Green water signals a change in water chemistry or biological balance, and its safety for swimming depends entirely on the cause of the color change. This tint is a warning sign requiring careful assessment before entering the water. The source of the green—whether a natural lake or a treated swimming pool—determines the potential health risks. In natural bodies of water, the coloration often points toward microorganisms, while in pools, it usually signals a chemical imbalance or mineral issue.
Identifying Harmful Algal Blooms (HABs)
The most significant health risk associated with green water in natural environments like lakes, ponds, and slow-moving rivers is a Harmful Algal Bloom (HAB). These blooms are typically caused by cyanobacteria, often called blue-green algae, which are photosynthetic bacteria. Cyanobacteria thrive in warm, nutrient-rich water and multiply rapidly, creating a dense, discolored surface layer.
A bloom may appear as a thick surface scum, resembling spilled paint, pea soup, or cotton-like clumps. The color can range from vibrant green to blue, red, or purple. It is impossible to visually determine if a bloom is toxic, as non-toxic blooms can look identical to dangerous ones. Some blooms may not even be visible on the surface, instead gathering several inches below the water line.
The danger from HABs comes from the release of cyanotoxins, which are produced by certain cyanobacteria species. These toxins can affect the liver, kidneys, or nervous system, depending on the specific type involved. Exposure occurs through ingestion, direct skin contact, or inhaling water spray containing the bacteria.
Ingesting water contaminated with cyanotoxins can lead to gastrointestinal issues, including nausea, vomiting, diarrhea, and abdominal pain. Larger amounts may cause liver or neurological damage. Contact with the skin or eyes can result in allergic dermatitis, rashes, itching, or conjunctivitis. Neurotoxins, such as anatoxins, can cause tingling, numbness, and progressive muscle paralysis, and are particularly dangerous to pets.
Non-Biological Reasons for Green Coloration
Green water in a controlled environment, particularly a swimming pool, frequently signals chemical issues rather than a dangerous natural bloom. The most common cause is a lack of disinfectant, which allows common green algae to grow quickly. This makes the water cloudy and unsafe due to the presence of bacteria, typically when free chlorine levels drop below the recommended 1-3 parts per million (ppm).
Another frequent cause is the oxidation of trace minerals, most often copper, which tints the water a light, clear green. Copper can enter the pool from corrosive, imbalanced water interacting with copper plumbing or heating elements, or from copper-based algaecides. When this copper oxidizes, usually after a chlorine shock, it turns the water a distinct tea-green color.
Seasonal factors can also temporarily give water a greenish hue, notably the presence of pollen or fine organic debris. Pollen is a naturally yellow-green tint that, when abundant, can settle in the water and give it a greenish appearance. While pollen is generally harmless, it signals that the filtration system may not be running efficiently or that the water is accumulating organic matter. High pH levels, typically above 7.8, can also reduce chlorine effectiveness, indirectly promoting algae growth and green color.
Immediate Safety Checks and Official Guidance
Before swimming in any green-colored water, perform several immediate safety checks, regardless of whether it is a pool or a natural water body. Always look for posted advisory signs from local authorities, such as the health department or park service, which may warn of bacterial or HAB risks. The simplest rule is to check water clarity: if you cannot see the bottom in the shallow end of a pool or in knee-deep natural water, avoid entering due to the risk of unknown contaminants and potential drowning hazards.
Note any unusual odors, such as a strong, foul, or chemical smell, which can indicate high concentrations of bacteria, decomposing organic matter, or chemical imbalances. For natural water bodies, if you suspect a HAB, avoid contact and immediately report your observation to the local health department or waterway manager for testing. This is the only way to confirm the presence of cyanotoxins.
If you have already made contact with water you suspect is contaminated, rinse off immediately with clean, fresh water. Monitor yourself and any pets for symptoms such as skin irritation, gastrointestinal distress, or neurological signs like weakness or disorientation. For swimming pools, use a test kit to ensure the free chlorine level is within the safe range of 1-4 ppm and the pH is between 7.0 and 7.8 before allowing entry. If a pool has been shocked, wait until the chlorine level drops into the safe range, which can take 8 to 24 hours.