Turbidity and Suspended Matter
This physical property measures light-scattering by suspended particles in water. Turbidity is related to the suspended solids (SS) concentration in water bodies, but several factors, including the number of particles, their shapes, and their surface properties, affect light scattering in addition to the mass of particles measured by SS. Consequently, there is no general relationship between turbidity measurements and SS concentrations, although good correlations may be found for specific water bodies where only small variations occur in the aforementioned factors.Many techniques are available to measure turbidity, but they do not all yield the same results; typically, each technique measures a different aspect of particle-caused light scattering. Today, turbidity of natural water samples is most often determined by direct measurement of light scattered at 90° from incident light. This is done with field and lab instruments called nephelometers calibrated with suspensions of “formazin,” a synthetic polymer rather than natural suspended particles. Formazin has an important advantage over natural substances: it forms particles that are highly reproducible in size, shape, and surface properties. Consequently, they yield reproducible amounts of light scattering.
Results are reported as operationally defined formazin turbidity units or FTU. Turbidity levels in pristine lakes are ~ 1 FTU; algal-rich eutrophic waters may have up to 10 FTU. Turbidity levels in reservoirs and rivers affected by agricultural drainage, urban runoff or soil erosion can reach hundreds or even thousands of FTU.
Because turbidity is an optical property, it is feasible to derive equations to retrieve turbidity data from satellite imagery; indeed, for reasons described below, turbidity results might be more accurate than satellite-derived SS estimates. Several empirical models are available in the literature, and they tend to be similar in form to SS retrieval equations. In previous work our group found high correlations of turbidity with reflectance in the red-edge band (~ 705 nm), which is available on Sentinel-2 satellites (Olmanson et al. 2013).
Natural waters contain small suspended particles in the size range (diameter) of < 1 μm to a few mm. Under normal conditions, turbulence keeps such particles suspended on timescales of hours to days (or even longer), but they eventually settle by gravity and are deposited to lake or river bottoms. Suspended solids (SS), also called suspended particulate matter (SPM), is a very broad term that includes a wide variety of inorganic and organic particles derived from many sources in watersheds and within water bodies themselves. External (allochthonous) sources of inorganic (or mineral) SS include soil and stream bank erosion and resuspension of bottom sediments by wind-induced turbulence or by bottom-feeding fish. Internal (autochthonous) sources of organic SS include primary production of algae, microbial food webs based on algal production, zooplankton and fish feeding/excretion activities, and decomposition of aquatic vegetation. External sources of organic SS include runoff from agricultural and urbanized lands, wind-blown leaves and other products of partial decomposition of terrestrial vegetation.
The sum of the mineral and organic SS sometimes is referred to as “total suspended solids,” or TSS. TSS concentrations range from ~ 1 mg/L in pristine lakes to tens of mg/L in eutrophic lakes and hundreds of mg/L in rivers and impoundments in watersheds with highly erodible soils and stream banks. TSS is measured gravimetrically by weighing the residue dried at ~100oC that was collected on a glass fiber filter (nominal pore size of 0.45 μm) from a known volume of water. VSS, a measure of organic SS, is the difference between the dried weight of residue and the weight after firing the filter at ~550oC. The ash remaining after firing, called the fixed residue, represents the mineral SS.
Through its central role in scattering and absorbing incoming light, SS is the main factor that determines the clarity of water in most lakes and rivers. The extent of light scattering and absorption by SS depends not only on its mass concentration, but on the number of particles and their shape and surface properties. These factors vary widely for SS in different water bodies and even over time in a given lake.
For reasons described above, analytical equations to retrieve SS or its mineral and organic fractions from satellite imagery are not possible, but empirical relationships have been reported using the scattering peak at ~ 705 nm and band combinations in the NIR or green regions, where plant pigments absorb minimally. In previous work, members of our group found strong relationships between reflectance at 705 nm and SS (r2 = 0.77-0.93) using airborne hyperspectral imagery on optically complex waters of the Minnesota, Mississippi, and St. Croix Rivers in the Minneapolis-St. Paul area (Olmanson et al. 2013). We also developed a retrieval equation (r2 = 0.80-0.90) for volatile suspended solids (VSS) using the ratio of reflectance at 705 to 670 nm, and a two-term equation consisting of the band at 705 nm and the 705:670 nm reflectance ratio worked well to predict mineral SS.
Olmanson, L. G., M. E. Bauer, and P. L. Brezonik. 2013. Airborne hyperspectral remote sensing to assess spatial distribution of water quality characteristics in large rivers: The Mississippi River and its tributaries in Minnesota. Remote Sens. Environ. 130: 254-265. DOI:10.1016/j.rse.2012.11.023.