MPUMALANGA LAKES DISTRICT (also called MLD)
Background:
At present there is a huge demand for coal. More than 270 unique pans and dams exist around Chrissiesmeer. There is a need to mine some of the lakes and/or the immediate vicinity – remove the coal and then rehabilitate the lakes, dams and vicinity afterwards. On the other side we have experts that are predicting a catastrophe.
Introduction to the position and location of the dams and lakes.
Pans are scattered across South Africa stretching from the Northern Cape Province up to the Mpumalanga Province. Our focus is on Chrissiesmeer and the surrounding area with its unusual high density of lakes and its own uniqueness. Chrissiesmeer is the largest lake in this area. There is a remarkable difference between the lakes in the western and Mpumalanga regions. The Western side is drier, have saline deposits, and are large. The pans in Mpumalanga are more or less oval and the drainage system is more or less a close system. In the vicinity of these lakes you will find a number of major rivers such as the Vaal River , the Komati River , the uMpuluzi River and the Usutu River.
Introduction to the geology characteristics of this area.
This location is underlain by rocks of the Ecca Group of the Karoo Supergroup and is formed by the Vryheid Formation. These rocks consist mainly of sandstones – they are huge and consist of very narrow joints. The rock strata (layer of rock) are more or less horizontal. In this area there is no evidence at present that the strata at the lakes are different from the immediate location surrounding the lakes and we have to assume that these lakes are not formed by a geological event.
These pans have bedrock on their western sides and sand on the eastern side. Professor John Wellington which also made a detail study of these lakes concluded that all these pans were linked in the past with an eastward drainage system – probably connected to the ancestral uMpuluzi river system. Head cutting by the Vaal River diverted this water away from the easterly flowing rivers and the drainage system became obsolete. Sand blown from the river beds and slopes then divided the network even further and it isolated the drainage system into a system which we know today. During the ice age which occurred in the Northenhemisphere exceptionally dry circumstances with western winds were experienced in South Africa. It is logically to assume that these have caused a reduction in the vegetation at that time.
Hydrological functioning of the pans
The pans are fed by water in three different ways. In the form of surface water, suface run off water and water that will seep into the soil and will form ground water which seeps into the lake .
The rock layer underneath the pan and the surrounding area consist of mainly sand stone on top of the coal layers. This rock is a low porosity rock with very narrow joints. Due to the limited storage capacity for this water it will transmit water slowly. The water will collect in the soil (the regolith ) and in the cracks of the bedrocks.
One of the features of these lakes is that the soil contains amounts of iron oxide called ferricrete which acts as a barrier to the downward flow of the groundwater. The Regolith layer is also thicker higher up on the slope and thinner on the slopes nearby the lake itself. Ground water will flow downward towards the pan due to the forces of gravity. This process is slow and the thinning of the regolith closer to the lake will cause groundwater seepage at this lower level. This process explains the hill slope effect that surrounds the pans in this area. The water that seeped out will slowly flow across the wetland towards the pan. There could be some leaking between the lakes which could explain the difference in saline concentrations.
On the output side water evaporation and transpiration of the plants are the only known sources of water exiting the dams. Exception such as heavy rain could cause a spill over effect with some of these lakes. The rate of water loss is more than the rate of filling and this should lead to a progressive increase in the concentration of salts but with these lakes it is not the norm.
Rainwater and the surface run off water have very low dissolved solid contents. The regolith is very sandy and is highly leached and contains relatively few minerals which can interact with the water. The flow of the ground water and the interaction between rocks and soils is relatively low. The ground water contains little solids and is in the region of about 130 parts per million. Ground water emerging against the slope and that runs into the lake add very little dissolved solids. Most of the solids inside the pan are generated by this process.
Environmental impacts and aspects, threats and countermeasures
Professor Frank Hodgson was involved in a study to determine the effect of opencast coal mining. The result of his studies can be used with the exception of one difference - the lakes are a closed system and the river he studied is an open system with a dilution effect.
Most of the coal rock contains the mineral called pyrite. Pyrite is a combination of iron and sulphur. In this area the pyrite is trapped within the sand stone strata and is stabilized due to the fact that water and air (oxygen) are excluded in a large degree. In the MLD the sulphuric acid will be produced as a normal process as the rock weathers but one must remember that the strata is dense with very narrow joints and water ingression is slow. The chemical reaction that is producing the sulphuric acid is slow enough for the groundwater to dilute it and to wash it away.
During mining this sandstone layer is removed in order to get to the coal. The rock mass which isolated the pyrite will now be broken up in pieces. During rehabilitation the open pit area will be filled with this rock mass. Rain can now enters this rock mass easily and it can accelerate the formation of sulphuric acid and iron sulphate. This is reaction process number one. Eventually this contaminated water will reach the surface and then the second process will occur. At the surface there is a lot of oxygen and red oxide will now be produced and even more sulphuric acid. The surface water will now change into a red to orange colour which is typical of sites where Acid mine drainage effects is experienced.
According to the research of Hodgson, leaching will occur and metals could be released such as manganese, copper, zinc, cobalt and cadmium. Some rocks (calcite and dolomite) can neutralize the sulphuric acid but this will cause the effect that the water is now polluted with calcium and magnesium sulphates. In our case the balance between high pyrite content and a small amount of rocks that can have a neutralizing effect is not effective.
The research of Hodgson proved that the sulphuric acid can flow into the disturbed sandstone displacing the fresh groundwater. Once the voids are filled the water will emerge on the land surface containing extremely high levels of sulphuric acids and sulphates at a figure of between 2000 and 3000 parts per million of dissolved sulphates. We use the SABS 241 standard as the standard for potable water. If one refers to this standard you will note that the maximum allowable is 200 parts per million. The sulphate at the surface will create more acid and the formation of red oxide. The typical PH of this water is in the region of 2.5. Most plants and aquatic life will cease to exist.
To control this effect clay layers could be used in the backfill. The clay layers can not seal the pyrite but can only delay the negative action. The lakes are isolated and any sulphates entering them will stay there. Any pyrite that is disturbed will continue to produce sulphuric acid for many a century. Groundwater will become unpotable. In the long term these unique lakes will be destroyed if they are mined with contamination at the source of 5 major rivers.
To log this cache email the cache owner with the answers to the following questions. Please do not include any answers in your log. Also please indicate your personal opinion in your log about the mining of this area. “Should we mine this location and forgot about the uniqueness, the bird and aquatic life or should we preserve the area?”
Questions:
Q1: Explain the term pyrite and the potential impact of this mineral on the lakes if not controlled properly?
Q2: What reaction will occur if the sulphate from the pyrite in the ground water is seeping onto the surface?
Q3: What does the word regolith indicate?
Q4: Explain the term ferricrete and its function?
Other lakes worth visiting
S26 17.420 E30 13.063 (Chrissiesmeer)
S26 12.642 E30 11.516 (Te Vredenpan and a generator of PEAT, first phase of forming coal)
S26 21.455 E30 14.889 (Another good example)
If you want you can follow the road from Chrissiesmeer, turn left when you reach the tar road and then left on the next gravel road back to Chrissiesmeer.
Precautions:
· Most of the lakes can be viewed from safe public gravel roads and therefore please do not enter any private property.
· If there is any lightening in this area please stay in your vehicle.
· Also keep an eye open for any potholes.
Intellectual rights belong to:
Professor Terence McCarthy, School of Geosciences , University of the Witwatersrand, Johannesburg
Professor Bruce Cairncross, Department of Geology, University of Johannesburg
Professor Jan-Marten Huizenga, Department of Geology, University of Johannesburg
Allan Batchelor 3 Wetland Consulting Services (Pty) Ltd.
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