A total coliform positive means a contamination pathway exists into your well, not that you are drinking something dangerous — most coliform is harmless. A positive E. coli is different: it means human or animal waste is entering the supply. Shock chlorinate, then retest. If it returns, you need continuous disinfection.
Two very different results
A lab report saying “coliform present” and one saying “E. coli present” are not
the same finding, and conflating them causes both panic and complacency.
Result
What it means
Total coliform positive
“a contamination pathway exists between a source of bacteria…and the water supply”
E. coli positive
“much more serious… it indicates that human or animal waste is entering the water supply”
Penn State Extension’s water team — Bryan Swistock, Stephanie Clemens and
William Sharpe — are careful about the first one: “Most types of coliform
bacteria are harmless to humans” and “drinking water that is contaminated
with coliform bacteria does not always cause illness.”
That is not a reason to ignore it. Coliform is an indicator: it tells you
something can get in. What gets in next time may not be harmless.
The standard is absolute regardless — “the drinking water standard requires
that no coliform bacteria be present in drinking water.”
Shock chlorination is a test as much as a treatment
The usual first step is shock chlorination: “introducing a high concentration
of chlorine to the water for a short time.”
What matters is the retest afterwards, because it distinguishes two
different problems:
Follow-up tests negative → likely a one-time contamination event, now
cleared.
Bacteria return → “you will need to consider a continuous disinfection
treatment system.”
That fork is the whole decision. Skipping the retest and buying a UV lamp
immediately means paying for continuous treatment you may not need — or worse,
installing it while the actual pathway stays open.
When UV works, and when it does not
UV is effective and low-maintenance in the right conditions. Two published
limits define them:
Concentration.“UV light units are not recommended for water supplies
where total coliform bacteria exceed 1,000 colonies per 100 mL.”
Clarity.“The untreated water entering the unit must be completely
clear and free from any suspended sediment or turbidity to allow all of the
bacteria to be irradiated by the light.”
The second is why UV sits last in the treatment sequence, after iron
removal and sediment filtration. A UV lamp installed ahead of particulate
removal has bacteria hiding in the shadow of suspended solids. It will still
draw power and still show a green light.
Test annually, because nobody else will
Penn State Extension recommends all private supplies be tested for total
coliform every year.
Private wells fall outside the enforceable standards that cover public water
systems. No utility runs this test, no report arrives, and nothing tells you
when a casing seal fails.
Where bacteria treatment sits relative to everything else is worked through in
the sizing method, and the rest of the cluster starts at
well water treatment.
The bottom line
Treat a coliform positive as evidence that a pathway exists, and an E. coli positive as a reason to stop drinking the water now. Shock chlorinate, then retest — that retest is the actual decision, because a clear result means a one-off event and a repeat means you need continuous disinfection. UV works well within its limits and not at all outside them: not above 1,000 colonies per 100 mL, and not on water carrying sediment. Step up to a well contractor when the pathway itself is the problem, such as a failed casing or cap.
Who this is not for
UV is not the answer for every positive. It is not recommended above 1,000 coliform colonies per 100 mL, and it does nothing at all if the water reaching it is cloudy. It also does not fix the pathway — if surface water is entering through a failed casing or cap, disinfection treats the symptom while the route stays open.
Questions
What does a positive coliform test mean for well water?
That a contamination pathway exists between a source of bacteria and your water supply. Penn State Extension notes most types of coliform are harmless to humans and that contaminated water does not always cause illness — the finding is about the route in, not the organism. The drinking water standard nonetheless requires that no coliform be present.
Is E. coli in well water worse than coliform?
Considerably. A positive E. coli result "is much more serious than coliform bacteria alone because it indicates that human or animal waste is entering the water supply." Treat a coliform positive as a pathway to investigate; treat an E. coli positive as an immediate stop-drinking situation until it is resolved.
Will a UV light fix bacteria in my well?
Sometimes. UV units are not recommended where total coliform exceeds 1,000 colonies per 100 mL, and the water entering the unit must be completely clear — any suspended sediment or turbidity shields bacteria from the light. UV also sits last in the treatment sequence for that reason.
How often should I test my well for bacteria?
Every year. Penn State Extension recommends all private water supplies — wells, springs and cisterns serving an individual house — be tested for total coliform bacteria annually. Nobody does this for you; private wells fall outside the enforceable standards that cover public systems.
