking a well under a broken cap is money burned. If the well is over 50 years old, get a licensed contractor out. I am not a plumber and certainly not a driller, and casing integrity 40 feet down is not judged from the surface.

How well water shock chlorination actually works

You fill the whole system with chlorine strong enough to kill what is living in it, hold it for hours, then flush. Published targets run from 50 to 200 mg/L of free chlorine, and that spread is real disagreement, not sloppiness. Extension bulletins give a range. Several state health departments print a single figure near 100 mg/L. Drillers facing iron bacteria slime go higher, because biofilm shields the organisms underneath it.

Behind that disagreement is a trade. Strong solutions punch through biofilm faster, but chlorine much above 200 mg/L attacks pump seals and brass fittings, and can pull enough iron and manganese from the formation to leave you running black water for a week. Where you land depends on what is coating the inside of your well.

The volume math is where people go wrong, and erring low means you disinfected nothing.

static water level = 60 ft
water column = 180 - 60 = 120 ft
6-in casing holds 1.47 gal per ft
well volume = 120 x 1.47 = 176 gal
pressure tank, heater, pipes = 80 gal
total to treat = 256 gal

target 100 mg/L free chlorine
6% bleach = 60,000 mg/L
256 x 100 / 60,000 = 0.43 gal
so 2 quarts, rounded up
at 200 mg/L, double it

other casings: 4-in = 0.65 gal/ft
8-in = 2.6 gal/ft

Your driller’s log gives depth, diameter and static level; many state geological surveys keep copies by parcel if yours is lost.

The procedure, and why one clean retest is not enough

This is an outline, not a substitute for your state health department’s written procedure. Get theirs: several states publish a one-page PDF better than anything I could write.

• Bypass any softener, carbon filter, RO unit or iron filter. Concentrated chlorine wrecks softener resin and strips carbon media in one pass.
• Mix the bleach into 5 to 10 gallons of water, pour it down the casing, then circulate with a hose from an outside spigot for 15 to 20 minutes to wash the casing walls.
• Open every fixture, hot and cold, one at a time until you smell chlorine, then shut it. Outside spigots and the ice maker line too. Then let it sit 8 hours minimum, ideally 12 to 24, with no water use at all. Plan it around a Saturday.
• Flush outside first, onto gravel, away from lawns and any drainfield. Water at 100 mg/L kills grass and knocks down septic tank biology.

Safety, briefly and seriously. Never mix bleach with an acid cleaner or with ammonia. Wear gloves and eye protection. Anyone with a compromised immune system, an infant on formula, or a pregnancy in the house belongs on bottled water until two clean retests are in hand.

Then wait. Residual chlorine suppresses bacteria for days, so an early sample tests the chlorine, not the well. Take the first sample once you cannot smell it at any tap, usually 7 to 10 days. A clean result there says the disinfection worked, not that the well is clean, so retest at 2 to 3 weeks and again at 2 to 3 months, timing one for right after a heavy rain. A well watertight in August can leak in April.

Samples run $25 to $60 depending on the lab and on county subsidies, so budget $75 to $180 for the sequence, all from a state-certified lab. Our guide to what to test for and how often covers the annual panel. Positive again after a competent shock and a wellhead repair? Stop shocking and hunt for the pathway.

UV disinfection as continuous treatment

Nothing gets added to the water, which is most of the appeal. A UV system is a stainless chamber holding a low-pressure mercury lamp inside a quartz sleeve. Water flows around the sleeve, takes a dose of 254 nm ultraviolet light, and the light scrambles the DNA of bacteria, viruses and protozoa so they cannot reproduce.

Dose is intensity multiplied by contact time, in millijoules per square centimeter, and NSF/ANSI 55 splits systems in two. Class A units are validated at 40 mJ/cm2 for water of unknown or contaminated bacteriological quality, which is your situation. Class B units deliver 16 mJ/cm2 and only polish water already known to be safe. Buy Class A. The gap is a few hundred dollars, and you are shopping because you do not trust the water.

The dose is delivered at a rated flow, which is what trips people up. Push more gallons per minute through the chamber than it was validated for and contact time falls, dose with it, and organisms come through alive with no warning on the panel. Size to peak demand: a three-bathroom house wants 10 to 12 gpm.

UV needs clear water or it does nothing

Ultraviolet light only works on what it can reach. Makers want turbidity under 1 NTU, because a bacterium riding a silt grain is shielded from the lamp. A 5-micron sediment filter ahead of the chamber is standard practice, and going finer costs flow and pressure, a trade-off covered in our piece on choosing a micron rating without strangling your flow rate.

Iron and manganese absorb UV and plate onto the sleeve as a film that dims the lamp. Most makers cap iron at 0.3 mg/L and manganese at 0.05 mg/L. Above that you need iron removal ahead of the UV chamber, a second system with its own cost.

Hardness furs up the sleeve the way it furs up a kettle element, and the limit usually quoted is 7 grains per gallon, 120 mg/L at the standard 17.1 conversion. At 17 gpg you will be wiping constantly. Softening only to protect a lamp is strange, but if you were leaning that way already, softener sizing on well water with iron present changes the arithmetic.

UV does nothing for nitrate, arsenic, lead, PFAS or hardness. Chemicals alongside the bacteria mean a second treatment train, usually point-of-use RO at the sink.

What UV costs, and when it is worth buying

Equipment for a Class A whole-house unit at 8 to 12 gpm runs roughly $400 to $1,100, the spread being chamber quality, a solenoid valve that stops flow when the lamp fails, and a real intensity sensor rather than a timer. A plumber adds $250 to $900, the top of that band being a rerouted supply line or a new outlet. Call it $600 to $2,200 installed, plus pretreatment if your iron or hardness demands it.

Running costs are small and not optional. Lamps fade steadily and get replaced yearly while still glowing, $80 to $180 depending on chamber length. The sleeve wants wiping every 6 to 12 months and replacing every 2 to 5 years, $40 to $120. Draw is 30 to 60 watts around the clock, so $30 to $100 a year at typical US rates.

Most wells that come back positive once do not need any of this. They need a new cap gasket and two quarts of bleach. Worth saying plainly, because a bacteria result frightens people into buying equipment the afternoon the news lands.

My rule of thumb: one positive after a storm or a pump job, with a visible defect you can fix, is a shock-and-retest situation. Two or more across seasons, with a sound wellhead, means the pathway is underground and continuous treatment is the honest answer. Between those sits a judgment call your county health department will make better than a website: they have the local geology, and nothing to sell you.