Turbidity is a measure of the relative clarity of water: the greater the turbidity, the murkier the water. Turbidity increases as a result of suspended solids in the water that reduce the transmission of light. Suspended solids are varied, ranging from clay, silt, and plankton, to industrial wastes and sewage.

Sources of Turbidity
High turbidity may be caused by soil erosion, waste discharge, urban runoff, abundant bottom feeders (such as carp) that stir up bottom sediments, or algal growth. The presence of suspended solids may cause color changes in water, from nearly white to red-brown, or to green from algal blooms.

Changes in Aquatic Life
At higher levels of turbidity, water loses its ability to support a diversity of aquatic organisms. Waters become warmer as suspended particles absorb heat from sunlight, causing oxygen levels to fall (Remember, warm water holds less oxygen than cooler water). Photosynthesis decreases because less light penetrates the water, causing further drops in oxygen levels. The combination of warmer water, less light, and oxygen depletion makes it impossible for some forms of aquatic life to survive.

Suspended solids affect aquatic life in other ways. Suspended solids can clog fish gills, reduce growth rates, decrease resistance to disease, and prevent egg and larval development. Particles of silt, clay, and organic materials settle to the bottom, especially in slow-moving stretches of rivers. These settled particles can smother the eggs of fish and aquatic insects, as well as suffocate newly-hatched insect larvae. Material that settles into the spaces between rocks makes these microhabitats unsuitable for mayfly nymphs, stonefly nymphs, caddisfly larvae, and other aquatic insects living there.

Sampling Procedures
Turbidity can be measured using a simple device called a Secchi disk, or a more precise instrument known as a turbidimeter.

A Secchi disk is an 8" diameter (23 cm.) black and white disk attached by a chain or rope that is marked in foot increments. Because Secchi disk measurements are based upon the disk being lowered until it disappears, it cannot be used in rivers which are shallow or have low turbidity. In these cases the Secchi disk reading may need to be estimated as accurately as possible.

It may be difficult to use the Secchi disk in fast river currents because the current will push the disk downriver, preventing an accurate measurement. A weight may have to be added to the disk in this situation. A turbidimeter is an optical device that measures the scattering of light, and provides a relative measure of turbidity in nephelometer turbidity units (NTUs). Secchi disk measurements and turbidimeter results can be roughly equated.

Turbidimeters are relatively expensive, but are the most accurate method for measuring turbidity. If multiple schools are involved in a watershed monitoring program, then one turbidimeter might be purchased for the program. Samples from throughout the watershed could then be read on the one turbidimeter. It may also be possible to use a local college's turbidimeter. Another option is to use an inexpensive turbidity test available from the LaMotte Company. Like the original Jackson Tube, this test involves viewing an object located at the end of a tube; in this case, the object is a black dot rather than a flame. As the turbidity of a sample increases, the dot becomes increasingly blurred. The turbidity of the sample is then compared with an identical amount of clear water to which a standardized turbidity reagent has been added.

Turbidity Testing Procedures: Secchi Disk

1. Lower the Secchi disk from a bridge, boat, or dock into the water until it disappears. It is important that the disk travels vertically through the water and is not "swung out" by the river current. Note the number of feet/inches on the chain or rope.
2. Drop the disk even further (until it disappears) and then raise it until you can see the disk again, Note the number of feet/inches on the chain.
3. Add the results of step I and step 2 and divide by two. This is your turbidity level using the Secchi disk.

The Relationship Among Feet, JTU'S, and NTU's
The Secchi disk measurement in feet has been roughly correlated with Jackson Turbidity Units (JTU's). These units were based upon a standard suspension of 1000 parts per million diatomaceous earth in water. By diluting this suspension, a series of standards was produced.

Jackson Turbidity Units (JTU's) are the application of these standards to the original device for measuring turbidity called the “Jackson tube.” The Jackson tube is a long glass tube suspended over a lit candle. A sample of water was slowly poured into the tube until the candle flame as viewed from above could no longer be seen. This device is no longer used because it is not sensitive to very low turbidities.

A turbidimeter measures turbidity as nephelometer turbidity units (NTU). Instruments such as the turbidimeter that measure the scattering of light are called nephelometers. Both NTU's and JTU s are interchangeable units. They differ only in that their name reflects the device used to measure turbidity.

Please note that the weighted curve chart in Calculating a WQI uses NTU's/JTU's. Because feet have been roughly correlated with JTU'S, the numerical value obtained from the weighted curve chart should be viewed cautiously when using a Secchi disk. The weighting factor for turbidity, however, is quite small (0.08) and does not affect the overall water quality index as heavily as most of the other water quality parameters.