**1. Introduction**

Mining has been the mainstay of many economies across the globe for centuries. There is evidence, for example, of copper mining in Cyprus from as early as 4000 BC and from the Rio Tinto deposits in Spain from 1200–1500 BC [1]. However, along with the economic growth spurred by mining came unprecedented environmental pollution. The contamination of water resources by high concentrations of metals, non-metals and radionuclide elements has been reported from Spain and Portugal [2], to Australia [3] and South Africa [4]. Groundwater may be contaminated by direct infiltration of leachate from mine tailings and other mine wastes or following under‐ ground disposal of mine wastes [5]. Contaminated groundwater then recharges surface water with acidic metal-laden water (acid mine drainage) (**Figure 1**).

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Figure 1.** A river in the west of Johannesburg, South Africa, contaminated by acid mine drainage from disused under‐ ground mines. Note the orange colour which is due to deposition of iron flocs on the river bed.

Acid mine drainage (AMD) is formed via a cascade of reactions (Equations 1–4) when sulphide minerals are exposed to oxygen by mining [6]. The process begins when oxygenated water percolates through the finely divided tailings and pyrite is oxidised to ferrous iron (Equation 1) and then to ferric iron (Equation 2). Ferric iron which is soluble at pH below 3.5 then acts as an additional oxidising agent for pyrite (Equation 3). Above pH 3.5, ferric iron precipitates as Fe(OH)3 (Equation 4); a reaction that is able to buffer the pH of AMD at pH 2.5–3.5 [7].

$$\text{FeS}\_2 + ^7\text{/}\_2\text{O}\_2 + \text{H}\_2\text{O} \rightarrow \text{Fe}^{2+} + 2\text{SO}\_4^{2+} + 2\text{H}^\* \tag{1}$$

$$\text{Fe}^{2+} + ^1/\_4\text{O}\_2 + \text{H}^+ \rightarrow \text{Fe}^{3+} + ^1/\_2\text{H}\_2\text{O} \tag{2}$$

$$\text{FeS}\_2 + \text{Fe}^{3+} + \text{l}2\text{H}\_2\text{O} \rightarrow \text{l}2\text{Fe}^{2+} + 2\text{SO}\_4^{2+} + \text{l}6\text{H}^\* \tag{3}$$

$$\text{Fe}^{3+} + 3\text{H}\_2\text{O} \rightarrow \text{Fe(OH)}\_3 + \text{\textdegree\text{H}^+} \tag{4}$$

This sustained acidity leads to the dissolution of other sulphide ores hence the presence of ions including Ag, Au, Cd, Co, Mn, Ni, Hg, Mo, Se, U, Th and Zn in mine drainage [4] and metalladen water percolates through the tailings heaps to recharge groundwater. A conceptual model of this process was supplied by Tutu et al. [4] (**Figure 2**). In this model, ingress through the dump by oxygenated water results in the oxidation of tailings and the dissolution of elements followed by a downward movement of these dissolved ions into groundwater. This chapter focuses on the transformation and mobility in groundwater, of inorganic contaminants originating from mining activity. Here, groundwater encompasses water in aquifers below tailings dumps as well as that in pores within tailings (pore water).

**Figure 2.** A conceptual model of the downward movement of elements through tailings dumps into groundwater (Af‐ ter [4]).
