Tuesday, October 21, 2014

Turbidity and Suspended Solids

One measure of water quality in our lakes and streams is turbidity.  Turbid is a characteristic or condition of fluid that is opaque, murky or cloudy, deficient in clarity.  It is caused by fine particles or solids suspended in the fluid.

Queensland Department of Environment and Heritage Protection
Sediment is a common component of solids suspended in natural bodies of water, often originating from soil loosened and exposed by human activity, such as construction or farming. Weakness in laws for the control of erosion and runoff, as well as lax enforcement, are ongoing causes of impairments to our surface water resources.

Sediment (along with toxins and pathogens) is transported to lakes and streams by runoff after heavy rains. Some of that runoff becomes overflow from storm or combined storm/sanitary sewers.

The U.S. Environmental Protection Agency outlines the negative effects of turbidity as follows: 

Higher turbidity increases water temperatures because suspended particles absorb more heat. This, in turn, reduces the concentration of dissolved oxygen (DO) because warm water holds less DO than cold. Higher turbidity also reduces the amount of light penetrating the water, which reduces photosynthesis and the production of DO. Suspended materials can clog fish gills, reducing resistance to disease in fish, lowering growth rates, and affecting egg and larval development. As the particles settle, they can blanket the stream bottom, especially in slower waters, and smother fish eggs and benthic macroinvertebrates.
There is an aesthetic value in water clarity as well.

Christine Kemker has written a comprehensive treatise on turbidity. [Kemker, Christine. “Turbidity, Total Suspended Solids and Water Clarity.” Fundamentals of Environmental Measurements. Fondriest Environmental, Inc. 13 Jun. 2014. Web. < http://www.fondriest.com/environmental-measurements/parameters/water-quality/turbidity-total-suspended-solids-water-clarity/ >.]

A few excerpts will serve as an introduction:

Total suspended solids (TSS) are particles that are larger than 2 microns found in the water column. Anything smaller than 2 microns (average filter size) is considered a dissolved solid...
The turbidity of water is based on the amount of light scattered by particles in the water column 2. The more particles that are present, the more light that will be scattered. As such, turbidity and total suspended solids are related. However, turbidity is not a direct measurement of the total suspended materials in water...
Total suspended solids, on the other hand, are a total quantity measurement of solid material per volume of water 6. This means that TSS is a specific measurement of all suspended solids, organic and inorganic, by mass. TSS includes settleable solids, and is the direct measurement of the total solids present in a water body. As such, TSS can be used to calculate sedimentation rates, while turbidity cannot 1,6...
Pollutants such as dissolved metals and pathogens can attach to suspended particles and enter the water 2. This is why an increase in turbidity can often indicate potential pollution, not just a decrease in water quality. Contaminants include bacteria, protozoa, nutrients (e.g. nitrates and phosphorus), pesticides, mercury, lead and other metals 17. Several of these pollutants, especially heavy metals, can be detrimental and often toxic to aquatic life 26...
For those concerned about water quality, Kemker’s piece is worth reading in its entirety.


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