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

Wilge River drains a relatively small area to the east of Pretoria and, primarily due to the continual release of
water from the dolomite and limestone formations, the streams and rivers are all perennial. Minor discharges of
treated domestic effluent from Bronkhorstspruit and Cullinan supplement flows in the Wilge River.
The sub-catchment contains one relatively large water supply dam (Bronkhorstspruit Dam), which supplies
domestic and industrial water to the town of Bronkhorstspruit, as well as irrigation water to the extensive areas
of irrigation downstream of the dam. In addition, the sub-catchment has numerous small farm dams that are
used to trap runoff in seasonal channels and supply this water for livestock watering.
There are also a very large number of small wetlands located along almost every stream and river in this subcatchment
(Marneweck et al., 2001). These are formed at the uphill side of protruding dolerite formations
(typically dykes and sills) that dip gently to the south. These dolerite formations act as impermeable barriers
and water collects on their uphill sides, saturating the soils. The downstream end of each wetland is usually
marked by a dolerite exposure where the water reaches the ground surface and flows over the dolerite barrier.
This system of inter-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 subcatchment.
5.2.1.2 Geology
In the upper reaches of the Wilge River, the geological features are characterized by the presence of extensive
dolomite and limestone formations surrounding and a small area of Karoo Supergroup rocks consisting of
consolidated layers of carbon-rich shales and intercalated arenaceous strata. Further downstream, centering
on the Bronkhorstspruit Dam, the sub-catchment is underlain by large areas of porous unconsolidated and
semi-consolidated sedimentary strata, principally quartzitic sandstones, and an area of Dwyka tillite. Close to
the confluence of the Wilge and Olifants rivers, a small area of acidic and intermediate intrusive granites of the
Waterberg Group underlies the northern portion of this sub-catchment.
The dolomite and limestone formations provide excellent aquifer characteristics and overly thick layers of
carbon-rich dolerites and shales of the Karoo Supergroup.
5.2.1.3 Pedology, agriculture and land use
Soils in the sub-catchment can be divided into five main groups:
• Moderate to deep sandy loam soils on flat and undulating terrain overlying dolomite and limestone in the
upper reaches of the catchment;
• Moderate to deep sandy loam soils lining long stretches of the Wilge River valley in its middle reaches;
• Moderate to deep clay loam soils over much of the middle portions of the sub-catchment (located away from
the river channels), overlying the more porous unconsolidated sedimentary material; and
• Moderately shallow to moderately deep, clayey loam to clay-rich, fine-grained soils over most of the lower
reaches of the sub-catchment; and
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