Many people wonder why pool water often has a bright, vibrant blue color. The blue hue of pool water can range from a pale, icy blue to a deep, rich blue. While some of the theories behind blue pool water are myths, the most common explanation is that the chlorine used to sanitize pools can react with organic contaminants to produce compounds that absorb red and yellow light, making the remaining light appear blue. However, the connection between chlorine and blue pool water is more complex than it may seem.
Does Chlorine Itself Cause Blue Water?
Chlorine is a commonly used disinfectant and sanitizer for pools. When chlorine is added to pool water, it does not directly interact with water molecules to cause a color change. Chlorine solutions appear clear and colorless when highly diluted in water.
In its elemental form, chlorine is a greenish-yellow gas. However, chlorine gas rapidly breaks down into hypochlorous acid and hypochlorite ions when added to water. Hypochlorous acid and hypochlorite do not themselves impart any blue color when dissolved in water at typical sanitizing concentrations.
So, chlorine does not directly react with water to form colored compounds. This means chlorine alone cannot account for blue pool water.
Chlorine Byproducts and Contaminants
Instead, blue color in pools results from chlorine reacting with nitrogen-containing organic contaminants that enter pool water from the environment and swimmers.
When chlorine is added to pool water, it acts as an oxidizing agent and can react with nitrogen compounds like ammonia, urea, amino acids and other organics. These reactions produce chloramines and other nitrogen-chlorine compounds.
Many chloramine compounds preferentially absorb red and yellow wavelengths of visible light. By absorbing warm colors, the remaining light transmitted through pool water appears blue. Different specific chloramines can result in different shades of blue.
The major chloramines responsible for pool bluing are:
- Monochloramine – pale blue
- Dichloramine – blue-green
- Nitrogen trichloride – deep blue
Higher levels of these chloramines lead to a more intense blue color. Factors like chlorine and nitrogen concentration, pH, and sunlight exposure affect chloramine formation and decomposition rates.
Organic precursors enter pools from external environment sources like dirt and plant matter. Swimmers also introduce nitrogen compounds in sweat, urine, mucus, cosmetics and more. The bather load affects the amont of nitrogen and organic precursors.
More swimmers means more nitrogen compounds for chlorine to react with, increasing chloramine production and blue color. Chloramine levels also build up over time after each swimmer introduces a small amount of nitrogen into the pool.
Role of Cobalt in Pool Bluing
Some pool operators intentionally add cobalt compounds to enhance and stabilize the blue color. Cobalt forms complexes that complement the color produced by chloramines.
Cobalt ions bind to organic ligands and reinforce chloramine color. Cobalt concentrations around 0.5 to 1 ppm can provide a strong blue color.
However, cobalt has been banned in some jurisdictions due to potential toxicity and carcinogenicity concerns. Many pool codes now prohibit or restrict the use of cobalt.
Myths About Blue Pool Water
There are a few common myths regarding what makes pool water turn blue:
- Myth: The blue color comes from copper in the pipes or water supply.
- Fact: The miniscule amounts of copper that could leach from pipes does not cause bluing. Properly balanced pool water prevents significant corrosion and copper leaching.
- Myth: Blue water comes from the plaster or fiberglass pool surface.
- Fact: Pool plaster and fiberglass gelcoat do not impart any blue tint on their own.
- Myth: Blue water indicates the pool has too much chlorine.
- Fact: High chlorine alone does not make pool water appear blue. Blue color comes from chlorine byproducts.
While these factors do not directly contribute to blue water, they can affect water chemistry, chloramine formation and overall water appearance. But the blue hue itself results from chlorine reacting with nitrogen compounds.
Water Clarity and Particle Density
In addition to chloramines, water clarity also influences apparent color. Particle-free water transmitted light better. This enhances the scattering effect that makes the water blue.
Cloudy water scatters light in all directions, reducing the dominance of blue light. So good filtration and coagulation improves bluing.
Conversely, turbid water with algae growth appears greenish from chlorophyll pigments. The green tint overwhelms the underlying blue color.
How to Adjust Pool Water Color
While pool water turning blue is normal, you can take steps to control the intensity of the color:
- Minimize contaminants entering the pool from external sources.
- Limit bather load to control organic and nitrogen input.
- Maintain proper chlorine sanitizer levels.
- Ensure adequate chlorine to nitrogen ratio.
- Oxidize chloramines with chlorine shock treatments.
- Use cyanuric acid to stabilize and extend chlorine activity.
- Lower pH to increase hypochlorous acid for more effective sanitization.
- Run filtration system to remove particles and maximize water clarity.
- Brush surfaces and vacuum debris from the pool bottom.
- Backwash filters when pressure rises to flush out captured particles.
- Use clarifying or flocculating agents if needed.
Balancing your pool water chemistry provides the best way to control blue color and clarity.
Health Effects of Colored Pool Water
While vibrant blue water may be aesthetically pleasing, colored pool water also indicates the presence of chloramine sanitizer byproducts.
At typical levels in pools, chloramines themselves do not pose significant health risks for most swimmers. However, some sensitive individuals may experience minor respiratory irritation with repeated exposure to chloramines.
The major concern is that colored water signifies reactions between chlorine and contaminants to form a wide range of disinfection byproducts (DBPs). The specific DBPs depend on the organic precursors present.
DBPs like trihalomethanes and haloacetic acids are linked to increased cancer risk and other health effects with long-term exposure. However, concentrations in pools are generally low if chlorination and recirculation systems are properly maintained.
The best way to minimize exposure to concerning DBPs is to prevent their formation in the first place. Adding chlorine stabilizers, maintaining ideal chlorine and pH levels, providing adequate circulation and filtration, and limiting contaminants are key to reducing DBPs.
Conclusion
The common blue color of swimming pools results from chlorine in the water reacting with nitrogen-containing compounds introduced by swimmers and the environment. Specific chloramines selectively absorb longer wavelengths of light, causing the remaining light rays to appear blue. Additional factors like water clarity, particle density, and intentional color enhancers also influence the intensity and hue of blue. While vibrant blue pool water has an aesthetic appeal, it also indicates the presence of chlorination byproducts that should be minimized for health. With proper pool management and chemistry, the blue color can be controlled while maintaining adequate disinfection.