Sulphur dioxide and nitrogen oxides emitted into the air as industrial wastes can travel thousands of kilometres before they are returned to earth as acid rain. Even remote areas of Scandinavia are affected in this way. Lime in the form of powdered limestone is deposited into affected rivers or spread by helicopter. Limestone, or calcium carbonate, is alkaline and counteracts the acidity. It is also applied the raise the pH of soils with excessive sodium.
Sweden has the world's most comprehensive liming programme. It spends around $24 million a year to treat 5,000 of its 85,000 lakes and hundreds of rivers. Norway, in a similar position to Sweden, has nonetheless decided against large-scale liming because it would divert political attention from the cure - reducing air pollution at its source. Many countries in Europe are also cautious about using lime because of the environmental damage it can cause. Spreading it on bogs around lakes or rivers kills mosses and lichens that act as sponges. By destroying the bogs, sudden floods can occur. The dilemma of whether or not to lime is particularly acute in Wales where acid rainfall is unpredictable and one third of the fallout occurs on less than 5% of wet days. Dosing rivers with the right amount of lime at the right time is extremely difficult, and the risk of overdosing very real.