What Are the Effects of High Chlorine Levels in a Gunite Pool?

Pool pH Levels

• pH is a measure of acidity versus alkalinity. A pH reading below 7.0 is considered acidic, and above 7.0 is considered basic or alkalinic. When a swimming pool water’s pH level increases above 7.6, it is too alkalinic. When the pH level decreases below 7.4, the water is too acidic. The website Pool Care suggests, “The most desirable level for pools and spas is between 7.4 and 7.6.” These levels must be kept in balance in any pool whether it is lined with Gunite or vinyl.

Chlorine and pH Levels

• Too much free chlorine can cause a pool’s pH to rise to levels above 7.6. High pH causes the calcium in the water to combine with carbonates in the water resulting in calcium carbonate. The calcium carbonate can form into tiny floating particles. Paradoxically, high doses of chlorine can also cause the pool’s pH level to fall to unacceptable levels (below 7.2.)

High pH and Pool Cloudiness

• Small calcium carbonate particles caused by the combination of water alkalinity (high pH) and excessive calcium in the water gives it a cloudy appearance. This water quality effect can occur in all swimming pools, including Gunite pools. The cloudiness may dissipate by itself or may be eliminated with a clarifying agent. Clarifiers do not reduce the alkalinity but cause the particles to clump together and sink to the bottom where they can be vacuumed up. Continuous filtration will also usually clear up the water as well.

Staining and Pitting

• Using large doses of chlorine, or shocking, can also reduce the pool’s pH. Pool shock is used to remove the combined chlorine, organic waste and contamination and to reestablish a positive level of chlorine for sanitization. The water is acidic and can corrode metals, eating away at copper fittings and heat exchangers and leaving metal oxides to stain pool surfaces, including Gunite pool-shell plaster. Pool water acidity can also cause the water to dissolve the calcium found in a Gunite pool's plaster layer, causing pitting of the plaster surface.