Investigating a spatial approach to groundwater quantity management using radius of influence with a case study of South Africa

Investigating a spatial approach to groundwater quantity management using radius of influence with a case study of South Africa Paul Seward1*, Yongxin Xu1 and Anthony Turton2 Department of Earth Sciences, University of the Western Cape, Private Bag X17, Bellville, 7535, South Africa 2 University of the Free State, PO Box 339, Bloemfontein, 9300, South Africa 1 ABSTRACT The purpose of this paper is to investigate whether a simple, spatially-based approach to groundwater sustainability using radius of influence should be used to replace the pervasive, yet deprecated, ‘natural recharge water balance’ volumetric method. Using South Africa as a case study, the radius of influence methodology was shown to be scientifically practical, to provide plausible results, and to be permissible under the country’s water laws. The approach also provides better indicators for institutions involved in groundwater management, and remains conceptually correct at all scales. However, further research is recommended on more robust alternatives to the Cooper-Jacob equation for determining radius of influence. Keywords: groundwater, sustainability, spatial, water balance, indicators, institutions INTRODUCTION Groundwater development creates a range of benefits and a range of consequences that depend on how intensively development occurs (Custodio, 2002; Pierce et al., 2013). Sustainable groundwater development (hereafter abbreviated to ‘sustainability’) represents a subjective, value-driven decision on the trade-off between these benefits and consequences for a given situation (Llamas et al., 2006). It is the job of hydrogeology to input objective information for the subjective decision-making process and to provide objective information to guide the implementation of the chosen sustainability scenario (Seward et al., 2006; Gleeson et al., 2012). The default information provided by hydrogeology is a ‘pump-the-recharge’ water balance (Balleau, 2013). This default creates serious problems because it (i) ignores the spatial and temporal aspects of sustainability (Theis, 1940; Bredehoeft, 2002); (ii) does not encompass the whole range of sustainability benefits and consequences (Kalf and Woolley, 2005; Pierce et al., 2013), (iii) is not even an indicator of the sustainability of any particular benefits/consequences option (Seward et al., 2006), and (iv) fuels the misconception that there is a single, numerical answer to sustainability (Balleau, 2013; Rudestam and Langridge, 2014). Existing approaches to this problem are (i) attempting to solve it by using the capture principle instead of natural recharge as the conceptual basis for monitoring, modelling, and adaptive management (Bredehoeft, 2002; Maimone, 2004), (ii) disputing that there is a problem (Zhou, 2009), and (iii) ignoring it (Balleau, 2013). While monitoring, modelling and adaptive management might seem like a reasonable solution in principle, in practice many countries do not have the combination of scientific and institutional capacity to implement this solution. A management/governance approach is therefore needed that has a sound conceptual basis and is readily implementable in practice. Instead of simple, but dubious, approaches based on allocating natural recharge, or ‘complicated’ but correct approaches of incorporating capture using adaptive management, this paper proposes a simple spatial approach using well-spacing and radius of influence to ensure that any new abstraction is sited far enough away from what shouldn’t be captured. The purpose of this paper is to test the proposed solution scientifically and legally using South Africa as a case study, and to test the solution institutionally using the generic concept of indicators. CONTEXT * To whom all correspondence should be addressed.  +2721 959 2911; fax: +2721 959 3126; e-mail: Received 28 April 2014; accepted in revised form 17 December 2014. This investigation depends on concepts related to groundwater ‘safe yield’ and groundwater ‘sustainability.’ These concepts have been debated for a century (Lee, 1915; Gleeson et al, 2012; Pierce et al., 2013; Rudestam and Langridge, 2014). A thorough history of the evolution of these concepts and their associated problems is provided by Kalf and Woolley (2005). It is difficult to find any definition of groundwater safe yield or sustainability that does not include some subjectivity or ambiguity. Even the ‘purely hydrological’ definition of Lee (1915 p. 48) contains the term ‘dangerous depletion of storage reserves.’ The seemingly irreverent definition of Lohman (1972 p. 62): ‘The amount of groundwater one can withdraw without getting into trouble ‘where ‘trouble may mean anything under the sun,’ highlights the subjectivity of safe yield and could well be applied to the more modern concept of sustainability. It would appear that the ecological impacts were not considered as part of ‘getting into trouble’ in the early definitions of safe yield, but are included in sustainability (Alley and Leake, 2004). The position of this paper is that the lists (Pierce et al., 2013; Llamas et al., 2006), commonly attached to what groundwater safe yield or sustainability should include, are merely attempts to bring ‘anything http://dx.doi.org/10.4314/wsa.v41i1.10 Available on website http://www.wrc.org.za ISSN 0378-4738 (Print) = Water SA Vol. 41 No. 1 January 2015 ISSN 1816-7950 (On-line) = Water SA Vol. 41 No. 1 January 2015 71 Figure 1 Cone of depression versus capture zone under the sun’ down to manageable limits, and that ‘sustainability’ is ultimately a subjective, value-driven, ‘political’ choice. The fact that it is subjective does not, however, detract from the importance of the concept of sustainability, nor does it abrogate physical science from providing the best possible inputs. These scientific inputs include acknowledging that groundwater sustainability has strong spatial controls. Spacing between wells, depths of wells and proximity to the recharge zone will determine how much water can be taken out an aquifer (Thomas, 1951). Proximity to existing wells, wetlands and streams will determine the extent of the consequences of utilising new wells. These spatial effects are explained by the capture concept (Lohman et al., 1972), whereby water sustainably pumped from wells is matched by reduced discharge and/ or increased recharge (Theis, 1940). The ‘capture’ concept or principle (Lohman et al., 1972) as used in basin yield and well-field yield determinations, is related to, but has a clearly distinct meaning from, the ‘capture’ zones as used in contaminant hydrogeology (Javandel and Tsang, 1986; Shafer, 1987; Zhou 2011, Asadi-Aghbolaghi et al., 2011). In order to prevent confusion, the meaning of the two different types of ‘capture’ will be briefly discussed. In both meanings of the term ‘capture’ a pumped well is involved and something is being captured. In contaminant hydrogeology, the capture zone refers to the zone fr (...truncated)