Assessment of Aquifer
Vulnerability in South Africa
Report No: 1432/1/07
Dec 2007
Executive summary
This report presents the main findings of the WRC-funded four-year
project on “Improved methods for vulnerability assessments
and protocols for producing vulnerability maps, taking into account
information on soils”, abbreviated as AVAP. The project
fulfils one of the key recommendations of a strategy on groundwater
protection research which was prepared by Sililo et al. (2001). For
detailed information, the reader is referred to the enclosed CD which
includes all deliverables of the project (Appendixes 1 to 25).
Aquifer vulnerability to contamination comprises two components:
unsaturated zone vulnerability and saturated zone vulnerability. For
the unsaturated zone, AVAP defined vulnerability as the ease with which
groundwater (at the water table) may become contaminated by a
contaminant source at the surface or in the unsaturated zone. For the
saturated zone, vulnerability is defined as a function of the period of
time after contaminating activities have ceased that a given
contaminant can be detected in groundwater plus the volume of the
aquifer throughout which the contaminant is above a preset
concentration. The decision-maker could focus on either one of these
vulnerabilities or in combination depending on the type of decision to
be made. AVAP developed new approaches for assessing aquifer
vulnerability.
Unsaturated zone
vulnerability
For the unsaturated zone, the “AQUISOIL” (Aquifer
Vulnerability Soil Assessment) approach and a modified DRASTIC
approach, called “EUZIT” (Excel-based Unsaturated
Index Tool) were developed. Both approaches can be used for assessing
vulnerability at the water table.
AQUISOIL focuses on the soil zone as the “first line of
defence” but can also be applied to the whole unsaturated
zone down to the water table. AQUISOIL comprises three types of
vulnerabilities which are rated for each single layer and weighted
according to thickness; i.e. chemical, hydraulic and climatic
vulnerability. With regards to chemical vulnerability, a new soil
classification was developed which relates sorption to selected soil
properties (pH, clay and organic carbon content, CBD extractable Fe and
Al, and the sum of exchangeable cations for more than 170 soil samples
representing major kinds of diagnostic horizons and materials in the
South African soil classification), by means of so-called chemical
envelope equations. These envelopes predict the upper limit of
contaminant sorption to be expected at a specified value of each soil
property and are based on the application of quantile regression (also
referred to as stochastic frontier analysis) which provides a
systematic method of determining ranges of a determinant variable over
which sorption can be either potentially maximal or inevitably minimal.
Chemical vulnerability classes have been defined accordingly for the
retention of the following categories of contaminants: cationic
(metals), anionic and non-polar organic. Hydraulic vulnerability is
derived from a permeability index which is based on soil texture and
the presence or absence of a cemented layer or horizon. The climatic
vulnerability uses a leaching or recharge index calculated with one or
other of the classical algorithms that take into account factors such
as rainfall, surface runoff, profile storage, etc. AQUISOIL has been
incorporated in a spreadsheet programme.
EUZIT is a spreadsheet-based modified DRASTIC (Aller et al., 1987)
approach. The rating of the unsaturated zone is based on a combination
of factors that contribute to the likelihood of contaminants reaching
the saturated zone following the path of aquifer recharge. The factors
considered are the unsaturated zone thickness, hydraulic properties
(vertical hydraulic conductivity), flow mechanism (preferential and
matrix), travel time, recharge (based on well established methods),
slope, contaminant sorption and decay (degradation). Any of these
factors can be disabled depending on site-specific conditions and the
weightings for each factor are user defined. EUZIT also allows for a
multi-layer unsaturated zone. The rating scores for each factor are
combined in the main menu to yield the rating of the impact of vadose
zone on aquifer or groundwater vulnerability. The EUZIT programme draws
the rating for sorption from the chemical vulnerability part of the
AQUISOIL spreadsheet programme. The vulnerability of an aquifer to
contamination originating at the soil surface can be assessed using
data that are readily available, that can be calculated or that can be
estimated using the information provided in the database of EUZIT.
Saturated zone
vulnerability
For the saturated zone, two approaches are proposed to determine its
vulnerability to contamination: one approach that uses (numerical)
geochemical and reactive transport models and a second, generic,
approach that generalise contaminant transport by an analytical
equation. AVAP concentrated on the first approach and evaluated for two
common aquifer types in South Africa: coastal primary and dual porosity
aquifers the aquifer vulnerability to inorganic contamination from
landfill leachate, industrial effluent and acid mine drainage, assuming
identical physical properties and contamination at the water table.
PHREEQC-2 (Parkhurst and Appelo, 1999) was used for the calculations.
Vulnerability of the saturated zone to contamination is expressed in
terms of spatial impact and persistence of the contaminant. Realistic
spatial and temporal scales are derived from the linking of PHREEQC-2
with a single or multi-species groundwater flow/transport model such as
MT3DMS (Zheng and Wang, 1999). For the modelling of organic
contamination, PHT3D (Prommer, 2003), BIOPLUME III and the LNAPL Guide
(American Petroleum Institute, 2004) can be used for simulating
reactive transport of BTEX (considered representative of petroleum
hydrocarbons), and the BIOCHLOR code (Aziz et al., 2000) for
chlorinated ethenes.
GIS-based vulnerability
assessment
A Geographical Information System (GIS) enables the compilation of
vulnerability maps combined with other infrastructural information so
that the results can be easily related to by planners and decision
makers. Two GIS approaches were developed to determine unsaturated zone
vulnerability as part of the AVAP project: EUZIT incorporated in a GIS
and the modified UGIf model.
EUZIT was integrated in a GIS to assess the variation in groundwater
vulnerability across an area. By building the input datasets into a GIS
and converting them to grids the EUZIT algorithms can be applied in GIS
using the “map calculator” functions. Within the
GIS it is also possible to carry out a sensitivity analysis to optimise
the weightings (and ratings) per factor. Depending on the scale of the
application and the degree of spatial variability, the possibility does
exist, however, that the initial spatial variability is
“lost” due to the relatively broad ranges within
the rating classes. One could refine the rating classes to enhance
spatial variability in vulnerability. Further work is thus recommended
on refining the ratings based on the scale of the study and data
availability and on scientifically motivating the weightings.
The UGIf model, which estimates recharge fluxes of organic pollutants
(BTEX) in an urban environment (Thomas et al., 2001), was modified to
include screening level models for vulnerability assessment such as the
Attenuation Factor Model (Rao et al., 1985), the Leaching Potential
Index Model (Meaks and Dean, 1990), the Ranking Index Model (Britt et
al., 1992), and a simple approach to assessing intrinsic vulnerability
of conservative contaminants. The three screening level algorithms of
UGIf require a combined grid containing attributes of average recharge
rate (m/day), soil moisture or volumetric water content, vadose zone
depths (m), and the retardation factor values. The UGIf model was made
suitable for South African conditions through the incorporation of
representative land types. The software packages required to run UGIf
model are ArcView GIS 3.x version (ver 3.1 or 3.2 or 3.3) and its
extension Spatial Analyst.
Both EUZIT and UGIf deal exclusively with the vulnerability of the
unsaturated zone. Both approaches are encouraged, as they present
decision-makers with a spatial representation of unsaturated zone
vulnerability.
Case studies
Two study sites were selected to illustrate the use of AVAP’s
approaches to the assessment of groundwater vulnerability: the
Goedehoop irrigation site near Secunda, and the Coastal Park waste
disposal site near Cape Town. It was found that the unsaturated zone
vulnerabilities are relatively high for both aquifers. From the smaller
travel times and higher leaching potential indices as derived from the
AQUISOIL and EUZIT spreadsheet tools and the UGIf model it can be
concluded that the unsaturated zone vulnerability to dissolved organic
contaminants (e.g. BTEX) of the primary Cape Flats aquifer at Coastal
Park is higher than the vulnerability of the dual porosity, weathered
zone aquifer at the Goedehoop site. This means that contaminants
generated at the surface are likely to reach the water table and
pollute the aquifer sooner at the Coastal Park waste disposal site. The
saturated zone vulnerability in terms of the spatial impact of
inorganic and organic contamination is expected to be higher at the
Coastal Park waste disposal site whereas the persistence is expected to
be higher at the Goedehoop site. More detailed modelling, accounting
for site specific physical properties, of the Cape Flats aquifer at the
Coastal Park waste disposal site is needed to enable a better
comparison of the saturated zone vulnerabilities of both aquifers.
Aquifer vulnerability and
decision-making
Aquifer vulnerability assessments form an important input to managing
the risk of water resource degradation. A framework to support
decision-making was developed to assist groundwater vulnerability
assessment practitioners in understanding the role of their assessments
in groundwater management and to assist them in the selection of
AVAP’s approaches to groundwater vulnerability assessment.
The framework highlights the fact that groundwater vulnerability
assessments serve as input to contaminant risk assessments, which will
in its part, contribute to a cost benefit analysis. It is the outcome
of the cost benefit analysis which will ultimately inform
decision-making. To guide the groundwater vulnerability assessor, a
table is presented which briefly summarises AVAP’s assessment
approaches in terms of their limitations, applicability, accuracy, ease
of use, and cost.
Copies of this report are available from the Foundation price
£25.00 less 20% for FWR members.