Overview
Turbidity is a measure of the amount of suspended material in the water. It describes the clarity of water. Suspended materials in water, such as clay, silt, and algae, reduce water clarity and cause turbidity. The units of turbidity from a calibrated nephelometer (turbidimeter), an instrument used to measure suspended material in water, are called Nephelometric Turbidity Units (NTU).
Influencing Factors
Turbidity increases due to: runoff from construction sites, farm fields, logging sites, runoff from urban areas with a lot of paved and impermeable surfaces, eroding stream banks, bottom feeding fish and burrowing animals, excessive algae growth, high velocity water such as stormwater, and windy conditions in shallow water areas.
A clear mountain stream might have a turbidity of around 1 NTU, whereas a large river like the Mississippi might have a dry-weather turbidity of around 10 NTUs.
The federal standard for high turbidity is 25 NTUs or greater. Turbidity levels are typically less than this, but will show spikes above this level due to factors such as those described above.
Importance
Turbidity is an important indicator of the amount of suspended sediment in water, which can have many negative effects on aquatic life. The suspended sediments that cause turbidity can block light to aquatic plants, smother aquatic organisms, and carry contaminants and pathogens such as lead, mercury, and bacteria.
Long-term turbidity monitoring is very important because it helps us measure the amount of soil erosion due to construction and agriculture. It also helps to determine the impact of our land-use practices and increasing urban development on water quality.
Measurement
Turbidity is measured by evaluating the amount of light scattered in water. Particles will scatter a light beam focused on them making it a meaningful measure of turbidity in water. Turbidity measured this way uses an instrument called a nephelometer with the detector setup to the side of the light beam. More light reaches the detector if there are many small particles scattering the source beam than if there are fewer. The nephelometric method reflects a precise measurement and is used by government agencies and by scientific researchers.