Turbidity is the state of relative clarity or cloudiness of water. Many factors can contribute to increased turbidity. The most obvious are suspended sediments consisting of various sand, silt, clay and gravel particles.
These may be introduced by soil erosion, urban runoff, bottom-dwelling aquatic creatures that stir up settled sediments, or human activities such as mining, excavation, and dredging. Other possible sources include phytoplankton, discharge of wastewater, algal blooms, and CDOM (colored dissolved organic matter) from decaying plants and other organic material.
Why Measure Turbidity?
Although turbidity may occur naturally in some cases, clear water is generally considered to be one of the primary indicators of water quality. The potential adverse effects of increased turbidity are varied. For example, suspended sediments can absorb the sun’s radiant energy, warm the water, and, in turn, lower dissolved oxygen. In lakes, this can lead to stratification and hypoxic conditions.
Contaminates can cling to suspended particles, which can be a health concern in recreational water and lead to illness in drinking water systems. Turbid waters can limit the visibility of species that rely on sight for feeding and even clog fish gills, thus restricting respiration. They can also block out sunlight that aquatic plants need for photosynthesis.
Sudden drops in turbidity can impact aquatic ecosystems as well. Lower turbidity leads to increases in light attenuation and temperature–both of which can change the local ecosystem, vegetative growth rates and habitat suitability for native species.
Lower turbidity can also be indicative of lower levels of production in an aquatic system. Declines in phyto- and zooplankton and other microorganisms that contribute to water clarity are foundational in food webs.
Precipitation can cause temporary declines in turbidity when there is a lack of runoff from surrounding land, but invasive filter feeders like zebra mussels can have much more permanent impacts on the lake waters. These changes can endanger native species and reshape ecosystems when historically high-turbidity waters transition.
How Is Turbidity Measured?
Most field turbidity sensors are based on optical technology, which relies on nephelometry to measure turbidity. Nephelometry indicates that the sensor measures light scattered at an angle, most often 90°.
To do this, sensors have a light source, typically an LED or tungsten lamp, which emits light from the sensor face. The light scatters off of particles in the water in all directions, and a photodetector located perpendicular to the light source responds to light scattered at 90 degrees.
Nephelometric turbidity sensors typically report values in Formazin Nephelometric Units (FNU, sometimes also called Formazin Turbidity Units, FTU) and are calibrated using formazin-based standards.
Nephelometric temperature sensors emit light and measure scattering at a 90° angle to determine turbidity.
FNU sensors must be designed based on a standard set forth by the International Organization for Standardization called ISO 7027. This standard calls for a monochromatic near-infrared light source at a wavelength of 860 nm +/- 30 nm (830-890 nm). The monochromatic light source is less affected by colored particles and, thus, maintains its accuracy in waters affected by CDOM.
How to Select a Turbidity Sensor?
When selecting a turbidity sensor for use in natural waters (lakes, rivers, streams, coastal and ocean), the most important considerations are measurement range, accuracy, and cleaning of sensor optics.
A wiper mechanism that cleans the sensor optics before taking a measurement may be effective in removing obstructions and maintaining measurement accuracy. However, long-term deployments in brackish and marine saltwater environments are particularly susceptible to biofouling. The addition of copper or an antifouling nanocoating can extend deployment times if permitted, but a periodic cleaning regime will likely be required to avoid light path obstructions.
Illustration of relative turbidity levels.
Clear water lakes and other pristine environments may be best assessed using low-range turbidity sensors with a measurement range of 0 to 40 FNU. However, most commercially available sensors provide adequate accuracy with extended ranges as well. Source water monitoring systems can use similar sensors, but drinking water distribution will require a very low-range sensor with adequate resolution at 5 FNU and under since drinking water should typically have a turbidity of less than 1 FNU.
What to Consider When Preparing a Turbidity Sensor?
Calibration is the most significant factor that affects the accuracy of turbidity measurements. Always perform calibrations by following the sensor manufacturer’s recommendation. Some manufacturers may suggest a zero calibration in addition to a higher range calibration.
Since turbidity is a measure of suspended sediments, be sure calibration standards are well mixed and do not contain entrapped air. Air bubbles can refract light and provide erroneous readings. Always agitate the probe in the standard to ensure a fresh sample is directly in front of the optics, and always activate the wiper to clean any debris and knock off any air bubbles. Allow adequate time for the readings to stabilize and then calibrate.
After calibration, rinse and move the sensor to a clean tap water sample, agitate the probe, activate the wiper, and monitor the sensor performance. The expected reading should be near zero.
Negative readings may indicate that the sample is cleaner than the zero calibration standard or that the sensor accuracy range spans below zero. Return the sample to the higher range calibration standard and confirm that the reading is still reasonably close to the value. If avoiding negative readings is important, a small offset can optionally be applied to the zero calibration.
A quality control (QC) check after calibration, prior to deployment, and periodically during the deployment should be performed. Select a method that will provide confidence that field measurements will be reliable and accurate. The most basic QC check is simply putting the sensor in clean tap water. Remember to always agitate and wipe. Turbidity readings should be very low (just a few FNU or less) and stable in a clean sample.
Erroneous readings and data spikes during long-term deployments should be expected with turbidity sensors as stray particles, filamentous algae, interference from aquatic creatures, and other biofouling make this a challenging measurement. Some manufacturers have implemented data averaging and spike rejection algorithms to present smoother and more representative results.
Various sensor designs have longer and shorter optical paths. Intuitively, a longer path seems better as it will measure suspended particles in a bigger sample. The downside is that the optical path may actually see the side or bottom of the sample container. As a result, a bigger sample container may be required, which leads to more calibration standard consumption.
Often, a short optical path is a better choice. Sunlight also plays a role in turbidity measurements and calibrations, as 860 nm light is present in both the sensor light source and sunlight. Most manufacturers have implemented a sunlight rejection algorithm, which can be as simple as measuring turbidity with and without the light source and reporting the difference.
For best turbidity measurement results, select a sensor with integrated cleaning, perform careful calibrations, and do as many QC checks as possible.
How to Deploy a Turbidity Measurement System?
Turbidity sensors can be deployed from either shoreside or open water deployments. Measurement depth(s), type of water body, availability of suitable mounting structures, and project specifications will often dictate which type of deployment is most appropriate.
Buoy-based deployments allow for flexible placement of turbidity sensors in open water environments.
For example, excavation work on a river bank may lend itself to establishing a shoreside turbidity sensor on the same bank where the work is being performed. On the other hand, if the goal of a project is to measure turbidity deposited by a river into a lake or estuary, a buoy-based system deployed at the midpoint of the river outlet might be more appropriate.
Turbidity sensors may be securely deployed in pipes that allow flow through to the sensor in mast-mounted systems.
A shoreside system typically consists of a weatherproof data logger mounted on a pole or existing structure, if available. The turbidity sensor rests inside a deployment pipe that protrudes into the water and has perforated slots or holes to allow flow to the sensor for accurate measurement. Cable can be protected with conduit as it is routed to the data logging system.
It is important to consider potential water level changes when establishing such a system. If the water level falls, the sensor may sit in air and measure inaccurately. Conversely, if water level increases, this can produce large forces that may damage the deployment pipe and cabling if not properly secured. Forces from swollen rivers can be enormous and carry heavy debris loads. Frequent inspection and debris removal is recommended.
Buoy-based systems contain a data logging unit secured within the hull of the buoy, which is normally powered from a battery with solar charging for continuous operation. If a single, near-surface measurement is acceptable, the buoy frame or a perforated deployment pipe provides a secure location to mount the turbidity sensor. For deeper measurement, the sensor can be mounted onto a line suspended from the buoy.
The buoy is secured in place with a single-point or multi-point mooring system depending on the sensor depth and environmental conditions. The mooring system must allow the buoy to move with any waves and water level fluctuations that the site may experience.
Depending on the reason for measuring turbidity, establishing a reference station for background/comparison measurements may be beneficial. In the example of excavation work on a river bank, a reference station upstream of the work area can help to separate turbidity released from the work area from turbidity occurring naturally in the river.
A string of turbidity sensors can be suspended from a buoy for applications requiring measurement at multiple depths.
A similar concept is often put into place on dredging and other construction projects where the work area is blocked off by a silt curtain (also called turbidity curtain, turbidity barrier, or silt barrier). One or more buoys measure directly outside the curtain to detect any sediment emissions through the curtain, while a reference station gathers comparative measurements from an area unaffected by the work to avoid false alarms and unnecessary work stoppages.
Some sites and measurement objectives may also benefit from measurements at multiple depths. For example, estuaries may have layered conditions with relatively fresh water at the surface layer, and saltier water at the bottom with a brackish transition zone.
Depending on the sources and dynamics of the environment, turbidity may also be layered, and sensors placed at multiple depths can help to distinguish relative levels and spreading. On a buoy system, sensors can simply be placed at the desired depths on a suspended sensor mooring line. Establishing multiple depths from a shore-based deployment may be more challenging, but pipes of different lengths may be able to be established next to each other in some cases.
Conclusion
Turbidity is one of the principal indicators of water quality and may be caused by various natural processes and human activity. Changes in turbidity levels impact ecosystems in different ways. Measurement of turbidity along with other environmental conditions can help to assess ecosystem health and study long-term changes.
Successful long-term monitoring depends on careful sensor selection, calibration, and system design. With proper maintenance including periodic cleaning and recalibration, a properly designed turbidity measurement system can provide many years of high-quality data.
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