Investigating acid rain

Acid rain affects the water quality of streams only if there is insufficient neutralizing capacity in the soils and rocks in the catchment. The streams most likely to be affected will be those where the catchment geology is of hard igneous rocks such as granite and these can be checked beforehand by looking at a geological map of the country.

It is possible though to check the acidity of rainwater and to see the influence of wind direction on the pH. To carry out such an investigation, a special rainfall collection device has to be prepared to ensure that the collected rainwater is not contaminated.

Rainwater collector

The ideal rainwater collector comprises a large, clean polythene funnel which is fitted over a polythene bottle. The collecting device should be fitted to a post about 1 metre from the ground and in an open area so that the rainwater is not firstly in contact with leaves from a tree. A particular problem that has to be dealt with is that birds may sit on the edge of the funnel and contaminate the rain water with their droppings! The birds can be deterred by putting a circle of chicken wire around the edge of the funnel as shown in Figure 42.

If you do not have a suitably sized polythene funnel, you can make an effective rainwater collector from mineral water bottles. These are very suitable because they were clean before the mineral water was put into them. You'll need two sizes of bottle, say a 5 litre one and a 500 ml one. The top part of the large bottle is cut off from the rest of it to form a funnel when it is inverted, whilst the bottom part of the collector is the smaller bottle which has also been modified to accommodate the funnel, as shown

Figure 42. Rainwater collector for acid rain study (1)
Figure 43. Rainwater collector for acid rain study (2)

in Figure 43. Again, this collector should be placed in an open space above ground level: it could be placed on a pile of bricks.

Rainwater should be collected from the rainwater collectors on each occasion that it rains. You may find that there is enough rainwater to analyse for both pH and sulphate, but if the amount is limited then just carry out a measurement of pH. On each occasion that you collect rain, make a note of which direction the weather is coming from. One way to find this out is to look at the weather map published in a daily paper; those in The Daily Telegraph or The Guardian are particularly good.

As you carry out the project over a period of time you will be able to compile a table of rainfall pH and sulphate content, and the wind direction. These data can be represented as a 'wind rose', as shown in Figure 44 for some rainfall collected in south-west Scotland.

Figure 44. Effect of wind direction on rainwater pH in Scotland Snow

On some occasions the precipitation will occur as snow. The snow is likely to accumulate in the funnel of the rain collector but the whole device can be brought indoors to allow the snow to melt and flow into the collecting bottle. You could also collect snow from an area which has not been disturbed. Simply go to the selected area and scoop the snow into a beaker that has previously been cleaned and rinsed with distilled or deionized water. As with rainwater, measure the pH and sulphate content and look for variations according to wind direction.

In some parts of the world where snowfall is frequent such as in Canada and Scandinavia, snow may be discoloured by pollutants that the flakes have collected during their formation. This discoloured snow is usually acidic because the pollutants causing the discoloration are soot particles from chimneys which have also emitted sulphur dioxide and nitrogen oxides.

You can check the snow sample you have collected for soot particles by first melting it in the beaker and then filtering it through a filter paper. The filter paper is then dried and examined for the presence of black particles. You can compare the cleanliness of snow which you have collected from the same area on different occasions and again see whether the wind direction affects the quality of the snow.

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