Hydrogeological assessment of Acid mine Drainage impacts in the West Rand Basin, Gauteng Province

Abstract

The comparatively short history of acid mine drainage (AMD) in the West Rand Basin (WRB) via mine water decant on the Randfontein Operations property (formerly Randfontein Estates Ltd) of Harmony Gold Mining Company west of Krugersdorp (Figure 1) is reasonably well documented (e.g. JFA, 2006; Coetzee, 2005). Decant first manifested on surface at a position very near borehole BH1 (Figure 1) on 27 August 2002 and later, on 3 September 2002, at the Black Reef Incline (BRI) shaft (Figure 1) some 200 m to the south (Du Toit, 2006). Initial estimates of the decant volume ranged between 7 ML/d in winter and 12.5 ML/d in summer (JFA, 2004). In early-2005, additional decant reported on surface at 18 Winze (Figure 1), an abandoned shaft to the east on the slope above the BRI (Coetzee, 2005). More recent estimates (Coetzee, 2005) put the rate of decant at between 18 and 36 ML/d. The subsequent development of a permanent water body in the Hippo Dam on the Tweelopie Spruit in the southern part of the Krugersdorp Game Reserve (KGR), together with the development of seeps and springs, reflects the more recent surface manifestation of mine void flooding and decant in the area. Prior to this, anecdotal evidence has it that the dam held water for only a few days before drying up, hence its other name of Dry Dam. The ramifications of decant for the subregion are enormous. The greatest focus in this regard is undoubtedly the Cradle of Humankind World Heritage Site (CoHWHS), which includes the home of “Mrs Ples” in the Sterkfontein Cave system. Of no lesser concern, however, are the downstream landowners and agricultural activities that are largely or wholly dependant on groundwater for potable and business use. In order to determine and implement the most appropriate acid mine water drainage management measure(s), it is necessary to first understand the hydrophysical environment that defines and informs the groundwater dynamic in the subregion. This dynamic includes the response of the groundwater regime to both natural and anthropogenic recharge mechanisms. The latter are predominantly mining related as might be associated with defunct underground workings, defunct and operational surface (opencast) workings and tailings dams. The interaction between surface water and groundwater represents another facet of this dynamic and, apart from AMD, also finds relevance in the discharge from two municipal waste water treatment works (WWTW), viz. the Randfontein WWTW to the southwest in the headwaters of the Riet Spruit, and the Percy Stewart WWTW on the Blougat Spruit to the northeast (Figure 1). The study reported herein explores this dynamic and ancillary issues by consolidating and comparing readily available historical data with “new” data sourced in early-2007