Water Quality Measurement

Measuring water quality is key to discerning appropriate uses and remediating problems. In fact, accurate water quality measurement, while basic in many situations, forms the basis for numerous crucial management decisions.

In this post, we will briefly explore the range of technologies and techniques used to monitor, measure, and evaluate water quality. In each section, we look at a single parameter and the particular methods and technologies that come into play measuring it—for example, sensor operation.

Algae, Chlorophyll and other Fluorescing Materials, and Phytoplankton

Algae, phytoplankton, and cyanobacteria provide food and oxygen for aquatic organisms and are an important aspect of water body health. That means that a phytoplankton imbalance can trigger an algal bloom, fish kill, toxic red tide, or other problems. It is possible to sample water and measure plankton counts in those samples, but this is difficult, imprecise, and time-consuming.

It is far more efficient and easier to use a chlorophyll sensor because all phytoplankton has chlorophyll A, which a sensor can detect in-situ. This makes real-time monitoring and logging of data possible, and continuous or long-term work a reality. However, a chlorophyll sensor has one weakness: it assumes the same levels of chlorophyll A among all algae and cyanobacteria, failing to identify specific species and giving only a rough estimate of biomass.

Nevertheless, chlorophyll sensors can show when a body of water is becoming eutrophic before it is visually apparent. These sensors work by estimating chlorophyll concentrations based on fluorescence, giving a reading in micrograms per liter (µg/l). Depending on the water body’s location and normal parameters, local managers will set healthy parameter limits for chlorophyll.

Blue-green algae, also called cyanobacteria, contain phycoerythrin and phycocyanin, and they are the only phytoplankton that does. This means these pigments are good indicators of cyanobacteria in a body of water. There exist both types of blue-green algae sensors, and they both rely on fluorescence.


Chromophoric dissolved organic matter (CDOM) is a valuable indicator of all the particles of matter that are floating around in water bodies. Only small, molecules with low molecular weight can permeate cytoplasmic membranes, and most dissolved organic matter is made up of particles that are too large to penetrate, so most dissolved organic matter is not usable right away in natural waters.

Even so, CDOM, containing everything from acids and dissolved soils, to lignin, metals, and decayed organic matter, is always in motion, influencing many processes in water. Light penetration, pH, salinity, and turbidity are all affected by fluctuations in CDOM, and aquatic life adjusts with these parameters.

CDOM binds with metals, fuels respiration, reduces transparency, and traps heat. High levels of organic matter in CDOM can trigger microbial consumption, and, in turn, increased oxygen demands as microbes consume dissolved oxygen as they work. Excessive production can end in oxygen depletion.

There are two common ways to measure CDOM. The first demands samples and lab analysis with spectrophotometers. The other method includes in-situ measurement using either a fluorometer or a water quality sonde equipped with a specialized sensor designed to handle CDOM. Either of these options can be deployed in the field alone or as part of a stationary monitoring unit.

Conductivity, Salinity and TDS

Conductivity means how capable water is of passing electrical flow. This capability is directly connected to the water’s concentration of conductive ions, which come from dissolved salts and inorganic materials like chlorides, alkalis, carbonate compounds, and sulfides. More ions mean higher conductivity, which is why distilled or deionized water is an insulator and seawater has very high conductivity. Typically, conductivity is measured in microsiemens per centimeter (μS/cm) or millisiemens per centimeter (mS/cm).

Salinity refers to the total concentration of all dissolved salts, but salinity is not typically measured directly. Instead, it is usually derived from the conductivity value and called practical salinity. Practical salinity can be derived based on a comparison of the sample’s specific conductance to a standard for salinity set by seawater. Because they’re based on conductivity values, salinity measurements are unitless, but they are often noted as practical salinity units (PSU).

Total dissolved solids (TDS) means the sum of all ion particles that measure less than 2 microns (0.0002 cm) in size. This includes dissolved salts and other electrolytes that comprise salinity concentrations, along with dissolved organic matter and other compounds. TDS is about equal to salinity in “clean” water, but in polluted areas or wastewater, TDS can also include organic solutes such as urea and hydrocarbons.

Dissolved Oxygen

Dissolved oxygen can be measured as a percent of air saturation or in milligrams per liter (mg/L). However, in some cases, researchers report DO in micromoles (μmol) or in parts per million (ppm). The relationship between milligrams per liter and the percentage of air saturation varies with pressure, temperature, and the water’s salinity. Therefore, you need to know your sample’s salinity and temperature to calculate DO concentrations from air saturation—or, in most cases, you can simply use an oxygen solubility chart.

There are three ways to measure dissolved oxygen concentrations. The more modern technique includes sensors, either based on optical or electrochemical principles. You attach the DO sensor to a meter for laboratory applications and spot sampling or to a process monitor, data logger, or transmitter for longer deployments.

Another possibility is the colorimetric method, which gives a basic estimate of your sample’s DO concentrations. High- and low-range DO concentrations each have their own technique within this method, and each is cheap and quick. However, this technique is both limited in capacity and prone to error when other substances are present.

The old-school technique for measuring dissolved oxygen is Winkler titration. However, although it is still used today, it is difficult to use, especially in the field. When properly used, it is very accurate, and the Winkler method in its seven modified forms is still in use today.

PAR and Total Solar Radiation

Solar radiation is measured in frequency (hertz, Hz) or wavelength (nanometers, nm). Energy increases with frequency and decreases with wavelength size, which simply means that light with shorter wavelengths—like UV radiation—has more energy. PAR sensors can measure this kind of energy.


Reported as values on a scale, pH readings are numbers between 0 and 14. These units represent the negative logarithm of the hydrogen ion molar concentration [-log(H+)] in the solution. Depending on how precise the measurement is, the pH value can be carried out to one or two decimal places.

Alkalinity can be expressed in microequivalents per liter (meq/L) or milligrams per liter (mg/L). When in mg/L, it refers to bicarbonate (HCO3–), carbonate (CO32-), or calcium carbonate (CaCO3) concentrations, although it is most common to see calcium carbonate.

Turbidity, TSS and Clarity

Turbidity, an optical property, is inherently difficult to measure in that it can be a subjective standard. Various units have been defined to permit comparison and standardize turbidity measurement. There are three modern techniques for measuring turbidity, and two for measuring TSS.

Turbidity is caused by colored materials and particles in water. It can be measured directly with a turbidity instrument such as a turbidity sensor or turbidimeter, or relative to water clarity. Water clarity techniques include a Secchi disc and are usually low-cost and quick, albeit limited in accuracy by whoever administers them.

Turbidity meters use optical scatter-detection techniques such as nephelometry (90-degree scattering) to deliver quick, accurate measurements of turbidity. Turbidity sensors also rely on optical technology, but can be placed directly into the source water, and don’t need a sample to measure turbidity continuously. Most data from turbidity sensors and meters are not inter-comparable, and turbidity units have no intrinsic meaning. Therefore, differences in the kind of suspended sediment and in-design details from instrument to instrument can alter a turbidity measurement.

Because turbidity is caused primarily by total suspended solids (TSS), measuring TSS by weight is the most accurate and common method to measure both TSS and turbidity. The method involves filtering, drying, and then weighing solids from a water sample, and it is time-consuming and difficult.

The second technique calculates continuous total suspended sediment measurements using acoustic Doppler meter backscatter. Instruments can also measure turbidity using backscatter, light attenuation, and surface scatter.

The Bottom Line

Whichever water quality measurements you’re working to secure, it helps to have the best equipment and advice. For more detailed information about water quality parameters and how they are measured, see our in-depth guide here.

Top image: Water Sampling. (Credit: Lisa Hoffmann (https://www.lisa-hoffmann.de/)[CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)]

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