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Turbidity

   

General Information: Turbidity is a unit of measurement quantifying the degree to which light traveling through a water column is scattered by the suspended organic (including algae) and inorganic particles. The scattering of light increases with a greater suspended load. Turbidity is commonly measured in Nephelometric Turbidity Units (NTU), but may also be measured in Jackson Turbidity Units (JTU).

The velocity of the water resource largely determines the composition of the suspended load. Suspended loads are carried in both the gentle currents of lentic (lake) waters and the fast currents of lotic (flowing) waters. Even in flowing waters, the suspended load usually consists of grains less than 0.5 mm in diameter (Table 1). Suspended loads in lentic waters usually consist of the smallest sediment fractions, such as silt and clay (Dunne et al., 1978).

Table 1. Size Classification of sediments (adapted from Friedman et al., 1992)

      Sediment class           Size (mm)
           Sand
               V. Coarse            1.5                          
               Medium               0.375
               V.Fine               0.094
           Silt
               V. Coarse            0.047                          
               Medium               0.0117 (no longer visible to the human eye)
               V.Fine               0.0049
           Clay                     < 0.00195
 
Numerical Categories: 
Designated Use        Acceptable Ranges

Recreation               5 NTU  (Awwa, 1990)
Aquatic Life          < 50 NTU instantaneously or
                      < 25 NTU for a 10 day average  (Harvey, 1989)
                      < 10 NTU for trout waters or
                      < 25 NTU for streams (non-trout waters) or
                      < 50 NTU for lakes and reservoirs (non-trout waters) 
                        (North Carolina Code, 2002)
Human Consumption     1 to 5 NTU (up to 5 NTU is allowed if the water supplier can demonstrate
                                  that this level does not interfere with:
                                  1) disinfection
                                  2) maintenance of a disinfecting agent
                                  3) microbiological determination  
                                    (North Carolina Code, 1992).)
Health Effects: Turbidity may be composed of organic and/or inorganic constituents. Organic particulates may harbor microorganisms. Thus, turbid conditions may increase the possibility for waterborne disease. (For more information on microorganisms, please refer to sections on bacteria, viruses, and protozoans.)

Inorganic constituents have no notable health effects.

Industrial Effects: Turbid water may not be suitable for use in industrial processes. The abundance of suspended solids may clog or scour pipes and machinery.

Recreational Effects: Highly turbid waters may be hazardous to the welfare of swimmers and boaters. Turbidity may obscure potentially dangerous obstructions such as boulders and logs. The organic constituents of turbid waters may harbor high concentrations of bacteria, viruses, and protozoans.

Environmental Effects: The series of turbidity-induced changes that can occur in a water body may change the composition of an aquatic community (Wilber, 1983). First, turbidity due to a large volume of suspended sediment will reduce light penetration, thereby suppressing photosynthetic activity of phytoplankton, algae, and macrophytes, especially those farther from the surface. If turbidity is largely due to algae, light will not penetrate very far into the water, and primary production will be limited to the uppermost layers of water. Cyanobacteria (blue-green algae) are favored in this situation because they possess flotation mechanisms (McCabe et al., 1985). Overall, excess turbidity leads to fewer photosynthetic organisms available to serve as food sources for many invertebrates. As a result, overall invertebrate numbers may also decline, which may then lead to a fish population decline.

If turbidity is largely due to organic particles, dissolved oxygen depletion may occur in the water body. The excess nutrients available will encourage microbial breakdown, a process that requires dissolved oxygen. In addition, excess nutrients may result in algal growth. Although photosynthetic by day, algae respire at night, using valuable dissolved oxygen. Fish kills often result from extensive oxygen depletion.

Turbidity is often largely due to suspended sediment in the water column. For more detailed information about potential effects of suspended sediment, please see the Sediment section.

Sources:

  1. Nonpoint source:
     

               
     

               
     

  2. Point source: Sewage treatment plants may discharge organics during sewage by-pass periods that contribute to turbidity. Dissolved nutrients released in treated wastewater may contribute to phytoplankton production.

Mode of Transport: Particulates can be carried by overland flow, or resuspended from the substrate by changes of speed or direction of the water current. Overland flow may also transport dissolved nutrients that contribute to phytoplankton growth.

Analytical techniques: (APHA, 1992)

  1. Nephelometric Method: Comparison of the light scattered by the sample and the light scattered by a reference solution.