Non-living organic staining
| Stain colour cheat sheet |
|---|
| • Green = Copper (Cu) • Red = Iron (Fe) • Yellow = Iron • Black = Iron, Manganese (Mn) • Brown = Iron, Calcium (Ca) • Grey = Calcium |
There are several different types of non-living organics that can end up leaving stains in swimming pools and hot tubs. As mentioned previously, stains that are green and brown are usually non-living organics and come from the tannins from leaves or other natural debris and often look like algae. For the most part, these are the easiest stains to eliminate.
Dark or reddish brown stains, which are typically metallic and generally stem from iron, copper or manganese, are somewhat harder to remove. A technique that works well for removing non-living organic stains is using the correct combination of chlorine (Cl) (elevate the level of chlorine to hit breakpoint) and scrubbing the area vigorously with a surface-appropriate brush. After 24 hours, add a concentrated form of a broad-spectrum enzyme as close to the discoloured area as possible, and brush again. The tannins will degrade and the stain will fade to the point where it is virtually undetectable.
Sources of metals
Stains that are not caused by non-living organics can result from the untreated presence of heavy metals in the pool water. These heavy metals can occur naturally in water (especially in well water) or may have been introduced into the pool water as a result of corrosion caused by improper water chemistry. Some copper algaecides can be another source of metal staining. Quality copper algaecides, however, are usually in a chelated or stabilized form and normally are not a problem as long as they are used as directed.
The use of ionization products can result in the introduction of too much copper and can also result in staining and/or discolouration. However, due to their complex nature, it is unfortunate a ‘silver bullet’ does not actually exist for metal stains. Pool/hot tub stain and discolouration removal can only be accomplished by a process of using the proper materials combined with the appropriate techniques.
Frequent testing is a good way to stay on top of any metals that are finding their way into the pool. Small, trace amounts of metals in water will build up over time and can lead to the staining and discolouration of the surface. Keep in mind, it is possible to have a metal stain and little to no metal in the water. This phenomenon can sometimes occur due to the fact the majority of metals in the pool are actually concentrated on the surface (hence the stain) and are low to non-existent when tested. Should this occur, proceed with the topical vitamin C (ascorbic acid [C6H8O6]) test.
Stain identification
The vitamin C test is a simple way to determine whether a stain has been caused by metals or non-living organics. This test involves placing a vitamin C tablet directly onto the stained area, and after a few minutes, if the stain noticeably fades around the dissolving vitamin C tablet, it is confirmed to be metal in nature and can be lifted. If there is no reaction, the stain may be non-living organic.
If the stain is suspected to be non-living organic, a small amount of sodium dichloroisocyanurate (NaDCC) (sodium dichlor) chlorine can be used to test it. This test involves applying a few ounces of sodium dichlor granules directly onto the stain (this may require some creative application methods, e.g. using a piece of polyvinyl chloride [PVC] pipe to apply the granules directly onto the stain, which has formed in the deep end of a swimming pool). Do not leave the sodium dichlor on the area for longer than one minute as a longer application may damage the pool surface with the effects of the chlorine. However, if a stain is non-living organic in nature, it will have faded significantly within 60 seconds.
Using the Langelier Saturation Index (LSI)
In addition to proactive testing (specifically targeting metals and hardness), the LSI should also be used to provide an even more detailed view of the pool water’s characteristics. The LSI is a calculated number used to predict the calcium carbonate (CaCO3) stability of water. It indicates whether water will precipitate, dissolve, or be in equilibrium with calcium carbonate. The LSI is expressed as the difference between the actual system pH and the saturation pH. For instance:
- If the LSI is negative, the water is under-saturated with calcium carbonate and will tend to be corrosive in the distribution system.
- If the LSI is positive, the water is over-saturated with calcium carbonate and will tend to deposit calcium carbonate forming scales in the distribution system.
- If the LSI is close to zero, the water is just saturated with calcium carbonate and will neither be strongly corrosive or scale forming.
Simply put, the LSI is a calculation that assesses the overall balance using the pH, alkalinity, calcium concentration, total dissolved solids (TDS), and water temperature of a water sample collected from the pool. There are many free online programs, as well as several mobile apps that utilize the LSI and are easy to use.
