Tough stain: Once mineral deposits

start to accumulate on a pool surface, it’s important to find

the cause before they create a permanent stain.
Tough stain: Once mineral deposits start to accumulate on a pool surface, it’s important to find the cause before they create a permanent stain.

Staining of cementitious surfaces is a problem in traditional and salt water pools. These stains can develop almost immediately or over time. When stains develop gradually, the pool owners may not realize the problem until it’s too late. 

Stains caused by leaves in the pool, metals in the source water, and exposed rebar will impact any type of pool. However, other causes of staining, such as galvanic corrosion and metals in the salt itself, are much more of an issue with salt water pools. Since salt water pools are unique, this article will address ways to help prevent stains in these systems.

Prime causes

The warning signs of staining in salt water pools include mild streaking down the sides of the pool, or discolored pool surfaces or water. Water discolorations can range from blue-green to dark brown. Interestingly, a significant contributor to staining can be the naturally occurring contaminants found in pool salt itself.

All salt molecules have the same chemical makeup — sodium chloride (NaCl). However, pool salt is not 100 percent pure sodium chloride; it contains different types and levels of impurities. Where the salt comes from and how it was produced — mined from underground salt deposits, mechanically evaporated, or evaporated from saline ponds (solar) — affects the levels and types of contaminants found. Manganese, iron and copper are responsible for the majority of staining issues.

The shape of the salt crystal is often an indicator of salt purity. Generally speaking, the more irregularly the salt crystal is shaped, the more impurities are either “locked” within the salt’s molecular structure or clinging to its surface. This is especially true of solar and mined salt, since these salts undergo little if any processing to remove naturally occurring contaminants.

On the other hand, some brands of mechanically evaporated salt are purer, having a more uniform, cubic shape. Mechanical evaporation involves solution mining and very high heat to produce salt from underground deposits. The high heat used to evaporate the salt actually eliminates many of the organic contaminants found in solar or mined salt.

Despite the additional processing it receives, some mechanically evaporated salt still contains high levels of stain-causing metals. Therefore, some manufacturers employ an additional purification step, called brine treatment, to remove these metals.

But even if high-purity pool salt is used, improperly applying it can also cause stains. If undissolved salt is allowed to remain on a cementitious pool surface, it can cause efflorescence, a type of staining. To put it simply, calcium carbonate is a major structural component of plaster, and it’s not very soluble in plain water; but high salt concentrations greatly increase its solubility.

In a salt water pool, the impact on plaster is minimal since the salt concentration is only about 3,200-3,500 ppm. However, the salt concentration in the immediate vicinity of an undissolved pile of salt on the pool floor could be well over 300,000 ppm. This concentration of salt is high enough to dissolve the calcium carbonate in the plaster, effectively weakening it.

In fact, the calcium carbonate quickly returns to its insoluble state as soon as it contacts pool water with “normal” concentrations of salt (i.e., 3,200-3,500 ppm). As the calcium carbonate falls out of solution, it attaches to and discolors surfaces. This discoloration is especially visible on colored plaster finishes.

Choose the right salt and add it properly

The greater the purity of the salt, the better it’s suited for use in a salt water pool. Ask your supplier to verify the manufacturing method and level of stain-causing metals in the salt you use — they should be able to guarantee its quality, or provide you with a product specification sheet showing the level of stain-causing minerals. Never use salt products not specifically designed for pool use, such as water conditioning pellets or rock salt. They contain more impurities, plus additives that should not be used in pools.

Even when using a high-grade pool salt, you should still follow best practices when adding it to the pool. For new pools, observe the 28-day waiting period before adding salt to ensure that the plaster cures properly. Then, add salt in the deep end — while the pump is running — and brush the salt until it is dissolved completely. In addition, consider adding a stain preventer when setting up a new salt water pool.

Test regularly

Staining due to metals in source water, corrosion of metallic equipment, or salt impurities is exacerbated by pH that is too high or too low. Weekly testing for pH and monthly testing for metals (if source water is  high in stain-causing metals or if pools contain copper heaters) is recommended.

Treat when needed

There are many stain removal products available that can be used in salt water pools. Some of the more advanced salt products also contain anti-stain agents, and some manufacturers offer performance guarantees with them.

Avoid phosphorous-based stain-fighters, since they break down into orthophosphates, which are nutrients for algae and promote the formation of phosphate scale in the chlorine generator. Physically removing dissolved metals can usually be accomplished with sequestering agents and filter aids. 

Fortunately, by following sound product application procedures and maintenance principles, stains can often be prevented. That way, customers can enjoy all the benefits of their salt pools without the worry of unsightly stains.