Prediction of Acid Mine Drainage Formation
James Manchisi and Sehliselo Ndlovu
Many countries have now enacted national legislation, signed international conventions and regional agreements and protocols that recognise the use of environmental impact assessment (EIA) tool as a key legal instrument to manage environmental impacts of development projects and policies (Maest et al., 2005; Morgan, 2012). The International Association for Impact assessment (IAIA, 1999) defines EIA as "the process of identifying, predicting, evaluating, and mitigating the biophysical, social, and other relevant effects of development proposals prior to major decisions being taken and commitments made". The specific forms of impact assessments may include environmental, social, health, sustainability, regulatory, human rights, cultural and climate change (Morgan, 2012). Thus, EIA process is crucial for identifying and predicting the potential impacts of projects such as mining on the biophysical and social environments. In addition, the EIA is used as an environmental management tool to develop environmental management plans (EMPs) as measures to mitigate impacts. The basic EIA steps include screening, scoping, impact prediction and evaluation, mitigation and follow-up studies for implemented projects to provide feedback (Noble, 2011; Castilla- Gomez and Herrera-Herbert, 2015). The potential for acid mine drainage (AMD) to form at mine sites is one of the key questions to be answered in an EIA process. A detailed discussion of the activities for each step in the EIA process is given by Noble (2015).
Despite many socio-economic benefits, the legacy of mining is mostly associated with many environmental impacts such as land degradation, solid waste disposal challenges, biodiversity loss, AMD, air and water pollution. This chapter focuses on the problem of AMD at mine sites and gives a critical review of its management through prediction. It is evident that mining, quarrying, excavation and mineral beneficiation activities to recover mineral-based products such as base metals, uranium, precious metals, coal and industrial minerals often expose sulphidic materials to the outside environment with the potential to form AMD (Rae et al., 2007). As part of EIA process, it is now a regulatory requirement for mine owners in most countries to predict the AMD potential for all types of mineral wastes and prepare management plans for future mitigation and to protect the environment at all stages of the mine life cycle (Maest et al., 2005; Price, 2009; Elaw, 2010).
The issue of AMD at mine sites has been a challenging environmental problem for many years. It is known to have contaminated soils, polluted water, affected ecosystems and destroyed biological resources (Banks et al., 1997; Bell et al., 2001; Gordon, 1994; Gray, 1997). Rae et al. (2007) classify AMD discharge as either acidic, neutral or saline (alkaline). Although the environmental impacts of acidic drainage are well known, the neutral and alkaline drainages may also be harmful and difficult to manage if they are metalliferous in nature, that is, if they contain elevated levels of dissolved metal ions. However, alkaline drainage is rare relative to acidic or metalliferous drainage (Rae et al., 2007; Price, 2009).
In the past, mining firms managed AMD problems by developing remediation plans only after its occurrence and subsequent impacts. The consequences of this reactive approach were extensive damage to natural resources and huge remediation costs. It was later realised that the practice was uneconomic and environmentally unacceptable, and so, the mining sector changed and started to focus more on proactive mitigation measures based on accurate prediction of drainage chemistry to prevent AMD formation and its impact (USEPA, 1994; Price, 2009). Furthermore, the interests for early assessment and detection for potential of mineral waste materials to generate acid using various prediction methods arose out of concerns of lag times associated with the occurrence of AMD and the costly lessons learnt from its long-term (perpetual) care over the years (USEPA, 1994; Price,
2009). In addition, the prediction of drainage chemistry was driven by the legal requirements in many countries such as the United States, Australia, Canada, etc., which stipulated that mining permits would only be granted if applicants included plans with practical methods to avoid AMD (USEPA, 1994; Price, 2009). This requirement led to increased efforts to develop techniques to predict the drainage chemistry and understand methods to avoid pollution (Lawrence et al., 1989; Ridge and Seif, 1998).
Lawrence et al. (1989) noted that accurate prediction of AMD formation would provide the basis for the development of effective waste management plans to prevent acid generation and/or allow for use of innovative methods to contain and treat unavoidable AMD. Other key benefits of predicting AMD formation include a reduction in environmental damages and remediation costs, and timely planning for mitigation facilities. Prediction also ensures a sustainable development of mineral resources by preventing impacts of AMD on water resources, aquatic and terrestrial life, vegetation, human life and livelihoods. The proposed proactive approach based on prediction agrees with sustainability principles and solid waste management hierarchy, in which the sustainability preference is to prevent AMD formation rather than treatment (Rae et al., 2007; El-Hagger, 2007; Lottermoser, 2011).
The aim of this chapter is to review the current understanding and recent developments in the AMD prediction methods, analytical procedures and any limitations that may arise during the characterisation of drainage chemistry at mine sites. In addition, the chapter will assess the sustainable AMD management options that eliminate risks and/or prevent AMD formation through prediction tools.