As someone who works in water treatment I fequently receive questions about red, black, pink or cloudy appearances in drinking water. Contrary to what might be expected none of these colors are inherently dangerous to human health. They do however make the water look unappetizing. They are called asethetic water quality indicators. Well the colors themselves isn’t the indicator, what causes the water is. I will outline the causes below.
Red water is usually caused by the oxydation of iron and iron bacteria. To be a little more accurate the color is a reddish brown, if you see a red that belongs in a paint can then I highly recommend NOT drinking it. Iron oxydation (rust) is not dangerous at all. Many water sources contain iron naturally. Iron is prevalent in groundwater. The red color comes from iron particles rusting when they come into contact with oxygen in the water. The rusting is accelerated when the iron is introduced to chlorine. As you know chlorine is very commonly used as a disinfectant. When there is a lot of iron and a lot of chlorine then there can be a visible particles of rusted iron in the water. This looks really bad when you turn on the faucet but iron is something they add to mineralized bottled water and iron is a necessary element in proper nutrition.
Iron bacteria can enter the water at the source or if the water is stored in a metal container or watermain. Wells can become contaminated with iron bacteria. When they do, read this to know what to do about it.
Black colored water is not to be confused with black water which is a term used for sewage. Sewage is often a yellowish brown, unless it has gone septic and then it is very black and very smelly. An odorless black tint to water is usually due to manganese. Manganese behaves a lot like iron does except it oxidizes a lot slower. Water stored for a couple days or more will turn black if there are high levels of manganese in the water. Manganese is more often found in groundwater than in surface water. Sometimes it wont be noticeable in the water. It will however be noticable as a black stain on appliances and reservoir walls.
Pink water comes from potassium permanganate (KMnO4). Permanganate is a treatment chemical used to help oxidize iron and manganese. When too much is added the water turns pink. When a lot is added then the water turns purple. The pink isn’t dangerous to human health. It is hower alarming to see pink coming out of a faucet. To read how to use potassium permanganate as a disinfectant read this.
To remove iron, manganese and permanganate is accompolished with greensand filtration. Don’t let the name fool you, greensand is black in color. Greensand is chemically activated to remove oxidized minerals from water.
A yellowish tint (sometimes brown) to the water IS potentially dangerous. Yellow tea colored water is indicative of organic material in the water. Organic material is mostly non living particles but it also includes bacteria, viruses, and other pathogens. Sewage is also this color. So beware of yellowish water.
Not many different things can cause a truely cloudy appearance to water. Turbidity is sometime said to be “cloudy” but it is caused by suspended particles blocking light from passing through. Usually turbidity is also colored at the same time. Unless the particles are white in color, then turbidity isn’t cloudy, it is dirty.
Cloudy water is caused from dissolved gasses (usually oxygen) in the water getting released. This happens when the temperature in the water is significantly different than the temperature of surrounding environment. Since large bodies of water are slower to heat up and slower to cool down, this difference happens every spring and fall. It is called reservoir turnover. The way to test if it is just dissolved gasses in the water is to let a glass sit for five minutes. All bubbles of gasses will disappear and the water will look and taste normal. If the water is still cloudy after five minutes, then the problem is caused by turbidity and it must be removed by filtration.
Not everything that can happen to drinking water is dangerous. Reddish tints from iron and blackish tints from manganese are natural and harmless. This article should help you determine when visual changes to the water are cause for alarm and when they can be ignored safely.