We have an upstairs bathroom that we've gutted and are having some contractors remodel. The toilet is the only thing that hasn't been touched. We had a plumber come and redo all of the source and drain lines for the new sinks and the shower. The other day, he turned off the water for the entire day to do his work and turned it on when he was done.

That evening, we turned on one of the downstairs baths, and blue water came out for about 2-3 seconds, then it became clear. We tried other sinks and saw the same thing. My immediate fear was that this was somehow the water from the blue bleach tablet we had in the upstairs toilet and that somehow the sewage drain line water from the toilet was making its way into the source line. I'm no plumber, but this seemed improbable to me since they are separate lines and direction the water pressure would probably not allow it to go throughout the entire house the way it did.

The plumber is a trustworthy guy who has done a lot of work for us before. He was pretty astonished when I showed him the blue water coming out of the downstairs bathroom faucet. He rechecked the lines and felt that it was unlikely that the sewage water was getting into the source line, but had no solid explanation for the blue water. He said it could be that the source valve for the toilet tank was somehow letting water back into the source line, but he seemed skeptical. He also said that it could be a coincidence of timing and that maybe the city's copper line has some corrosion which could possibly cause blue water. He suggested getting the city to come check the water quality.

We caught some of the water in a glass to see how blue it is. I apparently can't seem to smell anything, but my wife assures me that it smells like the bleach tablets. We we're going to try to put some food coloring of a different color in the toilet and see if that color comes through. We're also going to experiment with whether it happens only after we flush the toilet.

Any other suggestions? Has anybody seen something like this before?

  • 1
    Did you ever figure out what the issue was? Commented Jul 26, 2021 at 15:51

4 Answers 4


By the way, it is not at all easy for this to be sewage getting into your water lines, but it might be a problem of siphoning back into your house lines from the toilet tank itself. If so, it is still a bad thing that really must be repaired. I like the idea of a couple of drops of food coloring as a test of this. It might happen only when the toilet is flushed, and the faucet for the sink is also open.

If that test shows nothing, then redo the test, but this time, turn off the water pressure into your home at the main supply. (There will be a shutoff valve in your basement.) Now, open the valves in your downstairs faucets. This will create a siphoning effect, trying to suck water back from the toilet supply tank.

Since this has happened only once, my guess is it happened when the water pressure for your home was turned off. This allowed water to siphon back into the water supply lines from the toilet tank. An old valve in the tank might explain that.

Regardless, if you confirm this is what happens, then I would add an anti-siphon valve (also known as a back-flow prevention valve) into the water line to the tank. This is a spring loaded one-way valve, that allows water to flow into the tank, but not the other way.

Could this be a copper corrosion issue as the plumber claimed? This seems unlikely for that to have happened since you have not seen it before, but anything is possible. If you have that much copper in your water that is leaching out of the supply lines, this would be something to worry about. So if you do the food coloring test, and there is no sign of backflow from the tank, then I would get a water test done for copper. In fact, a quick check on Amazon finds a home water test kit that includes a test for copper in your water.

  • 4
    +1 for the tank water leaking back into the plumbing when the water main was shut off. Normally if your toilet had a leaking fill valve, supply water would leak into the toilet. But with the supply turned off, tank water could leak back into the supply, and diffuse into all your house plumbing. I'd replace that valve and then run all the plumbing in your house for half an hour or so (run washer & dishwasher empty too), then see if the problem has gone away. Commented Nov 7, 2010 at 13:23
  • 11
    Another check is to look when the water is shut down, with the downstairs faucets open. Does the water level in the tank drop? If so, then it is siphoning back. You would definitely replace the valve in the toilet tank if so.
    – user558
    Commented Nov 7, 2010 at 13:39

We had blue water coming out of our bathtub faucet for a number of months. I talked to people at our local plumbing company. They had no answer. I gave up searching the blogs on the internet. This is what I did to solve the problem. Our gas hot water tank was almost 15 years old, past it's useful life. I bought a new tank. I first removed the cold water copper inlet pipe from the wall to the top of the tank. It appeared OK. Then I loosed the pipe fitting on the hot water outlet on top of the tank. Black water started to seep out. After removing the pipe I stuck my finger in the pipe end. It came out totally black. It stained my finger. The black O ring seal had totally deteriorated. It was leeching a black/blue stain into the bathtub water. I replaced the tank and the flexible copper tubing to the tank. Problem solved. Try this first. Turn on the cold water to the bathtub. See if it comes out clear after a few minutes. Then turn on only the hot water. If it starts to come blue, then you may have a problem with the hot water pipe coming our of the hot water tank.


I wonder if there is some pipe that has had water sitting in it for a while. I saw this at my grandmother's house where there were bathroom taps that didn't get used for months at a time. It was so that blue my initial reaction was that it was blue toilet bowl cleaner contaminated from a cistern.

Blue or green water is caused by the corrosion of internal copper piping. Generally, the water discolouration is accompanied by a metallic taste. This is a complex problem that at elevated levels of copper can have health implications. it is caused by the release of copper from copper pipes into the water. While low levels of copper are essential for good health, the National Health and Medical Research Council (NHMRC) have specified a health limit of 2 mg/L for copper levels in drinking water. Water with a copper level greater than this amount should not be consumed or used for food preparation. Water with levels this high is usually seen as cloudy or blue/green with blue particles present. Sometimes, after boiling the water in a kettle or saucepan, the water or particles may change to black/brown and settle on the water surface.



This is just a random guess, but if the plumber used pex piping, is it possible for the piping to leech some coloring into the water it contains (this only makes sense if you did use pex piping, and it is blue).

Not the answer you're looking for? Browse other questions tagged or ask your own question.