Turbidity Measurement And Monitoring In Water Quality Analysis

What Is Turbidity?

Simply put, turbidity is the measurement of water clarity (i.e., transparency). Suspended particles – such as silt, algae, plankton, and sewage – can cause water to appear cloudy or murky. These particles scatter and absorb light rays rather than allowing light to be transmitted straight through the water.1

A higher turbidity reading represents cloudier and ‘thicker’ water with more particles throughout. When water is clear, it has low turbidity levels.

What Units Are Used for Turbidity?

Turbidity measurements are most commonly presented in Nephelometric Turbidity Units (NTU) or Formazin Nephelometric Units (FNU). While they are often used interchangeably, these units for turbidity are different – they represent the turbidity measurement method used.

NTU represents turbidity readings captured using a white light at a 90-degree detection angle. Sensors that use this type of measurement method are compliant with EPA Method 180.1.1

In contrast, FNU is the correct unit when using an 860 nm light (near IR) with a 90-degree detection angle. Sensors using this method are typically compliant with ISO7027.1

Check out the Light Source and the Angle of the Detector(s) sections to learn more!

We also encourage you to check out our technical note on Turbidity Units and Calibration Solutions, and our blog post on Turbidity Measurements: Tips and Precautions.

What Are Sources of Turbidity?

There are several ways particulates can get into a natural body of water, causing an increase in turbidity. The first is storm runoff.

As rain and melting snow flow across the landscape, particulate matter is picked up. This may be pollutants, dust, pet waste, and more in an urban environment. In a rural setting, this may be loose soil or leaves. As rainfall enters a water body, the velocity will increase, eroding riverbanks and causing additional sediment influx.

Spikes in turbidity can be tied to a number of different sources, many of which are listed below:

Wind erosion is a less common source of turbidity. Dust devils, tornadoes, and heavy wind can displace soil from the top of the ground and then suspend it in the air. The sediment will eventually drop out of the air and fall to the ground, potentially landing in a water body.

Coastal erosion is another source of turbidity. Waves naturally stir up sand from the bottom of the ocean and deposit it on the beach, but they also take sand from the beach back out to sea.

As land is cleared for building, rain events can cause loose sediment from construction sites to wash away. Silt fences are used to contain this runoff, but they aren’t always 100% effective, especially during significant storm events.

Dredging is digging sediment out of a channel to increase its depth. As shipping vessels grow in size, channels must get deeper. Dredging vessels must be deployed to fight natural channel sedimentation and ensure safe passage for barges. When dredging of a channel occurs, turbidity typically increases. If you’re really digging this topic, check out our blog on Real-Time Water Quality Data Used to Protect Ecosystem During Dredging.

Sewer discharge into waterways can occur during large storm events when combined sewer systems – sewers that collect rainwater runoff and wastewater – become overwhelmed, resulting in a direct release of sewage into water bodies. Combined sewer overflow (CSO) events often cause a spike in turbidity.

Animals can contribute to turbidity when their activities stir up silt, cause erosion, or release solid waste into the water.

Algae in water can be another source of turbidity, as the growth of these organisms prevents sunlight penetration into the water column. Not only does this increase turbidity, but it can also negatively impact predators that rely on sunlight to pursue their prey.3

Turbidity vs. Total Suspended Solids (TSS)

We can’t talk about turbidity without mentioning total suspended solids (TSS). Both are impacted by particles suspended in water, but they are fundamentally different measurements.

Turbidity is specifically looking at the clarity of the water. This is often quantified by determining the amount of light scattered by particles suspended in water. Size, shape, composition, and surface characteristics determine how a particle will scatter light. But the principle behind turbidity measurements is that, on average, increases in turbidity indicate an increase in suspended particles per unit volume of water.

Many turbidity sensors operate by shining a light beam into the sample solution and measuring the light scattered off particles – see How is Turbidity Measured? Typical units used for turbidity include FNU and NTU – see What Units Are Used for Turbidity? to learn more.

In contrast, TSS sensors quantify the concentration of suspended particles; the units used are milligrams per liter (mg/L). TSS is preferred over turbidity in applications such as wastewater, where it is critical to understand the amount of biological activity in the treatment system (i.e., the biomass). See the section Why Measure Turbidity in Wastewater? for more about this application.

It is possible to indirectly measure TSS with a turbidity sensor. Instruments like the YSI ProDSS can calculate TSS from a turbidity measurement if correlation coefficients are determined.

To establish a correlation between turbidity and TSS, turbidity data and corresponding samples must be collected at a sampling site. The samples need to be analyzed in a lab to determine a true TSS measurement (mg/L). Coefficients can then be calculated using the pairs of turbidity and TSS data. For the ProDSS, this is done with YSI’s Kor Software.

It’s important to note that correlation data must be collected for each unique sampling site, as this correlation is site-specific.

Learn more about turbidity, TSS, and a related parameter – total dissolved solids (TDS) – in our blog on Understanding Turbidity, TDS, and TSS.

Turbidity vs. Color

While the color of water can affect a turbidity measurement – colored particles can absorb the light bean used in some measurement technologies1 – turbidity is not a measure of water color.

Water can appear colored due to dissolved compounds or suspended particles. For example, tannins are dissolved organic acids that can give water a tea color. These leach into water when plant material – such as pine needles and tree roots – are slowly broken down into small particles that dissolve in water.4