Manganese in Well Water: Black Staining, Limits and the Fix

Black marks in the toilet bowl, stained laundry, or black deposits building up in the cistern and tanks: on a private well, that usually points to manganese. It most often dissolves naturally out of the rock, it often turns up alongside iron, and an ordinary cartridge filter won't stop it.

Is it manganese or iron?

The two often come together, but the colour usually gives it away. The HSE describes iron as turning water reddish-brown, and manganese as making it look black-ish. Both stain laundry and plumbing fixtures.

If your stains are mostly orange, read our guide to iron in well water. If they're blue or green, that's usually copper dissolved from pipework by acidic water, which we cover in blue-green stains and acidic water. Plenty of wells have iron and manganese together, so test for both.

What are the limits for manganese?

The European Union (Drinking Water) Regulations 2023, S.I. 99 of 2023, set 50 micrograms per litre for manganese. The HSE says drinking water should not be above this level, and that above it manganese can make water look black-ish, affect the taste and stain laundry and plumbing fixtures.

The health figure comes from the World Health Organization. In 2021 it set a provisional health-based guideline value of 80 micrograms per litre, to protect bottle-fed babies, the most sensitive group. It's provisional because of a high level of uncertainty in the science.

The HSE's advice, from its National Drinking Water Group:

• There is currently no scientific evidence of harmful health effects at levels up to 80 micrograms per litre.
• You should not drink water above 80 micrograms per litre on an ongoing basis.
• Babies in the womb, infants and young children are the most vulnerable, along with older people, people with liver disease and people who are anaemic.
• Water used to make up baby formula should not be high in manganese.

Why is there manganese in my well?

Usually it's natural. The HSE says water passing through soil and rock dissolves minerals containing manganese, and that this is the most likely source in drinking water. The EPA links high iron and manganese in Irish groundwater to low-oxygen conditions, and the WHO says levels are usually higher in groundwater that is acidic or low in oxygen.

The Geological Survey of Ireland (GSI) lists iron and manganese among the natural problems of sandstone and shaly limestone aquifers. A few examples from its county groundwater reports:

• Cavan: manganese was over the limit in 13 of the 30 results GSI examined, and GSI says raised iron and manganese would not be unusual in central and south-east Cavan because of the minerals in the bedrock.
• Wicklow: GSI calls high iron and manganese a major problem in the shale and sandstone rocks, with manganese up to 0.75 mg/L.
• Kilkenny: GSI said high iron and manganese at one supply was likely to come naturally from the sandstones, coals and muddy sandstones of the Castlecomer Plateau.

Not every case is natural, though. GSI notes that raised iron and manganese can also be a sign of pollution from organic waste, such as from farmyards, so test for bacteria at the same time.

Why a cartridge filter doesn't fix it

Underground, the manganese is dissolved, and the EPA notes that air turns it solid only very slowly. A sediment cartridge catches particles, so dissolved manganese goes straight through and turns up later as black deposits in tanks and stains on fixtures. The WHO says the right treatment depends on whether the manganese is dissolved or in particles. The answer is to turn it solid first and then catch it, which is what a proper manganese filter does.

How to remove manganese from well water

The WHO says manganese can be easily brought below 0.05 mg/L by oxidation followed by filtration, among other methods. For a house on a well, that means a backwashing filter on catalytic media, sized to your lab results.

1. Get the pH right

pH is the big one with manganese. The EPA's filtration manual gives a pH of 7.5 to 8.5 for manganese removal on catalytic media, compared with about 9 for sand filters using chlorine. If your water is acidic, as it often is on sandstone, granite and upland wells, the pH has to be raised so the manganese filter can do its job. Correcting acidic water also helps protect copper pipes.

2. Oxidation and filtration on catalytic media

A catalytic media filter, such as greensand or a manganese dioxide media, turns dissolved manganese solid and traps it in the bed. The EPA's manual says:

• Catalytic media must be regenerated, usually with potassium permanganate or chlorine.
• An oxidant should be treated as essential once manganese in the raw water goes over 250 micrograms per litre.
• The filter needs regular backwashing to stay effective.
• Overdosing potassium permanganate can turn the water pink, so dosing has to be set up properly.

On a house system the backwash runs automatically, with the waste sent to drain. Where iron is present too, the same filter normally deals with both.

3. Where a softener fits in

The WHO says a water softener or a greensand filter can be used at the point of entry to a home to reduce discoloured water and staining of laundry and fixtures. Where the water is hard as well, we plan the manganese treatment and the softener together from your lab report.

4. Reverse osmosis for drinking water

For drinking water at one tap, the WHO says reverse osmosis is the most effective and reliable treatment for manganese. Where there's a baby in the house, a reverse osmosis system at the kitchen tap gives an extra layer of protection.

5. Before any UV system

If your well also needs UV for bacteria, the manganese has to come out first. The EPA warns that iron and manganese can foul UV lamps, which stops the UV doing its job properly.

What to do next

1. Get a lab test. The EPA suggests testing for iron and manganese if your water is slightly discoloured. Ask for pH, iron, manganese and hardness, plus bacteria. An independent, accredited laboratory takes and analyses the sample. We don't test water ourselves.
2. Send us the report. We'll tell you what your water needs and give you an itemised quote. See our well water treatment page.
3. Check the grant. The council well grant covers 100% of approved treatment costs up to €1,000, but work must not start before the council approves it. Our private well grant guide has the details.

More on well water: iron in well water, rotten egg smell from well water and nitrates in well water.

Common questions

Is manganese in well water dangerous?

The HSE says there's no scientific evidence of harm up to 80 micrograms per litre, but you shouldn't drink water above that on an ongoing basis.

Why are my toilet and sink stained black?

On a well, black or black-ish staining usually points to manganese dissolved out of the rock underground. A lab test will confirm it.

Will a water softener remove manganese?

The WHO says a softener at the point of entry can reduce discoloured water and staining, but it also links poorly maintained softeners to higher manganese in homes on wells. The right set-up depends on your lab results.

Can I use my well water for baby formula if it has manganese?

The HSE says water used to make up formula should not be high in manganese. If your result is over 80 micrograms per litre, use another safe supply for feeds until the water is treated.

Why does my water have black bits after a softener or filter was fitted?

It may be overdue a service, or out of regenerant. Have it checked and serviced.