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‘Sustainable’ yield concept
A misconception of the meaning of the word ‘sustainable’ in terms of groundwater use seems to exist, leading to degradation of wetland and riparian ecosystems due to stream dewatering and groundwater depletion. Sustainable yield is often confused with ‘safe yield’, which assumes the attainment of a long-term balance between groundwater abstractions and annual recharge and often calculated as a percentage of the natural recharge (Sophocleous, 1997; Zhou, 2009). The concept of ‘sustainable’ yield was introduced to overcome the shortcomings of the safe yield idea.
In simple terms groundwater sustainability can be defined as development and use of groundwater in a manner that can be maintained for an indefinite time without causing unacceptable environmental, economic, or social consequences (Alley et al., 1999). Despite Bredehoeft’s (2002) provocative theory, known as the “water budget myth”, that sustainable groundwater development has almost nothing to do with natural recharge, it is still widely accepted that natural recharge is a very important factor in the assessment of sustainability (Sophocleous, 2005; Alley and Leake, 2004; Kalf and Wooley, 2005, Zhou, 2009). However, Bredehoeft (2002) reminded hydrogeologists that aquifers are more complex than an oversimplified water balance can accommodate. According to Devlin and Sophocleous, (2005) distinction should be made between the concepts of sustainable pumping rate (source yield) and sustainability (resource yield) and that recharge rates cannot be ignored in spite of the fact that sustainable pumping rates can be estimated without them.
Resource yield Aquifers are in a state of natural long-term equilibrium prior to pumping, and in order to maintain this equilibrium, groundwater abstractions must be balanced by increased recharge (increase in the amount of water entering the system) or reduced discharge (reducing the amount of water leaving the system) (Figure 2.14). The increase in recharge and decrease in discharge is termed ‘capture’ and equilibrium is achieved when it balances pumping. Thus, it is only after pumping that the change in recharge will contribute to the determination of a sustainable yield (Seward et al., 2006). When considering a groundwater basin as whole, all the individual abstraction points where ‘capture’ is possible, permissible and sustainable, will together define the basin yield (Seward et al., 2006). Abstractions from storage indicate on the other hand non-equilibrium conditions. Where the abstraction is greater than the difference between the maximum inflow (R+ΔR) and residual outflow (D) then an additional mining yield (change in S) is required to maintain the abstraction rate (Figure 2.14). It must be emphasized that the spatial distribution of these abstraction points strongly influences the total sustainable basin yield. Withdrawals from an aquifer might have a severe impact on individual ecosystems locally, but per square kilometre these withdrawals might be minor in terms of total recharge and discharge. Hence, a sustainable yield should account for small-scale local impacts and simultaneously consider the ability of an aquifer as a whole to recover from pumping stress (Maimone, 2004).
Many may argue that the “capture recharge principal” does not generally apply to basement aquifers as the resource yield is rarely achieved due to poor boreholes yields and low distribution densities. However, despite the relatively low (often less than 1 ℓ/s) borehole yields in the Limpopo crystalline aquifers, deeply weathered and highly fractured regions boreholes can have yields in excess of 5 ℓ/s. A number of exceptionally high yielding areas not known anywhere else in Africa within the basement aquifer system occur in the Dendron (Mogwadi), Baltimore and Tolwe regions (Figure 1.1). These aquifers have sustained (supposedly sustainable) large scale irrigation for the last few decades, suggesting high storage potential, high permeabilities and an interconnected fracture network with a major source of recharge. Seeing that abstraction rates far exceed vertical recharge rates which amounts to a few millimetre of the 400 mm annual rainfall, it can be argued that these aquifers are potentially recharged through lateral saturated flow from adjacent aquifers, sometime called inter-aquifer recharge.
Development of groundwater
Studies on crystalline basement aquifers have predominantly focused on the development of groundwater resources in thick regolith (i.e. weathered overburden) with dominant intergranular flow in mainly tropical to sub-tropical regions (i.e. western and southern Africa and South America) (Wright and Burgess, 1992). Early workers focused mainly on establishing a correlation between the yield of a borehole and 1) its depth, 2) the geology, and 3) weathering thickness (i.e. Houston and Lewis, 1988; Barker et al., 1992; McFarlane et al. 1992; Chilton and Foster, 1995). More recently, researchers have tried to identify the most important factor(s) in controlling borehole yields in crystalline terrain, in order to identify areas with higher groundwater potential (Mabee, 1999; Moore, 2002; Henriksen, 2003; Neves and Morales 2007). The influence of topography on borehole yield have been shown by many (i.e. McFarlane et al. 1992; Mabee, 1999; Henriksen, 1995) with the common result that wells located in valleys and flat areas show generally higher yields compared to wells located on slopes and hilltops. Although, specific rock types (i.e. granites, gneiss, schist) are in many cases the obvious factor in explaining the variation of borehole yields (Gustafsson and Krásný, 1994; Neves and Morales 2007), the influence is often supplanted by secondary features such as faults, fracture zones and dykes. As a result throughout the last decade the optimization of the location of wells in tectonically fractured areas throughout Africa, India and Brazil focused mainly on assessing the relationship between bedrock structure and groundwater production by analysing the position of wells in relation to lineaments (aeromagnetic data or Landsat images) (i.e. Greenbaum 1992; Fernandes and Rudolph, 2001; Owen et al., 2007; Solomon and Quiel, 2006; Henriksen and Braathen ,2006; Ranganai and Ebinger, 2008; Solomon and Ghebreab, 2008). These investigations are often based on tectonic models where the impact of original or present compressional stress on potential water bearing lineaments is assessed (Fernandes and Rudolph, 2001; Henriksen and Braathen, 2006; Neves and Morales, 2007; Owen et al., 2007). In some cases results showed that the majority of productive water wells are associated with lineaments parallel to the regional maximum horizontal stress in the area (Fernandes and Rudolph, 2001). It is therefore expected that fractures perpendicular to the regional compressive stress are closed and dry (Owen et al. 2007). However, in many cases authors could not establish a firm relationship between well yields and lineaments (Greenbaum 1992; Gustafsson, 1994) and in most cases, there is no direct evidence that the structures responsible for the flow correlate with mapped lineaments (Mabee et al., 2002). According to Sander (2007) this may be attributed to local factors such as fracture infilling, fracture connectivity or the poor knowledge of current stress regimes. It is recognized that lineament mapping is often subjective (Mabee et al., 1994) and depends on factors such as data quality, extraction technique and interpretation method. But when correctly interpreted lineaments are used in conjunction with a good understanding of local geology, tectonics, geomorphology, hydrology and aquifer characteristics, the most promising setting and orientation of fractures for future groundwater exploration can be identified (i.e. Solomon and Quiel, 2006; Galanos and Rokos, 2006; Ranganai and Ebinger, 2008; Solomon and Ghebreab, 2008).
1. INTRODUCTION
1.1. Problem statement
1.2. Investigation objectives
2. CHARACTERISATION OF CRYSTALLINE BASEMENT AQUIFERS
2.1. Conceptual framework
2.2. Groundwater recharge
2.3. Groundwater quality
2.4. Groundwater hydraulics
2.5. ‘Sustainable’ yield concept
2.6. Development of groundwater
3. HYDROGEOLOGICAL ASSESSMENT METHODS
3.1. Investigation approach
3.2. Groundwater recharge estimation methods
3.3. Geochemical investigation methods
3.4. Hydraulic testing
3.5. Assessment of factors controlling the occurrence of groundwater
3.6. Development of a regional conceptual model
4. DESCRIPTION OF STUDY AREA
4.1. Physiography and climate
4.2. Regional geology
4.3. Structural geology
4.4. Hydrogeological description
5. HYDROGEOLOGICAL CHARACTERISATION OF THE STUDY AREA
5.1. Characterisation of groundwater recharge
5.2. Geochemical description
5.3. Pumping test analysis
5.4. Groundwater sustainability (source vs. resource
5.5. Geological and geomorphologic influence on borehole productivity
6. CONCLUSION
6.1. Synthesis
6.2. Recommendations and suggestions for future investigations
7. REFERENCES
