the lower fox river watershed monitoring program    
University of Wisconsin Green Bay
University of Wisconsin Milwaukee

Turbidity / Transparency

The particulate matter carried by a stream determines its turbidity, or the relative muddiness or cloudiness of the water. Particulates in a stream consist of algae, sediment particles from erosion, coarse particulate organic matter (CPOM) such as leaves and twigs, and fine particulate organic matter (FPOM) that has been broken down by stream biota. Erosion is a natural geologic process. However, certain human activities such as farming, storm water discharge, and construction greatly increase the amount of erosion in a watershed. The increased sediment from these erosive activities blankets the stream bottom and destroys spawning areas and macroinvertebrate habitat. Sediment can also be resuspended into the water column by bottom feeders like carp or by walking through the stream. Suspended sediment blocks light needed by rooted aquatic plants, damages gills on fish and invertebrates, and decreases visibility for fish who must see their prey. Sediments can also carry adhered pollutants, such as heavy metals and phosphorus, into the stream.

There are several methods of measuring turbidity. The LFRWMP uses a transparency tube, which measures the depth at which a black and white crosshair pattern is visible at the bottom of a tube filled with stream water. Low transparency is highly correlated with high turbidity in streams. Another available method for measuring turbidity uses the Hach DR/850 colorimeter. This test reads the amount of light transmitted through the stream sample, and reports results in FAU, Formazin Attenuation Units. The most exact measurements of turbidity are made with a nephelometric turbidity meter. Turbidity meters report measurements in NTU, Nephelometric Turbidity Units, and have greater ability to determine lower levels of turbidity.

For the LFRWMP, measurements from the transparency tube should be reported in depth units of centimeters (cm). A transparency of about 25 to 35 cm is equivalent to about 25 NTU. A transparency of >60 cm is roughly equivalent to a turbidity of <10 NTU. A transparency of about 5 cm is roughly equivalent to a turbidity of about 200 – 300 NTU. A more detailed and robust relationship between turbidity and transparency for streams in Northeastern Wisconsin has not yet been developed.

Turbidity and transparency can also be related to total suspended solids and streamflow results for specific streams or rivers. Although general relationships have been reported, the relationship must be established on a site-by-site basis.