Turbidity and Suspended Matter

Turbidity is a measure of light scattering caused by suspended particles in natural waters. It is a measure of water clarity, and high levels of turbidity generally indicate poor water quality. Turbidity usually is reported in units of formazin turbidity units (FTU) or the equivalent nephelometric turbidity units (NTU).

Background information and how turbidity is measured.

Total Suspended solids (TSS) is the mass concentration (in mg/L) of particles small enough (< a few mm) to remain suspended in the water column by natural levels of turbulence and large enough to be collected on a filter with a specified pore size. There 
are many sources and kinds of SS, and high SS concentrations also are indicative of poor water quality.

Background information on TSS and
its measurement.

Click on the image to the right to see a sequence of Landsat 8 and Sentinel-2 images of western Lake Superior and the Duluth-Superior area from May and June 2019 showing widespread distribution of high TSS and turbidity levels in the lake that change rapidly over time in response to rain events. The main source of TSS and turbidity is the Nemadji River, which drains a watershed with highly erodible clay soils and flows into Allouez Bay at the southern end of the Duluth-Superior inner bay.

Turbidity in Minnesota Lakes

Turbidity is a commonly measured parameter in water pollution/water quality studies, but it is not commonly measured by limnologists in lake studies. Consequently, only limited information is available on turbidity levels
in Minnesota lakes. With support from the LCCMR, we recently developed satellite-based procedures usingSentinel-2 imagery to measure turbidity in lakes, and preliminary results are shown below. Work is underway to validate the model used to develop the maps below, and we expect to include turbidity data on Minnesota lakes in the LakeBrowser later this year.

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Suspended Solids in Minnesota Lakes

As part of the LCCMR project described for turbidity, we have developed a satellite-based procedure using Sentinel-2 imagery to measure TSS in lakes and large rivers. Preliminary results are shown below. In a previous study on rivers (see Rivers tab in Past Studies page), we developed a method to separate TSS into its two components, volatile or VSS, largely a measure of organic SS) and nonvolatile SS, the inorganic (or mineral) component of SS. Work is underway to apply this approach to Sentinel-2 imagery so that these two kinds of SS, which have very different sources and water quality Implications, can be distinguished across Minnesota lakes and included on the LakeBrowser.

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