Dissemination of Acid Mine Drainage

Hydrological processes control the mobilisation and dissemination of both AMD and the associated contaminants into other environmental compartments (Fosso-Kankeu et al., 2017; Santisteban et al., 2016; Oldham et al., 2019). The environmental compartments include soils, surface aquatic systems such as rivers, streams and reservoirs, and groundwater systems. A number of hydrological processes such as surface water runoff, sub-surface water flow, and groundwater flow mobilise and disseminate AMD in the environment (Chaubey and Arora, 2017). Surface water runoff and erosion mobilise and transfer AMD and associated colloids from various sources into surface aquatic systems such as rivers, streams and reservoirs (Ravengai et al., 2004; Masocha et al., 2020).

Groundwater flow, which is governed by Darcy's law, also transport AMD and contaminants from regions of high total hydraulic heads to those with low heads, in response to a total head gradient (Francisca et al., 2012). Surface water-groundwater interactions or exchanges provide the hydrological connectivity between surface aquatic systems and the groundwater systems (Chaubey and Arora, 2017). Therefore, the surface water-groundwater interactions facilitate the transfer of AMD and contaminants between surface aquatic systems and groundwater. Surface and underground cavities, holes, and cracks created during excavations, drilling, and blasting act as connecting and preferential pathways for the flow of AMD and contaminants.

Equations 1.5 to 1.9 show that hydrological fluxes/flows in both surface and groundwater systems are strongly coupled to contaminant transport (Francisca et al., 2012; Chaubey and Arora, 2017). Three key processes are responsible for contaminants transport by water (Francisca et al., 2012; Chaubey and Arora, 2017): (1) advection/convention/mass flow, (2) diffusion, and (3) dispersion. Advection or mass flow is driven by flowing surface water or groundwater flow, and flow velocity (Equations 1.6-1.7). Diffusion is governed by Fick's law, and transfers contaminants from regions of high to low concentrations, in response to a concentration gradient (Franscisca et al.,

2012). Dispersion is caused by turbulences and flow heterogeneities induced by spatial variability in porosity and hydraulic conductivity or permeability of groundwater-bearing rock formations. Diffusion and dispersion occur concurrently, hence, are often collectively termed hydrodynamic dispersion. The governing equations for mass flow and hydrodynamic dispersion have a flow velocity as a variable, indicating the importance of flow velocity in contaminant transport (Equations 1.5—1.7):

In Equations 1.5 to 1.7, the first and second terms on the right-hand side represent advection/mass flow and hydrodynamic dispersion, respectively. In these equations, V = water flow velocity (L/T), c = contaminant concentration (mg/L3), x = distance (L), and DL = longitudinal dispersion coefficient, calculated as a product of longitudinal dispersivity (aL) and velocity (V) (i.e., DL = aL x V), Rd = retardation factor, and X = first -order degradation or decay constant.

Depending on the nature of contaminants and biogeochemical conditions, two transport phenomena may occur: (1) reactive transport, and (2) non-reactive transport. Equation 1.5 depicts the governing equation for non-reactive transport of conservative (i.e., non-reactive) contaminants such as chloride. Non-reactive transport occurs when conservative contaminants such as chlorides only undergo transport without any reactions. Thus, barring the effects of dilution, the plume of a non-reactive contaminant is often characterized by a relatively fixed concentration along the transport pathway.

Reactive transport occurs when non-conservative (i.e., reactive) contaminants in AMD such as metals and nutrient ions (e.g., nitrates, phosphates, sulphates) simultaneously undergo transport and reactions. Typical reactive processes include retardation via adsorption onto solid matrices, and biochemical decay or degradation. Equations 1.6 and 1.7 show reactive transport, where the contaminant undergoes retardation (Equation 1.6) and decay (Equation 1.7), respectively (Francisca et al., 2012; Sadrnejad and Memarianfard, 2017; Sethi and Di Molfetta, 2019). An example of contaminants that undergo reactive transport is metals, which undergo adsorption on the solid matrix such as soils and sediments, a process that slows the transport processes. Other reactive contaminants include nutrients such as nitrates, phosphates and sulphates, which are taken up by microbes and aquatic plants during the transport process. Hence, depending on the nature of the reactions, the concentrations of reactive or non-conservative contaminants may undergo attenuation or increase along the transport pathway.

In summary, regardless of the nature of the transport phenomena (reactive or non-reactive), surface water and groundwater flows control the transport and dissemination of AMD and contaminants. The dissemination via hydrological processes explains the potential off-site environmental, human and ecological health impacts of AMD. Collectively, these processes demonstrate how hydrology controls the formation, mobilisation and subsequent dissemination of AMD and contaminants.

 
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