Developing a cost-effective method to characterise regional aquifer vulnerability in the Cape Flats
Date
2024-03
Authors
Journal Title
Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Aquifer vulnerability assessments are important tools in avoiding aquifer deterioration as they
indicate how likely an aquifer is to be adversely affected by land-based, anthropogenic
activities.” The aim of this study is to develop a simple, cost-effective method to characterise
aquifer vulnerability by combining and modifying existing methods. These methods are then applied
in the same study area to validate the cost-effective method. The Cape Flats Aquifer (CFA), located
in the Western Cape, South Africa, is used as a case study due to concerns over the deterioration
of groundwater quality to the well-utilised aquifer during the past decade.
Assessment of aquifer vulnerability in the CFA is achieved through a combination of DRASTIC-L
aquifer vulnerability mapping and wellhead protection area (WPA) delineation. DRASTIC-L is used to
identify areas that are most vulnerable to contamination. Steady-state wellhead protection areas
are delineated using finite difference numerical modelling (MODFLOW) and reverse particle tracking
(MODPATH) to determine the capture zones for major abstraction wells. However, the complex,
expensive, and time-consuming nature of numerical modelling highlights the need for simplified,
cost-effective methods that can be applied across South Africa. Inherent limitations and
redundancies with the DRASTIC-L method that were identified by previous studies and within this
study resulted in the development of the modified DRASTL method. Additionally, a new method for
cost-effective capture zone delineation using a combination of GIS and analytical equations is
evaluated. The results of DRASTL and the newly developed method for wellhead protection area
delineation are combined to cost-effectively characterise aquifer vulnerability in the CFA, which
is referred to as the DRASTL-WPA method.
Results from DRASTIC-L indicate that the CFA is very vulnerable. The DRASTL method shows a similar
spatial trend to DRASTIC-L; however, DRASTL is associated with a higher average vulnerability but a
lower tendency for very low or very high vulnerabilities. It can be seen that a very similar output
can be produced with fewer input parameters; however, validation is required to evaluate the
accuracy of the methods for the study area.
The cost-effective WPA delineation method yielded an intersection of greater than 70% with the
results from MODPATH given that an abstraction well is ~8 km away from a bedrock outcrop, and ~12
km away from the combined effects of bedrock outcrops and river boundaries. This is likely due to
the inability of GIS software to simulate complex hydrogeological
boundary conditions, as well as limiting assumptions of homogeneity and isotropy.
Additionally, one can expect an average 3.7 times overprediction of the capture zone size.
Several capture zones in the CFA correspond with the highest aquifer vulnerability highlighted by
the results of DRASTIC-L and DRASTL. Thus, it is suggested that land use practices are monitored or
restricted within these capture zones. Additionally, the capture zones delineated by the numerical
model or DRASTL-WPA can be used to define areas where in-situ remediation or beneficial management
practices should be implemented. Moreover, the numerical model can help with groundwater management
planning in the CFA to ensure the
sustainable use of groundwater resources.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (MSc)--Stellenbosch University, 2024.