If you have black or grey cloud-like stains on the floor of your in-ground vinyl liner, they are most likely caused by bacteria that is living in the ground under your pool. Bacteria sometimes show up years after a pool has been installed. We think the bacteria can be washed into your pool area by flooding rains or when a septic tank overflows.
There is little proven research on this topic but we understand the bacteria actually gets sunlight through the water, eats on some food source in the ground and then secretes a dye-like material that migrates through the vinyl and shows up as a stain on the floor of your pool. Often these spots appear grey, greyish green or light black in color. Once the spots appear, the damage to your liner is done and it’s irreversible. Algaecides and other chemicals used in the pool will have no effect on these stains since they can’t get to the source of the problem which is the bacteria in the soil.
So you live with stains as long as you can and then, when it’s time to change your liner, it’s also time to try and get rid of the bacteria to avoid a repeat problem with staining. No one knows precisely how to solve this problem but there are four methods that we understand may be effective. You may want to choose one or even try all four.
- The simplest way to attempt to kill the bacteria is to have your contractor spray the floor of the pool with liquid chlorine before installing your new liner. Chlorine bleach works fine and they will mist the floor of the pool using a garden sprayer three or four times.
- A second possible solution is to change the pH of the soil under and around the pool. This is done with a chemical called Copperas Iron Sulfate (FeSO4) which is available at tree nurseries or agricultural supply houses. The chemical is sprinkled on the ground around the pool and on the floor of the pool and then the ground is saturated with water for two to three days. The idea is to get the powder deep into the ground so it can change the pH of the soil and kill the source of the bacteria.
- Since bacteria need sunlight to grow, a third option is to eliminate the light source by putting a black sheet of plastic under the liner. This is not advisable if your yard has a high water table as rising water may float the liner and ball up the plastic underneath causing an even bigger problem than the stain.
- Finally, some companies are selling a roll-on epoxy-like stain barrier paint to cover the floor and walls of your pool. It’s like a liquid plastic that sets up hard to form a membrane intended to block out the light and prevent stains from getting to your liner. It is the most expensive of the options but some people claim it really works.
Discuss these options with your local pool professional and depend on them to suggest which method they have tried and found to be most effective.