IMPACT OF EMISSIONS FROM COAL COMBUSTION ON SOILS IN THE MPUMALANGA HIGHVELD REGION, SOUTH AFRICA

Earlier studies of acidic deposition have shown that soils, the major source of alkalinity in freshwaters, must become heavily acidified before the streams and lakes draining them also become acidified. The bulk of this research has been carried out in Europe and North America where the climatic conditions and soils are different from those in South Africa. The Mpumalanga Highveld region is the major coal-producing region in SA and is host to many manufacturing industries, petrochemical industries and coal-fired power stations.

We followed up a monitoring program that was initiated in 1996 with the objective of detecting changes in soil chemistry that may be related to acidic deposition. Soil samples were collected in a number of profiles at depth intervals of 10 cm along a 20km transect downwind of a coal-fired power station. The soils were characterized both physically and chemically in the field and laboratory. Soils in the region are deep, highly buffered, well drained and generally acidic. The average base saturation is 78 %, pH (KCl) ranges between 4.5 and 5.0, clay content ranges between 5 and 20 % and the organic carbon content ranges between 0.4 and 2 %. Saturated paste extracts of the soils showed that SO₄^2- and Ca²⁺ are the dominant anion and cation species, respectively.

The determination of pH, exchangeable acidity and S content of the soils confirmed earlier results indicating that there is no significant gradient in these parameters with distance from the power station and failed to provide any indication of a trend towards increasing acidification and sulfur enrichment with time. Leaching of sulfate, the buffer capacity of the soils and the possible co-deposition of fly ash emissions are suggested as possible explanations for the absence of detectable changes in soil chemical properties with either distance or time.