Following a request by two major banks, the Mine Water Research Group of the North-West University, conducted a desk-top study to assess how far underground infrastructure in the CBD of Johannesburg may be affected by rising mine water levels in the Central Rand. This follows the approval by Cabinet to allocate R225 million to mitigate effects of acid mine drainage (AMD), which in turn was based on a report of a Team of Experts to the Inter-ministerial Committee on AMD.
While the study by Prof. Winde and his team focused on the flooding risks of basement structures in the CBD, it also addressed a range of related issues. These include, amongst others, the identification of sources of water filling the mine void (ingress), factors controlling the rate of rise of the mine water table, and the expected volume of water overflowing from the flooded void (decant). Based on the evaluation of pertinent scientific and technical reports as well as primary data provided by the mining industry, the study also addressed a range of other risks possibly associated with the filling of the Central Basin.
The main findings of the Winde Report differ in a number of crucial aspects from the AMD report tabled to Cabinet. This includes:
- newly identified ingress sources,
- a slower rise of the mine water table resulting in a later date of decant (despite the unusual heavy rains in late 2010 and early 2011),
- a significant reduction of the expected decant volume (with possible implications for proposed treatment options)
- a number of flooding-related risks not considered previously have been identified
- and much less severe impacts of the untreated decant water on the quality of receiving streams.
A number of flooding-related risks not considered previously have been identified. This includes:
- the exposure of residents to the radioactive gas radon via shafts even before flooding of the Central Basin is complete
- the possible subsidence of tailings structures incorporated into infrastructure such as the M2 highway due to liquefaction of unconsolidated fill material in low lying reef outcrop zones.
In the light of the new findings the report suggest to explore possibilities for a more sustainable low cost, low-energy solution as opposed to the currently proposed high-cost high-energy, pump-and treatment-option likely to be subsidized ad infinitum by society.
On the other hand it is stressed that the conclusion that a qualified “do-nothing option” would not result in any flooding risk for the CBD nor in a significant degradation of receiving surface waters should not be used as an excuse to continue avoiding the much needed rehabilitation of mining-affected environments in one of the most densely populated areas of South Africa, especially as this is likely to simultaneously improve the quality of the decanting mine water in the long term.
Condensed overview on findings
Background
In August 2010, Prof. Winde, head of the mine water Research Group at the Potchefstroom Campus of the North-West University was approached, independently, by two major banking groups to assess the geotechnical risks to their buildings in Johannesburg Central Business District (CBD) posed by the rising water in the Central Basin Mine void. This request was in response to a number of partly sensational media reports based on the concern that acid mine drainage will severely affect the stability of the CBD infrastructure especially high rise-buildings with deep basements. In this regard the new Standard Bank administration building featured prominently. Other reports predicted adverse consequences that included the flooding of the tourist mine shaft at Gold Reef City, health effects, formation of sinkholes in the CBD due to AMD triggered solution of dolomite and large-scale pollution of rivers and drinking water sources. A variety of dates when the water will surface were predicted with most targeting early to mid 2012.
In response to the urgency portrayed in most reports Government established an Inter-Ministerial Committee (IMC) on Acid Mine Drainage (AMD) that, in turn appointed a team of 27 experts to advise Cabinet on a course of action. The team of experts reported back twice during late 2010, advising that the water level of the Central Basin should be maintained below what was termed ‘Environmental Critical Level’ by pumping from underground pump stations. The discharge would be neutralized before being released into receiving rivers. After the report was tabled to Cabinet in early 2011 a total of R225 million was immediately allocated to address the problem. This amount grew meanwhile to R400 million.
Much of the cited “AMD-problems” of Johannesburg stems from the gradual closure of mines along a 44 km long E-W running zone south of Johannesburg known as ‘Central Rand’ since the 1970s. After Durban Roodepoort Deep (DRD), located in the far western part of the Central Rand, closed its active underground operations in 1995, the only remaining active underground mine was the over 100 year old East Rand Proprietary Mines (ERPM) in the far eastern part of the goldfield. The successive closure of mines resulted in the flooding of those voids where pumping stopped as water naturally continued to flow into the generated mine void. Following an accident in late 2008 at the pumping shaft of ERPM as last pumping mine, the system of interconnected mine-voids (termed ‘Central Basin’) started to fill.
Scope of project
From August 201 to April 2011 a desktop study by the Mine Water Research Group evaluated available data from a number of studies done for different purposes. A scenario-based assessment model was developed to determine the risk a fully flooded mine void would pose to the underground infrastructure in the Johannesburg CBD.
Somewhat exceeding the scope of a desktop study, primary data supplied by the mining industry was also used, notably time series of pumping data and current mine water level measurements. The main parameters investigated included a variety of different elevation data sets, the mine void characteristics (volume, structure, shape, interconnectivity, closure rate) as well as the sources, pathways, quality and volume of the ingressing water. These parameters and their interrelationships were analysed with the ultimate objective to predict at what elevation the final mine water level in the completely flooded basin will be in relation to basements in the CBD assuming that no intervention will take place to keep the water artificially at a lower level (“do-nothing scenario”).
General findings
– As dolomitic ground instability, in the form of catastrophic sinkholes affecting central Johannesburg, featured prominently in media reports (even providing elaborate sketches on how this will happen) it needs to be pointed out that no dolomitic formations occur below the Johannesburg CBD.
– Since June 2010 the water level measurements across the Central Basin reflect a single water level that demonstrates a sufficiently high interconnectivity between the different sub-voids to allow for a single decant point (e.g. an open shaft) to control the water level of the entire Central Basin, provided that this decant point is large enough in diameter to accommodate the expected decant volume.
– The average water table rise since June 2010, when all sub-basins finally merged into a single large basin, decreased from an average of 0.55 m/d in the former central sub-basin to 0.37 m per day in the larger basin.
– The impact of rainfall on the rate of rise is less drastic and direct than previously suggested (exponentially rising rates were proposed) as directly rainfall-dependent ingress accounts for only a relatively small portion of the total ingress into the void. This explains the rather moderate effect even the exceptionally heavy rains that occurred since late December 2010 had on the observed rise of the mine water table causing a temporary increase by 37.5% (adding 0.12m/d).
– The lowest lying shaft that is still open and sufficiently connected to the Central Basin will serve as decant point. Provided that all ingress can overflow (i.e. the full decant volume be accommodated) this shaft will ultimately control the elevation of the final mine water level across the entire Central Basin. Based on an estimated decant rate of 30-40 Ml/d this is most certainly the case at all open shafts (i.e. shafts that have not been capped or filled or both). This decant point will be the Cinderella West Ventilation shaft at ERPM (near Boksburg) located at 1613.7 m amsl.
– Given the average water table rise per day, the elevation of Cinderella West shaft, and assuming that the exceptionally high 2010-2011 rainfall will not be exceeded the decant will start mid September 2013, more than a year later than previously predicted.
– In addition to water decanting from the shaft some mine water may diffusely seep out of the flooded mine void towards areas located below the decant level such as valleys of nearby streams. This is particularly likely to happen where transmissive geological features such as weathered dykes, fault lines, fractures etc. provide a pathway that hydraulically links the flooded void with the surface. The E-W profile in Figure 1 indicates some potentially affected stream valleys in the far east of the Central Rand.
– Where the Main Reef outcrop zone, which historically was mined from surface to a depth of approximately 40 m is topographically low enough, mine water rising in the underlying deep void may spill over and saturate the unconsolidated material which this zone was frequently filled.
– Where this results in liquefying the base of large tailings dams, or other structures it may pose a geotechnical hazard. As failures of slimes dams in the Central Rand area have occurred in the past and old tailings deposits have been incorporated to supports infrastructure (e.g. the M2 highway) possible risks associated with underground diffuse outflow of mine water into the disturbed outcrop zone should be investigated.
Key findings
– Flooding risk: Using the pile levels of the BSA tower East as the deepest of the Bank buildings considered in the Johannesburg CBD, it was calculated that the maximum elevation to which the mine water table can rise in the Central Basin mine void is 90 m below the base of these piles. For the new admin building of Standard Bank, which according to the latest issue of “You Magazine’ is already being flooded, the safety margin is 106 m. This was determined by using the surface elevation of the lowest shaft connected to the filling void (Cinderella West shaft of ERPM at 1613.7 mamsl) as the ultimate elevation of the water table in the completely flooded mine void. As this safety margin can accommodate all possible uncertainties associated with the data used and the conceptual model it was concu8lded that no risk of mine water flooding any part of the basement structure in the CBD of Johannesburg exists.
Figure 1 (Omitted)
– Estimated decant volume: Pumping volumes of the Central Basin mines and estimates for water volumes provided by various identified ingress sources indicated that decant volumes from natural and artificial sources will be in the order of 30-40 Ml/d. This contrast with 60-100ML/d cited in some scientific studies and most media reports.
It is expected that, as the mine void fills, some ingress sources will finally be below the recovered mine water table and thus no longer able to contribute to ingress. Therefore, post-flooding ingress volumes are generally expected to be lower than pre-flooding pumping volumes which are commonly used to estimate future decant rates.
The reduction of ingress is larger the higher the mine water table rises. Thus a decant level closer to surface may result in a (significant) natural reduction of the volume of water that needs to be treated reducing long-term treatment costs. If this concept of natural decant reduction were to be utilized, a balance between the benefits of reduced decant volumes/treatments costs and possibly increased risk to surface structures would need to be found. Considering only the partial reduction of the diffuse groundwater inflow from the fracture aquifer through the Main Reef outcrop zone an ingress reduction of some 4ML/d is estimated. This does not take into account the reduction of stream-loss which is likely to occur in some streams especially in the low-lying eastern part of the Central Rand.
The much reduced prediction of future decant volumes is likely to influence the choice of suitable treatment options as the feasibility of certain processes requires high volume throughputs.
– Newly identified ingress sources: the possible contribution of tailings reclamation activities on surface has, so far not been fully appreciated. Tailings reclamation is particular pronounced in the Central Rand, where it indeed started in the mid-1980s as this is the oldest of all goldfields where many old slimes dams with relatively high gold grades occur. Over the past 25 years Crown Gold Recoveries (CGR), the “world’s largest tailings reclamation operation’, alone mined some 200 million tons of tailings material in the Central Rand. The associated contribution to ingress is likely to be considerable as old tailings are hydraulically mined using high pressure water cannons. This introduces large volumes of additional water into a highly disturbed area where surface mining and subsequent filling resulted in exceptionally high infiltration rates. In some cases reclaimed tailings, along with process water, have for decades, been disposed of directly underground, increasing ingress while simultaneously reducing void space. The latter practice is proposed to be continued for coming years by Central Rand Gold (CRG) which aims to become the “world’s largest gold mine” by 2012 through shallow underground mining scavenging old voids of the Central Rand.
As most of the reclaimed tailings are deposited on a slimes dam complex at Nasrec, significant volumes of tailings seepage are generated adding an estimated 10 – 15 Ml/d to the ingress. As some of these slimes dams are placed directly on top of reef outcrop zones a high proportion of this seepage may flow directly into the underlying mine void. This also applies to many decommissioned slimes dams that cover large surface areas in the mining belt.
– Reduction of void volume: Confined to ultra-deep level mining for which South Africa is famous, the phenomenon of the gradual closure of stopes (the area where the actual ore body is removed) through the bulk expansion of highly stressed rock without the formation of fractures (this is termed ‘plastic flow’ or ‘creep’ in contrast to the brittle collapse of tunnels during which the overall volume remains constant as the filling of voids is counterbalanced by forming new fractures elsewhere) has so far not been considered in the context of mine flooding. Based on proxy parameters from two case studies (from the western and Central Basins respectively) the degree to which stope closure reduces the volume of the mine void has been estimated. Based on 2 completely different sets of data both case studies suggest a nearly identical volume reduction of approximately 75% of the original void volume.
As stope closure significantly reduces the exposure of unmined ore to oxygen and water as the main cause of AMD formation in underground mine workings, this process has also implications for the quality of the decanting water.
– Decant water quality: the quality of mine water decanting from the Western Basin has, using uranium (U) as the indicator, for the past 9 years improved from over 6000 µg/l during the initial phase of the decant to presently 100 – 200 µg/l U, much faster than anticipated. One of the reasons could be that less U is available to be mobilized due to stope closure in deeper parts of the void.
In addition, the pH of the Western Basin mine water directly at the outflow point is close to neutral suggesting that little U, and other heavy metals, can be liberated through acidic dissolution of minerals. This, in turn, means that the main pollution sources of void water are perhaps not located underground but on surface. This could include tailings and other mining residue deposits which cover a large percentage of the surface catchment above the void. If correct this has implications for the Central Basin, where much of the small headwater catchments of streams are also covered by mining residue, resulting in much of the ingress water arriving in the mine void already being polluted. A significant proportion of the void water is attributable to tailings-polluted stream water lost to the mine void especially where old surface diggings as well as transmissive geological features such as dykes, faults and fractures are crossed. Highly polluted seepage from slimes dams and sand dumps deposited on top or upgradient of the mined outcrops zones is also likely to contribute considerably to ingress.
– Decant-induced steam pollution: Where the mine void fills to levels above connected streams from which ingress is derived, the stream loss will be reversed preventing further ingress of polluted stream water. Various investigations conducted during active 4100µg/l). Therefore, the net pollution effect of decanting mine water diffusely seeping into nearby streams may not be as dramatic as predicted, as both waters, the decant and the polluted streams, display a similar poor quality.
This is quite different to the devastating effects observed at the West Rand where a non-perennial stream that was previously nearly unaffected by mining suddenly received all the decant. Much of the environmental damage observed in the Tweelopiespruit is caused by the spatial concentration of polluted water from an unnaturally large catchment area which is discharged into a single, small stream that has almost no dilution capacity. This, in turn, is caused by the mine void collecting infiltrating water that would otherwise drain into various streams across the entire West Rand and directing it via gradient flow towards a single outflow point (the Black Reef Incline shaft).
Assuming that much of the pollution of decanting mine water occurs on surface it may be possible to improve the decant water quality without very costly and difficult underground interventions but through remediation of mining-affected surface areas. This would simultaneously improve living conditions in many of the densely populated settlements which encroach onto the mining belt across the Central Rand.
Due to the above and the fact that during mining pumped (polluted) mine water was disposed of in nearby rivers, it is suggested that the Central Basin mine void decant, will in the long term, not have a much greater impact on the river systems than was the case during active mining. In fact, if the water in the mine void stratifies, as observed elsewhere, cleaner water may decant on surface after the initial flush of highly polluted mine water subsided and gradually improve the water quality of receiving streams.
– Groundwater pollution and associated radon exposure: Where the rising mine water will come into contact with the near surface aquifer, U contamination is at least initially to be expected. The associated radon risk needs to be assessed especially for informal settlements where the radioactive gas (formed ongoingly through the radioactive decay of uranium contained in the mine water) can easily accumulate in low-lying poorly ventilated shacks which often lack concrete floors that could limit a radon influx. As a leading cause of lung cancer in uranium miners, radon exposure constitutes a severe health risk.
– Radon (Rn) exposure via shafts: Since shafts are directly connected to the mine water and act as preferred conduits for equalizing barometric pressure differences between the void and the surface, it is likely that radon can reach the surface relatively quickly and well before it decays (the half life of Rn is 3.8 days) even in areas where the water table will remain deep below the surface. Thus radon is likely to already escape from the shafts well before the flooding of the mine void is complete. This renders shafts potential hot spots for radon exposure of surrounding areas. With over 100 shafts distributed across the ming belt the potential for radon exposure is considerable. The identification of affected areas may be difficult especially where old shafts have been covered with soil, or other material. As radon is odourless and the covered shafts invisible such spots are particularly dangerous for nearby residents.
– Decant impacts on dolomite: If AMD is allowed to decant freely from the Central Basin, it will have to travel, depending on where additional diffuse decant possibly occurs, for approximately 5 km in receiving streams before it reaches the nearest outcrop of dolomite. It has been proposed that increased rates of dolomite dissolution will result in the formation of potentially catastrophic sinkholes as experienced in the Far West Rand (FWR) goldfield. In this context it should be noted that the primary cause for sinkhole formation in the FWR is not the accelerated dissolution of dolomite by acidic mine water but the lowering of the groundwater table following the dewatering of karst aquifers in an attempt to reduce ingress into the underlying mine void. By lowering the groundwater table as sub-surface erosion base, surface water percolating through the soil cover can erode unconsolidated fines into deeper lying karst receptacles (caves). As the ongoing formation of the underground cavity is hidden from the eye by a stabile thin layer of naturally forming ferricrete (or tar/concrete in case of road and buildings) many of the sinkholes are only discovered when this thin arch on top suddenly collapses frequently resulting in catastrophes.
Since the lowering of the groundwater table and the associated increased rate of subsurface erosion is the actual source of sinkhole formation, sinkholes are unlikely to form south of the Central Rand where groundwater levels have not be lowered by mining.
However, if the current groundwater abstraction for irrigation exceeds the natural recharge, water levels in the dolomitic karst aquifer will drop increasing the risk of sinkhole formation albeit totally unrelated to the decant.
How far acidic mine water would indeed increase dolomite dissolution is unclear. Attempts by gold mines in the FAWR to use the abundantly available dolomite for neutralizing AMD failed as the dolomite chips were, within days, armored with inert coatings of iron hydroxides preventing any further dissolution of the dolomite chips.
– Flooding induced seismicity: Based on literature it appears that flooding induced seismicity is unlikely to exceed the magnitude of seismic events that occurred during active mining. This is supported by observations in the Western Basin where over the past 9 years since the basin is fully flooded no event was measured that exceeded the magnitude of previous events in this area. Compared to the FWR where active underground mining still continues, the overall seismic activity in the form of tremors appears to be significantly reduced.
For the Central Rand it is further to be considered that approximately two thirds of the Central sub-basin have already been flooded since 1975 without reports about associated increases in seismicity since then. As swarms of earth quakes appeared in early 2010 in the Augrabies National Park, i.e. far away from any flooded basin, the possible impact of natural seismicity should perhaps also be considered when interpreting temporally coinciding seismic and flooding events as attempted in the AMD report.
– Damage to business confidence and corporate image: With sensationalized coverage of AMD-related problems in Johannesburg not only by national but also international news media including the Washington Post, Le Monde and Die Welt concerns are raised that this may unduly affect the confidence of investors and do damage to the corporate image of businesses operating in Johannesburg. Apart from possibly discouraging foreign investors which is rather difficult to quantify, the exaggerated and often untrue media reports may have more tangible and direct effects on the property market and development plans for the CBD which still battles to overcome the inner-city decay.
Conclusions and recommendations
Given the shortage of water in Gauteng as the most water stressed province in South Africa that relies heavily on water imported from Lesotho at great costs, the anticipated decant from the mine void should be seen, not as a threat, but rather as an opportunity of using water which for a couple of years went unused to fill the void. Untreated acidic mine water has been used in the past by municipal sewage works in the Central Rand to aid nitrate digestion (sewage work Germiston: 4.2 Ml/d). Given the number of sewage works in Johannesburg and the volume of sewage to be treated, this alone could perhaps accommodate most, if not all, of the decanting water resulting in no treatment costs while saving clean water otherwise used for this purpose. This is but one example from a range of other possibilities that should be explored in order to arrive at low cost, low-energy solutions that are sustainable in the long term as opposed to the currently proposed high cost, high energy, end of pipe pump and treatment option likely to be subsidized ad infinitum by society.
In this context it appears that the AMD report to the Inter-Ministerial Committee and Cabinet, concerning the Central Rand, lacks a thorough analysis of available data and leaves many crucial aspects superficially covered. This includes key issues such as the volume of the expected decant, the compilation of sources of the ingressing/decanting water, water quality and relationship to rainfall, the rate of rise of the mine water table and date of decant as well as the spectrum of associated risks.
On the one hand it becomes difficult to avoid the impression that the report is a premature, somewhat hasty response to a largely media and interest group driven campaign that appears to have inflated, misrepresented and exaggerated possible risks associated with the filling of the mine void. It has been suggested, in public, that this may be aimed at creating a ‘knee jerk’ reaction by Government where a sense of urgency prevents sound scientific analyses and streamlines approval for an expensive ‘solution’ favoured by some.
On the other hand it needs to be stressed that the conclusion that a qualified ‘do nothing option’ would not result in any flooding risk for the CBD, nor in a significant degradation of receiving surface water, should not be used as an excuse to continue avoiding the much needed rehabilitation of mining affected environments in one of the most densely populated areas of South Africa.
In order to pro-actively address the identified risks of flooding-induced subsidence of structures in the low lying outcrop zones and the exposure of residents, especially n informal areas, to radon even before the void is completed flooded it is recommended to urgently conduct in depth investigations to quantify these risks.
