TURBIDITY

Turbidity is an optical measure of water clarity, which refers to the amount of light that is scattered by particles suspended in the water column. Scattered particles can be a range of materials such as sands, silts, algae, phytoplankton, sewage solids etc. The more scattering that takes place, the higher the turbidity of the water.

Turbidity is commonly measured in Nephelometric Turbidity Units (NTUs) of Formazin Nephelometric Units (FNUs); the different units are attached to two different methods of measuring turbidity and should not be used interchangeably, although they often are.

Turbidity is another common and valuable water quality parameter. Suspended particulate matter in water can negatively impact aquatic systems in many ways.

There are many different possible sources for turbidity – most lotic systems will experience fluctuations in their turbidity as a result of external influences such as run-off from storms, erosion by wind and water. Human activities can have large scale influences on turbidity with actions such as dredging and sewer discharges. Animals can also influence turbidity though erosion or defecation.

Turbidity has wide-ranging importance as it can affect the oxygen balance in a water source with particles displacing oxygen and causing a reduction . Particles can also influence water temperature, by absorbing solar radiation causing localised increases in temperature.

Increases in turbidity due to it’s impact on oxygen levels. Turbidity can cause local temperature increases through particles absorbing solar radiation, and warmer water holds less oxygen. The particles themselves also have the capacity to displace oxygen.

Another concern with increasing turbidity is the blocking effect it has on solar radiation reaching aquatic plants and primary producers. A reduction in sunlight can slow, and even completely halt, photosynthesis which can then have wide ranging implications throughout the food chain.

Loss of photosynthesis is not the only concern regarding aquatic animals. Increased particular matter in the water column can change fish behaviour and prevent spawning and migration. It is well documented that a sudden increase in turbidity can cause an entire spawning ground to fail.

– Turbidity and TSS are interlinked but very different parameters.

– Turbidity is a measure of water clarity which is determined by the amount of light that is able to pass through the water in question whereas TSS is a measurement that looks to quantify the concentration of the particles suspended in the water. TSS uses mg/L units and, like turbidity, is influenced by the size and shape of suspended particles in the water. By cross correlating turbidity readings with samples of suspended matter, a TSS reading can be outputted using a correlation coefficient-based algorithm.

– TDS are compounds that have been dissolved within the water column, for example minerals such as magnesium, chlorides and sulphates. TDS can affect the water clarity but it is not a measure of water clarity as, like TSS, it relates to quantifying particles within the water.

– There are many methods which have been developed for turbidity measurement. From the somewhat “old school” Secchi disk to more technologically advanced nephelometers and spectrophotometers.

– Proteus uses an optical turbidity sensor that captures scattered light based on a Formazin Nephelometric method. A measurement of the amount of light scattered by suspended particulate matter at 90 degrees to a beam of infrared light.

– Proteus is also capable of measuring TSS simultaneously, using the custom parameter function, which will calculate TSS in real-time. This requires some additional sampling at the site of interest.