South Atlantic Water Science Center

Due to a lapse in appropriations, sampling for Bacterialert has been temporarily discontinued. Additionally, USGS will not be able to respond to inquiries until appropriations are enacted. For more information, please see

Chattahoochee River BacteriAlert: Understanding Turbidity

Picture showing highly turbid water from a tributary (where construction was probably taking place) flowing into the less turbid water of the Chattahoochee River

Turbidity is the amount of particulate matter that is suspended in water. Turbidity measures the scattering effect that suspended solids have on light: the higher the intensity of scattered light, the higher the turbidity. Material that causes water to be turbid include:

What is turbidity?

Vials of water showing 3 levels of turbidity.

Turbidity makes the water cloudy or opaque. The picture to the left shows highly turbid water from a tributary (where construction was probably taking place) flowing into the less turbid water of the Chattahoochee River. Turbidity is measured by shining a light through the water and is reported in nephelometric turbidity units (NTU). During periods of low flow (base flow), the Chattahoochee River is usually a clear green color, and turbidities are low, usually less than 10 NTU. During a rainstorm, particles from the surrounding land are washed into the river making the water a muddy brown color, indicating water that has higher turbidity values. Also, during high flows, water velocities are faster and water volumes are higher, which can more easily stir up and suspend material from the stream bed, causing higher turbidities.

Measuring turbidity

Picture of a turbidity sonde - they are installed at the Medlock Bridge and Paces Ferry river sites.

Turbidity can be measured in the laboratory and also on-site in the river. A handheld turbidity meter (left-side picture) measures turbidity of a water sample. The meter is calibrated using standard samples from the meter manufacturer. The picture with the three glass vials shows turbidity standards of 5, 50, and 500 NTUs. Once the meter is calibrated to correctly read these standards, the turbidity of a water sample can be taken. Before on-site turbidity sondes were installed in the Chattahoochee, this is how turbidity was measured for the BacteriALERT project.

In the spring of 2002, sondes containing state-of-the-art turbidity meters (left-side picture) were installed at the Medlock Bridge and Paces Ferry sampling sites. The right-side picture shows a closeup of the meter. The large tube is the turbidity sensor; it reads turbidity in the river by shining a light into the water and reading how much light is reflected back to the sensor. The smaller tube contains a conductivity (electrical conductance of the water .. strongly influenced by dissolved solids) sensor (the two holes) and a temperature gauge (the metal rod).

Closeup picutre of the turbidity, conductivity, and temperature sensors on the turbidity sonde.

We are now measuring turbidity in "real-time" at the Medlock Bridge and Paces Ferry sites. Readings are taken every 15 minutes and are transmitted to the USGS database every hour at Medlock Bridge and every four hours at Paces Ferry. The current turbidities are displayed on the BacteriALERT home page. The BacteriALERT project is using turbidity as an indicator of the presence of E.coli bacteria, since the data collected so far indicate a relation between E.coli counts and turbidity.