The USGS Water Science School
It may be true that a bit of color in water may not make it harmful to drink ... but it certainly makes it unappealing to drink. So, color in our water does matter when it comes to drinking it, as well as in water for other home uses, industrial uses, and in some aquatic environments.
Pure water and color
Is pure water really clear? First, you will rarely see pure water as it is not found in a natural setting. The everyday water you see contains dissolved minerals and often suspended materials. But, for practical purposes, if you fill a glass from your faucet the water will look colorless to you. The water is in fact not colorless; even pure water is not colorless, but has a slight blue tint to it, best seen when looking through a long column of water. The blueness in water is not caused by the scattering of light, which is responsible for the sky being blue. Rather, water blueness comes from the water molecules absorbing the red end of the spectrum of visible light. To be even more detailed, the absorption of light in water is due to the way the atoms vibrate and absorb different wavelengths of light. The details are beyond the scope of this Web site, but Webexhibits explains this in much more detail.
Color and drinking water
If you have ever drunk water containing a bit of iron in it, you would know from the metallic taste left in your mouth that dissolved chemicals in drinking water can be less than desirable. Color in drinking water can be caused by dissolved and suspended materials, and a brown shade in water often comes from rust in the water pipes. Although water can contain contaminants, which are usually removed by water-supply systems, the plus side is that the water you drink likely contains a number of dissolved minerals that are beneficial for human health. And, if you have ever drunk "pure" water, such as distilled or deioninzed water, you would have noticed that it tasted "flat". Most people prefer water with dissolved minerals, although they still want it to be clear.
Have you ever gotten a glass of water from your faucet and the water is milky white water or hazy? This is almost always caused by air in the water. To see if the white color in the water is due to air, fill a clear glass with water and set it on the counter. Observe the glass of water for 2 or 3 minutes. If the white color is due to air, the water will begin to clear at the bottom of the glass first and then gradually will clear all the way to the top. This is a natural phenomenon and is caused by dissolved air in the water that is released when the faucet is opened. When you relieve the pressure by opening the faucet and filling your glass with water, the air is now free to escape from the water, giving it a milky appearance for a few minutes.
Color and water in the environment
Color in water you see around you can be imparted in two ways: dissolved and suspended components. An example of dissolved substances is tannin, which is caused by organic matter coming from leaves, roots, and plant remains (left-side picture). Another example would be the cup of hot tea your grandmother has in the afternoon. In the picture below the color is probably attributable to naturally dissolved organic acids formed when plant material is slowly broken down by into tiny particles that are essentially dissolved in the water. If you filtered that tannin-water in the picture the color would probably remain.
Most of the color in water you see around you comes from suspended material, as you can see in the right-side picture of a tributary contributing highly-turbid water containing suspended sediment (fine particles of clay, since this picture is in Georgia) to clearer, but still colored, water in the main stem of the river. Algae and suspended sediment particles are very common particulate matter that cause natural waters to become colored. Even though the muddy water below would not be appealing to swim in, in a way that water has less color than the water containing dissolved tannins. That is because suspended matter can be filtered out of even very dirty-looking water. If the water is put into a glass and left to settle for a number of days, most of the material will settle to the bottom (this method is used in sewage-treatment facilities) and the water will become clearer and have less color. So, if an industry wanted needed some color-free water for an industrial process, they would probably rather start with the sediment-laden water, rather than the tannin colored water.
Suspended material in water bodies may be a result of natural causes and/or human activity. Transparent water with a low accumulation of dissolved materials appears blue. Dissolved organic matter, such as humus, peat or decaying plant matter, can produce a yellow or brown color. Some algae or dinoflagellates produce reddish or deep yellow waters. Water rich in phytoplankton and other algae usually appears green. Soil runoff produces a variety of yellow, red, brown and gray colors.
Effects of color on ecosystems
Highly colored water has significant effects on aquatic plants and algal growth. Light is very critical for the growth of aquatic plants and colored water can limit the penetration of light. Thus a highly colored body of water could not sustain aquatic life which could lead to the long term impairment of the ecosystem. Very high algal growth that stays suspended in a water body can almost totally block light penetration as well as use up the dissolved oxygen in the water body, causing a eutrophic condition that can drastically reduce all life in the water body. At home, colored water may stain textile and fixtures and can cause permanent damage, as the picture of the sink above shows.
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