WRC Project No. K5/1432

Review of Groundwater Vulnerability Assessment Methods - Unsaturated Zone

Shafick Adams Nebo Jovanovic

Abraham Thomas Rian Titus

Riedewaan Anthony Alfred Majola

Haili Jia

Department of Earth Sciences Groundwater Group University of the Western Cape Private Bag X17 Bellville March 2004 7535

TABLE OF CONTENTS

EXECUTIVE SUMMARY ..................................................................................................ii

1 Introduction ...............................................................................................................1

1.1 Background .............................................................................................................1

1.2 Approach.................................................................................................................2

1.3 Report structure.......................................................................................................2

2 Framework for assessing groundwater vulnerability.................................3

2.1 Definition of key terms ...........................................................................................3

2.2 Assessing groundwater vulnerability......................................................................3 2.2.1 Process of groundwater vulnerability assessment...................................5 2.2.2 Key elements of vulnerability assessments.............................................6

3 The unsaturated zone .............................................................................................8

3.1 Overview of the unsaturated zone...........................................................................8

3.2 Case Study – Transport of inorganic salts ............................................................13

3.3 Unsaturated zone development for major Southern African lithologies ..............14 3.3.1 Alluvial Cover.......................................................................................17 3.3.2 Consolidated sedimentary cover ...........................................................19 3.3.3 Crystalline basement/regolith cover .....................................................22

4 Groundwater vulnerability assessment methods........................................26

4.1 Introduction...........................................................................................................26 4.1.1 Uncertainties associated with vulnerability assessments ......................26

4.2 Overlay and index methods ..................................................................................27

4.3 Process-based simulation methods .......................................................................27

4.4 Statistical methods ................................................................................................28

4.5 Characteristics of selected vulnerability assessment methods..............................28

5 Discussion and conclusion...................................................................................33

6 Bibliography ............................................................................................................38

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EXECUTIVE SUMMARY

This document reviews the framework for assessing groundwater vulnerability and the characteristics of the unsaturated zone. The unsaturated zone is important for retaining or attenuating any pollution threats to groundwater resources. The document also reviews the different methods available that can be applied to determine the likelihood of contaminants, passing through the unsaturated zone, reaching the groundwater resources as well as the behaviour of the contaminants within the unsaturated zone. Different methods were reviewed that include index and overlay methods, process based simulation models and statistical models. The document does not pretend to be a complete review of all available methods. The characteristics of the unsaturated zone for a few major lithological units in Southern Africa are also given. Due to the nature of the Southern African geology, 90% fractured rock with thin soil cover in most places, the vulnerability assessment methods need to be able to simulate or take into account the effects of preferential flow. The review of characteristics and processes occurring in the unsaturated zone and the review of models indicated the following knowledge gaps that need to be addressed in order to produce improved methods for groundwater vulnerability assessment:

a) Inclusion of multi-layer components for regolith material. b) Determination of hydraulic properties in the unsaturated zone (permeability,

porosity and water retention), due to general lack of data. c) Inclusion of preferential flow, in particular short-circuiting (fractures and rocks) and

funneling (lateral transport of water and contaminants). d) Inclusion of chemical properties (solubility, volatilization, sorption and degradation)

of specific contaminants and mixtures of contaminants. The following approaches could be applied to produce improved methods for groundwater vulnerability assessment:

1) Extension and improvement of the DRASTIC index method. 2) Use of numerical model(s) to improve the rating of the DRASTIC index method. 3) Simulation of best and worst case scenarios with suitable numerical model(s) for

average conditions or polygons.

The selected/improved/developed methods need to be tested at case study sites. For this purpose, conceptual models need to be developed for case study sites and a generic procedure was recommended 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. Finally, a comparison between the improved methods should be carried out for the case study sites.

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1 Introduction 1.1 Background This report contributes to a larger WRC funded project entitled: “Improved methods for aquifer vulnerability assessments and protocols for producing vulnerability maps, taking into account information on soils”. This report forms part of a four-part literature review on aquifer vulnerability. The aquifer vulnerability assessment methods that deal with the unsaturated zone, specifically the intermediate zone, are reviewed in this report. Water resources are vitally important for the future of humankind. A considerable portion of surface water resources such as lakes, rivers and stream flow comes from groundwater, where it contributes to important ecological habitats as well as surface drinking water supplies, industry, agriculture and recreation. Two-thirds of South Africa, including more than 280 towns and settlements, are largely dependent on groundwater for their drinking water supply and development. Groundwater in urban environments all over the world is becoming a natural resource of strategic importance owing to its limited availability, quality deterioration, increasing demand, and limited replenishment in the urban set up (Thomas and Tellam, in press). The degree of groundwater contamination depends on the intrinsic geohydrological characteristics and the physio-chemical properties of specific contaminants. Different types of pollutants are attenuated to a different degree depending on the characteristics of the site and speciation. Knowledge is therefore required on the properties of the porous medium through which the pollutant travels, the properties of the pollutant as well as the physical, chemical and biological processes (Sililo et al., 2001). This project is a natural outflow from the work of Sililo et al. (2001). The UWC group specifically focuses on the unsaturated zone properties with an emphasis on the intermediate zone. Knowing the characteristics of the unsaturated zone will assist the development of an aquifer protection strategy. A thorough understanding of the structure and hydraulic behaviour of the unsaturated zone is needed to assess the vulnerability of any aquifer. The Institute for Groundwater Studies (Bloemfontein), CSIR Environmentek (Stellenbosch) and the University of the Western Cape carried out a WRC-funded project aimed at the quantification of the impacts of groundwater pollution in urban catchments. In the absence of actual measurements of groundwater contamination, the priority contaminants were deduced from the sources occurring in South African urban areas, typical contaminant species emanating from these sources and hydrogeological properties. The simple model of Rao et al. (1985) was used for prioritization taking into account the properties of individual contaminants (half-life, sorption coefficient and Henry’s constant) and site-specific hydrogeological properties (groundwater depth and net recharge rate, air-filled soil porosity, volumetric soil water content at field capacity, soil bulk density and organic carbon content in the soil). The risk associated with these contaminants was assessed using a tiered approach, based on the type, number and level of management of the source, hydrogeological properties, exposure duration, toxicity and physio-chemical properties. The properties of the unsaturated zone and contaminant behaviour in this zone can be used to identify models and/or approaches that are able to predict the transport and fate of chemicals in the unsaturated zone. The behaviour of contaminants in the unsaturated zone will be assessed in this project.

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1.2 Approach This document reviews the current knowledge on the methods used to assess aquifer vulnerability to contaminants or pollutants mainly in the unsaturated zone. Descriptions of the main hydrogeological terrains occurring in Southern Africa are given. This is important, as it provides an input into the characterisation of the unsaturated zones in Southern Africa. The approach is to assess a broad range of methods available to the hydrogeologist. The report does not pretend to be a complete assessment of all available methods/tools/approaches used in vulnerability assessments. According to Van der Heijde (1996), the International Ground Water Modelling Center is informed every week of new computer codes addressing some aspects of fluid flow and contaminant behaviour in the subsurface. The report serves as a first screening tool for selecting approaches or tools for particular applications. The methods are discussed according to the following criteria:

Type of method; Scale of assessment; Reference location; Applicable environment; Cost and availability; Whether it is an intrinsic or specific assessment; and Geochemistry involved for the process based methods

Methods are then reviewed that apply to contaminant transport through the unsaturated zone (as per the deliverable of the project). The methods are described in terms of:

Description and background of the methods; The required inputs; The main assumptions; The relevance to groundwater vulnerability assessment; and Case studies.

1.3 Report structure This report is divided into 5 sections: Section 1 Provides a general introduction outlining the background of this project and

the approach for meeting the terms of reference. Section 2 Here the framework for assessing groundwater vulnerability is discussed with

particular reference to methods/approaches applicable to the unsaturated zone Section 3 This section deals specifically with the unsaturated zone and how the

unsaturated zone properties influence vulnerability assessments. The unsaturated zone characteristics for the Southern African environment are also discussed.

Section 4 Deals specifically with the various vulnerability assessment methods available, with specific reference to approaches and models applicable in the unsaturated zone

Section 5 In this section the main points of the review are discussed and suitable approaches and models that may be applicable to the next phase of the project are identified.

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2 Framework for assessing groundwater vulnerability 2.1 Definition of key terms Analytical model Mathematical solutions of the flow and/or transport equation for all

points in time and space. In order to produce these exact solutions, the flow/transport equations have to be simplified.

Conceptual model A simplified representation of how the real system behaves based on

analysis of field data. Deterministic model A model where all elements and parameters of the model are

assigned unique values. Intrinsic vulnerability The vulnerability of groundwater to pollutants by human induced

activities taking into account the inherent geological, hydrological and hydrogeological characteristics of a specific area and independent of the nature of the contaminants.

Macropore flow A form of preferential flow in which water flows along non-

capillary-size openings such as fractures, cracks, and root tubes. Model A simplification of reality in order to aid in understanding of and/or predict

the outcomes of the real system. Numerical model Solution of the flow and/or transport equation using numerical

approximations, i.e. inputs are specified at certain points in time and space which allows for a more realistic variation of parameters than in the analytical models. However, outputs are also produced only at these same specified points in time and space.

Preferential flow Non-uniform downward water movement along preferred pathways

that by-passes much of the matrix and includes funnel flow, unstable flow and macropore flow.

Specific vulnerability The vulnerability of an aquifer to a particular contaminant or suite of

contaminants taking into account the contaminant properties and their relationship with the various components of intrinsic vulnerability.

Unsaturated zone The zone in which the pore space contains two phases, water and air

- the zone between the land surface and the groundwater table. 2.2 Assessing groundwater vulnerability

In general, groundwater vulnerability assessments are aimed at determining the

tendency or likelihood for contaminants to reach a specified position in the groundwater system after introduction at some location above the uppermost aquifer (NRC, 1993). Groundwater vulnerability is a concept, not a measurable property. It is the tendency or likelihood of contamination occurring in the future, and thus must be inferred from surrogate information that is measurable. In this sense, a groundwater vulnerability assessment is a predictive statement much like a weather forecast, but for processes that

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take place underground and over long time scales (NRC, 1993). Vulnerability assessments help in the evaluation of the susceptibility of an aquifer to potential threats of pollution and the identification of corrective actions that can reduce or mitigate the risk of serious consequences from human activities on land. Vulnerability assessments combine the physical and chemical components of groundwater (i.e. hydrogeologic setting) with indicators of the nature and extent of potential contaminant sources to determine the potential impact of these anthropogenic influences on the groundwater quality (Hamerlinck and Arneson, 2003). Understanding the natural hydrogeologic and geochemical processes as well as the associated anthropogenic effects on a groundwater resource is required for complete scientific understanding of ground-water vulnerability (Focazio et al, 2002).

The potential for contaminants to leach to groundwater depends on many factors, including the composition of soils and geologic materials in the unsaturated zone, the depth to the water table, the groundwater recharge rate, and environmental factors influencing the potential for biodegradation. The composition of the unsaturated zone can greatly influence transformations and reactions. For example, high organic matter or clay content increases sorption and thus lessens the potential for contamination. The depth to the water table can be an important factor because short flow paths decrease the opportunity for sorption and biodegradation, thus increasing the potential for many contaminants to reach the groundwater. Conversely, longer flow paths from land surface to the water table can lessen the potential for contamination for chemicals that adsorb or degrade along the flow path. Groundwater recharge rates affect the extent and rate of transport of contaminants through the saturated zone. Finally, environmental factors, such as temperature, pH and water content, can significantly influence the degradation of contaminants by microbial transformations (NRC, 1993).

There are certain general geologic and hydrologic factors that influence an aquifer's

vulnerability to contamination as shown in Table 1, along with examples of features that lead to low or high vulnerability. Although these factors may look quite simple at first inspection, many of them interact in the environment to create more complex and subtle distinctions in vulnerability than the extreme situations in Table 1. In addition, many of these factors affecting vulnerability are highly variable and difficult to characterize over any given area.

Table 1: Principal geologic and hydrologic features that influence an aquifer's vulnerability to contamination (Source: NRC, 1993).

Feature Determining Aquifer Vulnerability Low Vulnerability High Vulnerability A. Hydrogeologic Framework Unsaturated Zone Thick unsaturated zone, with

high levels of clay and organic materials.

Thin unsaturated zone, with high levels of sand, gravel, limestone, or basalt of high permeability.

Confining Unit Thick confining unit of clay or shale above aquifer.

No confining unit.

Aquifer Properties Silty sandstone or shaley limestone of low permeability.

Cavernous limestone, sand and gravel, gravel, or basalt of high permeability.

B. Groundwater Flow System Recharge Rate Negligible recharge rate, as in

arid regions. Large recharge rate, as in humid regions.

Location within flow system (proximity torecharge or discharge area)

Located in the deep, sluggish part of a regional flow system.

Located within a recharge area or within the cone of depression of a pumped well.

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2.2.1 Process of groundwater vulnerability assessment

Assessing vulnerability is a dynamic and iterative process requiring determination of the purpose of the assessment, followed by selection of an assessment method, identification of the type, availability, and quality of data needed, performance of the actual assessment and use of the information gained from the assessment process to make decisions on groundwater resource management (NRC, 1993).

The first step in the process of vulnerability assessment is to identify the purpose of

the assessment. As shown in the flowchart (Figure 1), an assessment's purpose is determined by a variety of factors including the organization's groundwater policy goal, technical considerations such as the form of the output and the cost of the assessment, and institutional issues such as the time frame for the assessment and resource availability. Purposes of vulnerability assessments are many, ranging from improving information and education through analysing the impact of alternative groundwater policies, providing a tool for allocating resources, and guiding the decisions of land users or land use managers.

The next stage in the process of vulnerability assessment is to select a suitable

approach for conducting the assessment. Various methods are available for vulnerability assessment. This stage of the assessment process includes choosing a model or technique for the assessment, identifying the uncertainties inherent in the model and the data needed for the assessment, and testing the model and its assumptions.

Figure 1. The vulnerability assessment process (source NRC, 1993).

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Considerations regarding the availability and quality of the data required are directly linked to the performance of an assessment. These questions influence both the choice of technique for the assessment and the confidence of policy makers and regulators in making decisions based on the results.

Once an assessment is complete, various management actions are taken to protect groundwater quality or minimize contamination. Management actions range from altering land use practices, targeting resource allocations, or disseminating vulnerability information through an educational program to collecting additional data on factors relating to vulnerability or groundwater quality. Findings and recommendations on the use and improvement of vulnerability assessments and related research should also appear in the end of the process.

As the flowchart shows, the approach used to assess groundwater vulnerability is central to the process, but is also directly affected by inputs or considerations entailed by the purpose, data availability, and management use of the assessment. The selection and development of a method for vulnerability assessment is not simply a question of appropriate science, but also reflects concerns over the need for the assessment, the availability of suitable data, the level of uncertainty in the model or the data, and the impact of this uncertainty on the management actions resulting from the assessment (NRC, 1993).

2.2.2 Key elements of vulnerability assessments Key elements to consider in a vulnerability assessment for a particular application

include the reference location, the degree of contaminant specificity, the contaminant pathways considered, and the time and spatial scales of the vulnerability assessment. The reference location is the position in the groundwater system specified to be of interest. The groundwater table is the reference location used in most existing techniques. However, managers may determine that another reference location is more useful for their purposes. Vulnerability assessments may or may not account for the different behavior of different contaminants in the environment. Thus, there are two general types of vulnerability assessments. The first addresses specific vulnerability, and is referenced to a specific contaminant, contaminant class, or human activity. The second addresses intrinsic vulnerability and is for vulnerability assessments that do not consider the attributes and behavior of specific contaminants. In practice, a clear distinction between intrinsic and specific vulnerability cannot always be made. Contaminants can enter aquifers through a variety of pathways. Most existing assessment techniques address only transport that occurs by simple percolation and ignore preferential flow paths such as biochannels, cracks, joints, and solution channels in the unsaturated zone. The omission of preferential flow paths is likely a significant limitation of vulnerability assessments in many environments. Some overlay and index methods have attempted to address contamination that might occur in wells and boreholes by mapping those features in combination with the results derived from other assessment methods. The overall utility of a vulnerability assessment is highly dependent on the scale at which it is conducted, the scale at which data are available, the scale used to display results, and the spatial resolution of mapping.

The combination of these elements makes up a vulnerability assessment method. Inherent in any such combination will be scientific uncertainties associated with errors in data, errors in method, and potential misapplication of an approach to a given area.

The NRC (1993) list three rules that should be acknowledged for every groundwater

vulnerability assessment: 1. Most groundwater is to some degree vulnerable;

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2. Uncertainty is inherent in all vulnerability assessments; and 3. There is a risk that the obvious may be obscured and the subtle may become

indistinguishable.

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3 The unsaturated zone 3.1 Overview of the unsaturated zone

The unsaturated/vadose zone represents the top portion of the geological profile (Figure 2). This zone is subjected to weathering, erosion, pedogenic and other processes (Van Schalkwyk and Vermaak, 2000). The unsaturated zone is generally divided into three zones viz. (1) soil water zone (2) intermediate/vadose zone and (3) the capillary water zone. Most of the water in the intermediate zone is moving downward with some retained, but no in-situ use for it exists and cannot be recovered by man (Driscoll, 1989). Weathering profiles, soils and regoliths exhibit varying degrees of vertical complexity, it can vary from nearly homogenous stratigraphy to “layer-cake” horizonation to complicated patterns (Phillips, 2001). Spatial complexity in weathering profiles may be derived from the variability of the parent material and/or divergent regolith evolution associated with complex feedback relationships within weathering systems (Phillips, 2001). Phillips (2001) applied the state probability functions to weathered profiles to assess the extent to which spatially variable properties of weathering profiles are acquired or inherited.

Figure 2. Vertical zones of subsurface water.

In humid regions the unsaturated zone may be quite thin to absent. In dry regions very little water passes through this zone. In dry regions most of the water that infiltrates and percolates to the saturated zone occurs through streambeds and preferential pathways. In semi-arid Southern Africa numerous examples highlight the importance of streambeds and preferential pathways in groundwater recharge processes (Xu and Beekman (eds.), 2003).

Unsaturated zone studies in the past were mainly for water resource evaluation but have now shifted towards contaminant transport studies. The unsaturated zone and the confining layers above an aquifer are the main elements determining how vulnerable an aquifer system is. The unsaturated zone provides protection through (Harter and Walker, 2001):

Intercepting, sorbing and eliminating pathogenic viruses and bacteria; Sorbing and degrading many synthetic organic chemicals; and

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Attenuating heavy metals and organic chemicals through sorption and complexation with mineral surfaces within the unsaturated zone and through uptake into plants and crops.

The processes occurring in the intermediate zone are not always similar to those

occurring in the soil environment. For example, the hydraulic and therefore solute transport properties may be different, as well as the temperature, pH, microbiological activity, preferential flows, etc. depending on the nature of the subsoil.

Important attributes that need to be considered when dealing with contaminant

transport in the unsaturated zone are (Scanlon et al., 1997): Direction and rate of water movement; Spatial and temporal variability of water fluxes; The type of medium (porous or fractured) because of the higher potential for

preferential flow in fractured systems; The presence of vegetative cover as it removes much of the water from the surface

and; The surface topography as it controls the movement of water by concentrating

unsaturated flow in topographic depressions.

Flow and transport processes are inherently complex within the unsaturated zone due to the large variability of soils, sediments and rocks. Knowledge of the spatial variability in unsaturated flow is critical for realistic assessment of transport rates because spatially variable rates could allow contaminants to migrate more rapidly in some areas than in others, bypassing the buffering capacity of much of the unsaturated zone (Scanlon et al., 1997). In arid areas, the grade of consolidation and fissuring of the soil and the unsaturated zone can further reduce the accuracy of arrival times of contaminants to the water table (Collin and Melloul, 2003). In arid areas where fractured rocks are overlain by thick unconsolidated sediments the water fluxes decrease with increasing unconsolidated sediment cover (Scanlon et al., 1997). The physical properties of fractures must be known, to evaluate flow regimes, perform modeling calculations, and plan remediation where fractured rock strata are present in the vadose zone.

The travel time, attenuation capacity and quantity of contaminants reaching

groundwater depend on the subsoil that overlies groundwater, the type of recharge and the thickness of the unsaturated zone through which the contaminant travels (Sililo, 2001). Therefore, the unsaturated zone plays a major role in the transport and fate of contaminants from the surface to the water table. The mechanisms of the processes occurring in the unsaturated zone are not well understood, evidence of which was given in a number of applications (de Wit et al., 2001; Gaston and Locke, 2002; Logsdon et al., 2002; Vanderborght et al., 2002; Bergkvist and Jarvis, 2004). Tuller and Or (2002) suggested unsaturated flow occurs in completely filled pore spaces, in partially filled pores and grooves, as well as in films on solid surfaces. Some of the very dynamic processes involved in the unsaturated zone and related to groundwater recharge are evaporation (DePaolo et al., 2004), climate change (Glassley et al., 2002) and infiltration as indirectly affected by temperature (Lin et al., 2003). In particular, hydraulic properties of the regolith and unsaturated zone coupled with preferential flow processes require attention (Sililo et al., 2001), where little information is available, generally due to the cost and difficulty of taking measurements.

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In some cases, it is justified to assume that the properties of the subsoil are similar to the properties of the overlying soil. In other cases, however, different properties between soil and weathered bedrock were measured (Hubbert et al., 2001). The nature and properties of the unsaturated zone could be inferred from the nature of the soil and lithology.

Sililo et al. (2001) discussed the dominant processes and assigned hydraulic and chemical attenuation qualitative rating to subsoil horizons and materials, based on the South African soil classification (Soil Classification Working Group, 1977). This information, however, needs to be quantified and checked in the field. Campling et al. (2002) classified soil drainage into six classes (from excessively to very poorly drained) based on the local water table regime depth, determined through soil morphological indicators, soil auger hole and soil pedon observations. The use of above ground information for classifying soils is also not new. For example, Park and Burt (2002) attempted to correlate soil properties to the geomorphology of the terrain, but they found that the topsoil properties were better correlated with terrain attributes than subsoil properties. Similarly, Zhu et al. (2001) tried to correlate soils to environmental conditions for surveying purposes. Shirazi et al. (2003) developed soil ranks for specific ecoregions, based on the homogeneity of soil texture and rock fragments.

In the porous unsaturated zone, solute fluxes occur by advection and molecular diffusion (along concentration gradients) (Corey and Auvermann, 2003; Cheng et al. 2003; Magesan et al., 2003; Zhou and Selim, 2003). Molecular diffusion depends on the concentration gradient, the diffusion coefficient and the tortuosity of the medium. Advective transport can be quantified based on hydraulic conductivity and effective porosity, and it may occur in both longitudinal and transverse directions. Mechanical dispersion occurs due to the different velocities of water flux in the porous media. Molecular diffusion and mechanical dispersion are generally combined together to produce the hydrodynamic dispersion coefficient. The separate effects of molecular diffusion and mechanical dispersion can be described with the dimensionless Peclet number. A number of empirically determined, chemical- and site-specific parameters are therefore required to describe water and solute fluxes in the unsaturated zone.

In terms of solute flux properties in the soil, the most commonly used modeling parameters for generic contaminants are the sorption isotherms that describe the adsorption of chemicals on soil particles and organic matter, and the diffusion and dispersion coefficients derived from breakthrough curves. Sorption of organic contaminants is described with the organic carbon-partitioning coefficient (Koc), whilst volatilization and half-life are the other two specific properties most relevant to groundwater contamination. In addition, some organic compounds may be either insoluble or partially soluble in water, including a non-aqueous phase liquid (NAPL). The fluxes of NAPL chemicals in liquid and non-liquid phase can be estimated from the relative permeability and proportion of air, water and NAPL saturation in porous media. Berry et al. (2004) developed a model to simulate multiple fluid phases as well as chemical, particulate, and microbial transport with volumetric reactions and sorption. Yoon et al. (2003) and Wang et al. (2003) modeled the vapour exchange and sorption of NAPLs at different water contents. Pruess et al. (2002) used the theory of multi-phase liquid flow, heat transfer and solute transport to study leakage from nuclear waste storage tanks.

In the case of mixtures, the solubility product of inorganic chemicals changes depending on ionic strength of the solution, pH etc. (Sililo et al., 2001) Organic mixtures

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will also change the solubility of individual chemicals as well as sorption characteristics due to competition for sorption sites (Fetter, 1993).

For the regolith zone, the sorption isotherms can be used to describe the interaction between solution and media. Measurements of permeability and porosity of the media are required to determine the water fluxes, from which solute fluxes can then be estimated. Al-Raoush et al. (2003) developed a technique to extract pore network parameters from three-dimensional images of unconsolidated porous media systems. Tokunaga et al. (2003) discussed the importance of describing the moisture retention characteristics of gravelly material for water and solute flux estimation.

Typical values of water flow under saturation are 10-4 to 10-7 cm s-1 for clay and 10-2 to 10-3 cm s-1 for sand (Hillel, 1982). Under unsaturated conditions, these values decrease by orders of magnitude due to the exponential decrease in hydraulic conductivity, depending on matric potential. Therefore, the main process that causes movement of contaminants downwards in the unsaturated zone is infiltration. Infiltration of water is generally driven by huge suction gradients between the wetting front and the dry media. The wetting front is the zone that water invades advancing into an initially dry medium, with matric potentials typically just below saturation (between 0 and –2 J kg-1 or 2 kPa suction). Infiltration causes flushing of solutes downwards at the edge of the wetting front. In this way, a center of mass of solutes is generated and transported downwards during infiltration events. In terms of groundwater vulnerability to contaminants, water fluxes at low matric potentials (below –10 Jkg-1 or 10 kPa suction, corresponding to field capacity) and molecular diffusion of solutes are generally less important processes.

Approaches that are used to quantify downward solute fluxes include steady state and transient flow systems (Hillel, 1982). Steady-state flow systems imply that fluxes, gradients and water contents are constant with time. Therefore, they are easier to measure and model, but they may take a long time to stabilize. Transient flow systems imply variable fluxes, gradients and water contents in the unsaturated zone, which allows describing natural occurrences more accurately and in detail.

Sililo et al. (2001) indicated the need to identify chemical affinities between groundwater and overlying soil/regolith and their relationship to recharge characteristics. In the saturated zone, the same principles of water and solute fluxes are applicable as for the unsaturated zone, by making use of the hydraulic gradient and conductivity (Darcy’s law) as well as porosity. The hydraulic conductivity depends on the properties of the aquifer, as well as the density and viscosity (temperature) of the fluid (Brassington, 1999). Porosity depends on grain size and shape, degree of sorting, extent of chemical cementation and the amount of fracturing. Effective porosity is the amount of interconnected pore spaces, which are available for fluid flow. Values of porosity for a variety of materials was given by Brassington (1999), ranging from shale (6%) to peat (92%), and expressed as percentage of rock. Secondary porosity results after rock formation, due to rock cracking and dissolution (e.g. limestone). Movement of groundwater and fluxes in fractures was described by Fetter (1999). The specific yield is the volume of water draining from the aquifer by gravity. This can range from 3% for clay to 38% for dune sand. The fraction of effective porosity producing specific yield may vary from 0.07 for clay to 0.80 for coarse gravel and fractured micro-porous or non-porous rock (e.g. chalk, granite) (Brassington, 1999). Permeability determines how fast a fluid can move through the media. It can be classified as intergranular or primary (typical for unconsolidated deposits and weathered rock) and secondary (typical for fissuring and dissolution of consolidated rocks). Values of hydraulic

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conductivity or water permeability were given for a variety of unconsolidated sediments and rocks by Brassington (1999). These parameters can be used to calculate transmissivity, the average hydraulic conductivity multiplied by the full thickness of saturated aquifer.

The NRC (2001) identified five major issues that cause difficulties for estimating fluid flux and travel times through the fractured unsaturated zone;

Flow in unsaturated fractures may occur as either capillary flow or film flow; Preferential flow can occur in the unsaturated zone as a result of heterogeneities

and/or flow instabilities; As water moves through variably saturated fractures, a portion of the flow is

imbibed into the rock matrix; Solute transport in fractured rocks can exhibit complex behaviour that are difficult

to interpret; and The interpretation of environmental tracers has lead to the conclusion that

seemingly contradicts the initial conclusions based on classical hydrodynamic analysis.

Preferential flow within the unsaturated zone is important due to the greater

prevalence of preferential flow channels in the shallow subsurface. The effect of preferential flow makes transport difficult to predict and measure in the unsaturated zone. The different preferential flow types and their impacts on vulnerability assessments are discussed in more detail in the next section. Preferential flow

Micro- and macro-pore exchange fluxes, as well as preferential flow are complicating factors in estimating travel times of contaminants through the unsaturated zone. These have implications on groundwater as anomalies may occur, e.g. higher salinity in deeper layers of the unsaturated zone than in shallower ones, or unevenly distributed solute load reaching groundwater. Several models and experimental evidence can be found in the literature on water and flux exchanges between micro- and macro-pores in the unsaturated zone (Casey et al. 1999; Langner et al., 1999; Castiglione et al., 2003; Roulier and Jarvis, 2003a and b; Berry et al., 2004).

The extent of the effects of preferential flow on groundwater contamination is not fully understood. These effects were reported to be important in some cases (Sililo and Tellam, 2000; Jaynes et al. 2001), and negligible in others (van Schalkwyk and Vermaak, 2000). Hull and Bishop (2004) indicated that vertical flow could be variable depending on recharge. On the other hand, Dyck et al. (2003) measured relatively uniform deep drainage. Fetter (1993) classified preferential flow as short-circuiting, fingering and funnelling. Short-circuiting

Short-circuiting occurs due to movement of infiltrating water along preferential paths (e.g. plant roots, cracks and fractures, etc.). Sililo et al. (2001) assumed that preferential flow dominates in most fractured rock formations to the extent that water-rock interactions are negligible, but also suggested to include as much information as possible about the regolith and unsaturated zone in a simplified strategy for groundwater vulnerability assessment. Wood et al. (2004) proved that fracture networks in uncemented limestone create flow behavior not generally recognized in conceptual and numerical models (e.g. convergence of flow, pathway switching, and fluid cascades), where models tend to overestimate the interaction between fractures and matrix (Fairley et al., 2004). On

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the other hand, Hu et al. (2002) showed that liquid and tracers flowing through fractures penetrated into the matrix to a depth of several millimeters.

Smith (2004) reported that basalt lava fractures, tension cracks, lava tubes and rubble zones allow rapid infiltration and flow of water and contaminants. Mattson et al. (2004) reported that water movement in basalt formations is mainly gravity-dominated in preferential fracture pathways and rubble zones, and results in rapid vertical flow especially under conditions of positive hydrostatic head. Frazier et al. (2002) reported on similar preferential flow effects in fractured, weathered granitic bedrock. Saprolite weathered bedrock is generally known to have low permeability (Rasmussen et al., 2000), but rapid transport of contaminants may occur through fractures (McKay et al., 2002).

Contaminants can also be transported as adsorbed on colloids through macro-pores.

The fluxes of natural colloids and slaked fragments through macro-pores was discussed by Schelde et al. (2002), Rousseau et al. (2004), Nemati et al. (2003) and Nimmo et al. (2004). Washing of clay particles and accumulation in fractures was found to be the cause of granitic bedrock weathering (Frazier and Graham, 2000). Similarly, pathogens can be transported by preferential flow (Darnault et al., 2004). Fingering

Fingering occurs due to pore-scale variations in permeability, especially at boundaries where finer sediment overlies coarser sediment. Jury et al. (2003) reported that fingering particularly contributes to water flow in coarse-textured soils. Wang et al. (2003) proved that preferential flow occurs even in homogeneous soils through fingering. Kohler et al. (2003) found preferential flow originating primarily at the boundary between top- and subsoil as soon as the topsoil becomes sufficiently saturated. Funnelling

Funneling occurs whenever water is funneled on sloping impermeable layers, and concentrated at the end of these layers where it percolates vertically. Funneling is therefore typical for layered soil or sediment profiles. Pan et al. (2004) indicated that sediment layering could cause considerable lateral flow, which is then stopped by faults through which flow is then funneled. Nimmo et al. (2002) showed that low-permeability sediment layers in the unsaturated zone divert some flow horizontally, but do not prevent rapid transport to the aquifer, and that transport rates under these conditions may exceed 14 m d-1. Sovik et al. (2002) reported that knowledge about sedimentary structures in the unsaturated zone is important for monitoring of contaminant transport and for remediation purposes. O’Geen et al. (2002) discussed the effects of loess stratigraphy on unsaturated zone water movement.

3.2 Case Study – Transport of inorganic salts

A relevant case study of solute migration in the unsaturated zone was identified in

the literature search (Campbell, ???). In this study, the transport of inorganic salts (CaSO4 and MgSO4) originating from five years of irrigation with mine effluent was investigated. The aim of the study was to draw some conclusions regarding the efficacy of the soil and regolith as a repository for waste salts, and the potential for damage to groundwater quality. Undisturbed core samples were taken with a geological drill from two typical soils of the Mpumalanga Highveld at 200 to 300 mm depth intervals down to 3 m.

Chemical analyses indicated that salts were washed out by infiltrating rain water

down to about 2 m depth in the profile of a uniform sandy loam Hutton soil. The centre of

13

mass of salts, occurring at about 2 m depth, was evident from the chemical analyses. Non-contaminated land was used as comparative baseline. On the other hand, in the case of a sandy loam Bainsvlei soil, hydraulic attenuation occurred due to the presence of a plinthic layer at about 1 m depth. Salinity was high in the top 1 m soil layer, but salts were likely to migrate horizontally along the slope of the plinthic layer. The study also indicated differences in rates of transport between contaminant species. The bulk of calcium tended to migrate slower as CaSO4 has low solubility and tends to precipitate in the soil, whilst Mg was more mobile and high concentrations of Mg were measured deeper in the profile.

The outcome of this study indicated that contaminant transport in the unsaturated

zone depends on both the properties of the medium (e.g. depth, size and nature of layers) and the properties of the pollutant (e.g. chemical equilibrium reactions).

3.3 Unsaturated zone development for major Southern African lithologies

Several examples of conceptual models of unsaturated zone development are available in the literature for geological terrains. McElroy and Hubbell (2004) as well as Mattson et al. (2004) developed and tested a conceptual flow model for a waste site, located

on the Idaho National Engineering Laboratory (INEL), comprising basalt flows intercalated with sedimentary interbeds. Nolan et al. (2003) estimated steady, long-term recharge using pedotransfer functions of soil texture and interpolated the values with non-parametric methods to assess aquifer vulnerability.

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. 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. Regional maps of the geology, terrain morphology and soil cover are shown in Figures 3-5.

The vadose zones can vary significantly from site to site in their textural composition and hydraulic properties. In order to measure the capacity of the entire sequence of materials that overlie the saturated zone to transmit water, the thickness of the unsaturated zone and the hydraulic conductance of unsaturated material must be determined. Since the unsaturated zone varies in depth and composition across the country, it is important to know what chemical, biologic and hydrologic processes exist at a given location. Such an understanding is critical for predicting how contaminants will behave underground and what environmental impacts may occur. The next section only serves as a generic conceptual understanding of how the unsaturated zone will be constituted for the major lithological provinces of South Africa (Figure 3) and Table 2.

The characteristics of the unsaturated zone are also determined by the morphology of the landscape (Figure 4). Regolith and/or alluvial covers generally dominate in topographically lower lying areas such as plains, valleys and river channels. Where soil cover forms the unsaturated zone the soil cover may either be a result of in-situ weathering or the deposition of transported material (Figure 5). Where negligible soil cover occurs, the unsaturated zone is composed of mainly fractured or intact bedrock with primary porosity.

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Figure 3. Major lithologies of South Africa (Source: Department of

Environmental Affairs and Tourism). Table 2. Characteristics of major hydrogeological systems (Adapted from Foster et al.,

2000).

Hydrogeological environment

Type of deposit Mode of formation Distribution and thickness

Alluvial and colluvial formations - Inland - Coastal

Gravels, pebbles, sands, silts and clays

Unconsolidated detritus deposits in riverbeds and deltas, primary porosity/permeability usually high.

Areally extensive and of significant thickness.

Consolidated sedimentary aquifers

Sandstone, siltstone, mudstones, shales, conglomerates. Limestones

Compacted marine or continental deposits, degree of consolidation increases with depth/ages and reduces primary porosity/permeability but with significant fracturing. Derived from shell fragments/reef detritus, compacted and often with karstic fissures enlarged by solution.

Extensive aquifers of substantial thickness.

Crystalline basement Grading from fractured bedrock to residual clays

Deep weathering of igneous/metamorphic rocks usually producing mantle of moderate porosity/low permeability

Extensive, but aquifers of small capacity

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Figure 4. Terrain morphology of South Africa (Source: Deparetment of

Environmental Affairs and Tourism).

Figure 5. Soils map of South Africa (Source: Deparetment of Environmental

Affairs and Tourism).

16

3.3.1 Alluvial Cover Cape Flats Aquifer

The topographic low, located between Table Mountain and the Drakenstein and Hottentotsholland mountains and Atlantis in the north, is known as the Cape Flats. Large parts of the Cape Flats are covered by urban development.

The Cape Flats Aquifer (CFA) consists predominantly of Tertiary to recent unconsolidated sand deposits. This primary aquifer system occurs within the Sandveld Group and the various formations were deposited under shallow marine, lacustrine and aeolian conditions. Extensive but isolated calcrete horizons may serve as internal hydraulic barriers and result in localized perched aquifer conditions. The aeolian sand cover is delineated by the Springfontyn and Witzand Formations (Cole and Viljoen, 2001). The Springfontyn Formation (late Pleistocene to Holocene) are well-sorted, fine- to medium-grained, unconsolidated structureless quartzose sand and the Witzand Formation (late Pliocene to Pleistocene) are similar except that it is often cross stratified and contains marine shells (Cole and Viljoen, 2001). The deposition of sediments occurred initially in a shallow marine environment and subsequently progressing to intermediate beach and windblown deposition and finally to aeolion and marsh conditions. Table 3 indicates the lithostratigraphy of the Cape Peninsula.

The static water levels in the Cape Flats Aquifer fluctuate between 2 m to 5 m below ground surface. The water level is very close to the surface at the peak of winter rainfall. In the lower lying areas the water level reaches the surface during periods of high rainfall. The Cape Flats Aquifer is illustrated for various areas in Figure 6. The unsaturated zone develops in the unconfined aquifer and the thickness of the unsaturated zone is a function of the topography of the land surface and the underlying bedrock, the seasonal water level and the degree of interconnectivity with the underlying fractured rock aquifer. Calcrete and clay lenses or layers may create perched aquifers within the system. The composition of the sand, determined at the UWC borehole site, is generally well sorted with grain sizes ranging from 0.75 to 3.25 phi with minor amounts of granules, silt and clay fractions (Adams, unpublished data). Table 3. The Lithostratigraphy of the Cape Flats (Hartnady and Rogers, 1990).

Witzand FormationNA Recent

Langebaan Formation Pleistocene

Veldrif Formation Pleistocene

Springfontyn FormationNA Pleistocene

Varswater Formation Pliocene

Sandveld Group

Elandsfontyn FormationNA Miocene

Primary Aquifer

Unconformity False Bay dolerites (e.g. Logies Bay dyke) Jurrasic - Creataceous

intrusive contact

Pakhuis Formation Silurian

Peninsula Formation Silurian Table Mountain Group

Graafwater Formation Ordovician Nonconformity

Cape Point Intrusive Precambrian - Cambrian intrusive contact

Cape Granite Suite Cape Peninsula Batholith Precambrian - Cambrian Secondary Aquiferintrusive contact

Sea Point FormationNA Late Proterozoic Malmesbury Group Bloubergstrand FormationNA Late Proterozoic

Secondary Aquifer NA Not approved by SACS

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Figure 6. Cross-sections for various areas on the Cape Flats (Wright and Conrad,

1995).

18

3.3.2 Consolidated sedimentary cover Karoo Supergroup

The Karoo is almost completely characterised by sedimentary rocks with prominent dolerite dykes and sills due to their resistive nature to weathering. The lithology is dominated by fine to coarse grained sandstone, some shale, mudstone and siltstone. The western and the eastern part of the Karoo are covered with weakly developed soil types. They are highly erodible and the water infiltrates easily. The red, black and yellow clays dominate the northeastern parts. Infiltration of water is not as fast here and it limits the flow within the fractures. Sands dominate on the western edge of the Free State province and other areas in Mpumalanga. The erodibility there of these sands is moderately low. Dwyka Group

The Dwyka diamictite, tillite and shales have very low hydraulic conductivities. They range from about 10-11 to 10-12 m/s (Driscoll, 1986). They virtually have no primary porosity. This Group constitutes a very low yielding fractured aquifer and water is confined within narrow discontinuities like jointing and fracturing, they tend to form aquitards rather than aquifers. The aquifers in few sandstone bodies deposited in glacial valleys of the northern facies are of very limited extent. The Dwyka is not an ideal formation for the development of aquifers (Woodford & Chevallier, 2002).

Ecca Group The Ecca Group consists mainly of shales with thicknesses that vary from 1500 m in the

south to 600 m in the north (Woodford & Chevallier, 2002). The Ecca Group was deposited in a fluvial environment. Their porosities tend to decrease from 0.1 % in the north of latitude 28°S to less than 0.02 % in the southern and southeastern parts. On the other hand their bulk densities increase from about 2000 to 2650 kg/m3 from north to south. This gives the idea that the economically viable aquifers may exist in the northern parts of areas underlain by the Ecca shales. Rowsell and De Swardt (1976) found that the permeability of the sandstones are very low due to the fact that the sorting is poor and their primary porosities have been lowered considerably by diagenesis. Beaufort Group

The coarser grained rocks are found near the Cape Fold Belt (alluvial fan and braided stream environments), while mudstones, shales and fine-grained sandstones are found in the more distal central and northern portion (meandering river and floodplain environment) of the basin (Woodford & Chevallier, 2002). The sedimentary units in the Group therefore have very low primary hydraulic conductivities. They are both multi-layered and multi-porous with varying thicknesses. Hydraulic properties of aquifers are discontinued at the contact plane between two different sedimentary layers. The pumping of multi-layered aquifer causes the piezometric pressure to drop faster in more permeable layers than in the less permeable ones. It is therefore possible to mine layers completely without materially affecting the piezometric pressure in the less permeable ones. Furthermore, the coarser and more permeable sedimentary bodies are lens shaped. Stormberg Group

The sheetlike sedimentary bodies, the basal pebble conglomerates and coarse-grained sandstones make the Molteno Formation an ideal aquifer. The problem is that the Formation does not occur over a large area and it tends to form topographic highs. The largest part of the Elliot Formation consists of red mudstones, making it more of an aquitard than an aquifer

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(Woodford & Chevallier, 2002). The Clarens Formation consists almost entirely of well sorted, medium to fine grained sandstones deposited as thick consistent blankets (Visser, 1984), making it more homogeneous than any other Karoo formation. This should be ideal for an aquifer. However, it is poorly fractured with very low permeability, and can only store large quantities of water, which it cannot release quickly. According to Beukes (1969), the porosities ranges are:

Very fine grained sandstone --- 6.19 to 9.82 % Cross-bedded sandstones --- 8.87 to 10.75 % Average sandstones --- 8.46 %.

The Karoo dolerite intrusions

The dolerite dykes and sills were intruded into the sediments of the Karoo Supergroup. The dolerite sills and rings are by far the most common tectonic style in the Karoo basin controlling the geomorphology of the landscape to a large extent and they have the same geographical distribution (Du Toit, 1905; Du Toit, 1920). Many near vertical dykes are seen branching onto the sill and ring complexes or cutting through them (Woodford & Chevallier, 2002). There are transgressive oblique fractures that extend tens of metres away from the dyke into the country rock. Dyke orientation and density of intrusive areas also play a role, for instance, the higher the density the higher the yield (between 2 and 10 l/s compared to less than 1.5 l/s of low density areas). Table Mountain Group (TMG)

The extensive Table Mountain Group (TMG) comprises of Ordovician to Silurian arenaceous consolidated sandstone, shale, mudstone, tillite and a basal conglomeritic unit. The TMG represent a weathered and fractured secondary aquifer system. The aquifer geometry and groundwater flow in the TMG Aquifer system are controlled by the depositional and deformational characteristics of the Cape Fold Belt.

The dominant sandstones of Ordovician to Silurian age possess negligible hydraulic conductivity. The TMG aquifer outcrop and sub-outcrop area can be broadly divided into two interconnected (except where natural barriers like faults occur) domains, that is, the intermontane and the coastal domains. The different hydrological properties and potential of the formations within the TMG make these domains inhomogeneous (Rosewarne, 2002).

The deep fractures have high yielding conductivity and extend to great depths, thus promoting deep circulation of groundwater. Fractures, joints and bedding planes are open at surface and any soil covering tends to be permeable aeolian sands with calcrete layers, or alluvium. Their erodibility ranges from low to moderately high.

The medium to coarse grain sizes and relative purity of some quartz arenites, together with their well indurated nature and fracturing due to folding and faulting, enhance both the quality of the groundwater and its exploitation potential. Pakhuis-Cedarberg Meso-aquitard

It serves as a confining layer between the two main aquifers, namely the Peninsula and Nardouw aquifer systems. Peninsula Formation

It comprises of thick orthoquartzites with virtually no argillaceous zones and associated with brittle deformation (Jolly & Kotze, 2002). The thickness varies from 575 m on Table Mountain to about 1200 m eastwards, and the fractures are open to great depths.

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This is a prime target for drilling purposes. There are deep seated hydrotects (major regional fault systems extending over tens of kilometres that control the groundwater movement) interconnected with shallower intensively fractured zone present over most of the area. (Rosewarne & Weaver, 2002). The Peninsula aquifer outcrops at higher altitudes with lower shale. Joint systems within the Peninsula quartzites cut continuously across bedding and are less regular in length and spacing (Hartnady and Hay, 2002). Nardouw Formation

Here the quartzite weathers to produce a rough topography with weirdly shaped bare rocks, the surface for which many gullies and cracks are exposed. The aquifer has lower hydraulic conductivity and is prone to ductile deformation (Jolly & Kotze, 2002). Kotze (2000) concluded that the average density of Nardouw rocks is lower with shorter fracture length when compared to those of the Peninsula Formation. Matrix flow dominates the Nardouw aquifer and the transmissivity of the fractured and of more massive rock mass averages 50 and <1 m2/d respectively. Permeability perpendicular to bedding is low. The Lower Bokkeveld Formation

It comprises of three shales and three quartzitic units. Most boreholes in the Hex River Valley derive their groundwater from the Bokkeveld rocks (Rosewarne, 2002). Water level recovery is generally steep during winter months, but it is short lived, except those in the northern flanks of the valley, that recover fully each year no matter what the rainfall (Rosewarne, 2002).

Figure 7. Conceptual configuration of the unsaturated zone in consolidated sedimentary rocks.

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3.3.3 Crystalline basement/regolith cover

The term crystalline basement refers to igneous and/or metamorphic rocks, such as granites, gneisses, meta-quartzites, and basalts with negligible primary porosity. Regolith is defined as the solid product of intense in situ weathering (Howard and Karundu, 1992). The development of basement aquifers is notoriously complex, especially where a thinner weathered overburden is present (Lloyd, 1999; Chilton and Foster, 1995; Gustafson and Krásný, 1994; Rebouςas, 1993; Olofsson, 1993; Wright and Burgess, 1992; Commonwealth Science Council, 1990; Acworth, 1987). According to Butt et al. (2000) regolith has been forming continuously for over 100 Ma in parts of Africa, India, South America, SE Asia and Australia.

The degree of weathering is dependent on the lithology and the fracture intensity. Crystalline aquifers usually consist of a two-component system (Van der Sommen and Geirnaert, 1990; Chilton and Foster, 1995):

A weathered mantle rich in clays. This zone has high porosities but low permeabilities; and

Fractured bedrock zones with low porosities and much higher permeabilities. This zone is usually highly conductive.

Past weathering and erosion processes have resulted in the formation of a weathered

mantle or regolith, a zone of alteration products, normally more than 10 m thick and overlain by residual soil (Taylor and Howard, 2000; Chilton and Foster, 1995; Gustafson and Krásný, 1994; Acworth, 1987). Aggressive weathering and differential leaching has, through the downward movement of infiltrating waters, resulted in deep regolith profiles. Kaolinite, quartz and oxidized iron minerals generally characterize the residual soil. The soils are underlain by laterite beds or stone-lines and have a relatively high infiltration capacity as well as pathways for preferential flow in low permeability soils (Chilton and Foster, 1995). The regolith is a result of the infiltrating and acidic rainfall reacting with the host minerals, followed by leaching of the more soluble and mobile elements (or compounds) with contemporary as well as subsequent re-precipitation of the less mobile elements or compounds (i.e. the formation of Al/Fe oxides and kaolinite as examples).

In granitic rocks the weathered zone is usually uniform and less clayey than in schist and will show the intermediate zone of decomposed rock. Differential weathering occurs throughout the rock mass, as well as on fracture/joint planes. The transition zone is well developed in fractured zones (Van der Sommen and Geirnaert, 1988). Percolation of water from the surface to the weathered zone and into the fractured rock is complex. Basement aquifers have low porosities, permeabilities and generally high transmissivities.

Regolith characteristics change in response to different conditions over time, as shown by Butt et al. (2000). These changes are shown in Table 4.

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Table 4. Changes in regolith from the conditions of formation (Butt et al., 2000).

Uplift

Lowering of the water table. Irreversible dehydration and hardening of ferruginous and siliceous horizons. Increased leaching of upper horizons under more oxidising conditions. Increased erosion.

Down warping Water logging of lower parts of the landscape and imposition of reducing conditions. Decrease in erosion; increased sedimentation in valleys.

More humid climate Increased leaching and deeper soil development. Decreased erosion (due to denser vegetation).

Less humid climate Decreased leaching. Increased erosion.

Semi-arid or arid climate

Decreased leaching. Retention and precipitation of silica, alkaline earths and alkalis in silcretes, clays, calcretes, and salts. Increased erosion.

The mineral transformations in different weathering stages show the following sequences (Lan et al., 2003): 1. feldspar → sericite → hydromica → kaolinite; 2. pyroxene and hornblende → chlorite → montmorillonite → halloysite → kaolonite; 3. biotite → vermiculite → montmorillonite → kaolonite 4. quartz → silica → chalcedony → secondary quartz. The weathered zone in Namaqualand is dominated by a kaolinite rich overburden, indicating an advanced weathering stage. The formation of core stones within the weathered regolith profile is common.

Several classification schemes based on vertical profiles have been developed for basement aquifers (e.g. Taylor and Howard, 1999; Chilton and Foster, 1995; Wright, 1992; Acworth, 1987; UNESCO, 1984), mainly for humid regions. Figure 8 depicts the different classification schemes for weathered basement aquifers.

Due to its larger porosity the weathered/regolith zone acts as a reservoir that slowly feeds water downward into fractures in the bedrock (Daniel, 1996). The water level in the aquifers is directly related to the amount and duration of groundwater recharge. During periods of no or low recharge the water level recedes into the fractured bedrock aquifers and if a weathered overburden exists the water level will rise into the weathered zone or regolith aquifers (Figure 9 and Figure 10). Fractures exposed at the surface act as preferential flow paths. Infiltration at a point between soil and rock are dependent on the hydraulic conditions at that zone (Olofsson, 1993). If water enters a fracture above a point of saturation, the movement of the water will be predominantly in the direction of the dip of such fractures. Water will only enter a fracture after the fluid pressure exceeds the water-entering pressure of the fracture. Below the saturated zone, movement can be both vertical and horizontal. The lateral motion along the strike of the fracture would predominate (Ellis, 1909). Any connected series of joints will have a complex circulation. Nevertheless, the main circulation will be towards and along the fractures having the largest openings and the nearest discharge points, and in these fractures the general movement will be in the direction of a land slope. Percolation of the groundwater from the surface to the weathered zone and into the fractured rock is complex. Flow paths in fractured rocks are very complex and heterogeneous, due to its complex geometry (Karasaki et al., 2000).

23

A

BB Figure

8

. Weathered profiles for basement aquifers ((A) Taylor and Howard 1999, and (B) Chilton and Foster, 1995).

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Figure 9. Configuration of the relationship between the unsaturated and

saturated zone for hard rock aquifers (UNESCO, 1984).

Figure 10. Configuration of the weathered zone of a crystalline aquifer.

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4 Groundwater vulnerability assessment methods 4.1 Introduction

Several approaches or methods have been developed for assessing groundwater vulnerability. They range from sophisticated models of the physical, chemical and biological processes occurring in the unsaturated zone and groundwater regime, to models that weigh critical factors affecting vulnerability through either statistical methods or expert judgment. The National Research Council (1993) has classified these methods into three major classes: (1) overlay and index methods that combine specific physical characteristics that affect vulnerability, often giving a numerical score, (2) process-based methods consisting of mathematical models that approximate the behaviour of substances in the subsurface environment, and (3) statistical methods that draw associations with areas where contamination is known to have occurred (NRC, 1993). Lindström (1997) further divided the methods into the following assessment classes (1) empirical, (2) deterministic, (3) combined empirical and deterministic, (4) probabilistic and (5) stochastic. The assessments will be grouped in this document according to the NRC (1993) classification. With the aid of Geographic Information Systems (GIS), implementation of some of these methods becomes much easier as it can act as a computing environment for executing some types of assessments and for displaying the results of virtually all types of assessments. The index and overlay methods are now routinely performed within a GIS environment. GIS also allows quick assessments and adjustments of the results as well as integrating results from other methods/assessments for calibration and validation purposes.

The choice of model or approach to use will be dependent on (McMahon et al., 2001):

Study objectives; Level of accuracy required (dependent on how close the situation is to its action

thresholds and the sensitivity of the receptor); Stage of assessment (e.g. three tier approach of Xu and Reynders, 1995); Time scale of the project in relation to the potential risk to an identified receptor;

and Cost of constructing the model and the cost involved in getting additional

knowledge or confidence in the system behaviour to validate/calibrate the model. The unsaturated zone fate and transport models may be very simple equations

requiring minimal input or more complex models requiring much more detailed input. The strength of any unsaturated zone transport model is the conceptualization of (1) the relevant process, (2) the structure of the subsurface and (3) the potential events or scenarios that impact the behaviour of the modeled system (NRC, 2001). Conceptual models for partially saturated flow and transport in the fractured unsaturated zone environments are poorly developed and untested (NRC, 2001).

4.1.1 Uncertainties associated with vulnerability assessments

Vulnerability assessments will always be subject to uncertainties due to:

Lack of data; Errors in data used; Incomplete understanding of the environmental processes; Errors in aggregating information; Errors inherent to statistical measures of association;

26

The inclusion or exclusion of variables in most approaches is often arbitrary and based on expert opinion as to the weighting between factors.

The 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 observational data (Worrall et al., 2002).

Speed and robustness of numerical solutions (Ross, 2003) Lack of understanding of preferential flow processes. Dubus and Brown (2002)

investigated the uncertainties in water and solute fluxes due to preferential flow. Magnuson (2002) tested the uncertainties of a conceptual model based on scenarios of different infiltration boundary conditions and moisture characteristic curves for fractured basalts. Liu et al. (2003) simulated flow in the unsaturated zone using the Active Fracture Model (AFM).

• Spatial variability (Keller et al., 2002). Lu and Zhang (2002) developed a stochastic model for transient flow in heterogeneous, variably saturated porous media.

• The liquid fluxes should be well investigated. 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 measured while others cannot.

Only statistical methods allow for the computation of the degree of uncertainty while index and overlay methods do not include any measures of uncertainty. 4.2 Overlay and index methods

Overlay and index methods, are based on combining maps of various physiographic

attributes (e.g. geology, soils, depth to water table) of the region by assigning a numerical index or score to each attribute. These relatively simple applications assign a numerical index or rating to mapped physiographic and anthropogenic attributes of a region. The ratings are then combined to generate a composite sensitivity/vulnerability rating. The ratings can be considered equally or weighted according to the relative magnitude of their influence in the overall assessment determination. The most popular overlay and index method used all over the world is the DRASTIC method, developed by Aller et al. (1987), for vulnerability assessments at regional scales. The DRASTIC approach has led to the development of other methods using a similar approach but different input parameters. This was also the method used by Lynch et al. (1997) to map groundwater vulnerability in South Africa.

In the simplest case of these methods, all attributes are assigned equal weights, with no judgment being made on their relative importance. There are overlay and index methods that attempt to be more quantitative by assigning different numerical scores and weights to the attributes in developing a range of vulnerability classes, which are then displayed on a map (NRC, 1993). 4.3 Process-based simulation methods

Process-based simulation methods require analytical or numerical solutions to

mathematical equations that represent coupled processes governing contaminant transport. Process-based models for flow and transport in porous media are generally based on the Richards’ equation for variably saturated water flow and the convection-dispersion equation

27

for solute transport (Šimůnek et al., 2003). Methods in this category are many and range from indices based on simple transport models to analytical solutions for one-dimensional transport of contaminants through the unsaturated zone, to coupled, unsaturated-saturated, multiple phase, two- or three-dimensional models.

Process-based simulation models predict how long a contaminant will take to reach a given depth and/or the amount of contaminant by mathematically modeling the processes influencing contaminant fate and transport. It is important to note that computer models do not directly compute vulnerability but vulnerability is assessed as a function of what the computer model simulates. The complexity of the models can range from simple transport model indices to multi-phase, multi-dimensional modeling of contaminant movement through saturated and unsaturated zones. 4.4 Statistical methods

Statistical methods having a contaminant concentration or a probability of

contamination as the dependent variable form the basis for the third category of vulnerability assessment methods. These methods incorporate data on known areal contaminant distributions and calculate the probability of contamination by characterizing contamination potential for the specific geographic area using data from known contamination distribution in the area. The methods require a good regional database. 4.5 Characteristics of selected vulnerability assessment methods

Tables 5 to 7 lists the relevant characteristics of selected vulnerability assessment methods reviewed. Examination of the tables reveals some general similarities within the broad classes of methods. Overlay and index methods tend to be applied at small map scales covering large study areas, typically greater than 1:50000, whereas most current process-based models apply to problems at much larger map scales covering smaller study areas (NRC, 1993). Most overlay and index methods and most statistical methods refer to the saturated zone or water table as the reference location. In contrast, most process-based models have a floating reference location depending on the extent to which contamination is investigated through the unsaturated zone. For example, the reference location may be the bottom of the crop root zone for agricultural scenarios. Most overlay and index methods are designed to evaluate intrinsic vulnerability or have mixed specific and intrinsic utility, whereas, most process-based models and statistical methods are designed for specific classes of contaminants such as pesticides or nitrate. Most specific models for the fate of organics generally use three properties: (1) volatilisation, (2) soil-organic carbon sorption coefficient and (3) half-life. Specific models for pesticides can be adapted for other organics for which the three properties are known.

A complete review of some of the methods, listed in Tables 5 to 7, can be found in Appendix 1. Detailed description of the models listed in the Tables can be obtained from the sites listed below. Some models can be downloaded from the web:

http://www.epa.gov:80/ada/models.html http://www.gwsoftware.com/ http://www.ibmpcug.co.uk/~bedrock/gsd/progindx.htm http://www.lakes-environmental.com/lakeepa1.html http://www.mines.edu/research/igwmc/software/igwmcsoft/ http://www.rockware.com/

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Table 5. Selected index and overlay methods used in evaluating groundwater vulnerability to contamination.

Method ScaleReference location Characteristics Availability/Cost

Intrinsic Specific Geochemistry Reference

Index and Overlay Methods (Empirical)

DRASTIC Variable Groundwater Self programmable Intrinsic Generic contaminant Aller et al. (1987)

AHP-DRASTIC Variable Groundwater Pairwise comparisonmatrix; rating; Soil,unsaturated zone

Self programmable Intrinsic Generic contaminant Saaty (1980)_

GOD Variable Groundwater Lithological units Self programmable Intrinsic Generic contaminant Foster and Hirata (1988)

AVI Variable Groundwater Fractured rock Self programmable Intrinsic Generic contaminant Van Stempvoort et al. (1993)

SINTACS Variable Groundwater Unsaturated zone Self programmable Intrinsic Generic contaminant Civita (1994)

EPIK Variable Groundwater Unsaturated zone, karst areas.

Self programmable Intrinsic Generic contaminant Doerfliger and Zwahlen (1997)

SEEPAGE Variable Groundwater Self programmable Intrinsic Generic contaminantMoore (1988)

LeGrand Small Groundwater Rating Intrinsic Generic contaminant LeGrand (1983)

European Approach Groundwater Karst areas Intrinsic Generic contaminant WASP Site specific Groundwater Free - WRC Intrinsic Generic contaminant Parsons and Jolly (1994)

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Table 6: Selected process-based models that can be used in evaluating groundwater vulnerability to contamination. Method Scale Reference location Characteristics Availability/Cost Intrinsic

Specific Geochemistry Reference

Process-Based Simulation Models

PESTAN Large Soil Steady state; decay; hydrodynamic dispersion

Free - www.epa.gov

Specific Pesticides/dissolvedorganic solutes

Enfield et al. (1982)

MOUSE Large Groundwater, snow, surface water urban catchments Specific Nutrients Steenhuis et al. (1987)

PRZM/PRZM2 Large Soil Finite difference; convection-dispersion; noupward fluxes; multiple layers in unsaturated zone

Commercial. EPA Specific Pesticides Carsel et al. (1985)

RF/AF Variable Soil Homogenous area Specific Rao et al. (1985)

GLEAMS Large Soil 1-D, hydrology; erosion /sedimentation;piston flow

http://arsserv0.brc.tamus.edu /nrsu/glmsfact.htm

Specific Pesticides/nutrients Leonard et al. (1986)

CMLS/CMLS94 Large Soil CMLS94 can assess uncertainty Specific Non polar organic chemicals

Nofziger and Hornsby (1986)

RITZ Large Soil decay; volatilization; degradation, sorption,steady state; only downward movement

Specific Hazardous chemicals (Oils)

McLean et al. (1988)

BIOFT3D Site Soil/groundwater Finite elements; 3D; porous media; 3-phase flow; dual porosity

Specific NAPL; Draper Aden Environmental Modeling Inc. daem.info@daem.com

SESOIL Site Unsaturated zone; soil Volatilization; sorption; cation exchange;biodegration; hydrolysis

Commercial Specific Bonazountas and Wagner (1981)

VLEACH Variable Soil/groundwater steady state; soil adsorption; advection,sorption, vapor-phase diffusion, 3 phaseequilibration, decay; 1D

Free Specific DOC Ravi and Johnson (1993)

EPA CSMoS

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Method Scale Reference location Characteristics Availability/Cost Intrinsic Specific Geochemistry Reference

VS2DT Point Water Table Ion exchange, 2D; Nonuniform soil; finite difference

Commercial Specific Various USGS

MIKE SHE Variable Groundwater/surface water 3D; advection-dispersion; decay Commercial Specific

PELMO Site Soil Enhancement of PRZM; hydrology and transport; evapotranspiration

Specific Pesticides

HELP Specific Shroeder et al., (1984)

UGRPF Variable Water table Urban; recharge; pollutant flux; analytical and empirical

Available Specific Inorganics, BTEX Thomas (2001)

LEACHM Large Soil Suite of simulation models; finitedifference; 1D;

Available Specific Pesticides, salinity, tracers, nitates and phosphorus

Hutson and Wagenet (1992)

LPI Large Groundwater Pore water velocity; solute retardation; first order degradation rate coefficient

Specific Pesticides, non volatilechemicals

Meeks and Dean (1990)

OPUS Point Soil Infiltration, runoff, soil water, non-pointsource pollution; erosion, transport

Specific Pesticides and nutrients Smith (1992)

RUSTIC Large Groundwater Specific and Intrinsic

Dean et al. (1989)

UNSATCHEM Point Soil/groundwater Finite difference Free Specific Inorganics, pesticides

FAO-SWS Point Soil Finite difference Free Specific Major inorganics in soils Suarez et al., (2000)

HYDRUS1/2/ver5 Point Soil Finite elements, 1D, Free Specific Major Inorganics in soils, heat flux

Vogel et al. (1996)

CHEMFLO Point Unsaturated soil Finite difference; 1D, convection-dispersion equation

www.epa.gov Specific Chemistry Nofziger et al.

VIRTUS Point Soil Finite difference; 1D; virus adsorption, and degradation; transient state

Specific Viruses wrussell@ussl.ars.usda.gov

SWAP v. 2.0 Point Soil Water, heat and solute fluxes; deep drainage and leaching; preferential flow

Available Specific Generic contaminant Van Dam et al. (1997)

SWIM v. 1.0, v. 2.0 Point Soil Water and solute fluxes; deep drainage and leaching

USD 545 Specific Generic contaminant Ross et al. (1990)

MACRO Point Soil matrix Kinematic wave equation; Macropore flow Soil Science Dept. Swedish Univ. of Agricultural Sciences

Specific Pesticides, solublecontaminants, colloids

Jarvis et al. (1999)

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Table 7. Selected combined and statistical methods used in evaluating groundwater vulnerability to contamination.

Method Scale Reference location Characteristics Availability/Cost Intrinsic Specific Geochemistry Reference

Combined empirical process based models

DRASTIC-CLMS Large Pesticides Ehteshami et al., (1991)

DRASTIC-PRZM Large Pesticides Banton and Villeneuve (1989)

Method Scale Reference location Characteristics Availability/Cost Intrinsic Specific Geochemistry Reference

Statistical methods

Discriminant analysis Variable Groundwater Specific Pesticides Teso et al. (1988)

Regression analysis Small Groundwater Specific Chen and Druliner (1988)

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5 Discussion and conclusion

All environmental models are approximations of nature and thus do not provide absolute estimates of risk but conditional estimates based on multiple assumptions about source terms, environmental settings, transport characteristics, exposure scenarios, toxicity and other variables (Travis et al., 2001). However, when carefully developed and supported by field data, models can be effective tools for understanding complex phenomena and for making informed decisions (NRC, 2001). Numerous unsaturated zone analytical tools exist in the public domain most of which are based on equilibrium partitioning between the solid soil matrix and the soil pore water and are very similar with respect to the parameters that require input values.

A thorough unsaturated zone characterisation is essential when developing a

vulnerability assessment. Understanding, identifying and measuring the unsaturated zone characteristics will enhance the quality of the conceptual model for the behaviour and fate of the different contaminants. The unsaturated zone in Southern Africa is characterised by soil and strata with significantly different characteristics. The different characteristics include, among others, different depths, textures, mineral compositions and chemical properties resulting in different transport and attenuative characteristics. Discontinuities, in the form of fractures and bedding planes, make contaminant transport complex, especially in unsaturated zones consisting of different geological strata. Very few models can simulate these characteristics reliably.

The simplest unsaturated zone analytical tools estimate contaminant concentrations in unsaturated zone soil pore water from equilibrium partitioning equations and then use those aqueous concentrations as source input into a groundwater fate and transport analysis. Actual transport through the unsaturated zone is not estimated with this type of analytical tool.

The most commonly used models that estimate the migration of contaminants through the unsaturated zone are much more complex, requiring more input parameters. These are generally either infinite source or finite source analytical tools. These models require recharge rates so that a pore water velocity can be estimated as well as the vertical depth to groundwater or bedrock from the contaminated soil. An unsaturated zone finite source analytical tool is particularly useful in demonstrating how long it will take a contaminant to migrate from unsaturated zone soils to groundwater (if at all) and what the contaminant concentration (including the maximum concentration) will be in soil or soil pore water at various depths and at various times as migration occurs. More complex unsaturated zone models take into account mechanisms that would affect the vertical migration of contaminants toward groundwater (e.g. biodegradation, volatilization, etc.).

Travis et al., (2001) identified the following technical issues that are not addressed by

most models: 1. Complex terrains are often simplified and such simplifications affect the reliability

and accuracy of risk estimates, especially in localised areas; 2. Most models do not have the capabilities to handle contaminant transport in the

presence of nonaqeous phase liquids (NAPL’s); 3. Fractured and karst media are distinct hydrological and geomeorphological terrains

that allow for quick dispersion of contaminants through fast flow paths. Most models assume that the flow occurs through a semi-solid rock matrix. Fast flow

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leads to an apparent non-equilibrium situation with respect to the pressure head or solute concentration, thus limiting the definition of the initial boundary conditions of flow and reliable forecasts of contaminant transport in unsaturated media (Seiler, 2002); and

4. Models examine the partitioning, fate and transport of contaminants in different environments (i.e. soil, groundwater, surface water, air and biota). The models often do not allow for the total contaminant mass, accounting for degradation products, summed over all environmental media to remain constant.

“To be of use in environmental decision making, environmental models must have both scientific credibility and public acceptance” (Travis et al., 2001).

These two requirements are often contradictory. In order to be acceptable to the public

it must be very simple and to be scientifically defendable it often requires more complexity.

Any modeling approach must be determined by the objectives of the study, the availability of data and the complexity of the system and transport processes involved. The method that can be applied also relies on the available funding; the availability of data and the experience of the person/persons involved in the assessment.

From a technological point of view, the major restriction of unsaturated zone modelling were the problems of model operators, restricted result viewing and analyses capabilities of DOS codes and the lack of data to specify unsaturated zone parameters (Gogolev, 2002). Recently, packages were developed in a user-friendlier Windows environment (e.g. UnSat Suite Plus by Waterloo Hydrogeologic Inc.) (see also Appendix 1).

When selecting a particular model code the nature of the transport processes at the field site and the appropriateness of a particular technique needs to be considered. Overlay and index methods

Overlay and index methods often contain a layer or index describing the unsaturated or vadose zone. The unsaturated rating is based on a combination of factors that contribute to the likelihood of contaminants to reach the water table. Eimers et al. (2000) used the following factors, using spatial data layers in GIS, to determine the unsaturated zone rating: (1) vertical conductance of the unsaturated zone, (2) land-surface slope, (3) land cover and (4) land use.

DRASTIC is not particularly suited to fractured rock aquifers (Wei, 2001 and Garret, 1991). However, the DRASTIC approach is probably the most widely used method to assess regional groundwater vulnerability. The method can be adapted to suit most situations. The method can be modified to include or exclude parameters for a more robust assessment (e.g. Witkowski et al., 2003; Thirumalaivasan et al., 2003; Ehteshami et al., 1991; Banton and Villeneuve, 1989). Process-based methods

According to Evans and Maidment, (1995), sophisticated models may not always provide more reliable outputs. This is due to the fact that input parameters for these models are not always available and therefore have to be calculated indirectly or left out.

34

The MIKE SHE model appears to be good for the determination of groundwater vulnerability due to the fact that it takes into account contaminant migration, estimating flow paths and transport times in groundwater. MIKE SHE is an integrated modeling environment that allows components to be used independently and customized to local needs. The VLEACH model also has a solid foundation for the purpose of a groundwater vulnerability assessment.

The Urban Groundwater Pollution flux model of Thomas (2001) is a model that can be applied to the selected sites as the model code and expertise is with the Groundwater Group at UWC. The model has been demonstrated for an urban unconfined aquifer system in Birmingham, United Kingdom (Thomas, 2001, Thomas and Tellam, 2004b). Although the model is primarily developed for the Birmingham unconfined aquifer, it can also be extended to other similar cities provided all the basic inputs (mainly land use, hydrologic soil group, surface elevation, geology, meteorological data, EMC values, surface elevation, depth to water table and hydraulic properties of the geological units) are available. A detailed description of the model can be found in Appendix 2. Knowledge gaps and improvement of methodologies

There is a large number of modelling codes that can simulate surface water flow, subsurface water flow, and contaminant fate and transport. However, each of these codes is based on different assumptions chosen for the purpose of simulating specific scenarios. No single code can be used to simulate all possible situations. It appears that the intermediate zone was less researched than the soil and saturated zones (thereby the lack of information in the literature), and this project will help in making a major contribution towards understanding the processes and mechanisms occurring in the intermediate zone. In order to identify suitable models, detailed familiarization is essential in order to identify advantages and shortcomings. This implies not only knowledge of the theoretical basis of models (Appendix 1), but also knowledge of operation in order to learn models’ capability in terms of inputs and outputs.

For the purpose of improving groundwater vulnerability assessment, three approaches

can be recommended: 1. Inclusions of sub-ratings to improve ”I” of DRASTIC.

The following improvements could be included in DRASTIC, based on the knowledge gaps identified in the literature review:

a) Inclusion of multi-layer component, based on site-specific conceptual models.

b) Ranking based on determination/measurements of hydraulic properties for which equipment is available at UWC (permeability, porosity, water retention).

c) Inclusion of preferential flow and ranking:

- Short-circuiting (fractures and cracks) - Funnelling (lateral flow)

d) Inclusion of chemical properties and ranking based on geochemistry:

- Solubility: Data are available in the Excel database of the groundwater pollution inventory project.

- Liquid and non-liquid phase for organic chemicals

35

- Precipitation and other processes for inorganic chemicals - Volatilization: Data are available in the Excel database of the groundwater pollution inventory project. - Sorption: Data are available in the Excel database of the groundwater pollution inventory project. Sorption of selected contaminants could be measured with equipment available at UWC. - Degradation: Guidelines could be compiled for the estimation of half-life based on expected environmental factors (microbial activity, pH, temperature, water content etc.).

2. Use of numerical model(s) in the background to generate rating for ”I” of DRASTIC.

This option entails combining numerical model(s) with DRASTIC in order to account for processes in groundwater vulnerability assessment.

3. Simulation of best and worst case scenarios with suitable numerical model(s) for average conditions or polygons.

This option entails the use of process-based models to assess groundwater vulnerability. The selected/improved/developed models need to be tested at specific sites. The case

study sites should be selected based on the criteria that were listed during the AVAP workshop held on 03/06/2003 at CSIR (Stellenbosch). Additional criteria would include the availability of data (historic records for many years) and the feasibility of measurements aimed at completing data sets required for modeling. Mixtures of priority contaminants should be identified for the specific sites selected. For each of these sites, the following generic procedure can be used to describe the processes in the unsaturated zone relevant to groundwater contamination: i) Identification of groundwater depths and fluctuations. ii) Identification of unsaturated strata, based on soils, lithology and regolith data available. iii) Groundwater recharge. iv) Baseline of water chemistry (initial conditions). v) Identification of priority contaminants. vi) For stratum 1 to n:

a. Determination/measurement of porosity, permeability and hydraulic conductivity (intrinsic properties).

b. Preferential flow. - Short-circuiting - Funneling and lateral flow

c. Determination/measurement of contaminant sorption and decay (specific properties).

d. Multi-phase flow (e.g. non-aqueous phase liquids, NAPLs). e. Determination of dominant or average water and contaminant fluxes.

vii) Groundwater contamination and description of plume.

36

This procedure accounts for intrinsic properties of the media through which water and contaminants are transported, as well as the specific properties of contaminants. Some processes that are relevant at the micro-scale (e.g. hysteresis, fingering and molecular diffusion) were not deemed to be significant for the purpose of assessing groundwater vulnerability to contaminants. Finally, a comparison between the improved methods should be carried out for the case study sites.

Future work The following tasks are therefore recommended for inclusion in the work programme:

Task 1: Identification of specific sites to be used for model testing.

These sites will be used for testing improved methodologies for groundwater vulnerability assessment.

This task will last from July to August 2004.

Task 2: Identification of suitable models for groundwater vulnerability assessment.

Detailed familiarization and operation with models is essential in order to identify advantages and shortcomings, and to select suitable model(s).

This task will last from July to December 2004.

Task 3: Development of conceptual models for the specific sites selected.

A generic procedure will be used to describe the processes in the unsaturated zone relevant to groundwater contamination.

This task will last from September 2004 to December 2004.

Task 4: Improvement of methods for groundwater vulnerability assessment. This task entails development and adaptation of known methodologies in order to

improve groundwater vulnerability assessment. This task will last from January to June 2005. Task 5: Comparison of methods for groundwater vulnerability assessment.

This task entails the comparison of improved methodologies for groundwater vulnerability assessment, based on the product of task 4. The comparison of improved methodologies will be done for the specific sites identified in task 1.

This task will last from July to December 2005.

37

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