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Freshwater Acidification

Introduction

Acidification of freshwaters was a problem that was first identified in Scandinavia during the early 1970s, at which time many scientific studies were initiated. Since then the concerns that were voiced have been justified, and now thousands of lakes and rivers are known to be acidified. Areas that are most susceptible to acidification have an unreactive geology such as granite and a base-poor soil. Areas that are affected by acidification include Scandinavia, Central Europe, Scotland, Canada, and the United States. Lakes and streams that are generally regarded as acidified are very nutrient poor waters draining unreactive geology.

Ample evidence from chemical and biological studies of typical lakes prove that increased acidification has taken place. Diatom shells from lake sediments have allowed the course of acidification to be charted back through time. Diatoms are microscopic algae which live free floating in the water or attached to surfaces. They have hard shells of silica which are characteristic of each species. Diatoms are very sensitive to acidity, and their occurrence and proportions give good indications of pH levels. Evidence suggests that rapid acidification has been taking place at some sites for at least 100 years and is still occurring today.

Process

Acid rain can enter the water course either directly or more usually through the catchment. If the catchment has a thin, base-poor soil then acid water is passed to the lake. If the catchment has alkaline-rich soil then the acid rain is neutralised and so water entering the lake is of low acidity. In areas where a continual supply of base (alkali) cations is not assured then the gradual depletion of the bicarbonate in the lake means that the once stable pH will drop rapidly resulting in an acidified lake. Acidification can also occur in surges after snowmelt or drought; the first 30% of snow melt can contain 50 - 80% of the total acids in the snow. During drought conditions sulphur dioxide (SO₂) deposition onto the soil is reduced to sulphur and hydrogen; this is then re-oxidised in combination with rainwater to form acids. This is termed an acid pulse.

Effects

The effects of freshwater acidification are as follows.

• Carbon source changes from carbonate (HCO₃) to carbon dioxide (CO₂).

• Release of toxic metals.

• Phosphorous is retained.

• Freshwater fauna and flora gradually changes.

• Short-term pH depressions have direct toxic effects on susceptible organisms.

The onset of acidification brings about a clearer bluer water body due to the precipitation of humic substances. Whilst total biomass remains largely unchanged, the diversity drops considerably. Many algal species disappear, but some green filamentous algae are capable of mass proliferation in the extreme environment. The number of macrophytes in and around the water decreases, with Rush becoming the dominant species. White Sphagnum moss may invade lakes and form a thick green carpet over the bottom of the lake on account of the clearer waters allowing more light to reach the moss.

Soft bodied animals such as leeches, snails and crayfish are early victims, often being one of the first signs of the commencement of acidification. Few insect species are very resistant to acidification and species such as mayfly disappear even under moderate acidification. However, species such as dragonfly larvae, water beetle and bloodworms can grow abnormally large in their population size when competition is removed. Salmon, trout and roach are particularly at risk from freshwater acidification, pike and eel being relatively resistant. All life stages of fish are affected; the reproductive ability of adults, the survival of eggs and young fry. The death of adults at a low pH is caused primarily by the release of toxic metals such as aluminium. At pH 5 aluminium is at its most poisonous, being precipitated onto the gills of the fish in the form of aluminium hydroxide.

Causes

Acidification takes place most readily in areas where the natural geology is slightly acidic. Upland regions that have been subject to land-use changes over the last few decades are showing the signs of acidification.

Several factors affect acidity:

1. Action of atmospheric carbonic acids.
2. Formation of organic acids by humus podzolisation.
3. Podzolisation.

4. Livestock introduction into the catchment.
5. Use of nitrogen fertiliser.
6. Increased efficiency of drainage.
7. Dry deposition of air pollutants.
8. Wet deposition of sulphuric and nitric acids.

It will be a combination of the above factors that will lead to freshwater acidification. Natural acidification has been taking place since the last ice age, although the recent rapid acidification of many of the lakes can not be attributed to natural causes.

Restoration of Acid Waters

The only sure way to prevent further acidification of other susceptible water bodies is to reduce the emissions of acid pollutants. There is a relationship between sulphur emissions, deposition, sulphur in run-off and loss of alkalinity. If acidification of soils and freshwaters is to be prevented then sulphur deposition rates need to be reduced further. The technical means are available to reduce emissions, such as flue gas desulphurisation, low NOx burners, use of low sulphur coal and oil and increasing energy efficiency. At present the main way of reversing acidification in freshwaters is liming the water body or its surrounding catchment. The main liming method is to add the lime directly to the water body. However in the cases of certain lakes where the turnaround is very quick, the lime is added to the catchment. This has disadvantages though, the main one being that the lime can have an adverse effect on wetland species of plants. The advantages, however, are that the effects are longer lasting and metals are prevented from leaching into the lake water from the soil. The effects of liming are almost entirely favourable within the lake. The alkalinity of the limed lake is increased, the pH increased and heavy metal concentrations decrease back to within safe limits for fish life. The number of species of fish, benthic animals and plankton increases as does biomass production.

Summary

Freshwater acidification occurs in areas of high sulphur deposition and where soils are derived from granite or other rocks resistant to weathering. The magnitude of acidification in the last 150 years is a lot greater that in the last 100,000 years. It seems that atmospheric pollution is the major cause of acidification. There is evidence that in the past decade there has been a significant decrease in the acidity of rain and this is reflected by a small decline in acidity of some lakes.