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

connected wetlands provides an extremely important attenuation mechanism that ensures water is released
throughout the year, thereby maintaining perennial stream and river flows in this sub-catchment. However,
mining activities (blasting, ground clearing, overburden stripping, acid mine drainage) have damaged or broken
several of these dolerite formations in recent years. This has resulted in increased summer flows, reduced
attenuation and therefore lower winter flows, and a steady decline in water quality in downstream reaches
(Toerien et al., 1980; Johnson & Du Toit, 1997; Hodgson & Krantz, 1999; Marneweck et al., 2001).
5.3.1.2 Geology
In the upper reaches of the Little Olifants, Riet and Olifants rivers, the geological features consist almost entirely
of rocks of the Ecca Group and Dwyka Formation of the Karoo Supergroup. The Ecca Group rocks consist of
consolidated layers of silicified sandstone, shale and coal, together with interbedded mudstones, siltstones and
shales. The important Witbank and Highveld coal deposits located near to the town of Witbank form the centre
of this group. Rocks of the Dwyka Formation, principally tillite, shale and siltstones, underlie the Ecca Group
rocks. Extensive dolerite dykes and sills have intruded both of these formations.
The northern part of this sub-catchment is underlain by acid and intermediate intrusive formations of the
Waterberg Group, as well as by mafic and ultramafic intrusive formations. Small areas of dolomite and
limestone, as well as silicified sandstone are also found in the northern reaches of this sub-catchment.
5.3.1.3 Pedology, agriculture and land use
Soils in the sub-catchment can be divided into five main groups:
• Shallow to moderately deep sandy-clay loam soils on flat and undulating terrain overlying rocks of the Ecca
Group, principally shales and silicified sandstones, in the upper reaches of the catchment;
• Moderate to deep sandy loam soils lining long stretches of the Olifants River valley in its middle reaches;
• Moderate to deep clay loam soils over much of the lower portions of the sub-catchment (located away from
the river channels), overlying the more porous unconsolidated sedimentary material;
• Moderately shallow to moderately deep, clayey loam to clay-rich, fine-grained soils over granitic areas in the
lower reaches of the sub-catchment; and
• Dark grey to blackish, mottled vertisols located along low-gradient reaches of streams and river sections in
the upper reaches of the sub-catchment, forming characteristic wetland soils.
Most of the soils are highly suitable for commercial agriculture when sufficient water is available. Virtually all of
the areas with suitable soils, particularly the area downstream of the Loskop Dam, are contained within the
jurisdiction of formal irrigation boards or Government Water Control Areas. Further away from the main river
channels, and downstream of the coal mining operations in the upper parts of the sub-catchment, land use is
given over to small- and medium-scale livestock farming operations. A relative small variety of crops are
produced on the irrigated and rain-fed areas, primarily maize, lucerne, potatoes and sunflowers.
ccxcvi