What causes blue-green stains in a bathroom sink?

Blue-green stains are basic copper carbonate, the byproduct of copper pipe corrosion. When water is acidic (pH below 7.0) or has low alkalinity, it reacts with copper plumbing. The corroded copper deposits on fixtures wherever water contacts a surface and then dries, leaving behind that characteristic blue-green tint. Soft water from low-mineral geology accelerates the process.

Is copper in tap water dangerous?

At low levels, no. Copper is an essential nutrient and is safe at concentrations found in most homes. The EPA action level is 1.3 mg/L at the tap. Above that level, utilities are required to take corrective action. Short-term exposure to elevated copper can cause nausea, vomiting, and stomach cramps. Children and people with Wilson's disease, a copper metabolism disorder, are more sensitive and should test their water if staining is heavy or persistent.

How do I stop copper stains on my tub?

Removing stains without fixing the source is a temporary measure. For the stains themselves, a paste of Bar Keepers Friend or a commercial product like CLR applied directly works well. To stop new staining, you need to address the cause. If your water is acidic, a calcite neutralizing filter raises pH and reduces corrosion. A certified plumber can assess whether the pipe condition warrants more significant action.

Does low pH cause copper pipe corrosion?

Yes. Water with a pH below 7.0 is corrosive to copper. The lower the pH, the more aggressive the attack on copper pipes. Private wells in areas with granitic bedrock, such as parts of New England, the Appalachians, and the Pacific Northwest, often produce naturally acidic water. Some municipal supplies are also slightly acidic. A basic home test strip can check your pH in minutes.

What water filter removes copper?

An NSF/ANSI 53 certified filter is required for copper reduction at the drinking tap. Standard carbon block filters and pitcher filters that are only NSF 42 certified don't reliably remove dissolved copper. A calcite neutralizing filter installed at point of entry addresses the root cause by raising pH, which reduces how much copper the water picks up from pipes in the first place. Point-of-use NSF 53 filters handle what reaches the tap.

Blue-Green Stains on Fixtures: What Your Pipes Are Telling You

Blue or green stains on your drains, tub, and faucet bases are a signal your copper pipes are corroding. The staining itself is harmless to touch. The cause is worth understanding.

The blue-green color is basic copper carbonate, the same compound that gives the Statue of Liberty its patina. When acidic or low-alkalinity water moves through copper pipes, it leaches copper. That dissolved copper deposits wherever water contacts a surface and evaporates, leaving behind the characteristic tint.

Why Water Becomes Aggressive Toward Copper

pH is the primary driver.

Water with a pH below 7.0 is acidic. Acidic water is chemically aggressive, especially toward soft metals like copper. The more acidic the water, the faster copper corrodes. Low alkalinity compounds the problem, because alkalinity acts as a buffer that slows pH swings and reduces the water’s corrosive potential.

Private wells in certain geologic regions consistently produce acidic water. New England, the Appalachians, parts of the Pacific Northwest, and other areas with granitic or crystalline bedrock tend to have soft, low-pH groundwater because the underlying rock doesn’t contribute much buffering capacity. If you’re on a private well in one of these regions and have blue-green staining, low pH is the likely cause.

Municipal water can also be acidic, though most utilities adjust pH as part of treatment. If you’re on city water and have significant staining, it’s worth testing your pH at the tap, since pH can shift as water travels through the distribution system.

Where the Staining Shows Up

The most common spots are around drain openings in sinks and tubs, where water pools before draining. The copper deposits as the water evaporates. You’ll also see it at the base of faucets, around faucet aerators, and inside shower pans near the drain.

In severe cases, the water itself may have a faint blue tint, particularly in a glass of water that’s been sitting. That’s dissolved copper in visible concentration.

Health Considerations

For most people, the copper levels associated with minor staining don’t pose a health risk.

The EPA sets the action level for copper at 1.3 mg/L at the tap. Below that threshold, regulatory action isn’t required. Copper at lower concentrations is actually an essential nutrient, and trace amounts in drinking water are part of normal human intake.

But elevated copper, above 1.3 mg/L, can cause nausea, vomiting, and stomach cramps with repeated exposure. Children and infants are more sensitive than adults. People with Wilson’s disease, a genetic condition affecting copper metabolism, need to be particularly careful and should consult a doctor about their drinking water regardless of staining severity.

If you have heavy staining, young children in the home, or anyone with a copper metabolism disorder, a certified lab test is the right call. A basic water test from a certified lab will report your copper concentration at the tap. See best water test kits for options, or find a state-certified lab through your state health department.

What Doesn’t Work

A standard activated carbon filter won’t remove dissolved copper. Carbon block and pitcher filters that carry only NSF/ANSI 42 certification are rated for aesthetic contaminants, like chlorine taste and odor, not health-effect contaminants like copper. If you’re shopping for a filter to address copper, you need NSF/ANSI 53 certification specifically for copper reduction.

Ignoring staining while only cleaning it away also doesn’t work long-term. The staining will return within weeks because the source hasn’t changed.

How to Fix It

The most effective approach depends on what’s driving the corrosion.

If your water is acidic, a calcite neutralizing filter is the right starting point. Calcite is crushed calcium carbonate. As acidic water flows through it, the calcium carbonate dissolves slightly and raises the water’s pH and alkalinity. A properly sized calcite filter installed at point of entry can raise pH from 6.0 to 7.0 or higher and significantly reduce the corrosive potential of the water. These systems require periodic refilling of the calcite media, but otherwise require little maintenance.

For drinking water at a single tap, an NSF 53 certified point-of-use filter handles copper that has already leached into the water. This is a reasonable fix if you rent, if whole-house treatment isn’t in the budget, or if you want immediate protection for drinking water while you work on the broader issue.

If the staining is severe or the pipes show visible pitting or pinhole leaks, have a licensed plumber assess the pipe condition. Copper pipes that have been corroding for years can develop pinholes that cause concealed leaks inside walls. In that case, the calcite filter is still the right water treatment step, but you may also need pipe repair or partial re-piping with PEX, which doesn’t corrode.

Testing First

A $10 to $15 home test strip can confirm whether your pH is below 7.0. That’s enough to know whether acidity is the driver. For a precise pH reading and copper concentration, a certified lab test is more accurate and typically costs $40 to $75 for a basic panel.

If you’re on well water and haven’t tested in more than a year, blue-green staining is a good reason to run a full well water panel. See well water testing guide for what to include.

Sources: EPA Copper in Drinking Water | EPA Lead and Copper Rule