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Turbidity
Background Information
Turbidity refers to how clear or how cloudy the water is. Clear water has a low turbidity level
and cloudy or muddy water has a high turbidity level. High levels of turbidity can be caused by
suspended particles in the water such as soil, sediments, sewage, and plankton.
Soil can get
in the water by erosion or runoff from nearby lands. Sediments can be stirred up by too much
activity in the water, either by fish or humans. Sewage is a result of waste discharge and high
levels of plankton may be due to excessive nutrients in the water.
If the turbidity of the water is high, there will be many suspended particles in it.
These solid
particles will block sunlight and prevent aquatic plants from getting the sunlight they need for
photosynthesis. The plants will produce less oxygen thereby decreasing the DO levels.
The
plants will die more easily and be decomposed by bacteria in the water, which will reduce the
DO levels even further.
Suspended particles in the water also absorb additional heat from sunlight which will result in
warmer water. Warm water is not able to hold as much oxygen as cold water so DO levels will
decrease, especially near the surface.
Suspended particles are also destructive to many aquatic organisms. They can clog the gills
of fish and interfere with their ability to find food. They can also bury bottom dwelling creatures
and eggs. Suspended particles can transport pollutants through the water.
Test Procedure
There are several ways to measure turbidity. One way is with a Secchi disk which is
slowly lowered
into the water until it is no longer visible, then raised until just visible,
and lowered to just no longer visible. The depth at which the Secchi disk is no longer visible
the second time is recorded in meters. A Secchi disk is usually more appropriate for deep waters of lakes,
ponds, and rivers.
For shallow waters, a turbidity test kit that has a image at the bottom of a clear tube can be
used. This is the recommend method for this project. Sample water that is collected can be
added to the tubes to determine the turbidity level. The turbidity level is measured in Jackson
Turbidity Units (JTU). This method does not require students to go out into deeper water.
If using a meter to measure turbidity, the meter will most likely report in
NTUs, (Nephelometric Turbidity Unit). NTU and FNU (Formazin Nephelometric Unit)
are the USEPA-designated units of turbidimetric measurement.
They are based on use of a detector (meter) placed at 90° from the incident beam to detect scattered light, and are interchangeable units.
A clear mountain stream might have a turbidity of around 1 NTU, whereas a large river like the Mississippi might have a dry-weather turbidity of around 10
NTUs. These values can jump into hundreds of NTUs during runoff events. Therefore, the
turbidity meter to be used should be reliable over the range in which you will be working.
So, if using a Secchi disk for this testing parameter, please submit the results using units of meters.
If using a turbidity test kit, submit the results in JTUs, and if using a
meter, please report results in NTUs.
Unfortunately, there is not easy way to convert
these units.
What to Expect
A turbidity reading of 0-10 JTU or NTU is considered normal. A Secchi depth less of less than 1
meter indicates a high concentration of suspended solids.
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