If you see blue-green marks around a bathroom sink drain, along the bottom of a tub, or on a light-colored basin, copper is a common suspect. Many St. Louis homes built over the past several decades have copper supply pipes and brass fittings, and the marks often appear when metal in the plumbing slowly corrodes. The stain tells you that water is picking up copper somewhere in the system. It does not, by itself, tell you where the copper is coming from. This guide walks through what to look for and what you can check on your own before you call a licensed plumber.

What the stain is

Copper is a metal that can dissolve slowly into water, especially when the water is acidic or when it sits in pipes for long periods. The dissolved copper can leave a blue-green or greenish tint where water collects, dries, or pools. The marks usually appear near the faucet, around the drain, or under the spout, because that is where water lands and dries most often. Some homeowners also notice a faint line that follows the path of water down the basin.

Copper stains look different from hard water deposits. Hard water usually leaves white, chalky, or gray crust. Copper tends to leave a colored tint that gets greener after water sits. If your marks are white or crusty, the scale guides on this site will fit your situation better.

Look at the pattern first

Before you test anything, spend a few minutes noting where the stain appears. Write down the answers to these questions:

  • Does the stain appear only at the hot tap, only at the cold tap, or at both?
  • Is it around the drain, under the faucet, or on the floor of the tub?
  • Does it return soon after cleaning, or does it build up slowly over weeks?
  • Are other fixtures in the house showing the same tint?

A stain on the hot side only can point toward the water heater or the hot supply line. A stain on both sides suggests the incoming water is carrying copper before it reaches the fixture.

Run simple checks at home

Home checks will not tell you exactly what is in your water, but they can help you narrow the cause.

  1. Fill a clear glass from the cold kitchen tap and hold it against a white sheet of paper. Note any tint.
  2. Let a second glass sit overnight, then compare it with the first. A tint that grows with standing time can suggest metal in the plumbing.
  3. Use a basic pH test kit from a hardware store. Water that tests acidic is more likely to corrode copper.
  4. Look at the pipes where they enter your home, in the basement, crawlspace, or near the water heater. Copper pipe is usually smooth, bright, and rigid, and it often carries a label or stamp.

Clean the marks carefully

Clean the stain before you judge the cause, but photograph it first so you have a record. Use a mild, nonabrasive cleaner and a soft cloth. Avoid harsh acids on natural stone, and test any cleaner in a hidden spot before using it on a finish that scratches easily. If the stain comes back within a few days of cleaning, that is useful information, so note it.

When to have the water tested

Get the water tested by a certified lab if the stain returns quickly, shows up on several fixtures, or comes with a metallic taste. Ask the lab to test for copper and pH. If your home is older or you are unsure what material the service line is made of, also ask about lead. A lab report gives you exact readings that a home kit cannot, and those readings are what a plumber or water professional will need to recommend a next step.

Next steps for St. Louis homeowners

In St. Louis, the age of your home and the layout of its plumbing will shape the answer. Work through this order:

  1. Record the pattern of the stain, including which taps show it.
  2. Run the home checks and note the results.
  3. Order a lab test for copper, pH, and, if relevant, lead.
  4. Ask a licensed plumber to inspect any copper or brass lines that look corroded, pitted, or green at the joints.

Keep your lab reports and photos together. If you later talk with a plumber, a water professional, or your local water provider, those records will make the conversation shorter and more specific.