Technically speaking

WWT's technical editor Peter Minting reports on the decline of acid rain

Further evidence that acidification is on the decline in the UK has been found by a team of scientists at Reading University. Earlier this year scientists at the Institute of Freshwater Ecology found evidence for a reversal in the Lake District (see WWT February 2000).

Over the last two years, the Reading team repeated a survey which was carried out near Dorking between 1977-1982. The average pH under pine forest in the Tillingbourne catchment was found to have risen from 2.84 to 4.51. This is a significant change, considering that an increase in pH of 1 equates to a tenfold decrease in acidity. The team of scientists, led by Professor Richard Skeffington, believe the reduction in acidity is largely due to decreases in atmospheric pollution.

Air pollution can cause acid rain in a number of ways, the most important being the burning of fossil fuels which releases large quantities of sulphur dioxide and nitrogen compounds. Sulphur dioxide reacts with water to form weak sulphuric acid, and nitrogen compounds nitric acid. Some soils are able to buffer, or effectively neutralise the hydrogen ions responsible for the acidity. Trees can also neutralise a limited amount of hydrogen ions before they reach the ground.

In the Reading study, Professor Skeffington's team found total sulphate deposition had decreased by 82% and hydrogen ion flux in throughfall by 97%, with the tree canopy now able to neutralise nearly all the ions. The Tillingbourne catchment is acid-sensitive, i.e it has a low acid-buffering capacity. This is due in part to the peaty and sandy soil, which is more sensitive than say, chalky soils which are relatively alkaline.

The importance of reversing acidification cannot be underestimated for a number of reasons; when first brought to the attention of the UN in 1972, large tracts of pine forest in Scandinavia were seen to be dying. Pines are more susceptible than deciduous trees because they tend to grow on soils with little buffering capacity and have a shallow root mass. Changes in rainfall acidity can therefore have a rapid effect. Trees are not the only organisms to suffer; declines in fish, amphibian and insect populations have been seen in acidified lakes. Increases in sulphur and nitrogen deposition will also have a detrimental effect on the quality of groundwater and other water resources.

Although there is evidence in the Lake District and now in Surrey for a reversal of the acidification process, there is still a long way to go. In the Tillingbourne catchment, sulphate levels in run-off are more than would be expected from the rain data, probably because the recovery is being slowed by the large amount of sulphur still held in the soil.

Despite improvements in pollution control the amount of sulphate and nitrate in groundwater is still rising, with nitrate levels rising most significantly. It is also noticeable that the decline in nitrogen compounds in rain is nowhere near as marked as that for sulphur compounds - the decrease in concentration for sulphates in rain is 69% compared with 37% for nitrates. This could be partly explained by the changes in sulphur and nitrogen emissions in the UK. According to the DETR, sulphur emissions have declined by 67% since the 1970s, due to more power stations using natural gas instead of coal and a reduction in the household use of coal. The picture for nitrogen compounds is more complicated - industrial outputs have declined but this has been offset by an increase in road traffic. Overall, nitrogen emissions decreased in the UK by 26% between 1977 and 1997. It might be expected that nitrogen levels would be on the decline, as nitrogen deposition by rain is decreasing. But Professor Paul Whitehead, who worked on the project along with Professor Skeffington and Dr Tim Hill said: "Nitrogen levels in ground and surface water are still increasing, even though emissions are thought to have gone down. We do not really know why this is."

The team now wants to repeat the study over several years to answer this question and iron out any differences caused by unusual weather during the past two years. And although nitrogen emissions have decreased, there could be a localised effect operating in the Tillingbourne area. After all, the majority of nitrogen emissions are from traffic, and traffic in the south of England is still increasing.

The Reading team is now preparing a paper for a special edition of Science of the Total Environment, which will cover pollution across the Thames region.



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