Acid rain

Acidification of rain and snow may seem to be a recent environmental pollution problem. However, the phenomenon has been known for over a century since it was first noticed that buildings, trees and plants were damaged if they were downwind of chemical factories discharging acid fumes. The damage was mostly confined to periods of rainfall because of the removal of air-borne pollutants by rain droplets. At that time, the problem was a local one, confined to an area close to the factories. This was because the factory chimneys were relatively short and there was not widespread dispersion of the pollutants. The air was also polluted with the smoke from the individual coal fires in all the houses, as well as from small power stations adjacent to the towns and cities that burned coal to produce electricity.

Nowadays there has been more centralization of heavy industry, and electricity generation takes place at fewer larger power stations which use a greater variety of fuels than, say, 50 years ago. The waste gases from these industries and power stations are usually discharged into the atmosphere from high stacks (cover illustration 9) so the gases are dispersed much more widely.

The main acidifying gases are sulphur dioxide (SO2) and various oxides of nitrogen such as nitrous oxide (N2O), nitric oxide (NO) and nitrogen dioxide (NO2). These are collectively referred to as NOx. The SO2 originates mostly from power stations whilst road traffic is the main source of NOx. These gases undergo a series of chemical reactions with cloud water and sunlight to form sulphuric and nitric acids, as shown in Figure 21.

As a result of using high chimneys to prevent the fumes from factories and power stations affecting the local population, the polluting gases can sometimes cross national boundaries and get washed down by rain into a different country from that in which they originated. For example, the Norwegians and Swedes have shown that they receive ten times more acidity in rainfall from other countries than they produce themselves.1

Acid Rain And Global Warming

Figure 21. Chemical reactions of polluting gases in the atmosphere

Source: The Freshwaters of Scotland (eds Maitland, Boon and McLuskey).

Copyright John Wiley and Sons Ltd. Reproduced with permission

Figure 21. Chemical reactions of polluting gases in the atmosphere

Source: The Freshwaters of Scotland (eds Maitland, Boon and McLuskey).

Copyright John Wiley and Sons Ltd. Reproduced with permission

Much of this acidity originates from industrial areas in the UK. Similarly, Canada is a net importer of polluted air from the Midwest states of the USA and Japan receives acid rain from China.

The amount of acidity is expressed in terms of pH units on a scale of 0-14 where pH 7 is neutral. Values greater than 7 are increasingly alkaline whilst values less than 7 are increasingly acid. The pH scale is logarithmic so that a reduction of pH from 7.0 to 6.0 is a ten-fold increase in acidity.

Rainfall is naturally slightly acidic (about pH 6.0) because it dissolves carbon dioxide from the air to form carbonic acid. If rainfall has a pH of less than 5.6 it is regarded as being polluted with acid gases. Very polluted rain has a pH value of 3.0 or less.

The pH of water is usually measured with a pH meter as described in Chapter 12, but these instruments are a relatively recent invention. However, the Scandinavians claimed at a United Nations conference in 1972 that their rainfall became acidified by pollutants from other countries as early as the 1950s and in turn acidified many of their lakes. They did not have good chemical evidence for this because of the unreliability of pH measurements from that period, but produced other interesting testimony from an examination of their lakes. There are an estimated 15,000 lakes in southern Sweden that have been acidified by acid rain in the past 30 years! The change in pH of lake water over a period of time can be obtained by careful examination of the sediment at the bottom of the lake.2 The sediment contains the skeletal remains of tiny algae called diatoms which, when they were alive, lived by floating in the surface layers of the lake. There are many different types of diatoms and each of them has a different tolerance to the acidity in the water: at a pH of, say, 5.5, the water is suitable for one group of diatoms but if the pH falls to 4.7, it will be too acid for some of them but another group will thrive. The diatoms have a relatively short life, only a few days, and when dead their remains sink slowly to the bottom of the lake. Once there, they get covered with other debris over time. To obtain a record of the change in the acidity of a lake over a long time, scientists obtain cores of sediment from the lake by pushing a long tube into the mud. These cores are then frozen and sliced into segments. One part of the segment is examined carefully for the skeletal remains of the diatoms (Cover Illustration 10) and these are identified to see what type they are and their preferred pH. Another portion of the segment is 'dated' by looking at the isotopes present in it, e.g. by carbon dating. From this examination of the length of the core, any changes in pH over time can be obtained. As an example, Figure 22 shows the pH records of three lakes in different parts of the UK which have been acidified over time. These show that there was a rapid increase in the acidity (reduction in pH) in the period 1950-60 which

Increase Pollution After 1950

Figure 22. pH changes in three UK lakes according to the diatom records

Reproduced by courtesy of ENSIS, University College, London

Figure 22. pH changes in three UK lakes according to the diatom records

Reproduced by courtesy of ENSIS, University College, London coincided with the industrial expansion in western Europe and the construction of large power stations with their high stacks.

Although we have a number of lakes which have been acidified by acid rain in the geologically sensitive areas of the UK, the situation in Scandinavia is much more serious. In southern Norway, half the brown trout population has been lost by acidification in more than 2,800 lakes, whilst in Sweden, of the 85,000 lakes with a surface area greater than 2 hectares, 15,000 are acidified and, of these, 4,500 are fishless. There are two main reasons for this:

1. The prevalence of south-westerly winds which bring polluted rain from the UK, Germany and other countries in that direction.

2. The geology of the country which is mostly of granite, schists and quartzite which are slow weathering and resistant to erosion.

Although the phenomenon of acid rain has been known for some time in the Scandinavian countries and Canada, it has only recently been causing concern in the UK, especially in the northern part of Britain where acid soils and resistant rocks predominate. Figure 23 shows the areas in the UK where the surface water and ground water are susceptible to acidity because of the nature of the geology.

The prevailing wind in the UK is from the south-west which brings in uncontaminated rain from the Atlantic Ocean. Figure 24 shows a typical weather map with a depression centred off north-west Britain bringing in strong, wet south-westerly winds.

On a significant number of occasions, however, the weather is from the south-east because a depression can pass through the southern part of Britain. Figure 25 shows a weather map that produces this sort of weather pattern. The air movement round the depression draws in air from the near continent. Any rain that falls on Scotland is polluted from discharges into the atmosphere from the industrial areas around the English Midlands as well as from further south. An overall measurement of the acidity in rainfall in northern Britain shows that there is a gradient of acidity from high values (low pH) in the south-east and lower values in the north-west (Figure 26).

However when the geology of the area is considered (see Figure 23) the rocks in the south-east are seen to be softer and to have adequate neutralizing capacity to counter the effects of the acid rain. The rivers and streams in this area are not acidic. This is also a relatively dry area so the

Figure 23. Susceptibility of waters to acidification in UK

Source: Journal of the Geological Society 143. The Geological Society, Piccadilly, London.

Reproduced with permission

Figure 23. Susceptibility of waters to acidification in UK

Source: Journal of the Geological Society 143. The Geological Society, Piccadilly, London.

Reproduced with permission

Figure 24. Weather map for Figure 25. Weather map for south-east south-west winds in the UK winds in the UK

amount of rainfall is much less than in other parts of the UK. The problems are most pronounced in the Western Highlands, the Galloway Hills in south-west Scotland, the Lake District and parts of Wales. In all of these areas, the rainfall is higher than average so the loading of acidity (the amount of rainfall x the acidity) is greater. These areas also have resistant geology so any acidity is not neutralized before it enters the streams.

So what are the effects of acid rain on receiving streams? In Scandinavia, Canada and north-east USA, there is clear evidence which links acid rain to the death of fish and also making some lakes unfit for fish life.3 It is a particular problem at the time of the snow melt. Sometimes the snow is acidified by acidic pollutants if the wind is in a certain direction; it also gets discoloured because of soot particles which get trapped in the snow crystals. When this so-called 'black snow' melts, the acid in it rapidly reduces the pH of the melt water and kills the fish and other sensitive aquatic organisms.

In Scotland, north-west England and Wales, the effects are not so dramatic, but surveys carried out by water scientists have shown that, for example, some lochs in the Galloway Hills which were at one time good trout fisheries but are now fishless.4 The main problem with acid rain is not so much the acidity present (as H+) in the water but the excessive

Figure 26. Gradation of rainfall pH across the UK

amount of sulphate ion (SOf)which has been formed from the oxidation of SO2. (The measurement of sulphate is described in Chapter 12.) As the sulphate ion passes through the soil and over the rocks, it needs an 'ion pair' to maintain the rules of chemistry of electro-neutrality. In soils with base cations such as limestone, these ion pairs are calcium and magnesium, but in the areas where the rocks are resistant and the soils are lacking in base cations, the ion pair is aluminium which is present at a high level in rocks such as granite. The aluminium dissolves in the acid drainage and, if it exceeds a particular concentration, it kills fish by damaging the gills and the osmoregulatory mechanism which maintains the correct level of salts in the fish's bodily fluids. The aluminium also affects the larval stages of fish by reducing the action of an enzyme that dissolves the inner lining of the egg wall at hatching time, with the result that some larvae cannot emerge from their egg sacs. The result is that the number of fish fry is reduced and the population of fish slowly decreases. A characteristic of lakes affected by acidity is that the fish population is dominated by a reduced number of large fish, i.e. those that survived the hostile conditions, compared with a healthy lake where there are good representatives of all ages of fish.

It's not only the fish that are adversely affected by the low pH and high concentrations of aluminium. Earlier in this chapter, the impact of acidity on the microscopic diatoms in lakes was described. Another type of aquatic organism that varies in numbers according to the quality of the water, particularly in the flowing waters of rivers, is the invertebrates. They are discussed in more detail in Chapter 11 in relation to the impact of organic pollution on their numbers, but they can also serve as useful indicators of the acidity in rivers and streams.5 One group of invertebrates that are especially sensitive to increases in acidity are those that make a shell, such as the snails and mussels, or an outer protective shell called an exoskeleton. Examples of the latter type are shrimps and water fleas. The

Table 19.

Occurrence of selected invertebrates in acid streams

Mean pH

Group of invertebrates

Species absent

< 7.0

Crustacea

Gammarus pulex

< 6.0

Snails

Lymnaea peregra Ancylus fluviatilis

Mayflies

Baetis muticus Caenis rivulorum

Stoneflies

Perla bipunctata Dinocras cephalotes

Beetles

Esolus parallelepipedus

Caddis flies

Glossoma spp. Philopotamus montanus Hydropsyche instabilis Sericostoma personatum

< 5.5

Mayflies

Baetis rhodani Rhithrogena sp. Ecdyonurus spp. Heptagenia lateralis

Stoneflies

Perlodes microcephala Chloroperla tripunctata

Caddis flies

Hydropsyche pellucidula

reason why these invertebrates cannot survive in acid waters is because the low pH is usually associated with a lack of calcium and this element is an essential component for making a shell or exoskeleton. Shrimps and snails are rarely found in rivers where the pH falls below 5.5. There are other groups of invertebrates that also are affected by acidity to varying extents, such as the different types of mayfly larvae. Some of these can tolerate pH values as low as 4.5 whilst others cannot survive acidity less than pH 5.0 units. Table 19 shows the occurrence of some groups of invertebrates at different pH values.

The technique of collecting invertebrate species is described in Chapter 12, and it is thus possible to assess whether a particular upland river is affected by acidity, not only by the measurement of the pH, but also by examining the invertebrate animals present and seeing whether the absence of a sensitive group indicates that there have been 'pulses' of acidity brought in by acid rain clouds.

The cause and extent of acidification of rivers, lakes and lochs in the UK and elsewhere are now more clearly understood and some action has been taken to reduce the emissions into the atmosphere. At two of Britain's largest power stations the acid gases are neutralized before emission by passing them through fine limestone - so-called 'flue gas desulphurization' (FGD). Any future large power station must include FGD in its design. The amount of sulphur dioxide entering the atmosphere is slowly being reduced, as can be seen in Figure 27.

United Kingdom

Thousand tonnes ^ ,

— Total emissions

1 000 u—1—1—1—1—'—1—1—1—'—1—'—1—'—1—

1980 1982 1984 1986 1988 1990 1992 1994

Figure 27. Reduction in SO2 entering the atmosphere Source: The Digest of Environmental Statistics No 19. 1997. The Environment in Your Pocket. Crown Copyright is reproduced with the permission of the Controller of Her Majesty's

Stationery Office

This reduction is partly because of the neutralization of the gas at the power stations but also because of the switch to fuels with less sulphur in them. Figure 28 shows the relative proportions of fuel used for producing power in the UK each year since 1970 and the reduction in the use of coal and the preference for natural gas and nuclear fuel is clearly seen.

Tumors From Cell Phones

Figure 28. Proportion of fuels used for power generation in the UK,

1970-94.

Source: The Digest of Environmental Statistics No 19. 1997. The Environment in Your Pocket. Crown Copyright is reproduced with the permission of the Controller of Her Majesty's

Stationery Office

Figure 28. Proportion of fuels used for power generation in the UK,

1970-94.

Source: The Digest of Environmental Statistics No 19. 1997. The Environment in Your Pocket. Crown Copyright is reproduced with the permission of the Controller of Her Majesty's

Stationery Office

In contrast to the improvements in the control of acidity from power stations and other industrial sources, the amount of oxides of nitrogen is not declining and may even be slowly increasing. This is because of the steady increase in the number of vehicles on the roads. This is an issue dealt with in the next chapter

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