OVERVIEW OF THE IMPACT OF MINING ON THE ... - IIED pubs

neutralize each other – this situation is often referred to as an “antagonistic” or “counteractive” interaction.
Three typical wastes provide a simple example of this type of situation, namely: acid rock drainage, raw sewage
and power station ash. Individually, these wastes exert serious adverse impacts on aquatic environments; in
combination, however, they neutralize and offset each other so that their combined effect on the aquatic
environment is often innocuous.
In the context of this study, these examples of cumulative impacts are highly relevant as the three river basins
evaluated in this study all contain very large numbers of mining operations as well as a wide variety of industries
and other forms of land use. In combination, therefore, there is a large potential for the impacts of mining
operations to add to, or possibly even counteract, some of the impacts from these other activities.
2.7 The role and importance of natural weathering processes
Natural weathering processes exert a marked influence on soil formation and also on the types and quantities of
materials that enter aquatic systems from the surrounding landscape. Whilst climatic features as well as the parent
rock material, degree of vegetation cover and prevailing landuse, control the specific type and extent of weathering
processes that are active within a given area, weathering zones can be delineated on the basis of the degree to
which chemical or mechanical weathering processes are dominant. A useful index of weathering processes is
provided by a simple relationship between annual precipitation and evaporation (Weinert, 1964). This
dimensionless index, the "Weinert N" value, is calculated as:
12 x EJ
N = ────
PA
Where:
EJ = Mean evaporation (in millimetres) for the month of January, and
PA = Mean annual precipitation (in millimetres).
Where specific monthly evaporation data are scarce, taking 10% of the total annual evaporation can approximate
January evaporation values. The specific types of weathering processes that predominate in regions with different
Weinert N values are shown in Table 2.3.
Table 2.3: Weathering characteristics associated with different Weinert N values.
Weinert Index Value Weathering Characteristics
0.00 – 2.00 Predominantly chemical weathering
2.01 – 4.00 (Wet) Seasonal dominance of chemical weathering
4.01 – 5.00 Approximately equal chemical and physical weathering
> 5.00 Dominance of physical weathering processes
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