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Turbidity (with: Total Dissolved Solids, Total Suspended Solids in Passing) PDF Print E-mail

Turbidity, Total Dissolved Solids, Total Suspended Solids

You probably have noted that we only cover two of these three related measures. Well, the only reason we record TDS is because we are fortunate enough to have a meter that can record this value. But because we record turbidity in visually estimated JTUs (Jackson Turbidity Unit) it is hard for us to relate turbidity and TDS.

The Jackson Turbidity Unit is specific to the method we use to measure turbidity- looking down a column of water. This method is naturally imprecise, particularly on the low end and is actually only recommended in highly turbid waters. To be more precise and scientific about it the EPA recommends using a turbidity meter which measures in Nephelometric Turbidty Units (NTU.) It is impossible for us to do this because the cheapest turbidity meters I have seen run $800 and up!

Anyway, turbidity is a lack of clarity of the water due to the presence of small particles of solid matter suspended in the water. They are too small for gravity to pull them to the bottom against the force of the moving water. These solids are clays and soils and silts as well as organic matter from decaying plants and animals including algae. Silts dredged up from the bottom in a hard rain and fast flowing streams contribute to turbidity as does plankton. Don’t think plankton only exist in the ocean! (A measure of Total Suspended Solids does not include these creatures. Total Dissolved Solids was discussed in a previous entry. TDS consists of extremely small and ionized particles. The ionization- electrical charge- is what allows our meter to record them.)

Plants and animals need sunlight to grow. This is as true of the plants and animals in our streams and lakes as of those on the land. The solid particles in the water that comprise turbidity can reflect and scatter sunlight making it less effective for photosynthesis and for maintaining temperatures in deeper water. Consider also that gilled animals must filter this water through their gills to get oxygen. If those filters become clogged the animal will suffocate.

There are a number of factors that affect turbidity. Some of them are perfectly natural and have been happening forever. Other factors are relatively recent and the result of the actions and habits of you and me and human society in general. Fish like Carp and Catfish- bottom feeders- stir up sediments. As mentioned previously, dead and decaying plants and animals add particles to the water. Flooding can stir up bottom sediments and also, as the flood waters recede back within the banks, they bring particles of soil, vegetation, and other debris from the land. Algae also increase turbidity whether alive or dead. We previously examined some factors that increase algae populations; it is not always a natural occurrence.

High flow rates, the speed with which water moves, can affect turbidity. Fast flowing water disturbs bottom sediments and also contributes to another factor of turbidity- bank erosion. But bank erosion also contributes to channelization (the deepening and narrowing of a stream bed) which, itself, contributes to bank erosion, and they both contribute to turbidity! Bit of a vicious circle there.

While bank erosion is natural it occurs at an accelerated pace in highly developed areas like ours. Since so much of our watershed is paved rain water, rather than spreading out and seeping into the ground as it flows toward streams, gushes along the curb and into the storm drains, through the pipes and directly into the streams causing faster than normal flows and depths. In addition, the banks of streams in our watershed see a lot of human traffic which tamps down the soil and destroys vegetation on the bank. It may seem counter-intuitive, but highly compacted soil DOES NOT make for a stable bank. Soil in which plant roots can penetrate is best.

Effluent from wastewater treatment and septic systems also contributes to turbidity. The good news for us is that- per the comprehensive study published in 2002- this is practically a non-issue for the Silver Lake Watershed.

The measure of turbidity is not in itself a measure of the health of the water. Turbidity is a factor in the health of the creatures living in the water which help us determine the quality of the water. Turbidity also helps us to understand the quality and health of the lands beside our streams- the Riparian buffer zone. So, even with our limited ability and precision, it is worthwhile to perform our turbidity test. Chances are you’ll see real differences if you monitor after a heavy rain.

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When you return your data sheet I enter the information into an Excel spreadsheet to track and analyze the data over time. After enough time has passed and sufficient data has been collected I will share this with all of you and we will see what kinds of problems and trends we can discern.

In the meantime you might consider making and keeping copies of your data sheets and making comparisons and contrasts of the values you record. See if you notice that when one reading seems low another reading seems high, or the reverse, or the opposite. Look for patterns. See if you can think of reasons for these relationships, or whether they are relationships at all or simply coincidences.

If you can educate yourself, you can educate others. The spreading of knowledge and awareness is one of the greatest things you can do to protect our environment.

Thanks,

Vail

 
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