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Water turbidity – why it is an important water quality measurement.

What is Turbidity?

Turbidity is an optical feature of a liquid and a measurement of its clarity. Determining scattered light levels within a column of water indicates turbidity. It is an important measurement to take in assessing the quality of a body of water as both plant and animal growth depend on turbidity levels. Turbidity is also important for water safety and aesthetics. Measuring turbidity continuously and accurately is important. As such, operations monitoring drinking water sources and processes use turbidity as a simple indicator of water quality.

What increases water turbidity?

Suspended solids affect a liquid’s clarity by scattering light. Therefore more suspended solids increase light scattering, which indicates higher turbidity. A body of clear water will have low turbidity. Suspended solids can consist of inorganic or organic material, ranging from algae, clay, silt, dissolved organic compounds, gravel, and plankton. However, it is possible for such materials to be present in the body of water, but not suspended. For example, heavier particles such as sand will settle on a river bed if undisturbed through stirring. Such particles, when settled, do not affect water turbidity. This is an important distinction to recognise when measuring and discussing water turbidity.

Turbidity scatter illustrated
Water Turbidity – Suspended particles increase light scatter, indicating higher turbidity levels.

Sources of particles

Two broad categories separate the sources of particles affecting turbidity; human induced and natural sources.

Natural Sources

Natural sources of particles can range from clay, silt, sand, and algae. Its production occurs in the body of water through natural processes such as erosion or they are already present. However, the rate of natural particle production or removal can be affected by human activity. For example, mining operations may release chemicals or waste that cause alga to bloom.

Heavy rainfall events bring down sediments from upland. This process naturally introduces particles into the water body (again, human activity can increase or decrease this). If the sediments do not have the chance to settle then they will increase water turbidity. The heavy rainfall would also stir up any sediment on a river or lake bed, thus increasing turbidity.

Pond in a Japanese Garden. The water is undisturbed and un-stirred, however, there are suspended particles present causing some water turbidity. The perfect environment for Koi.
Human Sources

Human sources of sediments and particles are introduced from wastewater treatment facilities, runoff from mining activities, urban stormwater runoff, and agricultural activities.

Each has a different way of increasing turbidity. In the case of mining and agricultural activities, sediments and chemicals are released into rivers through erosion of ‘exposed’ land and runoff from tailings. Wastewater treatment facilities may release both chemicals and organic material into the waterways. These directly increase turbidity through increasing suspended particles and promoting the growth of algae by providing more nutrients.

For bodies of water near cities, stormwater runoff places a large role in water turbidity. Large rain events will remove loose particles off buildings and roads, carrying them into the nearby water bodies. The particles can consist of sand, dust, and various chemicals and substances emitted from vehicles (fuel, oils, brake dust, and exhaust fume deposits). Note, that many local authorities advise surfers and swimmers not to enter water bodies after large rains storms due to the increased concentration of harmful chemicals.

Why is water turbidity important?

High turbidity is not necessarily a bad thing, as low turbidity is not always good, it is very circumstantial. For example, most ecosystems depend on the level of nutrients and suspended particles within a body of water, to remain within a range. These nutrients supply organisms with food to grow. A thriving ecosystem relies on the organisms as their energy is transferred up the food chain.

A large rain event could cause two things, it may ‘cleanse’ the body of water by washing away suspended and settled sediments, thus starving the ecosystem of nutrients. Conversely, it may introduce a level of sediment or chemicals. The increased sediment may increase turbidity to the point that it blocks light to all but the top layers of a column of water, preventing plant and algae growth, it could also introduce and promote the growth of the ‘wrong’ organism. Should chemicals be introduced it could kill many organisms or cause algal blooms which in turn block light or drain the body of water of nutrients.

For human activity it is important to know and understand water turbidity as it allows effective water management by planning the changing of filters or the diverting of waterways to prevent contamination should pollution increasing event occur.

Greenspan turbidity sensor ts-1000
Water Turbidity Sensor manufactured by Greenspan.

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