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Turbidity and suspended solids

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Morris, 56267
Morris Minnesota 607 Pacific Avenue 56267

Muddy water, murky water, cloudy water. If you've ever used these terms to describe how a body of water looks, you're talking about turbidity. If you are standing in a foot of water and can't see your feet, you are standing in turbid water.


Turbidity is caused by particles suspended or dissolved in water that scatter light making the water appear cloudy or murky. These particles can include sediment - especially clay and silt, fine organic and inorganic matter, soluble colored organic compounds, algae and other microscopic organisms. In the Minnesota River Basin, which includes the Pomme de Terre, sediment is the primary contributor to turbidity.

Turbidity is measured using specialized optical equipment. Basically a light is directed through a water sample, and the amount the light is scattered is measured. The unit of measurement is called the Nephelometric Turbidity Unit (NTU). The greater the scattering of light, the higher the turbidity. Low turbidity values indicate high water clarity, and high turbidity values indicate low water clarity. You can try this for yourself at home. Get a small flashlight, a glass of water and some dirt. Shine the light through a glass of clean water and see what the beam looks like on the other side. Now stir in some dirt and shine the light through it again and compare what the light beam looks like. That is basically what a turbidimeter is doing.

Minnesota's water quality standard for turbidity for class 2B waters, which includes the Pomme de Terre, is 25 NTU. Any reading over 25 NTU is in violation of the standard. If at least 10 percent of the readings are over 25 NTU, the waterbody is placed on the MPCA's impaired waters list. Monitoring data over the last ten years indicates that 60 percent of the turbidity readings exceed the 25 NTU standard. Based on this information, the southern section of the Pomme de Terre, from Muddy Creek down to Marsh Lake, was placed on the list as impaired for turbidity in 2002.

Since turbidity is simply a measure of how light is scattered, we also need a measure of how much "stuff" is in the water. For this we measure Total Suspended Solid (TSS). This is a measure of how many milligrams of particles are in a one-liter sample of water. The more suspended solids in the water, the higher the turbidity reading will be. In the Pomme de Terre, the relationship of TSS to turbidity indicates a TSS concentration of 52 mg/L, approximately equal to the 25 NTU standard. TSS readings in the watershed routinely exceed 90 mg/L, and have gone as high as 230 mg/L of suspended solids.

High turbidity can significantly reduce the aesthetic quality of lakes and streams having a harmful impact on recreation, tourism and property values. Nobody likes to recreate in dirty water. Studies of lakeshore property show that property values can be influenced by water clarity readings. High turbidity can also increase the cost of water treatment for drinking and food processing. It can harm fish and other aquatic life by reducing food supplies, degrading spawning beds, and affecting gill function. Turbid waters mean fewer walleyes and more carp and bullheads.

Sediment often tops the list of substances or pollutants causing turbidity. Sources of sediment can include erosion from upland, riparian and stream channel areas. Human activities in agriculture and construction can accelerate this erosion. Stream and channel movement can also release sediment. Phosphorus from various sources can cause algal growth resulting in increased turbidities. Phosphorus sources may include wastewater treatment facilities, nutrient runoff from crop land, fertilizer runoff from nice green lawns, and other sources. Soil erosion on crop land has been a focus of soil and water conservation programs for many years. Urban stormwater runoff is also recognized as an important contributor of sediment.

If we look at the amount of Total Suspended Solids (TSS), and compare that number with the river flow, we can figure out how much sediment is actually being carried by the river. During an average year from April to September, over 11,000 tons of sediment flow down the Pomme de Terre River! That's our topsoil being carried down to the Gulf of Mexico.

In the next article, I will give a background on the TMDL process, how a waterbody is listed, and what has to happen once it is put on the list.

I would also like to invite everyone to the Pomme de Terre Watershed public meeting on Thursday, Sept. 18 from 7 to 9 p.m. at the Old #1 Bar and Grill at 412 Atlantic Ave. in Morris. Come and get more information about the turbidity TMDL, and get an update on the approved fecal Coliform TMDL and implementation plan, including focus areas and dollar amounts we are looking at.

Our website has been recently updated also, so be sure to check it out at

Shaun McNally is the Pomme de Terre Watershed Project Coordinator. He is located in the Stevens SWCD office in Morris - 320-589-4886 ext. 109.