A V A P

“Improved methods for aquifer vulnerability assessments and protocols (AVAP) for producing vulnerability maps, taking into account information

on soils”

WRC Project K5/1432

Aquifer vulnerability to pollution in urban catchments - A decision support system –

Phase I

Irené Saayman and Hans Beekman

Deliverable 2.6 March 2006

Contents

..............................................................................................................ii List of Figures...............................................................................................................ii List of Tables

..........................................................................................................................ii Boxes 1 ......................................................................................................... 1 Background

1.1 ......................................................... 1 Decision making and decision support1.2 .................................. 2 Main characteristics of a decision support framework1.3 ................................................................................... 5 South African context

1.3.1 ............................................................................................. 5 Legislation1.3.2 .......................................................................... 8 Institutional framework1.3.3 ............................ 10 Integrated Water Resources Management (IWRM)

2 .................................................................. 12 Understanding Aquifer Vulnerability

2.1 .................................................................................. 12 Defining vulnerability2.1.1 ........................................................... 12 Unsaturated Zone Vulnerability2.1.2 ............................................................... 14 Saturated Zone Vulnerability

2.2 ................................................................................... 16 Vulnerability indices2.2.1 ............................................................................... 16 Unsaturated Zone2.2.2 ................................................................................... 19 Saturated Zone2.2.3 ........................................................................ 22 Integrated approaches

2.3 ................................................................................................. 25 Uncertainty 3 ......................................................... 27 Decision support system (DSS) for AVAP

3.1 ........................................................................... 27 Development of the DSS3.1.1 ............................................................................... 29 Stage I – Scoping3.1.2 ..................................................... 29 Stage II – Vulnerabiity assessment3.1.3 ................................................................ 29 Stage III – Decision-making

3.2 ..................................................... 29 Coupling the DSS with the EIA process3.3 .............................................................................................. 29 Case Studies

3.3.1 .............................................................................................. 29 Secunda3.3.2 .......................................................................................... 29 Cape Flats

4 ................................................................. 30 Conclusions and Recommendations 5 ........................................................................................................ 31 References

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List of Figures .................... 3 Figure 1: The vulnerability assessment process (source: NRC 1993) Figure 2: Interaction between scientists and water resources decision makers on

groundwater vulnerability assessment tools (Focazio et al., 2002).......... 4 ........... 5 Figure 3: The three components that constitute risk (source: UNDP, 1994) Figure 4: Water resource protection measures within the Integrated Water

Resource Management framework of DWAF (Source: DWAF, 1999). .... 6 .......... 10 Figure 5: Water Management Areas (19) of South Africa (DWAF, 2004b). ............. 12 Figure 6: Schematic representation of unsaturated and saturated zones Figure 7: Simplified conceptual framework of the attenuation capacity of an

unsaturated zone................................................................................... 13 Figure 8: Conceptual framework of the spatial dimension of saturated zone

vulnerability (Campbell, 2005). .............................................................. 14 Figure 9: Conceptual framework of the factors influencing residence time of a

contaminant in an aquifer (Campbell, 2005).......................................... 15 Figure 10: Fluoride concentration against distance in the simulated coastal and

weathered zone aquifers at time = 360seconds (Campbell, 2005)........ 20 Figure 11: Lead concentration (moles/litre) and pH vs. time at specific distance from

the source in coastal and weathered zone aquifers (Campbell, 2005). . 21 ............. 23 Figure 12: Flowchart of ReSIS layer model (Conrad and Thomas, 2005).

Figure 13: Simplified flow chart for estimating pollutant mass fluxes at the water table using the UGiF model (Thomas, 2001). ........................................ 24

... 26 Figure 14: Complexity vs. uncertainty of the assessment (Focazio et al., 2002). ............... 28 Figure 15: Schematic overview of aquifer vulnerability decision-making.

List of Tables Table 1: Hydraulic attenuation: soil contribution to intensity of groundwater

recharge* (Sililo et al., 2001). ................................................................ 17 ................... 17 Table 2: Chemical attenuation: soil contribution (Sililo et al., 2001). Table 3: Unsaturated zone thicknesses and media type and resulting impact on

groundwater vulnerability (Adams and Jovanovic, 2005). ..................... 18 Table 4: Preliminary vulnerability assessment of CFA using unsaturated zone

information (Adams and Jovanovic, 2005). ........................................... 19 Table 5: Saturated zone vulnerability in coastal (primary) and weathered zone

aquifers (after Campbell, 2005). ............................................................ 22

Boxes ........................................................ 9 Box 1: Deciding on specialist input to EIAs.

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1 Background

1.1 Decision making and decision support Our society requires consistent supplies of good quality water. South Africa is a semi-arid country that already experiences significant levels of water stress (Beekman et al., 2005). Groundwater plays an increasingly important role in water supply. Increased economic development, however, threatens both quantity and quality of water resources. Effective decision-making involves the management of risk, through the identification, evaluation, selection and implementation of risk management actions. Deciding upon the actions appropriate to limit pollution risk to water resources requires the integration of ecological, socio-economic and physical considerations. This is a challenge for decision makers, as it requires the comparison of inputs from various specialist areas. Decision support systems help in the integration of such input in a relatively unbiased manner, while allowing users to test scenarios. The DSS, however, should not fully replace managerial judgment. Rather, it should assist and support decision-making, ultimately to create a more effective decision-making process. Aquifer vulnerability assessments form an important input to managing the risk of water resource degradation. They aim to protect groundwater quality by allowing development planners and regulators to incorporate a specific consideration of groundwater into their planning decisions (Campbell, 2004). In particular, it is a tool to assist with “the optimal distribution of future pollution sources” (Andersen and Gosk, 1987). Land-use planning that take cognisance of impacts on groundwater could contribute significantly towards the sustainability of groundwater resources. Making a decision on where to locate or allow an activity, however, is complicated by the hidden nature of groundwater, and the complex heterogeneous setting in which it occurs, and therefore decision-making will benefit from a decision support system. The confidence with which the outputs of vulnerability assessments can be used in decision-making is directly related to the availability and quality of the data used. These factors have a significant influence on the uncertainty inherent in the vulnerability assessment outputs. The decision-making process generates management actions that may not necessarily only address land use practices, but may target resource allocations, further data collection, and public behaviour. Decision-making, however, has to occur in the context of the existing legislative and planning policy context and societal values (e.g. environmental sustainability and the value of human life). Therefore, decision makers require appropriate understanding of the external decision-making environment in which they operate. This can be facilitated through the involvement of specialists, such as vulnerability assessment experts, in the decision-making process. In many countries, uncertainty in environmental vulnerability is dealt with through the application in decision-making of the precautionary approach, especially where there is uncertainty on the level of harm an activity may cause to human health or the environment (EEA, 2001). This is done even if “some cause and effect relationships are not fully established scientifically” (Raffensperger, 1999). Among other things, the burden of proof is

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shifted to the proponent of an activity. The process of applying the precautionary principle must be open, informed and must include potentially affected parties.

1.2 Main characteristics of a decision support framework Many different types of decision-making problems (e.g. appropriate timing of dam water release – Koutsoyiannis et al., 2003; and balancing fertiliser application with groundwater vulnerability – Chowdary et al., 2005) have been tackled through the use of decision support systems. Decision support systems are particularly well suited to the description of integrated systems (de Kok and Wind, 2003). This is because our understanding of the various components of such systems tends to vary, with some components well understood and easily quantified, while others can only be guessed at. The DSS becomes useful in that inputs can be changed, resulting in different scenarios, which can be used to illustrate possible outcomes. A scientifically sound DSS also offers decision makers a means to justify their decisions. This is an important component of facilitating effective decision making, as decision makers need to know that their decisions will be accepted at higher levels in the decision-making chain, and that decisions can stand-up to scrutiny in a court of law if challenged. Inevitably, trade-offs are made in decision-making. The outputs of the DSS must inform the decision-making process. Criteria used to arrive at the decision may be internal, or may be brought to the fore through a structured decision-making process, as happens in Environmental Impact Assessments (EIAs). Criteria used in trade-off decision-making include societal value, vulnerability, relative importance, equity and fairness (Saayman, 2005). DSS Design The design of the DSS starts with the analysis of a problem. Therefore, the design should be clear on how its outputs will address the problem (or a component of the problem). Decision support is meant to enhance the effectiveness of resource management. Most DSS approaches generate as their output scenarios, which are changed by altering the input parameters (e.g. Kukuri et al., 1998). In this way a range of scenarios is generated, varying between the best probable outcome and the worst. It should then be possible (in theory) for decision makers to calculate (to some acceptable degree) the costs and benefits associated with a particular management action. A generic outline of the components that constitute a framework for decision-making is presented by the NRC (1993 – Figure 1).

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Figure 1: The vulnerability assessment process (source: NRC 1993) The process starts with the identification of the assessment purpose. This is followed by the selection of a suitable approach, which is affected by considerations on the purpose, data availability, and management use of the assessment. Once an assessment is complete, various management actions may be taken to protect groundwater quality or minimize contamination (NRC 1993 – Figure 1). Components of an effective DSS Among the challenges to the use of decision support tools in environmental decision-making is the requirement that they are both scientifically credible and have public acceptance (Travis et al., 2001). These two requirements are contradictory, as public acceptance requires the DSS to be simple, while the need to have it scientifically defendable often requires more complexity (Adams et al., 2004). Decision makers have indicated that they prefer maps and graphs above tables and text in decision support tools (de Kok and Wind, 2003). Although decision makers also indicate that they prefer more detailed information, it was found that their ability to use information increases with a decreasing level of detail (de Kok and Wind, 2003). A certain degree of flexibility therefore needs to be built into the manner of outputs generated by decision support systems. For spatial type decision support systems de Kok and Wind (2003) suggest that users be allowed access to spatial information at different levels of detail, for example by means of zoom buttons.

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An optimal DSS is one in which the simplest model can be used to rank the management alternatives according to the objectives of end users (de Kok and Wind, 2003). To achieve this, insight is required in the way model uncertainty and sensitivity are propagated from a system’s input to the output indicators reflecting the management objectives. This requires an appreciation of the extent to which the choice for a particular level of detail affects the model uncertainty, and hence its ability to distinguish different management strategies (de Kok and Wind, 2003). The components that constitute an effective DSS include:

• The ability to forewarn of the consequences of human activity; • The representation of those consequences in a way that allows decision makers to

visualise and appreciate the magnitude (special, temporal, intensity) of the impact; and

• A technically sound and legally defensible process and outputs. Scientists can provide water-resource decision makers scientifically defensible information for the assessment of groundwater vulnerability. To the extent that uncertainties in the assessment can be elucidated either quantitatively or qualitatively, the scientific defensibility and ultimate usefulness of the product will increase (Conrad, 2004). Ultimately, successful groundwater vulnerability assessments blend scientifically defensible analyses used to meet science objectives with additional interpretations by water-resource decision makers to meet management or policy objectives (Figure 2). Therefore, careful communication and feedback between the water-resource decision makers and scientists are required during all phases of a groundwater vulnerability assessment from planning to interpretation of results. Figure 2: Interaction between scientists and water resources decision makers on

groundwater vulnerability assessment tools (Focazio et al., 2002). Linking with decision-making Vulnerability assessments are used as input to risk assessment studies. The three components that constitute risk are (UN:P, 1994 – Figure 3): (a) The hazard occurrence probability: i.e. the likelihood of experiencing a hazard at any

location;

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(b) The elements at risk: i.e. the people or physical elements that would be affected by the hazard if it occurred, and the estimated economic value; and

(c) The vulnerability of the elements at risk: i.e. how damaged the impacted elements would be following exposure to some level of hazard.

Development planners need to quantify the tangible aspects of vulnerability and loss to assist mitigation and preparedness planning.

Figure 3: The three components that constitute risk (source: UNDP, 1994) Decision support systems for aquifer vulnerability assessment will find its greatest application with decision makers in the land-use planning and water resource management fields. Mostly these decision makers will apply it through existing EIA processes, so that hydrogeologist will work with regulators to generate vulnerability assessments where potentially impacting activities are proposed. The need exists, therefore, to create a framework to enable the integration of vulnerability assessment outputs into EIA decision-making processes. Within such a framework vulnerability assessments will be well placed to contribute to groundwater sustainability by influencing development and land-use planning decisions. A challenge is to present the outputs of vulnerability assessments in a manner that makes its use in the EIA process practical, while enabling both decision makers and interested and affected parties (I&APs) to understand and appreciate its significance.

1.3 South African context 1.3.1 Legislation The Constitution of South Africa (Act 108 of 1996) gives all South Africans the right to an environment that is “not harmful to their health or well-being” and to have the environment protected for “the benefit of present and future generations”. The constitution also emphasises the need for cooperative governance between the different spheres of government. The need for cooperative governance applies especially to jurisdictions, such as water resources, ecosystems and environmental health that are not easily regulated through a single government department. Outside the constitution, the National Water Act (NWA - Act 36 of 1998) and the National Environmental Management Act (NEMA - Act 107 of 1998) are the two key pieces of legislation with direct relevance to groundwater protection. Other relevant pieces of legislation include:

• The Environmental Conservation Act (Act 73 of 1989) (ECA); • The Water Services Act (Act 108 of 1997) (WSA); and • The National Health Act (Act 61 of 2003) (NHA).

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The National Water Act (NWA) ments to protect and conserve groundwater. These include

• Licences and general authorisations; aste discharges;

ces;

sources.

esource based measures relate to the management of water resources. Its components are

The NWA introduces several instruboth source-based and resource-based tools (see Figure 4). Source Directed Controls are aimed at minimising, or preventing at source, the impact of developments or activities on water quality. They are principally targeted at the control of pollution point sources. Instruments employed in source-directed controls include (DWAF, 2004b):

• Standards to regulate the quality of w• Minimum requirements for on-site management practi• Requirements for minimising water use impacts; • Requirements for remediation of polluted water re

Rthe Classification, the Reserve, and the Resource Quality Objectives (RQOs). The Classification groups water resources into classes representing different levels of protection. It provides a framework for the protection and use of water resources, as both the ecological Reserve and the RQOs are functions of the resource’s management class. In order to maintain a water resource within an agreed management class, objectives are defined, which constitute the RQOs for that resource. RQOs could include any requirements or conditions that need to be met to ensure that the water resource is maintained in a desired and sustainable state (Colvin, et al., 2004).

Resource Directed Measures

Figure 4: Water resource protection measures within the Integrated Water Resource

Management framework of DWAF (Source: DWAF, 1999).

Vision for the resource

Set class of resource

Set Set Resource Quality Objectives Reserve

Determine allocatable resource

Draw up allocation plan

Draw up catchment management strategy

Call for licence applications

Evaluate licence applications

Issue water use licences

Audit compliance of licence holders

Monitor resource status

Review

check

check

Source Directed Controls

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Source-directed controls must be implemented on a differentiated basis, which takes into account the vulnerability and the importance of the affected resource. For each water management area (WMA), some level of resource directed measure determination has to be completed as a prerequisite for the implementation of source directed controls. DWAF advocates various levels of intervention for the protection of groundwater quality by source directed controls, including, the encouragement of self-imposed discipline, regulatory control, and the development of best practice guidelines (DWAF, 2000). Other measures that could contribute to the protection of groundwater include (DWAF, 2004b):

• Public involvement – public awareness is seen as the only permanent guard against degradation of groundwater resources. This requires the public to understand hydrogeological issues and appreciate the value of the resource;

• Land-use zoning – this is an effective source-based control that restricts potentially polluting developments on important or sensitive aquifer systems. Urban planners, for example, must be made aware of risks related to groundwater pollution and encouraged to plan town developments with due regard for hydrogeological issues; and

• Environmental management plans and environmental impact assessments – these should be mandatory for activities known to induce groundwater contamination, or in areas of important or sensitive aquifer systems.

National Environmental Management Act (NEMA) The NEMA (Act 107 of 1998) emphasizes the need for sustainable development, which “requires the integration of social, economic and environmental factors in the planning, implementation and evaluation of decisions”. Environmental Impact Assessment (EIA) and Strategic Environmental Assessment (SEA) are the tools favoured to achieve this. Guidelines on the components and requirements of hydrogeological specialist input to EIAs were published recently for use by the environmental authority of the Western Cape Province (Saayman, 2005). The guidelines emphasise the need for specialist hydrogeological input to the EIA process in instances where an activity has the potential to impact groundwater or groundwater linked ecosystems. The EIA guideline documents emphasise the fact that information must be presented in a way that makes it easy for the regulatory authority to make decisions. Regulations, published by DEAT (2004), list activities that require an EIA (“listed activities”). The protection of groundwater resources has received substantial attention in the drafting of these regulations, as many of the listed activities are specifically aimed at its protection. Strategic Environmental Assessment (SEA) Strategic Environmental Assessment offers a process that assesses regional or larger scale projects or developments in a holistic manner, by taking account of environmental, social and physical factors in the development of natural resources. The holistic nature of SEAs makes it ideal for National and Water Management Area (WMA) level planning of water resources development. Among the benefits of using SEAs in water resources management is the fact that it simplifies data into a form that makes it accessible to the public, regional opportunities and constraints are assessed, and the strategic decision making process is transparent and progress can be evaluated over time and corrective measures introduced where necessary (DWAF, 2001).

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The South African environmental legislative context, however, is characterised by fragmentation and at times overlap between roles and functions, which may lead to confusion. Added to this is the fact that a planned activity has to go through a host of regulatory requirements, often without any guidelines being available on which regulations need to be complied with and on which government authority to involve. 1.3.2 Institutional framework There are three tiers of government; each with its own set of functions and responsibilities. Some functions are shared among the different spheres of government, such as water resources management, environmental management and economic development. In the water resources management arena a regional sphere of government was created through the promulgation of 19 Water Management Areas (WMAs – see Figure 5). Government departments with an interest in water resources management include the national departments of Water Affairs and Forestry (DWAF), Provincial and Local Government (DPLG), Environmental Affairs and Tourism (DEAT), Agriculture and Land Affairs , Minerals and Energy (DME) and Public Works (DPW). Intergovernmental Forums is one way in which government has tried to promote cooperative governance between National and Provincial departments. The forums work towards the alignment of government policies, activities and programmes at both national and provincial levels. The large number of institutions and different spheres of government that regulate environmental and water resource related processes is in line with the adopted approach of devolving authority to the lowest possible level. Functions such as environmental management and pollution control, for example, are concurrent national and provincial functions. Provincial government is also responsible for assessing and considering development applications in terms of the requirements of the Environmental Impact Assessment Regulations (DEAT, 2005) and for giving input to spatial and development framework planning. The NWA has brought into being a range of institutions to enable water resources management at regional and local level. Overall, the Catchment Management Agencies (CMAs) will bear the responsibility for policing the adherence of end-users to permit conditions, but may delegate some functions to Water User Associations (WUAs) or Water Services Authorities (WSAs). At the local level management decisions may be made by locally constituted Water User Associations (WUAs). Land-use planning and regulation are performed by municipalities. Where water resources are shared with other countries international water management bodies (such as the Orange-Senqu River Basin commission) may be formed. Regional level water resources management must occur with interaction and cooperation between WMAs and the provincial government departments responsible for environmental matters, economic development, and housing. The supply side of water resources management is the responsibility of local authorities.

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Box 1: Deciding on specialist input to EIAs.

Environmental Impact Assessments (EIAs) provide decision-makers (government authorities, the project proponent or financial institutions) with information about the potential positive and negative impacts of a proposed development and associated management actions in order to make an informed decision whether or not to approve, proceed with or finance the development. The process requires specialist input on those components of the environment that could be significantly affected by the proposed activity. One of the challenges that EIAs face, is deciding when specialist input is required. The following flow chart has been designed to help project proponents and decision-making authorities decide on when specialist input may be required during the EIA process (Münster, 2005).

Approach to determining the need for, timing and role of specialists in the EIA process (Münster, 2005)

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Figure 5: Water Management Areas (19) of South Africa (DWAF, 2004b). 1.3.3 Integrated Water Resources Management (IWRM) The unity of the water cycle and the interdependency of groundwater and surface water require the integrated management of water resources. Only through understanding the links of the hydrologic systems, and sustainably exploiting them in a way which optimises their complementary differences, can we derive the greatest benefit from both resources (DWAF, 2004a). This requires research into the characteristics of groundwater and surface water resources and their interactions. If water resources management is to be effective it must be based on data rather than assumptions and beliefs. This requires investment in focused monitoring, reliable collection of long term data and quality assured storage in an accessible information system (DWAF, 2004a). In South Africa, groundwater occurrence and use is widespread, but highly localised. It is physically and economically infeasible to protect all groundwater resources to the same degree. Preventing all impacts on groundwater quality, would also not allow for much needed social and economic development. The protection of groundwater resources will have to be prioritised according to:

• The value of the resource; • The vulnerability of the resource; and • The risk of adverse impacts on human health and ecosystems.

CMAs should not operate alone on these issues and must cooperate with those authorities responsible for source-based control and land use decisions. Stakeholders and other IWRM

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decision makers need to understand the implications of groundwater- and land-use management decisions. This requires communication efforts from groundwater scientists to ensure that the public and the CMAs are fully empowered to protect and make the best use of aquifers (DWAF, 2004a). Improved public awareness and involvement is seen as the only permanent guard against the degradation of groundwater resources. Some degree of capacity development is required within institutions if participatory decision-making is to be achieved. The strategic value and importance of water dictates that part of the management remains a national matter, but the nature of aquifer systems requires groundwater management remains essentially a local issue (DWAF, 2004a). Land-use zoning is an effective method that restricts potentially polluting developments on important or sensitive aquifer systems. CMAs must ensure that urban planners (in municipalities), for example, are aware of risks related to groundwater pollution and must encourage them to plan developments with due regard for the nations’ water resources. This requires that water resource management institutions (at national, provincial, regional and local levels):

• Participate in the evaluation of EIAs and intervene when impact assessments identify the potential for neglect or damage.

• Participate in land-use planning and strive to influence the planning so activities with a high groundwater pollution risk are placed in areas with low or no groundwater potential.

• Enforce compulsory environmental management plans (including groundwater) for potentially polluting enterprises.

From the foregoing, it is clear that vulnerability assessments have an important contribution to make to the sustainable management of water resources. The efforts underway within the AVAP programme will produce improved vulnerability assessment methods that are suited to South African conditions. Note that for effective and efficient water resource management in this context, an appropriate understanding of aquifer vulnerability is needed, prior to developing a decision support system.

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2 Understanding Aquifer Vulnerability Both physical and chemical processes within the unsaturated and saturated zones are controlling the impact of contamination. Specifically, the residence time of a contaminant in moisture or groundwater, and the distance that it travels can be considered important measures of vulnerability.

2.1 Defining vulnerability Aquifer vulnerability to contamination comprises two components: unsaturated zone vulnerability and saturated zone vulnerability. 2.1.1 Unsaturated Zone Vulnerability Unsaturated zone vulnerability is defined as the ease with which groundwater may become contaminated by a contaminant source at the surface or in the unsaturated zone (Figure 6). It is controlled by the nature of the strata overlying the saturated zone and the nature of the contaminant or contaminant mixture (Campbell, 2004).

Figure 6: Schematic representation of unsaturated and saturated zones (Adams et al., 2004).

Factors controlling unsaturated zone vulnerability The ability of the unsaturated zone to attenuate and/or prevent any contaminant from reaching the saturated zone depends on the following factors (Adams and Jovanovic, 2005):

• The thickness of the unsaturated zone; • The hydraulic properties of the medium; • The amount of water moving through the unsaturated zone and reaching the water

table (recharge); • The dominant flow mechanisms (preferential pathways, matrix flow or a combination

of the two);

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• The sorption capacity of the media; and • The chemical characteristics of the pollutant (i.e. half-life, etc).

These factors are in most cases interdependent and assume relative importance from site to site. Unsaturated zone vulnerability is largely determined by the interplay between preferential and inter-granular flow, which controls the relative importance of chemical attenuation processes. The more preferential flow dominates, the less important chemical attenuation becomes, and vice versa. This is because preferential flow is relatively rapid and results in a relatively small surface area of contact between the infiltrating water and the solid matrix, compared with diffuse, inter-granular flow. A simplified conceptual framework of the attenuation capacity of the unsaturated zone (including the soil zone) is given below:

FLOW PREFERENTIAL…………………MIXED………………………..DIFFUSE

CHEMICAL ATTENUATION CAPACITY

LOW………………………………MEDIUM……………………………………HIGH

TOTAL ATTENUATION CAPACITY

LOW…………………………MEDIUM……………………HIGH Figure 7: Simplified conceptual framework of the attenuation capacity of an

unsaturated zone. Lower attenuation means higher vulnerability of the underlying groundwater. More recharge for a given mass of contaminant – i.e. a finite source – implies that the concentration of contaminant moving through the unsaturated zone will be more dilute, thus the impact on aquifer water quality less severe. However, the more recharge there is in a given period, the more rapidly a given mass of contaminant – dissolved or as mobile colloidal matter - will be transported through the unsaturated zone. The two processes usually do not impact on vulnerability in the same magnitude. In addition, the intensity and duration of rainfall events will determine the nature and magnitude of recharge. In its simplest form the principal geologic and hydrogeologic features that will influence an aquifer’s vulnerability from the unsaturated zone perspective are:

• Low Vulnerability – Thick unsaturated zone, with high levels of clay and organic material; and

• High vulnerability – Thin unsaturated zone, with high levels of sand, gravel or fractured rocks with high permeability.

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2.1.2 Saturated Zone Vulnerability Saturated zone vulnerability to contamination is considered a function of the period of time after contaminating activities have ceased that a given contaminant can be detected in groundwater and the volume of the aquifer throughout which the contaminant is above a preset concentration.

Factors controlling the spatial extent of contamination The same factors apply to the saturated zone as to the unsaturated zone, i.e. the greater the dominance of in-fracture flow as opposed to diffuse/matrix (primary) flow, the less influence chemical attenuation has. The flow velocity also has an influence – the faster groundwater is moving the further contaminated groundwater will move from its source in a given time period. The spatial distribution of the contaminant will also be controlled by the nature of flow in the aquifer. Fracture-controlled flow will, by definition, constrain the contaminant’s spread within the fracture network. Matrix flow will be constrained by spatial variations in hydraulic conductivity. Chemical diffusion is important at much lower flow rates. A conceptual framework of the factors controlling the spatial distribution of a contaminant in the saturated zone (aquifer) is depicted in Figure 8. The scales are relative and approximate, and the diagram indicates the relative effects of the contributing factors over a fixed period of time.

FLOW

FRACTURE ……………MIXED…………………..MATRIX (SECONDARY POROSITY) (PRIMARY POROSITY-CONTROLLED) FAST…………..SLOW FAST…………SLOW

LOW…………………HIGH Chemical attenuation capacity

LOW………………….HIGH Chemical attenuation capacity

LOW………………….HIGH

Chemical attenuation capacity

LARGE………………………………………………………………………SMALL

SPATIAL DIMENSIONS OF THE CONTAMINATED ZONE Figure 8: Conceptual framework of the spatial dimension of saturated zone

vulnerability (Campbell, 2005).

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Factors controlling the persistence of contamination Dilution The concentration of a contaminant is largely controlled by the rate at which dilution of a contaminant plume takes place in the aquifer. The main factor influencing dilution is the magnitude of recharge relative to the volume of the contaminated zone and recharge results in a change in the shape and extent of the contaminated zone. Faster flow velocities will also cause more dilution through dispersive processes. Conversely, low velocities result in less dispersion and, in extreme cases in fractured aquifers, contaminants may become trapped in dead-ends of fracture networks with low interconnectivity. Dilution would then be driven by chemical diffusion, and takes place very slowly. Chemical sorption Chemical attenuation is the capacity of aquifer solids to remove dissolved or suspended contaminants from groundwater and retain them. An increase in residence time is controlled by dissolution and de-sorption rates of contaminants adsorbed to the solid phase. A conceptual framework of the factors which influence the residence time of a contaminant in the saturated zone is shown in Figure 9.

RECHARGE LOW…………………………………………………………………………..HIGH

LOW…………………………………………………………………………..HIGH

DILUTION RATE HIGH……………………….…………LOW HIGH…………………….……………..LOW Chemical attenuation capacity Chem. attenuation capacity

LONG…………………………………………………………………………SHORT

RESIDENCE TIME OF CONTAMINANT IN AQUIFER Figure 9: Conceptual framework of the factors influencing residence time of a

contaminant in an aquifer (Campbell, 2005). Decay or decomposition Most organic compounds and other chemical species, such as for example the nitrate ion, are consumed by reactions which reduce their concentration in aquifers. It is possible to assess the rate of these reactions, and include this factor in a determination of residence time. Radioactive isotopes have limited life spans as a result of radioactive decay. The decay rate has a well-defined value which allows the maximum residence time of the isotope in an aquifer to be calculated.

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2.2 Vulnerability indices 2.2.1 Unsaturated Zone In South Africa, with its contrasting geological terrains, different unsaturated zones developed ranging from thick regolith sand to areas of very thin to little soil cover. The unsaturated zones are often made up of fractured rocks with very thin soil cover. The coastal zones often have an aeolian soil cover on top of the fractured rock aquifers. Note that approximately 90% of the subcontinent has fractured rock aquifers. The configuration of the land surface also influences the type of unsaturated zone that will develop. Vulnerability indices were developed for both the upper part of the unsaturated zone: the Soil Zone and the lower part: the Vadose Zone (see Figure 6). Soil Zone During infiltration through soils, many contaminants are naturally attenuated. An evaluation of the protective properties of soils is, therefore a critical component of vulnerability assessment (Sililo et al., 2001). A new, specific purpose, groundwater vulnerability classification system of South African soil forms is being developed based on (1) hydraulic attenuation characteristics using pedogenic information inherent in the current, general purpose, South African, soils classification, and (2) chemical attenuation characteristics of representative South African soils derived from pedogenic information and extensive laboratory batch experiments (Fey and Herselman, 2005). Both intrinsic and specific1 vulnerability are taken into account. Intrinsic vulnerability assessments can be used to give an indication of the vulnerability of aquifers to persistent, mobile contaminants while specific assessments can be used for contaminants that will migrate at a rate much slower than the average wetting front. Note that translating the existing South African soil classification system into one that caters specifically for groundwater vulnerability assessment will create a great opportunity in that existing soil maps can be converted into vulnerability maps without the need for large scale laboratory analyses and attenuation tests (Mongwe and Fey, 2004). Tables 1 and 2 show the vulnerability ratings in five risk classes, from maximal (=1) to minimal (=5) attenuation, for hydraulic attenuation and chemical attenuation (Sililo et al., 2001). Maximal attenuation represents minimal vulnerability of the saturated zone to contamination from a surface source, seen from a soil zone perspective. Three approaches were proposed for determining groundwater vulnerability:

• Combine and average the attenuation ratings (summing the hydraulic and chemical attenuation ratings and multiplying by 10 (in %) for each of the three contaminant categories and averaging the (risk) scores);

• Selection of the most critical of the two ratings; and • A hybrid approach based on averaging the results of the first two approaches.

This vulnerability assessment system is considered most useful for site-specific investigations, where the soil type has been accurately defined (Sililo et al., 2001).

1 Intrinsic vulnerability deals with hydrogeologic settings and the natural protection against contamination provided by physical characteristics whereas specific vulnerability is derived from the properties of specific contaminants and their behaviour in the subsurface environment.

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Table 1: Hydraulic attenuation: soil contribution to intensity of groundwater recharge* (Sililo et al., 2001).

Class Attenuation capacity and pedogenic inference 1 Maximal hydraulic attenuation: bare sheet rock; heavy crusting clays; steep slopes;

extreme aridity; minimal vegetation cover; shallow dorbank or calcrete horizons 2 Most calcareous and eutrophic clay soils; duplex and margalitic soils; lithocutanic

soils with steeper relief 3 Intermediate: mostly loamy, thicker eutrophic or mesotrophic soil profiles on gentler

relief 4 Dystrophic or mesotrophic loams and ferrallitic clays and loams on gentle relief 5 Minimal hydraulic attenuation: extreme water surplus sustained for significant

periods; sandy soil texture; absence of luvic or clay pan features in soil profile + vadose zone; regic sands of humid climates on level topography

* - Rainfall intensity less Runoff intensity less Evaporation intensity = Water available for deep drainage; then Recharge intensity (mm/day) = water available for deep drainage less Water storage capacity of soil profile. Table 2: Chemical attenuation: soil contribution (Sililo et al., 2001). Class Attenuation capacity and pedogenic inference A. Cationic contaminants ( inorganic and polar organic) 1. Maximal attenuation: Thick, clayey profiles especially margalitic soils; strongly

calcareous clays; eutrophic peats 2. 3. Intermediate: 4. 5. Minimal attenuation: Dystrophic sands low in humus

} all other soils

B. Anionic contaminants ( inorganic and polar organic) 1. Maximal attenuation: Deep, dystrophic, ferrallic clays 2. 3. Intermediate: 4. 5. Minimal attenuation: Eutrophic sands

} all other soils

C. Organic contaminants (non-polar) 1. Maximal attenuation: Deep humic clays and peats 2. 3. Intermediate: 4. 5. Minimal attenuation: Pure sands low in humus

} all other soils

Vadose Zone The following approaches were developed to produce improved methods for assessing aquifer vulnerability to contamination, from the vadose zone perspective, and in the South African context (Adams et al., 2004):

2• Extension and improvement of the DRASTIC index method (improvement of the “I” rating of DRASTIC, i.e. impact of the vadose zone).

2 DRASTIC (D=Depth to groundwater; R=Recharge; A=Aquifer media; S=Soil type; T=Topography; I=Imapct of the vadose zone; C=Hydraulic Conductivity; Aller et al., 1987) is an overlay or index vulnerability ssessment method which uses subjective ratings. It does not take into account preferential flow paths or fractured systems.

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o Inclusion of multi-layer component, based on site-specific conceptual models; o Ranking based on determination of hydraulic properties (permeability,

porosity, water retention); o Inclusion of preferential flow and ranking:

- short-circuiting (fractures and rocks) - funneling (lateral transport of water and contaminants)

o Inclusion of chemical properties of specific contaminants and mixtures of contaminants and ranking: - Solubility, volatilization and sorption: data are available in the Excel

database of the groundwater pollution inventory project. - Degradation: guidelines have been compiled for the estimation of half-life

based on expected environmental factors (microbial activity, pH, temperature, water content, etc.)

• Use of numerical model(s) to improve the “I” rating of DRASTIC. Numerical model(s) are combined with DRASTIC, and

• Simulation of best and worst case scenarios with suitable numerical model(s) for average conditions. Process-based models are used to assess vulnerability.

The selected/improved/developed methods were tested at case study sites. For this purpose, conceptual models were developed for case study sites and a generic procedure was followed to describe the processes in the unsaturated zone relevant to groundwater contamination. This procedure accounts for intrinsic properties of the media through which water and contaminants are transported, as well as the specific properties of contaminants. In other words, both intrinsic and specific vulnerability were assessed. Extension and improvement of the DRASTIC index method An example of rating the unsaturated zone thickness and media type as input to unsaturated zone vulnerability is given in Table 3. Ratings of other factors (e.g. flow mechanisms, sorption characteristics, etc.) that control the attenuation of contaminants in the unsaturated zone are described in Adams and Jovanovic (2005). Table 3: Unsaturated zone thicknesses and media type and resulting impact on

groundwater vulnerability (Adams and Jovanovic, 2005). Unsaturated zone

characteristics Thickness

Unconsolidated material overlying unconfined aquifer

0 – 30m 30 – 50 m > 50m Gravel 0 - 30m 30 - 50m > 50m Clean sand 0 – 15m 15 – 30m > 30m Silty sand 0 – 5m 5-15m > 15m Silt > 0.5m > 2.5 > 5 Clay

Consolidated fractured medium overlying unconfined aquifer 5 – 30 m 0 – 5 m > 30m

Leaky aquifers > 30m 5 – 30 m 0 – 5 m Vulnerability impact Low Medium High

Table 4 summarizes preliminary results of a vulnerability assessment of the Cape Flats (primary) Aquifer (CFA) based on unsaturated zone information. Clearly, the unsaturated zone above the Cape Flats Aquifer scores very low in its capacity to attenuate contaminants

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and has a high vulnerability class/rating. The CFA is thus generally vulnerable to most types of contamination (Adams and Jovanovic, 2005). Table 4: Preliminary vulnerability assessment of CFA using unsaturated zone

information (Adams and Jovanovic, 2005).

Factors Description Comments Impact Physical Characteristics

1. Sand, minor silt, clay lenses, heterogeneous, 1. Sieving analysis (UWC) Hydraulic properties Medium-High 2. Available information 2. K = 15-50 m/day, T = 30-980 m2/day

Thickness of the unsaturated zone 1-5m (average 3m) Some areas water table at surface High

Flow mechanism Matrix, Fingering 100% matrix flow assumed Medium 1.10-13% of MAP (Bredenkamp et al., 1995). 2. CMB = 10% MAP

1. Tritium profiling and CMB (Atlantis) Recharge/drainage High 2. CMB (UWC CAT Site) 3. 60-80 mm/yr

1. High silica, Ca sands. 2. The fraction of organic carbon of the sands (0.3-0.4%) is relatively low and sorption will thus be low (Sililo, 1997)

Peat layers can increase Sorption capacity Sorption capacity High

Chemical characteristics Thickness = 3m; effective porosity 25-40%, K = 15-50 m/day

Foster and Hirata (1995) equation: Travel time Gross surcharge: <1 hr High

Natural infiltration: <2 days

High travel time, low sorption, thin unsaturated zone

Medium to high Half-life Contaminant specific

Other High in shell fragments and calcite General

Vulnerability of an aquifer to contamination, originating at the surface, can be assessed using data that are readily available or that can be calculated. The main challenge is to find the interrelations between the different unsaturated zone variables at a specific location and assessing a vulnerability class. 2.2.2 Saturated Zone Two different, and for South Africa important, aquifer types were considered in assessing site-specific saturated zone vulnerability to contamination (Campbell, 2005): • Coastal primary aquifers, composed primarily of Recent/Quaternary (predominantly

unconsolidated) sediments, and • Dual porosity aquifers (mixed fractured/porous) developed in the weathered zone

overlying sedimentary or igneous rocks. A framework for establishing vulnerability indices was developed for these aquifer types based on three types of contaminant sources at the surface:

19

• Industrial effluents, with high levels of nutrient and trace elements, • Domestic landfill leachate, and • Acid mine drainage.

and two scenarios of leaching of the sources: • A continuous contaminant source, and • A single pulse of the contaminant source which is followed by a continuous injection of

uncontaminated source. Vulnerability can be interpreted in terms of spatial impact and persistence. Spatial component of vulnerability A continuous source of the contaminant (mixture) is injected into the aquifer. At a certain time (for example, the time to a breakthrough of a conservative solute such as chloride at a specific distance from the source), a snapshot is taken of the concentration of the contaminant of interest along the traveled distance or flow path. The distance (or in modelling terms: cell) furthest from the source in which the concentration exceeds a reference level (e.g. 5% of the input concentration, or natural background), is defined as the spatial impact. Figure 10 is an example of the spatial impact of contamination for the two aquifer types considered. Illustrated are the changes along a flow path (in cells) of fluoride from a continuous injection of industrial effluent at the breakthrough time of chloride (=360 seconds at the end of the flowpath). The fluoride concentration in the primary aquifer is generally higher along the flowpath. Note that the fluoride concentration has reached 5% of the injection concentration at a greater distance from the source (cell 6) than in the weathered zone aquifer (cell 5). In this case, the primary aquifer has a slightly higher spatial vulnerability with respect to fluoride.

0.0E+00

2.0E-05

4.0E-05

6.0E-05

8.0E-05

1.0E-04

1.2E-04

1.4E-04

1.6E-04

1.8E-04

2.0E-04

0 2 4 6 8 10 12

Distance (cells)

F- (molar) at t = 360 s Coastal Aquifer-zero CEC

5% of initial concentration

Weathered Aquifer

Figure 10: Fluoride concentration against distance in the simulated coastal and

weathered zone aquifers at time = 360seconds (Campbell, 2005).

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Time component of vulnerability – persistence of a contaminant Using the scenario of a pulse of contaminant mixture, followed by flushing, the residence time or persistence of a contaminant in different aquifers can be compared. Persistence is defined as the time it takes for the concentration of the contaminant at a fixed distance in the aquifer from the source to fall below a certain level, say for instance 5 % of the initial (input) concentration. Figure 11 is an example of the persistence to contamination for the two aquifer types considered. Illustrated are the changes in time of the pH and lead concentration from an injection of a pulse of acid mine drainage at a specific distance from the source in the aquifer. It can be seen that the lead concentration persists above the background level (about 1.5 micromolar) for about 1200 seconds of model time from the start, in the coastal (primary) aquifer, against about 1000 seconds for the weathered zone (dual porosity) aquifer. In this case, the primary aquifer can be considered more vulnerable (by about 20%) than the weathered zone aquifer.

0.0E+00

1.0E-05

2.0E-05

3.0E-05

4.0E-05

5.0E-05

6.0E-05

0 400 800 1200 1600 2000 2400 Time (s)

Pb (molar)

0 1 2 3 4 5 6 7 8 9

pH

Pb in Weathered Zone Aquifer

Pb in Coastal Aquifer

pH in Weathered Zone Aquifer

pH in Coastal Aquifer

Figure 11: Lead concentration (moles/litre) and pH vs. time at specific distance from the source in coastal and weathered zone aquifers (Campbell, 2005).

Comparing the saturated zone vulnerability of two aquifer types to contamination Based on 1D (PHREEQC - Parkhurst and Appelo, 1999) ‘generic’ modeling of the transport of contaminants in both types of aquifers for the three sources of contaminants, Campbell (2005) derived a set of vulnerability ‘values’ for the spatial and persistence indices. Table 5 shows a matrix of values for the indices which facilitates a comparison of vulnerability between the various options. In the primary aquifer for example, the spatial impact of ammonium is greater than that of fluoride. Thus, the primary aquifer is more vulnerable to the landfill leachate than for the industrial effluent. If we compare the primary with the dual porosity aquifer, the persistence to both contaminating sources is higher for the dual porosity aquifer, whereas for the spatial impact the opposite is observed. In other words, regarding the contaminant sources of landfill leachate and industrial effluent, the dual porosity aquifer is more vulnerable in terms of persistence than the primary aquifer but less vulnerable in terms of spatial impact.

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Table 5: Saturated zone vulnerability in coastal (primary) and weathered zone aquifers (after Campbell, 2005).

Aquifer Types Source Type & Contaminant of Concern

Dual Porosity (primary/fractured) Aquifer (Ecca Group, Karoo Supergrouo, Secunda)_

Unconsolidated Primary Aquifer (Cape Flats)

Major controls: dispersive processes

Major controls: cation exchange and dispersive processes Landfill leachate

AMMONIUM Spatial impact: 9 (in cells) Spatial impact: 5 Persistence: 1000 Persistence: 7000 Major controls: Precipitation of calcium fluoride and dispersive processes Industrial

Effluent (Secunda type) FLUORIDE Spatial impact: 6 Spatial impact: 5

Persistence: 1300 Persistence: 1600 Major controls: Precipitation of lead hydroxides, cation exchange and dispersive processes

Major controls: Precipitation of lead hydroxides and dispersive processes

Acid mine drainage with trace metals LEAD & Spatial impact: 2.5 Spatial impact: 3.5 ACIDITY Persistence: 1200 Persistence: 1000 2.2.3 Integrated approaches GIS-based algorithms were developed that incorporate the results of the unsaturated zone (soil and vadose zones) and saturated zone in determining aquifer vulnerability (Conrad and Thomas, 2005):

• ReSIS layer method (a revised DRASTIC method) – an index model capable of dealing with intrinsic vulnerability, and

• Revised UGiF model – a process-based model using analytical approaches which can deal with contaminant specific vulnerability

ReSIS A review of the DRASTIC method (Conrad, 2004; Conrad and Thomas, 2005) revealed that there is significant “double accounting” and that the 7 layers can justifiably be reduced to 3 significant layers corresponding to the three zones discussed previously, i.e. the soil zone (S), the vadose or intermediate zone (I) (beneath the soil zone and above the saturated zone) and the saturated zone (S). A fourth component that must be taken into account is that component of rainfall that replenishes the saturated zone or groundwater Re-charge. An acronym given for the revised DRASTIC method is ReSIS. The new method/model is still based on the same rated and weighted approach as the DRASTIC method, but provision is made for scalability of the data. For each of the input layers above, the recommended input is the site specific data collected by specialists. The model makes provision for the inclusion of coarser resolution data sets. This has the effect of increasing the uncertainty of the model to the level of the coarsest data set. It is recommended to use ReSIS for site-specific vulnerability assessments. Figure 12 describes the process in assessing groundwater vulnerability using the ReSIS model:

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Thin Thick Thickness/Importance

1 2 3 4 5

S 25

I 25

S 25

Re 25

VRPRELIM 100

WEIGHT

VRFINAL 100

Fracturing PFM (%)

Natural Recharge (Re)

Soil zone (S) Intermediate zone (I)

Saturated zone (S)

R A T E

Rat

ed n

atur

al re

char

ge Hydraulic vulnerability

rating (HVR) Max Min Hydraulic attenuation

1 2 3 4 5 1 1 2 3 4 5 2 2 2 3 4 5 3 3 3 3 4 5 4 4 4 4 4 5

Che

mic

al

vuln

erab

ility

ra

ting

(CV

R)

Min

M

ax

Che

mic

al

atte

nuat

ion

5 5 5 5 5 5

Figure 12: Flowchart of ReSIS layer model (Conrad and Thomas, 2005).

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• The Vulnerability Rating (V RATE) is calculated for the Hydraulic and Chemical attenuation per soil, intermediate and saturated zone layer

• The Vulnerability Weighting (V WEIGHT) is then varied according to the role played by the layer e.g. for underground tanks, soils weight will be low. The weighting is based on the relative importance of each of the three zones.

• A preliminary vulnerability rating is then obtained by applying a recharge factor to each layer (VR PRELIM = (V RATE * V WEIGHT))

• The final vulnerability rating is obtained by applying a preferential flow rate multiplier (VR FINAL = VR PRELIM * PFM). This preferential flow multiplier depends on the degree of fracturing and preferential flow paths within the study area.

UGiF model UGIf is a GIS based urban recharge pollutant flux model (Thomas, 2001). It is primarily meant for the estimation of groundwater recharge pollutant fluxes of specific pollutants viz. BTEX, nitrate and chloride to an urban unconfined, primary aquifer. The following processes (with their calculation method indicated in brackets) are accommodated for by the model:

• Infiltration and runoff (NRCS curve number method); • Evapotranspiration (UK Meteorological Office and Rainfall Evapotranspiration

Calculation System (MORECS) based on Penman-Grindley model); • Interflow (empirical index approach); • Volatilization (Henry’s law); • Sorption (distribution coefficient); and • Degradation (first order decay).

A simplified flow chart of estimating pollutant mass fluxes reaching the water table using UGiF is given in Figure 13.

Potential Recharge Pollutant Flux

Travel and Reaction Through Vadose Zone

Actual Pollutant Flux Reaching Water Table

Groundwater Recharge

Urban Land Use / Land Cover

Chemical Concentration

Figure 13: Simplified flow chart for estimating pollutant mass fluxes at the water

table using the UGiF model (Thomas, 2001).

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The UGIf model was revised to make it more suitable for South African conditions. Three new screening level models for vulnerability assessment were selected with their respective indices and incorporated in the model (Conrad and Thomas, 2005):

• The Attenuation Factor Index (AFI) - denotes mass emission of a chemical from the unsaturated zone to groundwater;

• The Leaching Potential Index (LPI) - ranks sites on the basis of their susceptibility to groundwater contamination;

• The Ranking Index model (RI) - denotes the vulnerability to groundwater contamination by a specific compound.

A simple approach to assessing intrinsic vulnerability of conservative contaminants was also included in the model. Another modification included the editing of the script of land use grid map preparation in order to accommodate for different land use / land cover types which occur in South Africa. The revised UGIf model requires a substantial amount of input data on both the soil and vadose zone. Note that pollutant fluxes are only calculated down to the water table. As such, the saturated zone component of vulnerability assessment is not taken into account.

2.3 Uncertainty Vulnerability assessments will always be subject to uncertainties albeit to a certain degree. The uncertainties include (Adams et al., 2004; Conrad, 2004):

Lack of data; Errors in data used; Incomplete understanding of the processes; Errors in aggregating information; Errors inherent to statistical measures of association; Arbitrary inclusion or exclusion of variables in most approaches and variables are

based on expert opinion as to the weighting between factors. Indices are typically not based on observations or measurements of groundwater

contamination and even when physically based models are used they are often prone to errors in model assumptions or in selecting the input parameters.

The approaches or models are rarely validated or tested against observed data. Lack of detailed understanding of preferential and fractured flow processes. • Spatial variability: even in the best designed studies with specially constructed

boreholes, there is variability in the concentration of chemical constituents among the water samples due to the inherent spatial and temporal heterogeneity of groundwater systems.

• If the direction of water movement or flux cannot be accurately predicted the likelihood of accurately predicting rates or patterns of contaminant movement decreases.

Some of the uncertainties or errors can be quantified while others cannot. Statistical vulnerability methods in particular allow for the computation of the degree of uncertainty while index and overlay vulnerability methods do not always include any measures of uncertainty. One should at several stages re-evaluate the balance among increases in

25

understanding of the system and resources needed (Figure 14) in meeting science objectives while accounting for sources of uncertainty (Conrad, 2004). It is important to avoid undue complexity and not to commensurate with the objectives of the assessment (quadrant I; Figure 14). On the other hand, too many simplifying assumptions introduced to an assessment in order to avoid complexity can create inaccurate depictions of groundwater vulnerability and a lack of scientific defensibility (quadrant IV). Figure 14: Complexity vs. uncertainty of the assessment (Focazio et al., 2002).

The challenge in designing an objective scientific approach for a groundwater vulnerability assessment is to find a reasonable balance among model (or assessment) complexity, resources required, and decreases in uncertainty gained while reaching predetermined science objectives (quadrant III; Focazio et al., 2002). In summary, it is important to make a reasonable assessment of the uncertainties involved in the vulnerability assessments. Data is likely to be scarce and of varying quality and allowances will have to made for this.

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3 Decision support system (DSS) for AVAP In this Section an outline is presented of a decision support system (DSS) for water resource decision makers on aquifer vulnerability. Main stages in the development of the DSS and its coupling with the EIA process will be described. Findings from two case studies in Secunda and Cape Flats will demonstrate the use and applicability of the DSS.

3.1 Development of the DSS Three main stages can be identified in the development of a DSS for aquifer vulnerability:

• Stage 1: Scoping • Stage II: Vulnerability assessment, and • Stage III: Decision-making

Figure 15 presents a schematic, though simplified, overview of aquifer vulnerability decision-making. For the scoping stage an analysis is made of the need for an assessment of the vulnerability of the particular groundwater resource to contamination. If deemed necessary, assessments of the vulnerability of the groundwater resource will be carried out. Vulnerability can be seen from different perspectives: from the soil and vadose zones’ perspective as the first line of defense up to the saturated zone where both spatial ‘spreading’ and the persistence of a contaminant determine the aquifer’s vulnerability to contamination (see Section 2). Other approaches integrate and/or lump the various findings on vulnerability. A detailed analysis is thus carried out during the assessment stage and requires specialist input. The findings of the vulnerability assessments form a basis for the derivation of management options and/or scenarios. These will form the starting point of the decision-making stage where analyses will be made of their costs and benefits and ultimately the formulation of management decisions and recommendations. Multi-stakeholder involvement is considered essential throughout the three stages.

27

Assess the vulnerability of

the groundwater resource.

DECISION-MAKING STAGE

Does the activity have the potential to pollute

groundwater?

Industrial activity

Sustaining ecosystems

Does the groundwater resource contribute to one or

more of the following?

Do strategic plans or strategies indicate that this resource may

be significant in future?

Yes No

No Yes

No

Yes

Agricultural supply

High

Low

Moderate

SCOPING STAGE

ASSESSMENT STAGE

Domestic water supply

Management options

Management decisions with

recommendations

No further consideration

of groundwater vulnerability is

required.

28

Evaluations of costs & benefits

associated with management options

Approaches as outlined in Section 2

Figure 15: Schematic overview of aquifer vulnerability decision-making.

3.1.1 Stage I – Scoping 3.1.2 Stage II – Vulnerabiity assessment 3.1.3 Stage III – Decision-making

3.2 Coupling the DSS with the EIA process

3.3 Case Studies 3.3.1 Secunda 3.3.2 Cape Flats

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4 Conclusions and Recommendations

…Phase II…

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