Numerical modeling of mine dewatering and flooding in the Evander Gold Basin, South Africa

Numerical modeling of mine dewatering and flooding in the Evander Gold Basin, South Africa Abstract K.T. Witthüser 1, M. Holland 1, T. Seidel 2, and C.M. König 2 1 Delta-H Water Systems Modelling Pretoria, South Africa; email: kai@delta-h.co.za, martin@delta-h.co.za 2 delta h Ingenieurgesellschaft mbh Witten, Germany; email: ts@delta-h.de, ck@delta-h.de The assessment and prediction of mine water rebound has become increasingly important for the gold mining industry in the Witwatersrand basin, South Africa. The cessation of dewatering lead to large volumes of contaminated surface discharges in the western parts of the basin. Towards the eastern extremity of the Witwatersrand basin the detached Evander Goldfield basin has been mined since the early 1950s at depths between 400 and 2000 meters below ground, while overlain by shallower coal mining operations. The hydrogeology of the Evander basin can be categorized by a shallow weatheredfractured rock aquifer comprising of the glacial and deltaic sediments of the Karoo Supergroup, while the deeper historically confined fractured bedrock aquifer consist predominantly of quartzite with subordinate lava, shale and conglomerate of the Witwatersrand Supergroup. The deep Witwatersrand aquifer has been actively been dewatered for the last 60 years with a peak rate of 60 Ml per day in the mid late 1960s. Modeling the impacts of mine dewatering and flooding on a regional scale as for the Evander basin entails challenges like the appropriate discretization of mine voids and the accurate modeling of layered aquifer systems with different free groundwater surfaces on a regional scale. To predict the environmental impacts of both the historic and future deep mining operations at Shaft 6, the detailed conceptual model of the aquifers systems and 3-dimensional model of the mine voids were incorporated into a numerical groundwater model to simulate the dewatering and post-closure rebound of the water tables for the basin. The presented model could serve as an example for the successful modeling of mine dewatering and flooding scenarios for other parts of the Witwatersrand basin. 1. STUDY AREA General Setting The Evander Gold Fields is located approximately 120km East-Southeast of Johannesburg, South Africa. Regionally, the topography is slightly undulating with surface elevations ranging from 1520 to 1700 meters above mean sea level and slopes generally lower than 10 %. The majority of the study area falls within the Waterval- and Klipspruit River catchment (quaternary catchment C12D), which drains southwards towards the Vaal River. The surface catchment is immediately south of the major surface water divide between the Vaal River basin to the south and the Olifants River basin to the north. The area receives approximately 670 mm of rain per year, with most rainfall occurring during summer. It receives the lowest rainfall in July and the highest in January. The monthly distribution of average daily maximum temperatures range from 16.4 C in June to 25.8 C in January. The mean (potential) annual evaporation amounts to 1700 mm/a. Geological Setting The principal economic horizon for the Evander Gold Fields is the Kimberley Reef Placer, which was deposited in a subsidiary basin (Evander Basin) during the early deposition stages of the Turffontein Subgroup Central Rand Group of the Witwatersrand Supergroup. The Evander Basin has undergone extensive structural displacement by primary and secondary faulting, with a predominant south down-

throw of the Reef which subsequently sub-outcrops against the Karoo Sequence at ±200 metres below surface. The basin extends to a maximum depth of approximately 2700 meters below surface. The Kimberley Reef dips to the North at angles varying between 25 and 70. Gold production in the Evander Gold Fields was started in December 1958 by the Winkelhaak Mine, followed by the Bracken- and Lesley Gold Mines and in early 1964 by Kinross Mine. Evander Gold Holdings now incorporates these four gold mines, however only Shaft 8 (Kinross) is still actively being mined. There are no exposures of Pre-Karoo rocks in the study area as outcrops are limited to Karoo dolerite and Ecca sandstone (Figure 1). Beneath the Karoo, rocks of the Transvaal, Ventersdorp and Witwatersrand Supergroups as well as the Archaean Complex (Pre-Witwatersrand) were identified in numerous exploration projects since the early 1900s (Jansen et al., 1972). Figure 1. Surface geology of the Evander Basin The generalized stratigraphy of the Evander Basin as compiled from surface borehole intersections, underground exposures and previous studies of the Evander area is given in Table 1. The sediments of the Karoo Supergroup were intruded by doleritic magma from a southerly direction to form thick sills and dykes. Three different ages of intrusions are recognized in the Evander Basin (Tweedie et al., 1986): Karoo age Intrusives (two phases of Karoo dolerites are present, an older intrusive, namely the B4 sill, mostly restricted to the surface and the younger B8 sill which occurs close to the Karoo-/ Transvaal Supergroup (dolomite) boundary and forms the lower boundary of the Karoo aquifer. Vertical offshoots (dykes) results in compartmentalization of the Karoo system (Figure 2). Ventersdorp age intrusives are present as near-vertical, north-south striking dykes dominate in the Evander Goldfield. Bushveld age intrusives occur as both dykes and sills of extensively altered medium to fine grained, ophitic diabases.

Table 1. Generalized stratigraphy of the Evander Basin (adapted from: Jansen et al., 1972; Harmony, 1994; Van Biljon, 2007; Burger, 2010). Max. Subgroup or Supergroup (System) thickness Description Series (m) Ecca Sandstone, shales and coal Karoo 50-300 Dwyka Conglomeratic sandstones and tillite Unconformity Pretoria Group + 275 Black shales with intercalated lava flow (Undifferentiated) Transvaal Chuniespoort 427 Dolomite and chert Black Reef 5 Unconformity Ventersdorp Klipriviersberg 1220 Witwatersrand Central Rand West Rand Conformable contact Quartzite with a poorly developed basal conglomerate Amygdaloidal, non-amygdaloidal and porphyritic andesitic lavas Turffontein 327 Quartzites, shale and conglomerates including the Kimberley Reef Johannesburg 295 Quartzites, conglomerates and lavas Greywacke, quartzite, poorly developed Government Reef conglomerates, tillites and magnetic + 430 shale Hospital Hill Magentic shale and quartzite Unconformity Achaean Complex (Basement)? Structural Setting Schists, gneisses, granites, phyllites and shales The Evander Goldfields lies on the southern edge of a northerly dipping arcuate basin (also referred to as a structural outlier to the main Witwatersrand basin) (Jansen et al., 1972, Tweedie et al., 1986). Its borders are defined towards the north by the intrusion of a mafic lobe of the Bushveld complex and towards the south and east by Archaean age Basement Complex (Figure 2). The basin is highly disturbed by normal faults. Towards the west of the Basin these faults strike northeast-southwest and swing to a north-south strike in the eastern parts of the Basin. Folding, associated with faulting, occurs along the eastern and western edges of the Basin. This has resulted in a major overturning of the succession in the east (in the proximity of Shaft 6). The south-eastern edge of the basin is truncated by a major ENEstriking fault which has a down-throw to the north (Figure 2). The faulting is as a rule post-ventersdorp and pre-transvaal in age. Although, a few faults have been reactivated post-transvaal and post-karoo times (Jansen et al., 1972), according to Van Biljon (2007) no evidence of major post-transvaal faulting was found that could potentially link the mine workings with the overlying dolomite and Karoo aquifers.

Figure 2. Geological features of the Evander Basin and conceptual cross-sections. Hydrogeological Setting Based on the hydrogeological studies (e.g. Van Biljon, 2007, Burger, 2010) and the developed conceptual understanding, two major aquifer zones can be differentiated; the upper weathered/fractured Karoo system and the deep aquifers and aquicludes of the pre-karoo rocks underlying the Karoo system. The Karoo groundwater systems comprises of 3 different aquifer zones, an unconfined or semi-confined shallow upper weathered zone (5 to 12 m below surface), a deeper semi-confined to confined fractured Karoo aquifer and artificial aquifers created by coal mining (the void zones). The permeability of the Karoo strata is generally low and as a result, the groundwater potential is classified as low. The deep aquifer system comprise of three major geological units, which split it into aquifers separated by aquicludes or aquitards. The Chuniespoort Group dolomites underlying the Karoo aquifer towards the north and east of the Evander basin can be regarded as an insignificant aquifer due to its lack of karstification. A thick succession of Ventersdorp lava forms an almost impermeable barrier between the overlying aquifers and the deeper Witwatersrand aquifer and mine workings. Within the deep fractured Witwatersrand sediments groundwater is encountered in the Evander Gold Mine workings as deep as 2 km below surface. Groundwater occurrence in the deeper aquifer is associated with connate water in the

Witwatersrand sediments and to a lesser extent with vertical seepage along discontinuities (smaller joints and faults), transferring water from the overlying Karoo aquifer or bordering Basement aquifers at distinct locations. Extensive mine development in the Witwatersrand sediments has formed an extensive artificial aquifer, that has a much higher porosity and permeability than the surrounding undisturbed Witwatersrand aquifer. The current understanding of the hydrogeology of the Evander basin suggests that the underlying gold mining operations pose no threat to the Karoo aquifer since the hydraulic connection appears to be limited to a few selected dolerite dykes. Accordingly, the Karoo aquifers have not been significantly affected over the past 60 years of gold mining in the region, even though large volumes of groundwater had been abstracted from these mines. A summary of the known hydraulic parameters of the aquifers and aquicludes dominating the hydrogeology of the Evander basin is given in Table 2. Table 2. Summary of hydraulic properties of the different aquifer systems in the Evander Basin (adapted from: JMA, 2002; Vermeulen and Dennis, 2009; Burger, 2010). Lithology Aquifer Hydraulic conductivity (m/s) Storativity Min Max Min Max Weathered/Fractured Karoo aquifer 1E-08 1E-06 1E-04 5E-03 Supergroup Coal Seam (Before mined) 1E-09 1E-07 1E-03 1E-02 Transvaal Dolomites Supergroup (Aquiclude) 1E-10 1E-08 1E-06 1E-03 Ventersdorp Ventersdorp lavas Supergroup (Aquiclude) 1E-14 1E-11 1E-06 1E-03 Witwatersrand Confined Fractured Supergroup aquifer 1E-10 1E-07 1E-06 1E-03 2. GROUNDWATER MODEL Objectives The objectives of the groundwater flow model are to predict the operational dewatering rates for Shaft 6 operation to enable mining in currently flooded parts of the mine, to predict the dewatering rates over life of mine including future mine development and to estimate post-closure flooding rates and levels. Selected Code To accommodate the complex underground mine workings and its interaction with the surrounding aquifers, the finite element (FE) modeling code SPRING (König 2011) was chosen. SPRING offers a socalled mine boundary condition, which allows to describe mines or separate mining fields within a finite element mesh and couple their hydraulic behavior to surrounding aquifers. The program was first published in 1970, and since then has undergone a number of revisions. SPRING uses the finite-element approximation to solve the groundwater flow equation and is able to simulate steady and non-steady flow, in aquifers of irregular dimensions, as well as confined, unconfined and unsaturated flow, or a combination thereof. Different model layers with varying thicknesses as well as out pinching model layers are possible. Model Domain The horizontal model boundaries coincide with the Evander Basin as delineated by Jansen et al. (1972) and Tweedie et al. (1986) (Figure 2). The northern boundary is associated with the Bushveld intrusive complex and towards the east and south by the Archaean age Basement Complex. The western boundary is related to the regional north-east striking fault system. The northern boundary closely corresponds to the major surface water divide between the Vaal- and Olifants river system, while the southern boundary

corresponds closely to the quaternary drainage catchment C12D. The model domain covers an area of 1020 km 2, 1375 km of surface drainages, a surface relief difference of 800 meters and a vertical thickness of up to 4600 meters. The domain was spatially discretized into up to five element layers (six node layers) of 140 543 elements each. In order to simulate the seepage into the mine voids and the subsequent potentiometric surface more accurately, the lower model layers were further vertically refined (split) within the mining areas to accurately reflect the mine developments. These locally refined layers pinch out beyond each mining area and allow a better numerical approximation of the potentiometric surface near the voids. The size of the triangular and rectangular elements (side length) range from approximately 20 meters within and around the mining voids as well as faults and dykes to a maximum of 250 meters further away from the areas of expected steep gradients. In accordance with the developed conceptual model, the upper model layer simulates the Karoo aquifer systems and the lower layers represent the deeper underlying aquifers and aquicludes. While the top elevation of the uppermost layer was based on a 50m x 50m digital elevation model, the elevations of the lower layers are aligned to the developed geological model (Figure 3). Figure 3. Conceptual three-dimensional view of the Deep Aquifer Model (vertically exaggerated). Model Calibration Around 260 water level measurements were used as calibration targets for the model. To ensure a potentially unique calibration, regional recharge estimates of between 3 and 5% of the mean annual precipitation were considered fixed and only hydraulic conductivity values varied within reasonable boundaries (Table 2). A reasonable root mean square error of 9.3 and a correlation coefficient R 2 between modeled and observed groundwater elevations of 80 % was achieved for the steady-state calibration. Following the steady-state calibration of hydraulic conductivities, a transient model calibration against observed historic mine inflows was performed.

Model Predictions The calibrated Evander Basin flow model was used to estimate (annual average) groundwater seepage rates into the mine void, focusing on existing and proposed future mine workings of Shaft 6. The following modeling scenarios were used to determine the operational dewatering rate for Shaft 6: 1. Inflows into existing workings of Shaft 6 while continuous dewatering of Shaft 8; 2. Inflows into existing workings of Shaft 6 while flooding of Shaft 8; 3. Inflows into existing and future workings of Shaft 6 while continuous dewatering of Shaft 8; 4. Inflows into existing and future workings of Shaft 6 while flooding of Shaft 8. The dewatering rate (i.e. mine inflows) for the existing and future Shaft 6 mine voids is estimated to be approximately 2.5 M m 3 /a (Table 3) The simulation results indicate only marginal increases of inflows into Shaft 6 if the neighboring Shaft 8 is decommissioned due to the fact that the flooding of the mine voids will take considerable time, i.e. beyond the life of Shaft 6 operations. In other words, Shaft 6 is for its operational life still within the existing cone of dewatering. Table 3. Simulated mine inflow rates for Shaft 6. Model Scenario Inflow m 3 /a Ml/d 1) Pump Shaft 6 and 8 943 400 2.58 2) Pump Shaft 6, stop Shaft 8 963 450 2.64 3) Pump Shaft 6 (future) and 8 2 469 700 6.77 4) Pump Shaft 6 (future), stop Shaft 8 2 499 700 6.85 In order to model potential post-closure impacts related to rebounding groundwater levels once active dewatering of the underground mine workings stops, it was assumed that pumping ceases at the end of the life of Shaft 6, i.e. after 31 years. The flooding of the underground mine workings and rebound of the regional groundwater levels is controlled by the volume of the mine voids as well as the hydraulic properties of the surrounding aquifer systems and driven by recharge of the shallow aquifer along with leakage into the deeper aquifer as well as deeper regional groundwater flow. For the post-closure model scenario, groundwater seepage into the mine voids was no longer removed from the model domain (water balance), but allowed to fill up the mine voids over time, i.e. the mine and ground-water levels are allowed to rebound freely. Interconnectivity of the Shaft 6 mine voids was assumed and accordingly equal water-levels within the void specified. The computation of the rebounding water table in the aquifer takes cognizance of interactions with the mine voids as well as unsaturated and saturated flow conditions. Once the active dewatering is stopped, modeling results indicate that it will take approximately 17 years for the water level in the mine itself to reach pre-mining water levels in the shaft and more than 60 years for the rebounding groundwater levels in the aquifer (Figure 4) to approximately stabilize at an elevation of 1588 mamsl.

Figure 4. Vertical cross-section of the simulated potential heads 50 years after mine flooding. With a rebound of the water levels, pre-mining regional flow directions towards other discharge areas are re-established and limit a further rise of water levels. Based on the current post-closure model simulations, groundwater will not decant from the top of Shaft 6 at 1631 mamsl. However, lower lying shafts (i.e. below 1588 mamsl) in the coal mining areas could potentially decant if water levels in the coal mines are not managed. 3. REFERENCES Burger, M. (2010) Investigaiton into the groundwater interaction between a deep coal mine and a deeper lying goldmine. Upublisehd M.Sc Thesis. Institute for Groundwater Studies, University of the Free State, Bloemfontein, South Africa. Harmony Gold Mining Company (1994) Kinross Mines Limited, Environmental Management Programme Report. Jansen H., Schalk FS., Leube A., Snyman AA., Steyn APA., De Jager FSJ. (1972) The geology of the country around Standerton with a contribution on the Evander goldfield. Geological Survey. Government Printer, Pretoria, South Africa. Jasper Muller Associates (JMA) (2002) Compilation of geology and groundwater inputs for the Middelbult block 8 EMPR SASOL coal volume I and II text and appendices. Report prepared for Sasol Mining (Pty) Limited. König, C. ( 2011). User s Manual Version 4.037, delta h-ingenieurgesellschaft mbh, Witten, Germany. Tweedie, E.B. (1986). The Evander Goldfield. Mineral Deposits of Southern Africa Vol. 1, Anhaeusser, C.R., and Maske, S. (eds). Geological Society of South Africa, Johannesburg, South Africa, 705-730. Van Biljon, M. (2007) Harmony Gold Mining Limited Evander operations: Geohydrological assessment of the groundwater table in Evander no. 6 shaft. Rison Groundwater Consulting. Vermeulen, P.D. and Dennis I. (2009). Numerical Decant Models for Sasol Mining Collieries, Secunda. Institute for Groundwater Studies, University of the Free State, Bloemfontein, South Africa.