"Dirty water" Return to list of pracs

Aim: This is an activity to begin a discussion about polluted water.

Equipment: The equipment includes...  

• Clear plastic tube or pipe;
• sample of pond water;
• A4 paper with a pattern in the middle (2cm x 2cm);

Method: (a) Construct a turbidity tube which can measure the degree of how dirty pond water is. It is a tall clear plastic tube that you place a sample of pond water in. You then look downwards through the top of the tube to see if you can the a pattern (any pattern) draw on a piece of paper which is placed underneath the turbidity tube as it stands on a table. The scale on the side determines the amount of turbidity. (b) The turbidity plates do the same thing. The length of the string under water measures the turbidity value also.

Background knowledge: As erosion occurs within a catchment tiny particles of clays, silts or small organic particles are washed into water bodies. Industrial wastes and sewage also can contribute particles. These tiny particles can be held by the water and are termed suspended solids. The faster the water is moving the more suspended solids it can carry. Suspended solids can be measured in milligrams per litre (mg/L) by filtering the water and weighing the dried residue. Turbidity is the result of suspended solids and is a relative measure of the clarity of water: the greater the turbidity, the murkier the water. Increases in turbidity reduce the transmission of light.

· Increased turbidity silts up stream habitats.
· The fine particles settle in downstream areas and make the bottom habitat unsuitable for many animals.
· Increased turbidity reduces light penetration through the water
· This reduces the amount of light for plant growth and thus limits growth of beneficial aquatic plants.
· Increased turbidity affects pollutants.

The most effective method of testing for turbidity is by using a turbidity tube. For this test you simply collect water and pour into the tube until you cannot see the markings on the bottom. Record the reading from the side of the tube. If possible, stand the tube on a white tile. These tubes are handy because they are cheap and simple to use. A seechi disc can also be used. For greater accuracy use a nephelometer. This instrument determines the scattering of light and is measured in standard Nephelometric Turbidity Units (NTU). Seasonal mean turbidity and suspended solids levels must not change by more than 10%.