Blue-green marks under the taps, a green ring round the plughole, a streak down the bath where a tap drips: that's copper, and it's coming out of your own pipes. The usual reason is slightly acidic water. It's mostly a private well problem in soft water areas, and worth sorting for the water you drink and for the pipework.
Quick answer
- What it is: copper dissolved from copper pipe and brass fittings, staining sinks, baths and toilets blue-green
- Why it happens: soft, acidic water attacks copper. Wells in granite, sandstone and peaty upland areas are the usual suspects
- The limits: copper 2.0 mg/L, and a pH between 6.5 and 9.5 (S.I. 99 of 2023)
- Today: run the kitchen tap for a minute first thing in the morning before drinking from it
- The fix: a neutralising filter that raises the pH on the whole supply, sized from a lab report. Very acidic water may need a dosing pump
What causes blue-green stains?
The World Health Organization (WHO) says dissolved copper can give water a light blue or blue-green colour and a metallic, bitter taste, and that it causes blue to green staining of sinks and plumbing fittings. WHO puts the start of staining at above 1 mg/L, half the legal limit. So stains are an early warning: they don't prove the water is over the limit, but they do mean copper is dissolving.
The copper rarely comes from the ground. Uisce Éireann says the main cause of high copper is water in contact with copper plumbing inside the property. All water is corrosive towards copper to some degree, but acidic water, and very alkaline water, dissolves much more of it.
| Stain colour | Usual cause | Read more |
|---|---|---|
| Blue-green | Copper from pipes and brass fittings, usually because the water is acidic | This guide |
| Orange-brown | Iron | Iron in well water |
| Black or dark brown | Manganese, often alongside iron | Manganese in well water |
Other signs that copper is dissolving:
- A metallic or bitter taste, worst first thing in the morning
- Hair taking on a green tinge after washing, which Scotland's drinking water regulator says has been widely reported on private supplies with high copper
- Leaks. Over the years corrosion thins copper pipe from the inside and can end in pinhole leaks, often in runs you can't see
Why soft, acidic well water attacks copper
pH is a scale from 0 to 14: 7 is neutral and anything below 7 is acidic. The Irish regulations set a range of 6.5 to 9.5, with a note that the water should not be aggressive, in plain terms that it shouldn't eat into pipes.
Groundwater gets its hardness mainly by dissolving limestone, as Geological Survey Ireland puts it, and that limestone also neutralises acidity. An EPA study of Irish groundwater found that only 2 to 3% of carbonate minerals in the rock is enough to bring the pH to near neutral. Where the rock has none, the water stays soft and acidic. The EPA's 2026 private supplies guidance adds that pH, alkalinity and low conductivity (few dissolved minerals) can all mean more metal dissolving from pipework.
The same EPA study shows which ground gives the most acidic wells:
- Sandstone and similar non-limestone rock: the lowest median pH, 6.03
- Granite and volcanic rock: a median of 6.07
- Other rock types: nearly neutral, with medians of 7.1 to 7.3
- Peaty uplands: the EPA notes natural water can go as low as about pH 4.5 in acid peaty upland areas
A median of around 6 means the typical well in those rocks sits below the 6.5 minimum. That's why pH correction comes up so often in Co. Wicklow and Co. Wexford, where well water is soft and often acidic. Acidic water can also dissolve old lead pipe, so in an older house test for lead too.
The limits
| Check | Limit | Type of standard |
|---|---|---|
| Copper | 2.0 mg/L | Chemical parameter |
| pH | 6.5 to 9.5, and the water should not be aggressive | Indicator parameter |
| Lead | 10 µg/L, falling to 5 µg/L from 12 January 2036 | Chemical parameter |
From the European Union (Drinking Water) Regulations 2023, S.I. 99 of 2023. Household wells are exempt from those regulations, but councils and the private well grant use the same values as the standard.
Is copper in drinking water harmful?
Uisce Éireann describes copper as an essential nutrient that can be toxic at high concentrations. WHO set its guideline of 2 mg/L, the same as the Irish limit, to protect against short-term stomach upsets. Nausea is the effect most often reported, along with vomiting, diarrhoea and stomach pain, usually within 15 to 60 minutes. WHO notes some uncertainty about long-term effects for sensitive groups, such as carriers of the gene for Wilson disease, a rare condition in which copper builds up in the body.
WHO also says drinking water that has stood in copper pipes can considerably increase copper intake, especially for babies fed formula made up with tap water. If you have a health concern, talk to your GP.
Run the tap first thing. WHO says copper is usually low in water that has been running, and can be much higher in water that has stood in the pipes overnight. Uisce Éireann says running the tap for at least a minute is an effective way to bring copper below the limit. Do it every morning, and after a few days away, before drinking or making up a bottle.
How acidic water is fixed
The fix is to correct the pH where the water enters the house, so it stops attacking the pipes.
| Treatment | How it works | Best for |
|---|---|---|
| Calcite neutralising filter | A vessel of calcite, a purified crushed limestone. The water dissolves a little of it on the way through, which raises the pH. It backwashes itself to stay clean | Mildly acidic water |
| Calcite and magnesia filter | The same, with magnesia (magnesium oxide) mixed in for a stronger correction | More acidic water |
| Dosing pump | A small pump feeds a measured dose of an alkaline solution, such as washing soda, into the supply | Very low pH |
| Reverse osmosis at the kitchen tap | Removes copper from the drinking water at that one tap | Drinking water only. It doesn't stop the pipes corroding |
Worth knowing about neutralising filters:
- Where it goes: after any sediment and iron filters and before a UV unit, so sediment from the filter doesn't foul the UV lamp.
- It adds a little hardness, as it dissolves calcium and magnesium into the water. On soft water that's rarely a problem.
- The media is used up slowly, so it's checked and topped up.
- Sizing matters. Too small and the pH barely moves. Too much contact and the water can become too alkaline.
Don't stop at the kitchen tap. A drinking water filter gives you copper-free water at one tap, but acidic water keeps eating the rest of the pipework, and that's where pinhole leaks come from. If the report shows acidic water, correct the pH on the whole supply.
What to test for
We don't test water ourselves. An independent, INAB-accredited lab does the analysis, and we specify the treatment from the lab report. Ask the lab for pH, alkalinity, hardness and conductivity, copper (and lead in an older house), iron and manganese, plus E. coli and coliforms, which the EPA says are the most important tests for any private well. Tell the lab about the stains. Some labs also offer a Langelier Index test, which shows whether water tends to corrode pipes: Fitz Scientific lists one for €60.
Then send us the report through our well water treatment page. Copper and pH are both drinking water standards, so where your council agrees treatment is needed, the private well grant may cover it, at 100% of approved treatment costs up to €1,000. Get the council's written approval before any work starts. Our private well grant guide explains how.
Blue-green stains on mains water?
It's rarer, as Uisce Éireann can control the pH of water leaving its treatment plants. If you're on a public supply and seeing blue-green staining, report it to Uisce Éireann. Its advice for copper is to run the tap for at least a minute before drinking and have a competent plumber check the pipework inside the house.
Blue-green stains on your well water?
Send us your lab report. We'll specify the right pH correction from your results and our own plumbers will fit it, nationwide except Donegal, Kerry, Leitrim and Sligo. Pinhole leaks too? We can repair the pipework while we're there.
5-star rated on Google
Common questions
Are blue-green stains dangerous?
The stain itself isn't, but it shows copper is dissolving from your pipes. Staining starts above about 1 mg/L and the limit is 2.0 mg/L, so only a lab test tells you where your water sits.
Will a water softener stop blue-green stains?
No. A softener removes hardness and doesn't raise the pH. Soft, acidic water needs a neutralising filter, and usually no softener at all.
Do I need to replace my copper pipes?
Usually not. Correcting the pH lets a thin protective layer form inside the pipes and slows the attack. Sections already leaking may need replacing.
How much looking after does a neutralising filter need?
Very little. It backwashes itself, and the media is topped up as it dissolves, usually at the annual service.
