HYPOTHESISING THE HIGHLY FRACTURED DEFUSE SOUTH AFRICAN TRANSVAAL AQUIFER
(SATVLA) 

MIKE BUCHANAN (2005)  

 Hampshire, United Kingdom.

IUCN WCPA Task Force on cave and Karst.

Cave Research Organisation of South Africa - CROSA

1. INTRODUCTION

In this document I explore the probability of a vast undermanaged 
subterranean cross border, sedimentary karst hydrological system . This will be referred to as The South African Transvaal Aquifer (SATVLA) for the purpose of this document. The naming will be subject to governmental influence due to the nature and size of the SATVLA upon acceptance.

This hypothesis can be considered the first ever attempt at describing the karstic system which makes up the subterranean hydrology of the Limpopo River catchment and drainage basin.

The SATVLA can be construed as one of the world’s largest subterranean sedimentary karst aquifers, after the Edwards Aquifer in the United States of America. This system has many notable karst basin features. Including a conforming sedimentary periphery, inwardly directing geologic strike angle towards the central basin. The slope angle of the karst varies between ten and fifteen degrees - A well defined, buried karst basin around and beneath The BIC. Inclusive of a well-developed, deep-seated hot spring network within the central basin is also evident (Bella Bella, Grobelarsdal etc lowest basin point amsl).

The SATVLA hosts the world’s oldest Neoarchaen dolomitic geology, including a one of a kind complete, magmafic, lopolithic infill of economic importance called the Bushveld Igneous Complex (The BIC).

“The Neoarchaean-Palaeoproterozoic Transvaal Supergroup geology was laid down during the Archaean geologic era which spans the period of time 2800 to 2500 million years ago; the period being defined chronometrically and not referenced to a specific rock section level. Oxygenic photosynthesis first evolved in this era and was accountable for the oxygen catastrophe which was to happen later in the paleoproterozoic era 2.4 mya from a poisonous buildup of oxygen in the atmosphere, produced by these oxygen producing photoautotrophs, which evolved earlier in the neoarchean era.” (1.Wikipedia encyclopedia).

From this, the stratigraphic subdivisions within the Transvaal Supergroup have been delineated. They are consolidated into five distinct groups, some containing various stromatolitic and oolitic assemblages throughout the sedimentary sequence. The lowest non sedimentary layer (Chuniespoort - Black Reef Quartzites) containing no known fossil remnants, stromatolites or oolites.The delineated sedimentary stratigraphy within the Malmani Dolomite Subgroup are named - the Oaktree, Monte Christo, Lyttelton, Eccles, and Frisco Formations

2. Description and history

The SATVLA is undoubtedly one of the worlds largest subterranean groundwater bearing dolomitic karst aquifers. Consisting of a multitude of separate intercommunicating porous sub systems via fractures and interbedded chert. All located in one dynamic mega groundwater system with a shallow stratigraphic strike angle. Thereby, low flow hydrologic gradient. This high altitude, highly fractured, diffuse groundwater karst system consists of an average conformity of 1500 - +- 3000 meters thick and is calcium magnesium carbonate or dolomite with blatant surficial karstification (CaMg(CO3)2). The dolomite incorporates stratified, interbedded silicate chert (fossiliferous interbedded chert) which provides a well defined base for dolomite karst land-form development.

The interbedded chert (Mohs scale 6-7*), has a higher density and Mohs hardness rating code than the dolomite (3-4). Silicate Chert is considered as a biologic stromatolitic remnant of the Archean development of this once inland sea, lake or water body. No one is quite sure how dolomite came to being. Many speculate that it was either laid down as dolomite due to the acidic constituency of the atmosphere many millions of years ago or the dolomite was once limestone that had a unique transformation which gave it the magnesium carbonate either by volcanic super-heating or meteoric impact. All concepts will be discussed latter.

*The Mohs scale was devised by Friedrich Mohs in 1812 and has been a valuable aid to identifying minerals ever since. (2. Andrew Aldern Geologist).

Karst aquifers are vitally important because they provide fresh potable water (this being a small part of the systems production). Twenty five percent of the earth’s human population depends on karst systems for drinking water. They are also seen as sites of natural habitat for numerous specialist feeders. Like bats, which remove vast quantities of invertebrates from these environments in their diet. Aquifers and their vulnerability to contamination by pollutants are reasons to seek conservation and an improved understanding of how karst systems work.

The primary reason for this hypothesis, is to explore the disparity between current anthropogenic interactions within this under managed resource. This is seen to be one of the largest threats to the under managed subterranean karst aquifer system. It is common knowledge internationally that the BIC is the world’s most economically viable geology, providing a large percentage of the world’s quality platinum and palladium as well as many other valuable heavy metals. Some of the most advanced deep mining technology is used to recover gold from it's contact periphery. This history is one of the prime reasons for a conflict of this nature to exist, challenging the balance between mineral resource over-exploitation and environmental sustainability. Gold was discovered in Johannesburg, South Africa in the late 1800’s and its demand grew rapidly because of mining technological advancements and the vast availability of valuable gold deposits around Johannesburg. Most major cities around the world evolved along an economically navigable river or coastal plain, Johannesburg has none of them. This city was founded and established because of its gold. It is 5500 ft above mean sea level (amsl). This phenomenon has a vast bearing on the understanding of complications facing the SATVLA which will be discussed latter.

The SATVLA is detailed in Council for Geosciences geological maps from as early as the beginning of the 1900’s. This was merely indicated as a dolomite belt. The nature of which was not quite understood at this stage. This belt was detailed as a surface exposed geology that happened to be on the contact of Johannesburg’s (JHB) viable granite dome which is the source of all the gold, Uranium, Lead (Pb), Molybdenum, zinc etc. More recently the sediments are well documented and defined throughout and under the BIC. The sedimentary dolomite belt border extends from Mogale City (was Krugersdorp) in the south and follows an oval pattern toward the northwest, toward the mining town of Ventersdorp , broaching the Botswana border. It takes a turn to head back out of Botswana in a northeasterly direction towards Thabazimbi, on towards Mokopane (Potgietersrus) Northwest Province of South Africa. It narrows superficially and creates part of the great South African, Drakensberg Escarpment heading down towards the Mozambique border, skirting the border by a few hundred kms. Then doubles back to the Johannesburg area culminating at the now world famous Cradle of Humankind World Heritage Site, Sterkfontein caves and surrounds (CHK WHS). The SATVLA karst lends itself to the creation of the CHK WHS, due the nature of the sedimentary karstic processes that were laid down some 2300 million years ago.

Some speculated that three to four billion years ago a vast inland sea had developed, possibly prior to the development of Gondwanaland Land, when the world had only one super continent. This inland sea bore testament to the earliest evolutionary phases of macro life and possibly some of the first living organisms that were starting to adorn our earth; Prokaryotic life, cyanobacteria or blue green algae grew into what we know as stromatolites. These biologic fossils can be witnessed in the stromatolitic ensemble that makes up the Transvaal Supergroup sedimentary succession today.

As volcanic activity and the natural cooling processes were settling down after earth’s evolution, the earth had a rapidly changing atmosphere which was much more acidic than today. Due to volcanic action and cooling processes the atmosphere would have been sulphurous and full of dust and ash from the changing earth. We can see this happened in the content of the most basal layer of the SATVLA. It is merely a thick layer of sediment that has no visible life forms encapsulated it (Nanobacterium cannot be ruled out). As atmospheric dusts settled on the inland sea, it sank to the sea floor and overtime grew to many hundreds of meters thick, encapsulating some of the most important events that occurred during our earth's evolutionary processes. This is well conveyed in the book written by Mc Carthy and Rubidge called The story of Earth & Life. ISBN: 1 77007 148 2.

The SATVLA is believed to have developed around two thousand three hundred million years ago as an inland sea prior to the Gondwanaland split. This hydrologically sound system was upset some 1300 million years ago when great intercontinental tectonic movement caused  volcanic, or a succession of volcanic events to take place, which in turn caused a warping or uplifting of this sedimentary sea. This changed the nature of the sea into geology that no longer was filled in by water but which was replaced by molten magma. Geologists have identified a number of different events that helped this process along, hence the variable economically rich mineral resources that are found within the BIC. The BIC filled in the SATVLA Sea basin remnant and was primarily responsible for eluding many geologic/hydrologic authors for centuries.

The size of the SATVLA is still not quite understood. However if one ponders the possibility that the area depicted on the Geosciences map is roughly 500 km by 250 km without incorporating other extra periphery based dolomitic land-forms that could have been, or are hydrologically connected and communicating, the SATVLA size could jump considerably, i.e. The Ghaap Plateau certainly, possibly and not ruling out the Namib karsts or western Botswana karst! However, for this document I shall focus on the sediments that have now been irrevocably buried by the BIC.

The surface fluvial catchment area has historically been recorded as the Limpopo basin.

3. TIMELINES - According To Geological History

1) TVL SEQUENCE laid down 2,300 million years ago. The Cango and UK limestone’s are +- 200 million years old with substantial conduit cavern development I.E. karstification. A clear indication that the SATVLA had well defined karstic and conduit formation prior to the BIC formation.

2) BIC formation started 2,054 billion years ago

3) Vredefort ASTROBLEME 2,023 million years (huge fracturing took place at this time)

4) Pilanesberg Volcano/s 1 300 million-year-old

5) The Tswaing/Soutpan Meteor Crater 200 000 years ago (again, huge fracturing of the karst and associated dykes would have prevailed,responsible for the mammoth collapse of all Pretoria region caves. The reason why they all demonstrate substantial instability and collapse)

6) Gondwana split 100 million years ago, possibly responsible for the uplifting of the southern side of the SATVLA

4.  Limpopo Basin profile

Statistics and background information: -
• Catchment area: Around 413,000 km²
• Rainfall: Average 530 mm per annum. Range: 200-1,200 mm
• Evaporation: Average. 1,970 mm per annum. Range: 800-2,400 mm per annum)
• Runoff: 5.5 x 109 m³ per annum or 13 mm per annum
• Water transfers: Water is transferred into the basin under 6 separate transfer schemes
• Population: 14 million
(Information provided by the Department of Agriculture, South Africa)

The Limpopo River Basin

“The Limpopo River flows over a total distance of 1,750 kilometers  It starts at the confluence of the Marico and Crocodile rivers in South Africa and flows northwest of Pretoria. It is joined by the Notwane River flowing from Botswana, and then forms the border between Botswana and South Africa and flows in a northeasterly direction. At the confluence of the Shashe river, which flows in from Zimbabwe and Botswana, the Limpopo turns almost due east and forms the border between Zimbabwe and South Africa before entering Mozambique at Pafuri. For the next 561 km the river flows entirely within Mozambique (the Limpopo flood plain) and enters the Indian Ocean about 60 km downstream of the town of Xai-Xai. The Limpopo river basin is almost circular in shape with a mean altitude of 840 m above sea level. It lies between latitudes 22°S - 26°S and longitudes 26°E - 35°E. The total surface area drained by the basin is estimated at about 415,000 sq km.”

“The Limpopo basin covers almost 14 percent of the total area of its four riparian states - Botswana, South Africa, Zimbabwe and Mozambique, and, of the basin’s total area, 44 percent is occupied by South Africa, 21 percent by Mozambique, almost 20 percent by Botswana and 16 percent by Zimbabwe.”

“In the southern (South African) portion of the basin, the Bushveld Igneous Complex forms an extremely important geological feature, and contains a very large proportion of the region’s mineral wealth. The geological features of this area consist mostly of basic mafic and ultramafic intrusive rocks, accompanied by extensive areas of acidic and intermediate intrusive rocks. At the southern and eastern periphery of this area, large dolomite and limestone formations occur, accompanied by extensive mineralization along their contact zones.” (4.Information from the Southern African Research and Documentation Centre)

What is not mentioned in virtually all publications is that the BIC covered the SATVLA’s sediments some 2,054 million years ago. This happened as a result of various volcanic eruptions and magmafic oozing taking place over a 750 million year period. The molten magma slowly filling in the TVL basin, thus obscuring the (karst) groundwater resource potential for many years to come.

5. Bushveld Complex

“The Bushveld Complex is largely igneous in origin and occurs in the northeastern region of the Province, from Brits and Rustenburg in the east to north of Zeerust and Swartruggens into Botswana. Of the three suites making up the Complex, only the peripheral mafic intrusive phase occurs in the North West Province. This consists of four main layers: the upper zone of gabbro, olivine, diorite to grandiorite with some anorthosites and magnetites. The main zone of 5 200m which consists mostly of gabbros, which gives rise to topographic features such as hills. Below this is the critical zone consisting of norites, anorhosites, pyroxenites and chromites, below which is the basal zone mostly of pyroxenite and peridotite. Platinum and chromite from this zone is mined extensively in the Rustenburg / Brits region. “

The Bushveld Igneous Complex (BIC)

“The Bushveld Igneous Complex (BIC) is the world's largest known layered intrusion and has an estimated area extent of 182,000 km2 .The BIC is an enormous, champagne glass-shaped body 370 kilometers across, with its center buried deep beneath younger rocks but with its rim exposed. It is composed of a series of distinct layers, three of which contain economic concentrations of platinum group elements (PGE). The principal PGE-bearing reefs are the Merensky Reef and the Upper Group 2 (UG2) Reef, which occur around the Eastern and Western sides ("limbs") of the BIC. A third PGE-rich layer, the Platreef, is found only on the Potgietersrus limb at the north-eastern edge. “

“The BIC contains an ultramafic to mafic unit (the Layered Series), up to 9 km thick, which outcrops as eastern, western and northern lobes surrounding a felsic core of largely granitic rocks. The Merensky Reef, which is the best known and most commonly exploited platiniferous horizon in the complex, can be traced for at least 240 km along strike and is estimated to contain 60 000 t of platinum group metals in its upper 1 200 m, as well as significant resources of cobalt, copper and nickel. The pyroxenitic Platreef horizon, north of Potgietersrus, is a wide zone containing PGE mineralisation, along with nickel and copper. Of major economic importance is the UG2 (Upper Group 2) chromitite horizon that is being increasingly exploited for its PGEs, particularly in the eastern lobe of the Bushveld Complex. This represents an even larger resource of PGEs than the Merensky Reef. The BIC also contains almost 70% of the world's reserves of chromite as well as significant resources of vanadium.” (North West Dept. Agriculture, Conservation and Environment, Mafikeng)

The entire BIC is underlain by Archean sediments which host many billions of Terra liters of subterranean  groundwater, some geothermal by nature. The slow moving young groundwater finds its way through the documented Transvaal dolomite sediments from its exposed periphery. Following the the ten degree strike angle circumferentially.  Flowing towards the deepest most central points, where this low flow emanates in the form of highly valuable fossiliferous groundwater, older than 20 000 years in age (J. Talma CSIR - Warmbaths/Bella-Bella). Which is being exposed to intensive, irresponsible Government sanctioned anthropogenic interactions, in the form of unsustainable, deep heavy metal mining, within and around the BIC. Inclusive of inputs from industrial and agricultural pollutants without authoritative knowledge backed check. All as a result of past ignorance around the true nature of this vast low flow hydrology. "The Compartmentalisation Theory".

There was never tight confined compartmentalisation as described and  documented historically by many notable Key Opinion Leaders and geohydrological authors. Simply, a vast volume of groundwater moving slowly in terms of geologic time and permeability varying exponentially at depth. Within one vast, highly fractured, defuse, mega Archean - Proterozoic Karstic Groundwater System that requires immediate remediative intervention without further delays.

THE SOUTH AFRICAN TVL AQUIFER and B.I.C.  

Click on picture for large image

With associated impacting events. A modified Council for Geosciences map with insert by Buchanan

References

1. Wikipedia encyclopedia - http://en.wikipedia.org/wiki/Transvaal_Basin

2. Andrew Aldern Geologist - pachamamatrust.org/f2/1_K/SGl_geology/Aa_INTRO_KSGl.htm‎

3. Mc Carthy and Rubidge The story of Earth & Life. 2005 - ISBN: 1 77007 148 2.

4. The Limpopo River Basin - www.sardc.net/imercsa/Limpopo/objective.htm

5. The Bushveld Igneous Complex (BIC) State of environment report 2002         - www.nwpg.gov.za/soer/FullReport/industrial.html

Click on the image above for a larger picture.

A Holistic Approach to Water Management of The Cradle for Humankind World Heritage Site.

2005

J. Groenewald(1), M. Adlem(2), M. Buchanan(3) and G. Eloff(4)
(1) Council for Geoscience, Private bag X112, Pretoria South Africa.
(2) Council for Geoscience, Private bag X112, Pretoria South Africa.
(3) P.O.Box 8947, Cinda Park, Boksburg.
(4) Gauteng Department of Agriculture, Conservation & Environment, PO Box 8769, Johannesburg.

Note:- This paper was extracted from a poster presented the International Association of Hydrology, Slovenia, 2005

Abstract

South Africa is host to a large succession of sedimentary carbonate rocks within the geological column. They range from Swazian (>3090Ma) to quaternary in age and are variably composed from pure calcium carbonate (limestone) to 46 percent magnesium carbonate (dolomite). The carbonate rocks of interest to this project is the impure limestone or dolomite rocks of the Malmani Subgroup (Fig. 1) which forms part of the Transvaal Sequence of sedimentary and volcanic rocks that was deposited in a generally shallow marine environment. This sequence was only moderately affected by tectonism. The Malmani Subgroup is divided into five formations on the basis of its general lithology, however the two formations in which karstification took place most frequently is the Oaktree and Monte Christo formations which forms the 900 m base of the Malmani Subgroup. These formations are playing host in recent times to the proclaimed World Heritage Site (WHS) for the Cradle of Humankind (COH), where the most productive and important palaeoanthropological foundings were made over the last sixty years.
Mining and industrial activities have caused pollution in the surface and subsurface hydraulic regimes over the last thirty years and has raised alarm to the authorities managing the World Heritage Site.
A project initiated by the Gauteng Department of Agriculture, Conservation and Environment (GDACE) is currently investigating the quality of the water interacting with these very important cave systems. Although this has only recently been finalized, work has already started to establish a baseline database on current water quality by the Council for Geoscience. This is only part of a program that will deal with managing the water for future sustainability not only by consumers and tourists in the area but also for the fauna and flora and the valuable cave systems. The following is merely a presentation of findings to date and proposed methodology for a holistic approach.



Introduction


At the request of the local environmental authority for the COH, Gauteng Department of Agriculture, Conservation and Environment (GDACE), a project was launched to compile a water management strategy and program to ensure the sustainability of the WHS. The main objective of this strategy will be to monitor, manage and remediate any damaging influence from cultural activities.
The Cradle for Humankind is an approximate 51 000 Ha (figure 2) area lying to the North West of Mogale City (Krugersdorp), spanning across provincial borders of Gauteng in the south and North West province to the north. The protection of this area is obvious for several reasons including most importantly, amongst others, conservation and tourism. Due to closing of mines to the south and obvious job losses occurring this area could yield several job opportunities in future balancing the equation. The mere abundance of pristine fauna and flora along with billion year old geological formations and fossils, most certainly warrants protection of this area for our future generations.
The land use in and adjacent to the area includes conservation, tourism, agriculture, small industries and mining. The most significant polluters from these are the mines and agriculture with the smaller industries playing a lesser role. Typically pollutants from these activity includes high salt loads (SO4-, Na+, Cl-, PO4-), heavy metals, and possibly some organic contaminants (DNAPL and LNAPL).
These contaminants are significant in some way or the other mostly in a destructive capacity and thus have to be quantified and qualified to minimize any of its associated processes and effects. This can only be achieved as part of a holistic approach. Water monitoring on surface is usually easy as the flow paths are known, groundwater on the other hand is much more difficult as the flow paths and its dynamics can not be seen, conduit establishment age does not mimic the terrestrial flow paths,  they can however be measured and conceptualized. It would therefore be vital to firstly understand the hydrologic and geohydrological regimes of the area as it interacts with the Lithology. Secondly to establish a base line data set, thirdly interpreting and understanding (conceptualizing), fourthly establishing a monitoring network and finally to manage and remediate the contaminated system.

History of Sterkfontein Caves

Sterkfontein is one of the world's most productive and important palaeoanthropological sites. It is the place where the very fist adult ape-man was found by Dr Robert Broom in 1936. This ancient cave system has over the years revealed a sequence of sedimentary infill deposits with fossils dating from about 3.5 to 1.5 million years ago, a period of time which spans the early development of the family of man - the hominids. In addition to almost 500 skull, jaw, teeth and skeletal fossils of these early hominids, there are many thousands of other animal fossils, over 300 fragments of fossils wood, and over 9,000 stone tools which include some of the earliest manifestations of human culture on earth. Some of the youngest deposits in the cave also contain fossils and tools from the period just prior to the emergence of modern humans, the period ca. 100,000 to 250,000 years ago.
The environment inside caves is fragile, and the locations of many caves are kept secret in order to protect them. The younger fossil-bearing sediment at Sterkfontein, about 1.5 to 2 million years old, contain stone tools that are the earliest cultural remains yet found in southern Africa. These are attributed to a more advanced species of hominid, such as Homo habilis ('handy man'). A cranium of  H. habilis was excavated from the upper levels at Sterkfontein in 1976. Many scientists consider H. habilis to be the ancestor of H. erectus, who was, in turn, the ancestor of H. sapiens, the single species to which all of humankind belongs.H. habilis appears to have co-existed with a second australopithcine species, a more heavily-built, specialised individual called A. Robustus, which was first described from Kromdraai, a few kilometres north of Sterkfontein, by Broom in 1938. A. robustus and early members of the genus Homo have also been found at Swartkrans and Gladysvale. The extraordinary richness of the fossil sites around the Sterkfontein area led to concentrated efforts resulting in the area being declared a Cultural World Heritage Site.

Possible causes of pollution in the area and most important chemical processes.

Sources of contamination

Storm water from mines, industrial and domestic areas
Slime dams and dump leachate
Domestic waste leachate
Treated sewerage waste released into the streams
Local septic tanks and informal pit latrines
Mine water AMD
Agricultural run-off and leachate
Site specific waste dumps (metal in caves etc.)
Leaking fuel storage tanks.

Most important chemistry processes

Dolomite dissolution (after Hodgson et al., 2001)
Dolomite dissociation is given as:
CaMg(CO3)2 0 Ca2+ + Mg2+ + 2CO32-

The solubility product for this reaction in pure water is 1.995 x 10. This is very small compared to the reaction in de-ionised water, where the solubility is 14 mg/L, which would increase with the hydration of CO2 . This id due to the reaction:
CaCO3.MgCO3 + 2HCO3 0 Ca2+ + Mg2+ + 4HCO3-

This indicates the importance of CO2 in natural karst development. Other acids also results in lowering of pH which results in charged metal ions in solution, for example the reaction of dolomite with sulphuric acid:
2H2SO4 + CaCO3.MgCO3 + 2HCO3 0 Ca2+ + Mg2+ + 2H2O + 2CO2 + 2SO4

Solubility also increases with ionic strength, i.e. as the salinity of the water increases, so the solubility of dolomite will increase. This is due to solution non-ideality. Thus in natural developing karst the solubility will be mainly controlled by chemical factors and physical factors such as precipitation and physiography.
The expected stoichiometric composition of CaCO3 to MgCO3 and Si4+ of the dolomites should be 53:43:2 percent (Booysen, 1981).Due to the chert banding in the Monte Christo formation there will be co-precipitation of Silica and also some iron, aluminium and manganese oxides due to the presence of siderite and rodochrosite.



Acid Mine Drainage (after Scott, 1995).

This phenomenon has already been known since 1903 during pumping from the gold mines. The Witwatersrand Supergroup sediments contain varying proportions of sulphide minerals, the predominant sulphide being pyrite (FeS2) and can constitute up to 3 % by weight of the rock. Other traces of suphide includes; pyrrhotite, arsenopyrite, chalcopyrite, galena, cobaltite and gersdorffite. Atmospheric oxidation initiates the reaction, the sulphidic component (S2- ) in pyrite is oxidized to sulphate (SO42- ), whereby acidity (H+) is generated and ferrous iron (Fe2+) ions are released, the reaction:

FeS2(s) + 7/2O2 + H2O → Fe2+(aq) + 2SO42-(aq) + 2H+Acidity (aq)

Once the reaction is initiated ferrous iron is oxidized to ferric iron (Fe3+) the reaction is speeded up by microbial catalysis, the reaction:

Fe2+(aq) + 1/4O2 + H+ Bacteria Fe3+(aq) + 1/2H2 O

The ferric iron is then hydrolysed by water to form the insoluble precipitate, ferrihydrate (Fe(OH)3) plus more acidity, the reaction:

Fe3+(aq) + 3H2 O Fe(OH)3(s) + 3H+

The oxidation of pyrite is the most acidic of all weathering reactions since four moles of acidity is created from one mole pyrite. In addition to reacting with oxygen, pyrite may also be oxidized by dissolved ferric iron to produce additional ferrous iron and acidity, the reaction:

FeS2(s) + 14Fe3+ + 8H2O → 15Fe2+(aq) + 2SO42-(aq) + 16H+Acidity (aq)

Thus once the acid generating reactions have started, oxygen is only necessary for the microbial oxidation, pyrite will continue to be oxidized even in the absence of oxygen by ferric iron. Therefore sealing or flooding of mines will not stop oxidation.
The lowered pH due to above reactions also has the ability to leach other sulphide minerals in the ore like Ni, Pb, Cu, As and leachable oxides which are uranium bearing. These will normally co-precipitate with the iron oxides.


Geology (after Martini et al., 2003)


Stratigraphy


The geological sequence in the Cradle area consists of sediments that have been deposited onto basement granites and greenstones. The Witwatersrand sediments containing gold bearing conglomerate layers were deposited on this basement rock. They were in turn unconformably overlain by the Ventersdorp Supergroup andesitic lavas, which in turn were unconformably overlain by the Transvaal Supergroup dolomites. They are finaly covered conformably by Pretoria Series shales and quartzites.
The Cradle Caves are developed in the Malmani Subgroup of late Archaean age (2,5-2,6 billion years), which have been deposited on the intracratonic Transvaal Basin.
The lithology consists essentially of shallow marine stromatolitic dolostone with a variable amount of chert. The dolomite mineral is typically rich in Fe and Mn (up to 3% combined). Its thickness reaches 1450m in the Sterkfontein area . Based on the abundance of chert, the subgroup has been subdivided into 5 formations.
The Oaktree Formation (180m thick) represents the basal unit, characterized by its very chert poor nature. The overlying unit, the Monte Christo Formation (700m thick) is rich in chert. It has thin but spectacular oolitic beds at its base. The strata dip about 30° to the NW.


Tectonic Activities


Tectonic activity resulted in open folds and block faults in the Witwatersrand sediments, which caused some of the sediments to outcrop to the south of the Cradle area and is where most of the mining activity took place. Folding and faulting is featured throughout the Transvaal dolomites as well and three events can be highlighted: (i) Folding and faulting due to the origin of the Bushveld complex +/- 2000 ma, (ii) folding and faulting during Pilanesburg intrusion +/- 1400-1300 ma and (iii) later movement associated with uplifting.
Dykes and sills from north striking to east-west striking of Pilanesburg and Karoo age are numerous in the area. A large variety of fractures are visible in the dolomite of which the strike and dip varies throughout the area.


Methodology


The following sampling procedure was followed according to Department of Water Affairs and Forestry (DWAF) criteria and standards. Measuring of infield physical parameters (EC, DO, Redox, pH, TDS, Sal and Temp.)
Measure infield Total Alkalinity (mg/l CaCO3) and Sulfates. Sampling for Major ions and Trace elements as follows: Major Ions: 2 x 250 ml plastic bottles, rinsed three times with sample water before filling (physical parameters are measured using 60 ml from one bottle and alkalinity and sulphates with 10 ml respectively). Both bottles are immediately cooled in a cooler bag to +/- 4 degrees and stored in a fridge thereafter until lodged at the Lab (usually within a day).
Trace Elements: 1 x (pre-rinsed with DI water) glass bottle is filled with 100 ml of water via a syringe filter -0.45μm (conducted even under the most strenuous conditions) and 5 ml HNO3 to prevent secondary reactions from developing. Gloves are worn during sampling and equipment is thoroughly de contaminated with cleaning solution and DI water before continuing.
The samples are then analysed by the CGS’s Laboratory where Ion chromatography (IC) is employed to determine seven different anionic species, mass spectrometer (MS) and inductively coupled plasma spectrometer (ICP) is utilized for the trace elements. The major ions are measured in ppm while the minor or trace elements are measured in ppb.


Results

To date 61 samples have been acquired throughout the southern portion of the Cradle area (figure 2). These were all analysed and checked for reliability with a partly developed excel spreadsheet based on work done by Hounslow (1995). Using the charge balance calculation for reliability, it was found that 83 % of the samples were within + or – 10 %. The other 17 % is due to very aggressive mine waters with low pH and possible organics in significant quantities; they also typically have high salt values and no bicarbonates.
All these samples are one off samples and seasonal variations most certainly can cause significant variations. However certain interpretations can already be made.
Three different water bodies were sampled, surface water, cave water bodies and boreholes, although the caves and boreholes are mainly the same water in the dolomite the latter just not allowing direct access. The springs sampled are mainly the end of the line, residence time wise for the groundwater in the dolomite but are essentially groundwater. A basic relationship between surface and groundwater were attempted with no implication as to dynamic processes. This is due to surface water being subject to rapid mixing and gaseous exchange, which is not necessarily the case with groundwater.
Also as a note, no datum levels has been measured up to date and will follow in future, it would seem that the cave water bodies are some times at different levels due to sealing by clay or silty layers making them separate entities (pools) in what seem to be very low energy flow systems. All samples were plotted onto tri-linear diagrams (Piper) and Durov, Expanded Durov, Schoeller and Stiff diagrams to interpret the data and infer relationships.

Surface and mine waters

The four direct mine related samples (Harm-01, Harm bypass-01, T-spruit and Sunken stream), taken from the mine decant point and the immediate downstream points is typical Witwatersrand mine type water (Scott, 1995 and Usher, 2001) as it plots in field IV on the Expanded Durov diagram (Figure 4). The reef that was mined previously is below the dolomite and the water therefore has to move through the dolomite zone before decanting on the surface and therefore dissolution of the dolomite occurs and therefore the high calcium and magnesium (figure 5-Piper) associated with the Acid Mine Drainage (AMD) or sulphide oxidation taking place in the old worked out shafts. The spruit sample plots in field VI indicating water from treated effluent (municipal waste disposal site) and is directly downstream from the sewerage works, typical sulphates along with sodium and chloride values. No mixing with mine water is evident here. The surface samples progressing downstream plot in field V and VI which would indicate mixing from high sulphate mine waters and sodium and chloride treated effluent water. These values seems to gradually dilute as mixed with fresher dolomitic waters along the flow path until it plot in field II of the Expanded Durov Diagram which indicates typical dolomitic water. The samples that plot in field I is probably freshly recharged water. The Stiff diagrams (figure 6), although masked by the high concentration of sulphate scale wise, indicates the same patterns as described above with the Piper and Expanded Durov diagrams and specific signatures are visible for these different type waters.
All indications are therefore that the mine decant water mixes with the downstream surface water although other factors such as effluent water as well as agricultural pollution also contributes to the surface regime. The Schoeller diagram shows the same pattern although not included here.

Way Forward

Phase 1-A desk study compiling all current information including, Climate data, precipitation from relevant stations, stream flow, land use, GIS maps (topography, dtm, geology, catchments etc.), reports from previous authors, in digital format. Basically all relevant information on Geohydrology, hydrology and chemistry of the cradle. Phase 2-All structures and stress directions should be mapped from the airborne geophysical data. DWAF will assist in mapping of local geological structures. Phase 3-Gravity surveys over specific caves to map the direction of passages to link the dynamics between caves. This should be done for the bigger systems to begin with and possible ad hoc work later on for drilling. Completing the baseline data for the project area. This data will then be used for compiling flow nets (piezometric heads) for the modeling and quality maps to assist with the establishment of a monitoring network. A complete borehole census (including springs and caves) of the cradle area to the north. This will include a first phase identification of all boreholes including data on general access, co-operation with land owners, gps – coordinates, casing height, equipment fitted, is sampling possible, water level, owner details etc. Phase 4-During this phase a survey team will measure the accurate positions of the surveyed boreholes captured in phase one with regards to its x, y and z position in Cartesian and geographical coordinates.
Phase 5-Sampling of all available boreholes for base line geo-hydrochemistry data. Phase 6-Natural isotope samples and carbon 14 dating for some points in network, to date or trace possible flow paths. Interpretation of all relevant data acquired to date to construct conceptual model of the cradle.
Phase 7-Possible drilling of extra boreholes to calculate aquifer parameters for input into numerical model. Pump testing of boreholes to obtain hydraulic aquifer parameters. Phase 8-Double ring infiltrometer tests to establish seepage rate of surface water bodies into the subsurface to calculate recharge. Phase 9-Recharge calculations from all acquired data, essential for numerical model. Compilation and construction of numerical model and calibration using FEFLOW and PMWIN software. A possibility (not necessarily required) will be to look at tracer tests to establish certain important flow paths. Phase 10-From the above database and results of the numerical model a network of sampling points covering a grid of the area as well as all relevant “hot spots” will be designed from newly drilled boreholes and existing ones.
Phase 11 -Frequent sampling according to above criteria for at least two years. Compiling a GIS map containing all liable polluters to the Cradle area. Following legal frame work to hold certain liable parties accountable.

Conclusions

The baseline chemistry data acquired to date is a very helpful tool to establish trends for future reference for pollution and state of the environment. Although it is not a full complete picture of the Cradle it does however yield a starting point from where future work can advance and concentrate. More seasonal and temporal data is definitely needed to complete the picture. With this data it is clear that three types of pollution is entering the system, those from the mining activities, Agriculture and small industries and waste disposal sites. Building a suite of data as part of a Geographical information system seems to be the only way to manage a system of this size. Monitoring of the system with properly planned positions from the current data will allow to pinpoint any future changes. Conceptualizing this system is going to be of utmost importance before final interpretations can be made.

References and Bibliography

Bredenkamp, D.B. 1986. Groundwater supply potential of dolomite compartments west of Krugersdorp , DWAF. Technical Report No. GH3440.

Drever, J.E. 1982. The geochemistry of natural waters. Prentice Hall, nglewood Cliffs. ISBN 0-13-272790-0.

Hodgson, F.D.I., Usher, B.H., Scott, R., Zeelie, S., Cruywagen, L-M. and De Necker, E. 2001. Prediction Techniques and Preventative Measures Relating to the Post Operational Impact of Underground Mines on the Quality and Quantity of Groundwater Resources.

WRC Report No. 699/1/01. ISBN 1-86845-701-X
Hounslow, A. W. 1995. Water quality data: Analysis and Interpretation. ISBN 0-87371-676-0.

J.E.J.Martini(1), P.E.Wipplinger(2), H.F.G.Moen(2) and A.Keyser(3). 2004. Contribution to the speleology of Sterkfontein Cave,Gauteng Province, South Africa.

Ligthelm, R. 1993. Die Hidrogeologie van Karstgebiede met Spesiale verwysing na Grondwater Besoedeling Suid van Pretoria. MSc. University of Pretoria.

Nazari, M.M., Burston, M.W., Bishop, P.K. and Lerner, D.N. 1993. Urban Ground-water Pollution: Case Study from Coventry, United Kingdom. Ground Water, Vol. 31, No. 3. pp 417-424.

Scott, R. 1996. Geohydrochemistry and Pollution. 3rd Edition, Institute for groundwater studies.