Activity sheet 3: Measuring turbidity
Your first question is probably,‘What is turbidity?’ Turbidity is how clear the water appears. If the turbidity is high you cannot see through the water to the bottom of the lake. It is murky and dark. This is caused by lots of solid particles suspended in the water. These particles can come from soil washed into the water or the sediments on the bottom being stirred up. If sewage is pumped into the water this can be another cause of high turbidity. So, you can see why it is important to know the turbidity of the water! Another cause of high turbidity is growth of plankton (microscopic organism) in the water.
So, if you want to know if it is safe to swim in the water, turbidity can be an important test.
Why measure turbidity?
As well as looking bad and murky, water that has a high turbidity has many other effects on the lake.
If the water turbidity is high then the suspended particles in the water will block the sunlight from reaching to the depths of the water. Plants need sunlight to carry out photosynthesis. Without photosynthesis there is less oxygen for the animals that live in the water. If the plants die, bacteria will break down the dead plant material. As there is more food for the bacteria they will increase in number and use up more oxygen.
Lots of particles in the water also absorb extra heat and the water may heat up. This results in less oxygen in the water as well.
Many animals that live in water have external (meaning on the outside) gills for breathing. If the turbidity is high the gills can become clogged and the animals cannot breathe.
So you can see there are lots of good reasons to have clear water.
How to measure turbidity
Turbidity can be measured using a turbidity tube. A turbidity tube is a long clear tube with a pattern on the bottom. Take a water sample using a bottle or a cup. Shake the bottle and then slowly pour the water into the tube until the pattern on the bottom disappears when you are looking from the top. Read the value below the water level. Do not average the values or make a guess between the levels. Record your results. Repeat three times.
An acceptable level for turbidity in a lake is 20 NTU or less. Turbidity tubes, like the one in the photograph below, come in two pieces that fit together to make one long tube. The units are found on the side.
The student below is looking into the turbidity tube to see if the pattern has disappeared.The closeup photograph shows the patternon the base of the tube.
Questions:
1 What instrument would you use to measure turbidity?
2 Outline how you would test turbidity.
3 What factors can lead to high turbidity?