Why do I have blue-green stains in my sink or tub?

Copper corrosion causes them. The EPA sets a secondary guideline for copper at 1.0 milligram per liter, and above that it lists two effects: "metallic taste; blue-green staining." The agency explains the link in its section on corrosion. It says the corrosion of iron and copper "may stain household fixtures and impart objectionable metallic taste and red or blue-green color to the water supply."

Penn State Extension describes the same pattern from the homeowner's side. "Copper can cause bluish-green stains in sinks and a metallic taste to water, especially when water is in contact with pipes for several hours (overnight)." That's why the first water of the morning tends to carry the most copper. It has been sitting in the pipes all night.

Where does the copper come from? Usually from inside the house.

• Copper supply pipes. The Minnesota Department of Health says older homes are more likely to have copper piping, and copper gets into water when the water moving through it is corrosive.
• Copper fittings and brass parts. Even plastic-piped homes use copper and brass. The same agency notes that most newer homes use PEX or PVC but may still have copper fittings or components.
• New copper work. Copper parts usually build a natural coating that keeps copper from dissolving. The Minnesota agency says copper plumbing less than three years old usually hasn't had time to build that coating yet. A fresh remodel can stain for a while.
• Hot water. The Minnesota agency says hot water releases more copper from pipes than cold water.

Many things make water more or less corrosive. Penn State lists pH, calcium, hardness, dissolved solids and temperature, and says soft, acidic water tends to be more corrosive. Phoenix water is hard. The city's 2025 report lists it at 10 to 17.6 grains per gallon, with a pH range of 6.6 to 8.8. So city-wide conditions are not the typical soft, acidic setup. When a Phoenix home stains blue-green, look at the plumbing in that house: its age, any recent copper work, and how hot the water heater runs.

### Where the stain shows up is a clue

A stain right under a dripping faucet is a common pattern, since each drop leaves a little copper behind as it dries. Fix the drip and the stain grows much slower. Our guide to fixing a dripping faucet covers that. Stains in the tub under the spout or rings in the toilet bowl point to the same metal. Stains in every fixture point to the supply piping, not one faucet.

Small amounts are fine for most people. The Minnesota Department of Health says eating or drinking copper "does not usually cause illness in most healthy people." Too much is another story. The agency lists headaches, vomiting, diarrhea, stomach cramps and nausea among the effects of too much copper. It says people with Wilson's disease and some infants under one year old are more sensitive.

The agency puts the line at 1,300 micrograms per liter. It says water above that "can be a health risk for everyone." The EPA uses the same number as an action level for public water systems. Per the Minnesota Department of Health, a system has to act if more than one in ten of the homes it samples come in above it.

Phoenix does well on this test. Its 2025 water quality report shows the city's 2024 home tap sampling, where nine in ten taps were at or below 0.3 parts per million of copper. The results ranged from 0.04 to 0.5, and zero of the 56 taps sampled were above the 1.3 action level. That's the city's water. Your own tap can differ, and the Minnesota agency says your home "may have higher levels of copper in drinking water than the homes your public water system tested." A blue-green stain is exactly the kind of sign that says your house might be one of those.

### Simple steps to cut copper exposure

The Minnesota Department of Health gives these steps.

1. 1Run the water first. If a tap hasn't been used for more than six hours, let it run for at least 30 to 60 seconds before drinking or cooking.
2. 2Drink and cook with cold water. Use cold water for baby formula too, since hot water releases more copper.
3. 3Test if it matters. The agency says testing is important if an infant or someone with Wilson's disease drinks the water. Use an accredited lab.

Blue-green stains can also hint at lead. The Minnesota agency says that if you see signs of corrosive water, lead may also be a problem. Phoenix's FAQ passes on the EPA's point that homes built before 1986 are more likely to have lead pipes, fixtures and solder. If your home is that old, test for lead at the same time. Our guide on testing home water for lead covers how.

Not always, but stains are one of the early signs. The Minnesota agency lists three signs of corrosive water together: pinhole leaks, pitting in pipes, and blue-green stains on plumbing fixtures. If you have stains alone, keep an eye on things. If you have stains plus small leaks, the corrosion has reached the pipe walls.

Here's how to read it.

• Stains only, in one spot: usually a drip or a single brass part. Fix the fixture and clean the stain.
• Stains in several fixtures and a metallic taste in the morning: copper is dissolving across the system. Test the water for copper and lead.
• Stains plus pinhole leaks or pitting: the copper itself is wearing through. Our page on pinhole leaks in copper pipes explains why they happen, and when a full repipe makes more sense than one more patch.

Penn State lays out the fix options. You can replace corroding parts with corrosion-resistant ones, treat the water to make it less corrosive, or remove copper at the tap. For the tap option, it lists distillation, reverse osmosis and activated alumina filters as acceptable methods. For a whole-home look at treatment, see our page on the signs you need water treatment.

Clean the stain, then fix the cause. Otherwise it comes back.

For the cleaning, follow the care guide from your sink or tub maker. Finishes differ, and a scrub that works on one can scratch another. When in doubt, start with the gentlest cleaner the maker allows.

For the cause, work down the list above. A dripping faucet needs a cartridge or washer. A hot water heater setting may be worth a look. Pipes with pinholes need a repair plan. If stains show up with brown or yellow water too, our page on whether brown or yellow tap water is safe covers the iron side. If you're not sure the problem is in your house at all, check whether bad water is the city's problem or yours.

HQ Plumbing & Air handles home plumbing repairs, including leaks and dripping fixtures, and installs water softening and filtration systems. We're licensed and insured, and we'll give you an upfront price before any work starts. The estimate is free.