Investigation of potential water quality and quantity impacts associated with mining of the shallow Waterberg coal reserves, west of the Daarby Fault, Limpopo Province, South Africa

Investigation of potential water quality and quantity impacts associated with mining of the shallow Waterberg coal reserves, west of the Daarby Fault, Limpopo Province, South Africa M Bester and PD Vermeulen* Institute for Groundwater Studies, University of the Free State, PO Box 339 Bloemfontein 9300, South Africa Abstract From South African and international experience, it is known that coal mining has a pronounced impact on surface and groundwater quality and quantity. The influx of water may be as low as 1% of rainfall for underground mines to as much as 20% for opencast mines. Such differences may influence the quantity and quality of surface water and groundwater resources at the local scale and further afield. The Waterberg coal reserves represent the only area with proven coal reserves which are still available for development in South Africa. These reserves are targeted for large-scale mining in the near future, and are situated in a relatively dry part of South Africa. In view of the low rainfall and limited surface water resources, the necessary level of safeguard measures to ensure the quantity and quality of existing water resources is unclear. Experience from other areas cannot necessarily be extrapolated directly. A scoping level study was performed to consolidate the existing information on the geohydrology and pre-mining water quantity and quality of water resources associated with the Waterberg coal reserves. New data regarding water quality and acid-base potential for the different geological areas (through field investigations) and geology and mining methods were obtained. Findings showed a significant likely impact on groundwater resources, with a potential for generation of acid mine drainage, although low transmissivities will in all probability prevent decant after back-filling is complete. Keywords: Waterberg Coalfield, water quality, water quantity, recharge, acid-base accounting, geohydrological modelling Introduction Objectives of the study Local South African and international experience in the field of coal mining has yielded the generally known fact that coal mining has a pronounced impact on surface water and groundwater quality and quantity. The influx of water into the mines may be as low as 1% of rainfall for deep board and pillar mines with no subsidence, to as much as 20% for some opencast mines (Hodgson et al., 2007). Such differences may influence the quantity and quality of surface water and groundwater resources at the local scale and further afield. The Waterberg coal reserves represent the only large area with proven coal resources remaining for development in South Africa. These resources have been targeted for large-scale mining in the near future. The Waterberg coal resources are situated in a relatively dry part of South Africa. In view of the low rainfall and limited surface-water resources, the necessary level of safeguard measures to ensure the quantity and quality of existing water resources is still unclear. Experience gained from other South African mining areas cannot be extrapolated directly as the area is unique in terms of the climatic setting and prevailing local conditions. The study aimed to investigate: • The different aquifers in the study area and their geohydrological parameters • Pre-mining water quantity and quality of water resources associated with the Waterberg Coalfield • The acid-generating potential of the geology found in the study area and buffering capacity of country rock. * To whom all correspondence should be addressed.  +2751-4012482; fax: +2751 4446538; e-mail: Received 8 April 2010; accepted in revised form 3 September 2010. Available on website http://www.wrc.org.za ISSN 0378-4738 (Print) = Water SA Vol. 36 No. 5 October 2010 ISSN 1816-7950 (On-line) = Water SA Vol. 36 No. 5 October 2010 This information will assist in: • Detecting potential problem lithologies with regards to higher acid generation potential • Predicting the impact of additional mines in the area • Determining whether the future mines would ever reach decant level using geohydrological modelling methods • Providing recommendations on future studies. Study area The study area is located in the Limpopo Province of South Africa, and forms part of the Waterberg Coalfield. The Limpopo province is South Africa’s northernmost province (see Fig. 1). The study area covers an area of more than 2 300 km 2. With regards to infrastructure there is presently 1 operational colliery and 1 operational power station located in the study area, with a second power station currently under construction. The area has a low rainfall with a dry climate, receiving only summer rainfall. The average annual rainfall for the area varies between 285 mm and 560 mm (SA Weather Service, 2008). The area is drained by 2 rivers, namely the Mokolo, 531 Figure 1 Location of the study area (outlined in green) running north-south, and the Limpopo, running roughly south/ west - north. Both rivers can be classified as non-perennial. The Limpopo River is an international water course which drains Botswana, South Africa, Zimbabwe and Mozambique. In terms of the National Water Act, 1998 (Act No. 36 of 1998: RSA, 1998), any authorisations for water use activities including disposal of waste associated with mining and power generation will be conditional upon preventing pollution of the river, in order to meet international obligations and conditions of any international agreements. It is necessary to ensure that future planned mining activities are located and configured to minimise pollution with particular consideration of international rivers and their requirements. Geology and mining Coal was discovered on the farm Grootegeluk while drilling for water in 1920. Iscor (Pty.) Ltd. began the development of the Grootegeluk Colliery in the 1970s, with the mine producing its first 200 kt of low-ash coking coal in 1980 (Snyman, 1998). The Waterberg Coalfield trends east–west, and is heavily faulted (see Fig. 2). It is composed of sediments of the Karoo Sequence and forms a graben structure. The Eenzaamheid fault forms the southern boundary, with rocks belonging to the Waterberg Group occurring south of this fault, while the northern boundary is delineated by the Zoetfontein fault with Archaean granites outcropping north of this fault. The Karoo sequence in the area has been faulted by 2 major faults with displacements greater than 250 m, namely the Daarby Fault in the north and the Eenzaamheid Fault in the south. Associated minor faulting within this graben is also apparent and recognizable lineaments have been interpreted over the regional area as well. The Daarby fault subdivides the coalfield into a shallow, opencast-mineable western part and a deeper north-eastern 532 part, across a displacement of some 400 m, trending NE/SW for a distance of approximately 25 km. The Zoetfontein fault was tectonically active before and during Karoo deposition, whi (...truncated)