What is it? Why is it important?
The word “turbidity” [Tn, units of NTU’s] has been used in a general sense to indicate the extent to which water lacks clarity. Turbidity is tightly linked to the optical aesthetics of water, and therefore the public’s perception of water quality. The public prefers water of high clarity for recreation and consumption.
Water supplies are generally regulated for turbidity levels (values < 1 NTU normally prevail; e.g., usually after treatment).
Turbidity has been criticized as an imperfect scientific measure, but it is widely used and applied as a measure of quality for water supplies. Further, it has been demonstrated to have utility in the estimate of Secchi disc transparency (another widely used measure of clarity), the concentration of total suspended solids (TSS), and the light scattering coefficient (an important optical characteristic). A number of scientists have recommended Beam Attenuation Coefficient (BAC) as better measure of “turbidity” than turbidity.
How is it measured?
Turbidity, until the recent development of probes such as these adopted at this site, was measured in the laboratory with a nephelometric turbidimeter. The unit of measure is nephelometric turbidity units (NTU). In nephelometers, a beam of light is directed along the axis of a cylindrical glass cell containing the sample. Light scattered by particles from the beam within a rather broad angle centered on 90 º is measured by a detector located on one side of the cell. The turbidity probe on the buoy is constructed in a similar manner as the nephelometer, except that the scattered light detector is located within the water as opposed to outside a glass sample cell. The accuracy of field probes, at the low turbidity levels of interest to water supplies, can not presently match laboratory instrumentation.
Origins
Turbidity in water bodies is caused by a heterogeneous (mixed) population of suspended particles, which may include clay, silt, finely divided organic matter (detritus), phytoplankton (microscopic free floating plants), and other microscopic organisms. In general, this population of particles in a lake is a composite of sediments received in tributary inflows, resuspended lake sediments, and particles produced within the lake (particularly phytoplankton). Thus, the variations in measured turbidity may reflect the dynamics of phytoplankton growth as well as tributary runoff (driven by the timing of rainfall events).
What to Look for in Our Systems?
High turbidity observations are expected to occur only irregularly, for short intervals in these lakes, associated primarily with meteorological events. Particularly high runoff events can result in large inputs of sediment and increases in turbidity . Skaneateles Lake is known to experience irregular short-term turbidity events in the northern portion ot hte lake following prolonged intervals of high wind speed from out of the south. These are caused by sediment resuspension.
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