Routinely checking the pipes below your sink is a vital part of keeping your home's plumbing running smoothly and safely. Green staining on pipes is more common on homes built before the 1990s when copper plumbing was common. A few different reasons account for why the pipes under your sink may be turning green, especially if they are copper, but the only long-term solution for each scenario may be pipe replacement.
Copper will turn green after prolonged exposure to oxygen, which is carried by the water. Many copper pipes are coated with a protective enamel, but this barrier may be broken down by various chemicals over time. Wipe the green copper piping down with white vinegar to remove most of the green material, but be forewarned that this is only a temporary solution. The green will return when the copper pipe is once again exposed to oxygen.
Partly responsible for green stains found on copper piping is acidic water. You'll also notice that the water coming out of your faucet may have a green tint that could potentially stain your sink. Some companies offer water softeners that plug right into your home's main plumbing network and remove many of the chemicals that lead to acidic water. The water from your tap should ideally have a neutral pH of around 7.0.
Another common cause of green staining on your sink pipes is copper erosion, which goes along with acidic water. The corrosive water will slowly wear down the copper piping over time, leading to green water and leaks. Check your pipes for a leakage and green streaks, which could be signs of dripping water. These pipes will probably require replacement if there is a leak.
Mold plays no favorites in regards to the type of piping you have and can appear on everything from copper to PVC plastics. Mold builds up in perpetually moist environments and comes in a variety of colors, ranging from red and black to blue and green. It can be hazardous to health if it is allowed to thrive. Wipe down the sink after every use to keep it dry, and ensure there aren't any leaks in the pipes under your sink that could create a mold-promoting environment.