Bacteria are the one problem in well water you can't see, smell or taste. The water can look perfect and still fail a test. If your lab report shows E. coli or coliforms, this guide explains what the result means, what to do straight away, and how to fix it for good. If you're on a public supply and want to know about boil water notices and cryptosporidium, there's a section on that further down.
Quick answer
- What it means: E. coli means the well has been contaminated by human or animal waste. Coliforms mean surface water is getting in. The standard for drinking water is zero of either in a 100 ml sample.
- Straight away: boil all drinking water from the well until it's safe, and speak to your local HSE Environmental Health Officer if you're worried about anyone's health.
- The fix: seal and protect the wellhead, disinfect the well with household bleach, then fit a UV disinfection system with a sediment filter in front of it.
- After that: retest, and keep testing at least once a year, as the EPA recommends.
What the result means
| On the lab report | What it is | What it tells you |
|---|---|---|
| E. coli | Bacteria that live in the gut of people and animals | Sewage or animal waste is getting into the well. Some strains can cause serious illness |
| Coliforms | A wider group of bacteria found in soil, vegetation and surface water | Water from the surface is reaching the well, so the well isn't properly sealed or protected |
Under Ireland's drinking water regulations, the limit for E. coli is zero in a 100 ml sample, and any coliforms found count as a failure too. There is no safe low number. A result of 1 is a fail, just like a result of 100.
What to do straight away
- Stop drinking the water untreated. The EPA advises boiling the drinking water from the well until you're sure it's safe. Boil it, let it cool and store it covered somewhere cool, such as the fridge, or use bottled water.
- Use boiled or bottled water for drinking, brushing teeth, ice, preparing food that won't be cooked and pets' water. For baby feeds, the HSE advises boiling bottled water too before you use it.
- Take extra care with babies, older people and anyone who is unwell.
- Ask for advice. If you're worried about anyone's health, the EPA says to contact your local HSE office and ask for an Environmental Health Officer.
Where the bacteria come from
- A septic tank or percolation area too close to the well, or not working properly
- Livestock, slurry or manure near the well
- Surface water running in after heavy rain, through a damaged cap, a cracked lining or a wellhead that sits below ground level
- Old, shallow or hand-dug wells, which are much easier for surface water to reach
The EPA says the best time to test is after heavy rain, when contamination is most likely. A well that passes in a dry spell can fail after a wet week.
Fixing it takes two things
1. Look after the well itself. The wellhead should be capped, sealed and standing above the ground, with animals and anything that could pollute kept well away from it. The EPA has a free online tool and guidance on protecting your well, and your local authority can advise on what yours needs. The well and pipework can then be disinfected with household bleach, which is called shock chlorination. That deals with the bacteria that are there today. It doesn't stop them getting in again, and the EPA is clear that a one-off disinfection can't replace a treatment system if the supply is often contaminated. Our guide to the rotten egg smell from well water walks through the EPA's shock chlorination steps.
2. Fit ongoing protection. A UV disinfection system goes on the main supply into the house. The water passes an ultraviolet lamp, which kills bacteria such as E. coli and coliforms as the water flows by. Nothing is added to the water and it doesn't change the taste.
What a UV system needs to work
- Clear water. Sediment shields bacteria from the light, so a sediment filter always goes in front of the lamp.
- Iron, manganese and hardness dealt with first. They coat the glass sleeve around the lamp and block the light, so any treatment for them goes ahead of the UV.
- The right size. It has to match the fastest flow the house will draw, with several taps and showers running at once, not the size of the house.
- A new lamp once a year. The lamp keeps glowing long after it has stopped doing its job, so change it on a date, not when it looks dim. Clean the sleeve when you do.
- Power at all times. If the UV is switched off, the water isn't being treated.
Why not a jug or tap filter? Ordinary jug filters and standard carbon cartridges are not designed to make contaminated water safe, and Uisce Éireann's own advice is that home filters do not guarantee your water is safe. For a well with bacteria, whole-house UV is the treatment.
Test again afterwards
Once the system is running, send another sample to the lab. A clear report is the proof that the treatment is working. After that, the EPA recommends testing a private well at least once a year for bacteria, and every three years for chemicals. Test sooner if the water changes in smell, taste or colour. Your local authority environment section or HSE Environmental Health Officer can point you to a laboratory.
On mains water? Boil notices and cryptosporidium
Public water supplies are treated and tested by Uisce Éireann. When a problem is found, such as cryptosporidium, a tiny parasite found in human and animal waste, Uisce Éireann reports it to the HSE and the EPA, and the HSE advises on whether a Boil Water Notice is needed. Cryptosporidium is highly resistant to chlorine, which is why boiling is the advice until the supply is fixed, for example by adding UV treatment at the plant.
If there's a Boil Water Notice in your area, Uisce Éireann's advice is:
- Boil water for drinking, drinks made with water, brushing teeth, ice, food that won't be cooked and pets' drinks. Bring it to a vigorous boil for 1 minute, for example in an electric kettle, then let it cool.
- Filtered water needs boiling too. Home filters do not guarantee the water is safe.
- Showering, bathing and flushing are fine, but make sure children don't swallow bath water.
- Baby feeds: use tap water or bottled water that has been boiled for 1 minute and cooled, or ready-to-use formula.
- Check for notices on water.ie or call Uisce Éireann on 1800 278 278.
A Do Not Consume notice is different. It's issued when a contaminant is found that boiling can't remove, and boiling the water will not make it safe. Use bottled water until the notice is lifted.
On a public supply, there's no need to fit UV to your house. If you'd like better-tasting drinking water day to day, see our drinking water systems and our guide to whether Irish tap water is safe to drink. Keep following any notice, as a filter is not a substitute for boiling.
What to do next
We don't test water ourselves. An independent, INAB-accredited laboratory does that. Send us the report through our well water treatment page and we'll specify the treatment your water needs, from the sediment filter and any iron or manganese treatment through to the UV unit. Our own plumbers fit and service it. If the water also stains, read our guide to iron in well water.
Failed a well water test?
Send us your lab report and we'll specify the right UV and pre-filtration for your well. Our own plumbers fit and service it, nationwide except Donegal, Kerry, Leitrim and Sligo.
5-star rated on Google
Common questions
Does boiling kill E. coli?
Yes. Boiling makes water from a contaminated well safe to drink, and the EPA advises it until the well is sorted. It's a stopgap, not a long-term fix.
Will shock chlorination fix it for good?
Not usually. It kills the bacteria in the well and pipes on the day, but if surface water or waste keeps getting in, the bacteria come back. That's why UV is fitted as ongoing protection.
Does a UV system remove anything else?
No. UV only kills bacteria and other microbes. It doesn't remove iron, manganese, hardness, smells or chemicals. Those need their own treatment, fitted ahead of the UV.
How often should I test my well?
The EPA recommends at least once a year for bacteria and every three years for chemicals, ideally after heavy rain. Test sooner if the water changes.
Is there a grant for well water treatment?
The EPA says your local authority can give information on grants for improvement works, treatment or a new well. Ask them before you go ahead.
