Turbidity

Image of a steep coastal embankment where sand and soil are washing into the lake creating cloudy water along the coastline.
Photo Credit: Megan O'Brien

Increased turbidity can be caused by the erosion of sand and soil into the water.  

What is turbidity?

Turbidity often is used as a proxy for water clarity. Technically, what turbidity measures is the intensity of light scattered by particles in the water sample at 90° incident to a light source. Particles such as clay, silt, sand, algae, plankton, microorganisms, and other matter suspended in the water scatter the passage of light through water. Particle suspension is dictated by both the size of the particle and the energy of the system.

To the naked eye, turbidity appears as cloudy or muddy water. However, it differs from the color of the water. For example, tannin-rich waters that flow through peaty areas may have a dark color, but low turbidity. 

Turbidity (NTU)

Turbidity is reported in units called Nephelometric Turbidity Units or NTUs. The Hydrolab sensor’s range is 0–3,000 NTU, but likely your measurements will be on the lower end of this range.

A turbidity graphic that show varying degrees of water clarity in small bottles of illustrated water samples. At 250 NTU the water is brown and opaque. At 10 NTU the water is translucent and lightly tinted yellow.
Source: NCPEDIA; Creative Commons BY-NC-SA

These illustrations are examples of turbidity standards. The numbers under the bottles range from 250 NTU to 10 NTU.  The lower the number, the lower the turbidity. 

Why do we measure turbidity?

Turbidity is important to measure because at certain levels (typical­ly higher) it can impact a water­body. For instance, high turbidity reduces the amount of sunlight pass­ing through water from the sur­face. Less sunlight can reduce the rate of photosynthesis.

Suspended particles also absorb heat, so high turbidity can raise the water temperature. This, in turn, can reduce the concentration of dissolved oxygen, since warm water holds less dissolved oxygen than cold water. Suspended materials can clog fish gills and deposited sediments can smother fish eggs and aquatic insects.

Illustration of two waterbodies one with clear water and plants growing; the second with cloudy water and dead plants.
Graphic Credit: USGS

What affects turbidity measurements?

Suspended materials can enter the water by natural or human-caused means, such as soil erosion of a riverbank, or urban and agricultural runoff from surrounding land. Turbidity often increases sharply during and after a rainfall since the energy of falling and flowing water is the primary way that sediment gets dislodged and carried into rivers.

Particulates also could be introduced into a waterway directly from storm or wastewater discharge pipes. In addition, biological activity can increase turbidity such as from excessive algal growth or from bottom feeders such as carp stirring up bottom sediments.

Turbidity Factsheet

right-click to download turbidity fact sheet
Download the Factsheet

Turbidity Videos

An empty mussel shell in the foreground of a rocky coastal embankment and the water is not very clear.
Photo Credit: Megan O'Brien

Turbidity can vary by water body, weather, and seasonal changes.