Blue-green stains in sinks, tubs and showers
The short answer
Blue-green staining is dissolved copper, and the copper is coming from your own pipes. Acidic water — pH below about 6.5 — slowly dissolves the inside of copper plumbing, and the stain is the evidence. It is not a cosmetic problem: the same process eventually produces pinhole leaks. The fix is an acid neutraliser, not a cleaner.
Health & safety
If you have blue-green staining and young children or an infant in the house, test specifically for copper using a first-draw sample. The EPA action level is 1.3 mg/L, and children are more sensitive to copper than adults.
What actually causes it
Several different problems produce this same symptom, and they need completely different fixes. Work down the list — each one names the check that rules it in or out.
- 1
Acidic water dissolving copper pipe
Low-pH water is chemically aggressive toward copper. It strips metal from the inside wall of the pipe continuously, carries it to your fixtures, and deposits it as the characteristic blue-green residue. Common in areas with granite bedrock, sandy soils and high rainfall.
How to tell
A pH test below 6.5, and staining that appears everywhere in the house rather than at one fixture. Worst where water sits — the overflow of a rarely used bath, the bottom of a shower. - 2
New copper plumbing still passivating
Freshly installed copper sheds a little metal before an internal oxide layer forms. In water of normal pH this settles within months.
How to tell
The plumbing was installed recently, the staining is mild, and a pH test comes back in the normal range. It should be fading, not intensifying. - 3
A grounding or stray-current problem
Electrical current travelling through the plumbing accelerates corrosion dramatically. Uncommon, but it produces the same staining with normal pH water, and it will destroy pipework far faster than acidity alone.
How to tell
Normal pH but severe, worsening staining — especially with pinhole leaks appearing in a cluster. This is one to put in front of an electrician, not a water treatment dealer.
Why this is a plumbing problem, not a staining problem
It is easy to file blue-green stains under 'annoying' and get on with your life. That is the wrong instinct, because the stain is a byproduct of your pipes being consumed. The metal in the sink came out of the wall.
Left alone, acidic water thins copper from the inside until it perforates — the pinhole leak. These do not announce themselves. They weep inside walls and under floors, and the first sign is often a stain on a ceiling or a jump in the electricity bill from a pump cycling to feed a leak. Repiping a house is a five-figure job; an acid neutraliser is not.
There is also a health dimension, though a secondary one. The EPA sets an action level of 1.3 mg/L for copper in drinking water. Most households with visible staining are well below it, but if you have blue-green stains and small children in the house, testing specifically for copper is worth doing rather than assuming.
Confirming it before you spend anything
Two tests settle this. Both are cheap, and doing them in the right way matters more than which lab you use.
Test the pH first — a strip is adequate for this, since you are looking for a range rather than a precise figure. Anything below 6.5 confirms acidic water. Then test for copper, and here the sampling method is the point: take a 'first draw' sample from a tap that has been sitting unused overnight. That water has had the longest contact with the pipe and shows the worst case. A sample taken after running the tap will read far lower and tell you very little.
If pH is low and first-draw copper is elevated, you have your answer. If pH is normal but copper is high, look at grounding and stray current instead — a neutraliser will not help.
The fix, and why order matters here
The standard treatment is an acid neutraliser: a tank of calcite — crushed limestone — that the water passes through, dissolving a small amount of calcium carbonate and raising the pH toward neutral. It is passive, needs no power, and the media is simply topped up once or twice a year.
For very low pH, or where the required flow is high, a soda ash injection system is used instead. It doses sodium carbonate into the line with a small pump. More precise and more capable, but it needs power, solution mixing, and regular attention.
One consequence to plan for: a calcite neutraliser raises hardness as it works, because it is dissolving limestone into your water. If you already have hard water and were planning a softener, size the softener after the neutraliser is in, not before — the numbers change.
The order point is broader than that. An acid neutraliser belongs early in the treatment train, ahead of any iron filter, because oxidising media barely functions below pH 7. Fit an iron filter into acidic water and it will underperform from the day it is installed, and the usual conclusion is that the filter was faulty.
Getting the existing stains off
Copper stains lift with a mild acid. White vinegar and a non-scratch pad will handle light staining on porcelain and enamel. Heavier deposits respond to a paste of cream of tartar and lemon juice, or a commercial copper stain remover.
Avoid abrasive powders on acrylic or fibreglass — they scratch, and the scratches then hold stains better than the smooth surface did. And as with iron, there is little point cleaning until the water is treated.
The treatment this points to
If this turns out to be the cause, here is where the equipment sits in the treatment train. Order matters — each stage protects the one after it.
- 1Acid neutraliserBefore any oxidising filterRaises pH out of the corrosive range. It must come before iron filtration because oxidising media barely works below pH 7.
Frequently asked questions
- What causes blue-green stains in a sink?
- Dissolved copper from your own plumbing, caused by acidic water with a pH below about 6.5. The water slowly dissolves the inside of copper pipes and deposits the metal at your fixtures.
- Is water that leaves blue-green stains safe to drink?
- Usually, but it should be tested rather than assumed. The EPA action level for copper is 1.3 mg/L. Take a first-draw sample from a tap unused overnight, which represents the worst case, and test that.
- Will a water softener fix acidic water?
- No. A softener removes hardness minerals and does not raise pH — in fact softened water is marginally more aggressive toward copper, not less. You need an acid neutraliser or a soda ash injection system.
- How much does an acid neutraliser cost to run?
- Very little. A calcite tank has no power draw and no moving parts; the ongoing cost is topping up the media once or twice a year, typically a bag of calcite. Soda ash injection costs more to run because of the pump and the chemical.
- Does acidic well water damage a water heater?
- Yes. Low pH attacks the tank lining and consumes the anode rod faster, shortening the heater's life. It is one of the less visible costs of leaving acidic water untreated.