Acid mine drainage

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Chapter 2 LITERATURE REVIEW

 Introduction

After a detailed desktop study, examination of aerial photographs/topocadastral maps and interviews, five main contributors to the contamination of the Wonderfonteinspruit were identified. These are acid mine drainage from the decant point, informal townships, the Flip Human sewage works, the tailings dam and some agricultural activities taking place along the river. The literature review focuses on the different impacts possibly resulting from the contaminators. Possible mitigation methods that have been applied throughout the world are also highlighted.

Acid mine drainage

Acid mine drainage (AMD), or acid rock drainage (ARD), refers to the outflow of acidic water from (usually) abandoned metal mines or coalmines. ARD occurs naturally within some environments as part of the rock weathering process but is exacerbated by large-scale earth disturbances characteristic of mining and other large construction activities, usually within rocks containing an abundance of sulphide minerals (Wildeman, Gusek & Brodle, 1991). This chapter will only focus on AMD as it is the main contributing factor to the deterioration of the Wonderfonteinspruit water quality.

Impacts

Sub-surface mining often progresses below the water table, so water must be constantly pumped out of the mine in order to prevent flooding. When a mine is abandoned, the pumping ceases, water levels rise and flood the underground portions of the mine. After being exposed to air and water, oxidation of metal sulphides (often pyrite, which is primarily composed of iron-sulphide) within the surrounding rock and overburden generate acidity. AMD is characterised by high sulphate concentrations, high levels of dissolved metals and the pH is generally <4.5. The chemistry of the oxidation of pyrites, the production of ferrous ions and subsequently ferric ions, is very complex, and this complexity has considerably inhibited the design of effective treatment options. Although a host of chemical processes contribute to AMD, pyrite oxidation is by far the greatest contributor.
The solid pyrite, when introduced to oxygen and water, is catalysed to form iron (II) ions, sulphate ions, and hydrogen ions. The hydrogen ions bind to the sulphate ions to produce sulphuric acid (Hedin, Watziaf & Naim, 1994). In some AMD systems temperatures reach 50°C, and the pH can be as low as 3.6. Figure 2.1 illustrates acid mine drainage.
Figure 2.2 illustrates the so-called ‘Yellow boy’ in a stream receiving acid drainage from surface coal mining. When the pH of AMD is raised past 3, either through contact with fresh water or neutralising minerals, soluble iron (II) ions hydrolyse to form iron (III) hydroxide, a yellow-orange solid colloquially known as ‘Yellow boy’. ‘Yellow boy’ discolours water and smothers plant and animal life on the streambed, disrupting stream ecosystems. The process also produces additional hydrogen ions, which can further decrease the pH (Hedin, 1994).
The iron hydroxides (‘yellow boy’) that precipitate in pond treatment systems may be a valuable resource. The idea to recover the iron has been around at least since the turn of the century and is now becoming a reality. The iron hydroxides deposited in passive pond treatment systems are consistently very high in iron content, and are being proposed for use in sewage treatment, as pigment, as colorants in construction material, and other uses. Research is currently being conducted into the feasibility of using ‘yellow boy’ as a commercial pigment (Sikora, Behrends, Brodie & Bulls, 1996).

Remediation

In research done worldwide five main treatment options were identified for acid mine drainage namely:

• Carbonate neutralisation,
• Ion exchange,
• Active treatment with aeration,
• Precipitation of metal,
• Constructed wetlands.

These five treatment options will be discussed in more detail in order to establish the suitability of these treatment methods for this study (these treatment options will be analysed and the best alternative will be selected as remediation method in Chapter 4).

Carbonate neutralisation

Generally, limestone or other calcareous strata that could neutralise acid are lacking or deficient at sites that produce acidic rock drainage. Limestone chips may be introduced into sites to create a neutralising effect (Ziemkiewicz, Skousen & Lovett, 1994). Where limestone has been used, such as at Rheidol in mid Wales, the positive impact has been much less than anticipated because of the creation of an insoluble calcium sulphate layer on the limestone chips, coating the material and preventing further neutralisation.

Declaration
Acknowledgements 
Abstract
Abbreviations 
List of Figures 
List of Tables 
Chapter 1 INTRODUCTION
1.1 Background to the study
1.2 Motivation for the study
1.3 Mining background
1.4 Study area
1.5 Aim
1.6 Objectives
1.7 Research methodology
1.8 Organisation of chapters
Chapter 2 LITERATURE REVIEW 
2.1 Introduction
2.2 Acid mine drainage
2.3 Informal settlements
2.4 Sewage works
2.5 Tailings dam
2.6 Agricultural activity
2.7 Conclusion
Chapter 3 DATA COLLECTION AND DISCUSSION OF RESULTS
3.1 Introduction
3.2 Impacts
3.3 Water quality
3.4 Water volumes
3.5 Impact of additional water
3.6 Conclusion
Chapter 4 EVALUATION OF REMEDIATION METHODS 
4.1 Impacts identified through the literature review
4.2 Best remediation option
4.3 Conclusion
Chapter 5 RECOMMENDATION FOR APPROPRIATE MITIGATION METHODS 
5.1 Wetland capability
5.2 Chemical remediation
5.3 Wetland design
5.4 Site specific wetland requirements
5.5 Positioning of wetland
5.6 Delineation
5.7 Other mitigation methods
5.8 Conclusion
Chapter 6 CONCLUSION
6.1 Introduction
6.2 Summary of the study
6.3 Concluding comments
References 
Appendix
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