The long-term impact of intermine flow from collieries in the Mpumalanga Coalfields

English: Mines fill up with water after closure. As a result, hydraulic gradients develop between them and different hydraulic water pressures are exerted onto peripheral areas or compartments within mines. This results in water flow between mines, or onto the surface. This flow is referred to as intermine flow. Intermine flow as a concept includes the quantity and quality of the water. The significance of this project lies in: • The establishment of a Geographic Information System (GIS), detailing the various aspects around intermine flow for the Mpumalanga Coalfields. • The identification and discussion of management options to minimise the long-term impact of intermine flow on the environment. • The provision of this information to the industry and government for the assessment of future liabilities and impacts. The total area investigated constitutes 26 000 km2 with the mine lease areas amounting to 4250 km2 • This covers all of the collieries in Mpumalanga Province from where information could be obtained. Information available in the South African coal-mining industry suggests that mines fill up with water and decant after closure. This usually occurs within 10 y. At the more isolated collieries, rebound of the water level may take up to 50 y. Apart from the fact that mine water is saline, low pH-values may also be encountered. Mining has been on all five coal seams in the area. Statistics on mined areas are provided below: See table in full text. Sufficient connectivity exists between mines and the surface to allow interflow of mine water. These connections are in the form of opencast mining, shafts, prospect boreholes and subsidence structures. Interflow of water on coal seam levels is also possible in most of the collieries due to the extensive nature of mining, particularly in the Witbank and Secunda areas. Pathways in mines dictate flow of underground water, rather than natural flow paths. Many critical areas of potential intermine flow have been identified, where significant quantities of mine water will transfer from one mine to the other. The combined impact in terms of anticipated water that will be available for interflaw or to decant from the various mines is in the order of 360 MLld with a sulphate load of 660 t/d. For illustrating the significance of these volumes, the total volume to decant from these mines is in the same order as the annual natural run-off into the Witbank Dam. The range of quantities and qualities for individual mines will vary significantly, depending mainly on the mining method employed. Many other site-specific factors also play a role. Management options to cope with intermine flow are applicable on either a catchment or mine basis. Those on a catchment basis are: • The reduction of the number of decanting points through the interconnection of mines. • The control of decanting positions through interconnection of mine workings. • The conjunctive treatment of decanting mine water at convenient locations. On a mine basis, the following actions will markedly reduce the volume of water or the amount of salt to be discharged: • Design long-term water management schemes taking cognisance of neighbouring mining activities. • Design the mine lay-out to retain as much of the mine water as possible in the underground workings whilst mining. • Investigate coal barrier characteristics and design coal extraction accordingly. • Minimise water ingress into mines to reduce water volumes, if required. • Mix mine water of various qualities to achieve the best possible quality before flood discharge. • Minimise salt loads by: • Flooding mine workings as soon as possible. • Flushing flooded mines. • Utilising the natural neutralisation potential of the coal and rock. Examples of intermine flow already exist on an alarming scale in the Mpumalanga collieries. This study shows its impact on surface and also at barrier pillars in individual collieries. Mines should incorporate mine-water interflaw into their planning phase and ensure that this fits in with activities of their neighbouring mines.