A positive coliform result means surface water is reaching your well through a physical breach. Shock chlorination clears the contamination, finding and fixing the breach prevents its return, and UV provides ongoing protection — but only on clear water.
What a coliform result actually tells you
Total coliform bacteria are common in soil and surface water and mostly harmless in themselves. That is precisely why they are used as an indicator: their presence in a well means surface water is getting in somewhere, and whatever else is in that surface water is getting in with them.
E. coli is a different result. It indicates fecal contamination specifically, and it is an immediate safety issue — stop drinking the water, use bottled or boiled water, and treat it as urgent.
Find the breach, because that is the actual problem
Disinfection without fixing the cause is a temporary measure. Common breaches, roughly in order of frequency:
- A failed or missing well cap. The cheapest possible fix and a genuinely common cause. Look at it.
- A cracked or corroded casing, letting shallow groundwater bypass the depth the well was drilled to reach.
- Insufficient casing depth on an older well, especially one drilled before current standards.
- A septic system too close, or a failing one.
- Surface water pooling around the wellhead — grading that directs runoff toward rather than away from it.
- Recent flooding, which is why post-flood testing is standard advice.
Shock chlorination
The immediate response. A strong chlorine solution is introduced into the well, circulated through every fixture in the house until chlorine is detectable at each, left to work for a period, then flushed out.
Then retest, and retest again a few weeks later. A well that tests clean immediately after shocking and positive a month later is telling you the breach is still open.
If you are on a septic system, be aware that flushing a large volume of chlorinated water into it is not ideal for the bacteria doing the work in there. Discharge where practical.
What UV does
A UV system passes water past a lamp emitting ultraviolet light at a germicidal wavelength, which disrupts the DNA of bacteria, viruses and protozoa so they cannot reproduce. It is fast, it adds nothing to the water, it changes no taste, and it produces no by-products.
Its limits are equally clear:
- It removes nothing. Organisms are inactivated, not filtered out. Their remains stay in the water.
- There is no residual. Anything entering the plumbing downstream of the lamp is untreated. This is the opposite of chlorine, which persists.
- It does nothing for chemicals. Nitrate, arsenic, iron, hardness — all unaffected.
- It needs clear water, which is the requirement that dictates its position in the system.
Why UV goes last
UV works by light reaching the organism. Turbidity, iron and sediment all scatter and absorb that light, and organisms can shelter in the shadow of a particle. A UV system on cloudy or iron-bearing water is running and not disinfecting, which is the worst possible failure mode because everything looks fine.
So UV sits at the end of the train, after sediment filtration and after any iron or manganese removal:
- Sediment
- Neutralizer, if pH is low
- Iron, manganese and sulfide removal
- Softener, if hard
- UV
Systems publish a required maximum turbidity and iron level for exactly this reason. Check your test results against them.
Living with UV
- The lamp is replaced annually even though it still lights. Output degrades well before failure, and a lamp that glows is not necessarily a lamp that disinfects.
- The quartz sleeve needs periodic cleaning, because mineral scale on it blocks the light just as effectively as turbid water.
- It runs continuously and uses electricity accordingly. A power cut means untreated water, so flush after an outage.
What we are not going to do here
We are not going to recommend a specific UV system, because sizing depends on your peak flow rate, your water clarity and your well's specific test results, and getting it wrong produces a system that appears to work and does not. This is one of the few categories where a local water-treatment professional is genuinely the right call — and bacterial contamination is not a category to guess in.
Questions people actually ask
Does UV make well water safe to drink?
It inactivates bacteria, viruses and protozoa, which addresses the microbiological risk. It does nothing for nitrate, arsenic, lead, iron or hardness. Safe requires the right combination of treatment for everything your test found, and UV is one component of that.
How often do UV lamps need replacing?
Annually, even though the lamp still lights. Germicidal output degrades substantially before visible failure, so a glowing lamp past its service life is producing light that no longer disinfects reliably.
Can I put UV before my iron filter?
No. UV needs clear water — iron, sediment and turbidity scatter and absorb the light, and organisms can shelter behind particles. UV always goes last in the treatment train, after everything that removes particles.
Do I still need to shock the well if I install UV?
Yes. UV treats water passing the lamp; it does nothing about contamination already established in the well, the casing or the plumbing upstream. Shock chlorinate first, fix the breach, then install UV as ongoing protection.