Module 4 - physical and chemical parameters
Waterwatch Australia Steering Committee
Environment Australia, July 2002
ISBN 0 6425 4856 0
Turbidity: opacity or muddiness caused by particles of extraneous matter; not clear or transparent.
In general, the more material that is suspended in water, the greater is the water's turbidity and the lower its clarity. Suspended material can be particles of clay, silt, sand, algae, plankton, micro-organisms and other substances. Turbidity affects how far light can penetrate into the water. It is not related to water colour: tannin-rich waters that flow through peaty areas are highly coloured but are usually clear, with very low turbidity. Measures of turbidity are not measures of the concentration, type or size of particles present, though turbidity is often used as an indicator of the total amount of material suspended in the water (called total suspended solids). Turbidity can indicate the presence of sediment that has run off from construction, agricultural practices, logging or industrial discharges.
Suspended particles absorb heat, so water temperature rises faster in turbid water than it does in clear water. Then, since warm water holds less dissolved oxygen than cold water, the concentration of dissolved oxygen decreases.
If penetration of light into the water is restricted, photosynthesis of green plants in the water is also restricted. This means less food and oxygen is available for aquatic animals. Plants that can either photosynthesise in low light or control their position in the water, such as blue-green algae, have an advantage in highly turbid waters.
Suspended silt particles eventually settle into the spaces between the gravel and rocks on the bed of a waterbody and decrease the amount and type of habitat available for creatures that live in those crevices. Suspended particles can clog fish gills, inducing disease, slower growth and, in extreme cases, death.
Fine particles suspended in water carry harmful bacteria and attached contaminants, such as excess nutrients and toxic materials. This is a concern for drinking water, which often requires disinfection with chlorine to kill harmful bacteria.
Turbidity is affected by:
Regular turbidity monitoring may detect changes to erosion patterns in the catchment over time. Event monitoring (before, during and immediately after rain) above and below suspected sources of sediment can indicate the extent of particular runoff problems.
Turbidity can be measured using a Secchi disk, a turbidity meter or a turbidity tube. The turbidity tube is adequate for most purposes, but if your waterways are generally very clear a turbidity meter may be more suitable. The Secchi disk is useful only in non-flowing, relatively deep water.
Turbidity is best measured on-site in the field, but if necessary it can be measured later, within 24 hours of sampling, provided the sample bottles are filled completely, leaving no air gap at the top.
A Secchi disk allows you to measure the water's transparency. The clearer (less turbid) the water, the greater the depth to which the disk must be lowered before it disappears from view. This is why Secchi disks are not useful in shallow water.
The main advantage of Secchi disks is that they are cheap and easy to use.
The Secchi disk is a black and white 20 cm diameter disk which is attached to a long tape measure or cord marked in metres. Ask your Waterwatch coordinator how to obtain a Secchi disk.
Lower the disk into the water until it disappears and then raise it until it reappears. The depth (as indicated on the tape) at which you can see the disk is the Secchi disk reading (see Figure 4.6).
Turbidity is a relative measure. It is usually expressed as nephelometric turbidity units (NTU) or as metres depth. Other units, such as formazin turbidity units (FTU) or Jackson turbidity units (JTU), are specific to particular types of turbidity meter and their methods of calibration. They are not absolute measures. (They can be converted to NTU by calculation, if you need to, and the method may be explained on the instrument.)
Turbidity meters measure the intensity of a light beam when it has been scattered by particles in the water. They are effective over a wide range - from 0 to 1000 NTU.
The turbidity tube reads turbidity by absorbing light rather than scattering light, so it overestimates turbidity in samples that are highly coloured and underestimates turbidity in samples containing very fine particulates, such as clay. However, it is very simple to use and gives good comparative measures.
The turbidity tube is a long thin clear plastic tube, sealed at one end with a white plastic disc with three black squiggly lines on it (seen when looking down the tube). The tube has a scale marked on the side. Your Waterwatch coordinator can tell you how to obtain a turbidity tube.
*The scale is non-linear (logarithmic) and there are gaps between numbers. When the water level is between two numbers, record the value as less than the last number. If you can see the wavy lines when the water is at the top, record the result as 'less than 10 NTU'.
Wash the turbidity tube thoroughly with tap water and ensure the tube is kept clean and free from contamination.
No calibration is required and the tube reads from 10 to 400 NTU.
Make sure the Secchi disk is clean. Have a second person check the result. Check the accuracy of markings on the Secchi disk cord. Perform the test in shade if possible.
Undertake a field replicate test every 10 samples.
For river monitoring, Secchi disks have limited use because the river bottom is often visible. Also, the disk is often swept downstream by the current, making accurate measurements impossible. You may consider using a Secchi disk if you wish to monitor the clarity of a lake or deep slow moving river or estuary, and the water is too clear - i.e. less than 10 nephelometric turbidity units (<10 NTU) - for accurate turbidity tube readings, and your group cannot afford to buy a turbidity meter.
To avoid error make sure the turbidity tube is clean and free of scratches. Perform the test in shade if possible. Have a second person check the result. Note if highly coloured water is present, because it may elevate the readings. Test a field replicate every 10 samples.
Natural (or background) turbidity levels in waterways vary from <1 NTU in mountain streams to hundreds of NTU during rainfall events or in naturally turbid waters. Turbidity is affected by river flow, so be sure to measure the river flow when you collect your sample.
Interpreting turbidity readings requires information about the natural turbidity in your area. There are large variations in turbidity in Australian river systems; inland rivers tend to be naturally more turbid than coastal rivers. Find out the normal range in your catchment from your Waterwatch coordinator, natural resource management agency or local council. Then, over a series of measurements, build up a picture of the turbidity and its variation in your own waterbody.
If, on any particular sampling occasion, values that differ markedly from those expected for that time of year or flow rate you should contact your Waterwatch coordinator and ask about the relevant trigger values discussed in the revised national water quality guidelines (ANZECC/ARMCANZ 2000).