Ameliorating acidification: liming
One way of counteracting freshwater acidification is by adding neutralising agents to the water. The primary effects of liming on water quality are to increase the pH, alkalinity, acid neutralising capacity (ANC) and Ca2+, and decreases in toxic metal levels. Effects on N and P less conclusive, with increases, decreases and no change all recorded in various studies. Benefits include increased fish reproduction, increased density of sensitive species and recolonisation of eradicated species, with an overall increase in community complexity.
The dissolution of lime in catchments is dependent on solution pH, particle size, application rate and degree of dilution. Retention time and efficiency depend on the chosen compound and application method. In experiments on peat from northeast Scotland, surface application of CaPO4 was found to be the most efficient of a range of treatments (Sanger et al., 1993). In upland soils, calcium released from the dissolution of lime moves very slowly through surface organic horizons and surface-applied lime can therefore be relatively immobile. In addition, during dry weather or prolonged periods of excess surface runoff, applied lime may be almost completely bypassed.
Liming is often considered the most effective method in upland catchments for amelioration of both soil and surface water acidity and for counteracting Al toxicity (Sanger et al., 1993). Applications have declined over the last 20 years, however, since UK Government subsidies for lime purchase were retracted. Norway and Sweden have adopted large-scale liming as a national strategy (Henrikson and Thornelof, 1995). Powdered CaCO3 is the most common liming agent used in these operations. Liming does not usually restore lakes to pre-acidification levels, either chemically or biologically.
Undesirable effects of liming can include damage to terrestrial ecosystems, where lime is applied to land or whole catchments, particularly on wetlands. Direct liming of British lakes is generally inefficient, because of the short residence time. Although liming catchments rather than watercourses may be more efficient, catchments need up to 100 times as much lime as lake water (Woodin and Skiba, 1990). Targeting specific parts of catchments may be more successful, but in severely affected areas those parts that are targeted, often support blanket bog communities. Here even low levels of liming can cause widespread and long lasting detrimental impacts on Sphagnum species, with indirect effects on invertebrates and mammals (Farmer, 1992).
References:
Farmer A.M. (1992) Catchment Liming and Nature Conservation. Land Use Policy, 9: 8-10.
Henrikson L. Hindar A .Thornelof E. (1995): Freshwater liming. Water Air And Soil Pollution, 85: 131-142.
Sanger L.J. Billett M.F. Cresser M.S. (1993): Assessment By Laboratory Simulation Of Approaches To Amelioration Of Peat Acidification, Environmental Pollution 81: 21-29
Woodin S. Skiba U. (1990): Liming Fails The Acid Test. New Scientist 125(1707): 50-54.
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