Diagram showing the effect of drainage water from a tailings dam into the subsurface layer, water rises to the surface creating a marshy area 

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... the scarcity of surface water causes small towns and settlements to rely largely on groundwater sources for potable water. There are very little, if any, mining activities that do not have an impact on groundwater in some way. The impacts vary with the different stages of mining, i.e.  Operational phase: Dewatering of a mining area, groundwater exploitation for use in the mining operations, construction of tailings dams and waste rock dumps which can pollute the underlying groundwater system.  Closure and post-closure phases: Flooding and decanting of mines, acid water drainage and associated water quality problems. The impact of waste facilities should also be included due to drainage into the groundwater system. Until the early 20 th century, virtually all tailings were simply released in the closest watercourse. Where mining continued for a number of years this could sometimes lead to surface water features such as rivers, dams and even estuaries becoming clogged with tailings. This in turn had a devastating impact on the riparian zone and as such prompted the development of alternative tailings disposal methods. By the 1940’s the sedimentation dams, also known as tailings dams were becoming common practice in the mining industry world-wide [6], although this practice has been used in South Africa from the outset of gold mining on the Witwatersrand. Though tailings dam failures are amongst the most spectacular causes of pollution, they are not the most common or widespread. The alteration of regional water flow patterns (as a tailings dam can form a recharge point) and pollution thereof is far more common. A typical example is the re-vegetation of tailings dams to reduce surface erosion and stabilization of the topsoil in these dams. This rehabilitation however does not reduce the leaching of contaminants into the groundwater system or surface environment. The tailings dam design and the nature of the tailings plays a large role in the impact of the tailings dam on the groundwater system. These effects can be summarized into the following main groups:  The inclusion of a lining beneath the dam.  The long-term rehabilitation plan of the tailings dam.  The chemical and physical properties of the tailings.  The water level in the tailings dam.  The hydraulic conductivity of the tailings dam. Tailings are predominantly fine-grained with a grain size of less than 1 mm. The hydraulic conductivity often falls into the same range as those of clay and silt (1 x 10 -8 – 1 x 10 -3 m/d). Tailings conductivities generally range from 1 x 10 -1 – 1 x 10 -5 m/d. The magnitude of the groundwater related issues is not only related to the design of the tailings dam and types of tailings but also to the nature of the subsurface. These effects can be assigned to different parameters which influence the groundwater system:  Recharge, directly from precipita- tion or artificially via the tailings dam aids in the movement of pollutants and dilution.  The consolidated, unconsolidated or karst rock matrices that serve as water- bearing units. The fractures that occur in the rock matrix must also be taken into account.  The unsaturated zone which is the portion of the rock matrices which occurs between the base of the tailings dam and the groundwater (piezometric) level [1]. All infiltrating water must pass through this zone before it reaches the groundwater system. The deeper the groundwater level the less chance it has of being polluted due to attenuation of possible pollutants by means of chemical reactions, dilution etc. In many cases a freatic water level develops underneath the tailings dams if it is in operation for many years and a confined layer is present between the groundwater level and the original ground surface.  The hydraulic conductivity of all subsurface media has a direct influence on the rate at which water and associated pollutants can transverse the aquifer. Examples of related hydraulic conductivity effects include:  When water moves through the tailings dam, the rate at which it moves is Marshes can occur in the vicinity of a tailings dam if the underlying soil and rock media have a very low hydraulic conductivity when compared to that of the tailings dam. Seepage at the toe of tailings dams is also a consequence of low depended on the water level in the dam and the hydraulic conductivity of the tailings. If the hydraulic conductivity is too low then the water might not be able to drain through the system fast enough, causing seepage along the banks of the tailings dam, see Figure 1. This is exacerbated by anisotropic hydraulic conductivity, often 2 to 10 times greater in the horizontal direction than the vertical owing to the layering arising from the deposition practice. If a lower hydraulic conductivity layer is overlain by a higher conductivity layer, seepage can occur at the contact of these two layers (see Figure 1). hydraulic conductivity below the dam. An example of this is shown in Figure 2. The water collected in the marsh can contain high levels of dissolved solids and other polluted species. Groundwater levels in the vicinity of tailings dam can also increase if there is a barrier such as a dyke in the vicinity of the tailings dam and the water seeping into the groundwater system is at a faster rate than the movement of the groundwater see Figure 3. In this scenario water can flow either into deeper aquifer systems or redirect into mining operations flooding the mining area with contaminated water.  Some gold tailings dams with high pyrite content develop an impermeable thin layer with an extremely low infiltration rate of < 0,05 m/d comparing to others with an infiltration rate of > 0,24 m/d [5]. In the case of other tailings dams the infiltration rate could be as high as 0,5 m/d. In order for a mine to obtain a mining license, an environmental impact study has to be performed on the area. The most important step in this assessment is site characterization. Site characterization can be seen as the process whereby all relevant data concerning the site and the problem at hand is collected and interpreted to form a conceptual model. The following questions must be addressed during site characterization:  What is the spatial extend and height of the tailings dam?  What is the piezometric level in the tailings dam?  What are the composition, size and hydraulic conductivity of the tailings?  Is the tailings dam lined, if so what are the properties of the lining? Is the tailings dam capped, if so what are the properties of the capping?  What are the properties (including hydraulic conductivity) and thickness of the unsaturated zones?  What is the piezometric level of the groundwater system?  What is the hydraulic conductivity of the groundwater system?  Are there any aquifer boundaries and, if so, what type of boundaries?  Is there any groundwater abstract- tion is the vicinity of the site?  What is the natural groundwater recharge? Once this information is available then a conceptual model for the site can be developed. The conceptual model forms the basis for understanding groundwater related problems. It is important at this point to highlight the effect of data shortages in the conceptual model and gathering of data if it is needed to complete the analysis framework. Once all required data have been collected the conceptual model can be refined and assessments made. There are numerous tools available for assessing the impact of a tailings dam on the groundwater system. Two of the most widely used tools Risk assessments and Mathematical models. Firstly, a risk can be defined broadly as the probability that an adverse event will occur in specified circumstances. There are numerous commercial risk assessment packages such as Risk Workbench that can be used to perform groundwater related risks. However, the Institute for Groundwater Studies at the University of the Free State has developed a fuzzy logic based risk assessment tool (South African Groundwater Decision Tool - SAGDT) that can be used to determine the impacts of mining including tailings dams on the groundwater system [3]. This freeware can be obtained from the Department of Water Affairs and Forestry at the following website: www.usersupport.co.za. The main usage of this software is to include pre-selected weights into the system for specific hydrogeological parameters which can then be evaluated to determine the risk factor to the groundwater system. Secondly, mathematical models are generally divided into analytical and numerical models. There are many analytical models based on Darcy’s Law that can be used to determine seepage of water and associated pollutants; however these models are usually idealized and empirical in nature. MODFLOW is one of the most-used numerical models to simulate the impacts of tailings on the groundwater system. If the conceptual model is correct, the results of the assessments based on this modeling package are usually of a high quality, however there are major assumptions that are usually overlooked when using this package. i. It assumes that the tailings dam and the groundwater system are directly connected, since MODFLOW only takes into account saturated flow conditions. ii. In order to simulate the unsaturated flow more accurately numerical model software such as FEFLOW should be considered. iii. Many of the models developed are for porous flow conditions and therefore the simulation of fracture zones is quite difficult or impossible in some cases. iv. Similarly there are many assumptions on which the mass transport models are based. These assumptions are usually neglected and this can cause some serious errors in the results obtained if the movement of pollution is considered. A vital step in determining the correctness of the evaluation process is the calibration of the data which can include further site monitoring processes, updating and review of simulation results. The ...