Turbidity can occur naturally in surface water, as sand, clay, silt and other organic matter can enter a water body through erosion or seasonal changes in flow. It’s important to monitor baseline values of turbidity to establish what the natural variation is for the area and compare these trends to effects of stormwater runoff, droughts, effluent discharge and other significant events.

Monitoring turbidity helps tell the story of activity in the area–whether natural or manmade–and how it impacts environments and communities. Natural events such as heavy rains, melting snow and runoff can displace sediment, potentially harming water quality. Additionally, operations such as construction, agriculture, mining, logging and dredging can generate pollutants or disturb sediment and harm surrounding environments if not carefully managed. And when floods or stormwater runoff pass through industrial areas, they can carry contaminants from these operations into natural water sources.

Increased turbidity also plays a big role in whether water bodies are suitable for recreational purposes; people generally don’t want to swim, boat or fish in areas with cloudy water. 

But turbidity is not just an aesthetic concern. Changes in turbidity in lakes, rivers, wetlands and other environments are an important indicator of water quality and an important factor for the health of plant and animal life. High turbidity can decrease the amount of sunlight able to pass through the water to plants, decreasing photosynthesis and creating a ripple effect that will impact the rest of the food chain.

Turbidity also plays an important role in source water monitoring. Increased turbidity in a drinking water reservoir needs to be carefully monitored for contaminants so operators can adjust the process to account for higher levels of bacteria, sediment and metals or nutrients. Harmful algal blooms are also a concern when monitoring source water, as an overabundance of algae in reservoirs can dramatically change the quality of water entering a plant.


Environmental monitoring stations are often set up downstream of effluent release to study the impact of the wastewater treatment process on surface water. Monitoring turbidity near where effluent meets a natural water body helps verify that the treatment process has been effective and effluent discharge meets or exceeds the quality of pre-existing conditions.

In coastal monitoring, turbidity is more often created by an abundance of organic matter, such as plankton or algae. Plankton is the most common source of turbidity in the ocean, and many scientists monitor for turbidity to assess the availability of plankton as a food source for other creatures. Dredging operations can also increase turbidity in coastal environments. Turbidity must be closely monitored during dredging to assess and minimize impacts on aquatic life and ecosystem health.

Monitoring for turbidity during and after storm events helps coastal communities stay informed and accurately assess erosion risks. Beaches that lose too much sand are more vulnerable to hurricanes, flooding and other natural disasters. Environmental scientists and aquaculturists in coastal areas also keep a close eye on turbidity as an indicator of runoff from surface water sources, to identify and mitigate the impact of any bacteria, nutrients or sediment that accompany them.

Turbidity in groundwater is typically quite low, less than 1 NTU/FNU if the system is not disturbed. Because of this low baseline, turbidity higher than 1 NTU/FNU in groundwater often indicates that an event on the surface is affecting the aquifer. Impacts from drilling and blasting, earthquakes or surface water infiltration can all raise turbidity levels in groundwater.


The other cause of increased turbidity in groundwater are changes in flow rates, gradients or direction of flow. These changes can result from pumping, injection or purging before collecting groundwater samples.


Turbidity is often measured during a well purge to indicate when conditions have stabilized enough to begin sampling. Turbidity spikes when pumping begins; a stabilized turbidity value indicates the well screen is fully purged and water quality is representative of the surrounding aquifer.

Customers want clear drinking water, but the role of turbidity in the water treatment process goes beyond aesthetic expectations. Microbes, bacteria and other pollutants can attach to particles suspended in water. This connection makes them more difficult to remove and can complicate the disinfection process, leading to greater health risks. For example, in the United States,  plants that source drinking water from surface water are required to monitor turbidity during the process and maintain a value below 1 NTU in finished water. Many plants also monitor source water for turbidity before treatment begins, to more effectively identify and address levels of bacteria, sediment, metals or nutrients entering the plant.


Monitoring turbidity during the drinking water treatment process can provide a benchmark of a filter’s effectiveness and indicate when it might need to be replaced. And high turbidity can point to pipe damage or other issues with infrastructure in a distribution system.

Wastewater applications more commonly measure total suspended solids (TSS) or mixed liquor suspended solids (MLSS) rather than turbidity. TSS plays an essential role in managing the activated sludge process, as the right level of TSS helps create a positive environment for microbes to consume and break down waste while avoiding system overload. TSS is also important in managing the sludge/water interface to detect a rising sludge blanket and monitor settling characteristics of the sludge.


That said, turbidity is a good indicator of process performance and monitoring turbidity at the point of effluent helps identify if adjustments are needed. If high turbidity is observed, it can indicate problems with the settlement process, floating sludge or sludge blanket overflow, over- or under-dosing of chemicals or high flows affecting the treatment process.


For more on the difference between turbidity and TSS, visit "What is the difference between measuring turbidity and measuring suspended solids?"

Measuring turbidity with a Secchi disk involves lowering a disk into a water body. The disk is attached to a line marked with depth measurements. The operator lowers the disk until it disappears and notes the depth marked on the line. This depth (known as the “Secchi depth”) is then recorded. The Secchi depth is inversely related to turbidity; clear water will have a high Secchi depth, while turbid water will have a low Secchi depth.


While Secchi disks are a low-cost, easy-to-use method, they work only in deep water, where the disk is not visible along the bottom of the water body. They’re also more susceptible to the effects of variable conditions like light, waves and obstructions in the water, and are vulnerable to human error. Ideally, turbidity readings taken with a Secchi disk should be collected at the same time each day by the same person each time to eliminate variations in eyesight and height.

The Jackson Candle Method is one of the earliest methods of measuring turbidity. Turbidity is calculated by observing a candle flame through a glass tube of water, adding water to the tube until the flame is no longer visible. If only a little water obscures the flame, the sample is high in turbidity. If a lot of water is needed, the sample is clearer. The Jackson Candle Method is easy to set up and use but can result in inconsistencies in measurement with different operators.

The turbidity tube (sometimes known as a transparency tube) was invented in Australia in the 1990s to allow for turbidity measurement in shallow streams where Secchi disks were ineffective. Turbidity tubes are plastic tubes approximately 100 cm tall with a miniature Secchi disk printed on the bottom. The tube is filled with water. If the Secchi disk is not visible, the operator presses a release valve near the bottom of the tube to slowly let out water until they can see the disk. They record the value printed on the side of the tube at this depth. Often, operators will take two measurements with a turbidity tube and average the depths to arrive at the most probable measurement. Depths are then correlated to NTU values using a table produced by the tube’s manufacturer.


Turbidity tubes are easy to use and can be transported to the field to avoid time delays for lab sample analysis. Tubes also come pre-calibrated, so turbidity correlations are easy to calculate. But, like the Secchi disk and Jackson Candle Method, readings can be affected by factors like operators’ eyesight, human error and environmental conditions.

Optical sensors come in benchtop units (spectrophotometers) or portable probes for field measurement (turbidimeters). Unlike Secchi disks, the Jackson Candle Method and turbidity tubes, optical sensors present consistent values no matter who uses them.

Optical sensors automatically calculate turbidity values based on the difference in emitted light and detected light. All optical turbidity sensors contain a light source to emit light and a photodetector, which receives that light and calculates a turbidity value based on the calibration of the sensor.


There are two methods optical sensors use to measure the difference between emitted and detected light. Some optical sensors calculate this difference based on how light scatters when it comes into contact with particles in the sample. Other optical sensors calculate the difference based on how much the emitted light diminishes between the light source and the photodetector.

Three other components further differentiate methods of optical turbidity measurement: the type of light emitted into the sample (white light or LED), the number of photodetectors present and the angle at which the photodetector(s) are positioned in relation to the light source.


For more detail on the types of optical turbidity sensors, their methods and components, see “What are common types of optical turbidity sensors?”

Acoustic doppler meters are an established method of flow measurement. Recent research has introduced their potential for monitoring turbidity as well, by using acoustic velocity backscatter data to derive suspended sediment values. This method of measurement is similar to optical sensors, but with sound. Sound waves also scatter when they encounter particles in a water sample. Just like optical sensors include a photodetector to calculate turbidity based on the ratio of returned light to emitted light, acoustic doppler meters include a transducer that calculates turbidity based on the ratio of returned sound frequencies to emitted sound frequencies. However, acoustic doppler meters are not a direct measurement of turbidity and require correction for absorption and diffusion of sound waves across distance.


For a detailed look at this method of measurement, read the “Sediment and Erosion Techniques” chapter from the USGS’ Application of Hydraulics.