The only sure way to prevent acidification of freshwater bodies is to reduce the emissions of acid pollutants in the first place. There is a relationship between sulphur and nitrogen emissions, acid deposition, acids in water run-off and loss of alkalinity. If acidification of freshwaters is to be prevented then the deposition of sulphur and nitrogen needs to be reduced. The technical means are available to reduce emissions, such as flue gas desulphurisation, low nitrogen oxides 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. Liming of water directly, however, causes aluminium and other metals to come out of solution and fall to the bottom of the lake, causing toxicity problems for organisms living on the lake bed. Lime can be added to the catchment, although this 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 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 and plankton increase as does the total production of living matter.
Acidified lakes in Sweden have been restored in the short term by liming. Each year thousands of tonnes of limestone are sprayed on Swedish lakes and watercourses, by means of trucks, boats or helicopters. Liming on such a large scale, however, is expensive, costing million of pounds.
Liming provides only a temporary solution, hence it is far better to attack the source of the problem by reducing emissions of acidifying pollutants.