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Turbidity: Description, Impact on Water Quality, Sources, Measures - A General Overview Water Quality/Impaired Waters #3.21 • March 2008 any Total Maximum Daily Load projects in Minnesota Maddress turbidity. Turbidity in water is a measurement of how cloudy or Soil erosion on crop murky it is. In your espresso or latte’ you land has been a focus want high turbidity. In your lake or of soil and water stream, probably not. In either case, the conservation programs for many substances resulting in high turbidity years. Urban may not be intrinsically harmful, but stormwater runoff is their effects can be. Too much caffeine in also recognized as an the evening can affect sleep. Too much important contributor algae or sediment in lakes and streams of sediment, whether from construction can make them unsuitable for recreation sites, runoff from and aquatic life. impervious surfaces, A report of the European Inland Fisheries or other sources. What Is Turbidity? Advisory Commission lists five ways that Turbidity is caused by particles suspended fine particles can have a harmful impact on or dissolved in water that scatter light freshwater fish: making the water appear cloudy or murky. • acting directly on fish, killing them or Particulate matter can include sediment - reducing their growth rate, resistance especially clay and silt, fine organic and to disease, etc.; inorganic matter, soluble colored organic • preventing successful development of MPCA Area Offices: compounds, algae, and other microscopic fish eggs and larvae; Rochester area: organisms. In the Minnesota River, • modifying natural movements and 507/285-7343 migrations; Mankato area: sediment is the primary contributor to 507/389-5977 turbidity. In a shallow lake in August, it • reducing the amount of food available; Marshall area: may be algae. In a northern Minnesota lake and 507/537-7146 it may be tannin released by the breakdown • affecting the efficiency of methods for Willmar area: of organic material. catching fish. 320/214-3786 Detroit Lakes area: 218/847-1519 Impact of Turbidity Turbidity Sources Brainerd area: High turbidity can significantly reduce the Sediment often tops the list of substances 218/828-2492 Duluth area: aesthetic quality of lakes and streams, or pollutants causing turbidity. However, 218/723-4660 having a harmful impact on recreation and any watershed has multiple sources of the Metro area: tourism. It can increase the cost of water pollutants or physical features that can 651/296-6300 treatment for drinking and food processing. affect water clarity. These can be divided Toll-Free Number: It can harm fish and other aquatic life by into natural or background, and human- 800/657-3864 reducing food supplies, degrading induced sources. Natural sources can spawning beds, and affecting gill function. include erosion from upland, riparian, stream bank, and stream channel areas; wq-iw3-21 Minnesota Pollution Control Agency • 520 Lafayette Rd. N., St. Paul, MN 55155-4194 • www.pca.state.mn.us 651-296-6300 • 800-657-3864 • TTY 651-282-5332 or 800-657-3864 • Available in alternative formats however, this is difficult to measure due to agriculture Turbidity is affected by and development activity. Human activities can several factors in water: accelerate erosion. Tannic acids often associated with presence of dissolved peat and bog areas cause water to be colored resulting in and suspended solids, turbidity. Algae that grow with nourishment from size and shape of nutrients entering the stream through leaf decomposition particles and or other naturally occurring decomposition processes can composition of the also be a source of turbidity. Stream channel movement particles. Water quality can also release sediment. measurements that can help in the Phosphorus from various sources can cause algae growth characterization of resulting in increased turbidities. Phosphorus sources turbidity include total may include wastewater treatment facilities, nutrient suspended solids, runoff from crop land and other sources; and bottom volatile suspended sediment. Organic matter from sewage discharges, solids, total dissolved especially during treatment plant bypasses, can solids, suspended sediment concentration, chlorophyll a, contribute to turbidity. Soil erosion on crop land has and particle size analysis. Other factors such as flow, been a focus of soil and water conservation programs for sediment source and composition, algal species and many years. Urban stormwater runoff is also recognized sediment transport characteristics can also provide as an important contributor of sediment, from important information in characterizing the turbidity construction sites, impervious surfaces, or other sources. present in water. Measuring Turbidity Turbidity Water Quality Standard Turbidity is measured using specialized optical Minnesota’s water quality standards include a turbidity equipment in a laboratory or in the field. A light is number as a measure of whether a water body meets its directed through a water sample, and the amount of light designated uses: scattered is measured. The unit of measurement is called a Nephelometric Turbidity Unit (NTU), which comes in Classes (and descriptions) Turbidity (NTU) several variations. The greater the scattering of light, the 1B (drinking water) 10 higher the turbidity. Low turbidity values indicate high 2A (cold water fishery, all recreation) 10 water clarity; high values indicate low water clarity. 2B (cool/warm water fishery, all recreation) 25 2C (indigenous fish, most recreation) 25 Measuring water Transparency Tube transparency and The relationship of TSS and transparency to turbidity Total Suspended across many streams indicates that 25 NTU is Solids (TSS) also approximately equal to 58 mg/L for TSS and 20 can be used to centimeters of visibility through a T-tube. However, the predict turbidity relationship between turbidity and TSS can vary greatly values. Secchi in individual streams or even locations within a stream. disks in lakes and transparency tubes More Information in streams provide Protocol for Turbidity TMDL Development: a simple and low- www.pca.state.mn.us/publications/wq-iw1-07.pdf cost method for measuring water Minnesota River Basin Data Center: clarity. These are http://mrbdc.mnsu.edu/mnbasin/wq/turbidity.html widely-used in Water on the Web: citizen lake and http://waterontheweb.org/under/waterquality/ stream monitoring Minnesota River Turbidity TMDL Project: programs. Laboratory analysis is necessary for www.pca.state.mn.us/water/tmdl/project-mnriver- measuring TSS in milligrams per liter. turbidity.html Turbidity – A General Overview • Water Quality/Impaired Waters #3.21 • March 2008 page 2 .
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