Preventive Techniques for Acid Mine Drainage
This section of the chapter discusses and evaluates, in detail, the preventive techniques for AMD formation. Typically, these techniques are designed to exclude any of the three components that are essential in the formation of AMD: oxygen, water and microorganisms (Kuyucak, 2002; Pozo-Antonio et al., 2014; Park et al., 2019). In other words, prevention of AMD generation mainly requires protection of sulphide-bearing minerals from air, water and microorganisms (Kefeni et al., 2017). It must be noted, however, that the nature of the sulphide- waste materials and the conditions of the mining site dictate the choice and implementation strategy of the preventive techniques (Kuyucak, 2002).
Control of Water Movement
Water is the most important media responsible for the transportation of contaminants and thus all measures aimed at curtailing AMD generation should be concerned with the control of the flow of water (Akcil and Koldas, 2006). The techniques of controlling the flow of water restrict the movement of water through potential acid-producing sulphide materials. In other words, the control technologies rely on the prevention of the entry of water into the AMD source (Barton-Bridges and Robertson, 1989). In this regard, a number of measures can be employed to control the movement of water. Firstly, surface water can be diverted from flowing through potential acid- producing sulphide materials. This can be achieved through the use of well- designed and waterproofed pipes, ditches and/or channels (Kuyucak, 2002; Pozo-Antonio et al., 2014). In addition, surface water diversion may involve the construction of drainage ditches or impervious channels that are able to move surface water quickly across acid-producing sulphide materials (Skousen et al., 2019). Secondly, groundwater can be prevented from infiltrating into sites that contain acid-generating materials using intercepting structures such as grout barriers (or curtains) and reinforced concreate walls (i.e., slurry walls) or even diversion ditches (Skousen et al., 2000; Kuyucak, 2002; Akcil and Koldas, 2006; RoyChowdhury et al., 2015). The use of grouts to separate acid-producing rocks and groundwater has also been recommended by Skousen et al. (2019) who states that "injection of grout barriers or curtains may significantly reduce the volume of groundwater moving through backfills". Thirdly, using underdrains and sealing layers, hydrological water seepage into acid-generating waste can be prevented (Akcil and Koldas, 2006). Fourthly, for open pit mines, it is appropriate to flush or drain water rapidly from spoil heaps before it accumulates to high levels (Akcil and Koldas, 2006). Finally, it is also important to use properly designed slopes and steps in order to minimise the infiltration of water into the mining waste (Kuyucak, 2002; Pozo-Antonio et al., 2014). Despite all the measures to control the flow of water, Sahoo et al. (2013) state that "surface barriers can achieve substantial reductions in water flow through piles, but generally do not control AMD completely".