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University of the Western Cape (2007)

A preliminary understanding of deep groundwater flow in the Table Mountain group (TMG) aquifer system

Netili, Khangweleni Fortress

Titre : A preliminary understanding of deep groundwater flow in the Table Mountain group (TMG) aquifer system

Auteur : Netili, Khangweleni Fortress

Université de soutenance : University of the Western Cape

Grade : Magister Scientiae - MSc (Earth Science) 2007

Résumé
The Table Mountain Group (TMG) Aquifer is the second largest aquifer system in South Africa, after dolomites. This aquifer has the potential to be a significant source of water for the people of the Western Cape. The occurrence of hot water springs in the TMG in relation with the main geological fault systems in South Africa shows that deep flow systems do exist. Little is known about these deep aquifer systems in South Africa (i.e. flow mechanisms). To close the above-mentioned knowledge gap, this study was initiated. The current study gives a review of some of the aspects that need to be considered when distinguishing deep groundwater from shallow groundwater. These involve the use of hydrochemistry, temperature in relation to groundwater circulation depths and the general concepts of confined and unconfined aquifers. Temperature is regarded as the most suitable dividing line between shallow and deep groundwater for this study. The use of this dividing line in the TMG is limited by the lack of data regarding the distribution of the geothermal gradients. The modelling package TopoDrive was used to simulate groundwater flow on a steady state with the aim of assessing the influence of the Cedarberg aquitard (C/S) on the deep flow. This was achieved by means of particle tracking, and multiple simulation runs with variable (i.e. from K=1.0E-15 – K= 1.0E-02) hydraulic conductivity (K) of the C/S. Simulation results show that C/S with K value of 1.0E 02 can allow shallow groundwater from the Nardouw Aquifer to recharge the deep Peninsula Aquifer. The lowest K value of 1.0E-15 makes the C/S to act as a barrier, a situation where there is no water exchange between the upper and the lower deep aquifers. This situation is not realistic in the TMG since the area is structurally disturbed. Flow paths with long travel times of about 405 years were estimated with the use TopoDrive. These ages are not correlating with the age estimated Carbon-14 data. These isotopic analyses have estimated several thousands years for most of thermal spring water in the TMG. The reason for this may be the fact that the groundwater model presented here does not account for all the complexities of the fractured rock aquifer. These include structural setting, distance from recharge to discharge point, and permeabilities of the aquifers. This analysis demonstrates the high level of uncertainty involved in calculating travel times from the recharge area to discharge area, given our current limited knowledge of the appropriate parameter values. It is therefore recommended that further detailed research studies be conducted on the aspects of deep groundwater reviewed in this thesis.

Mots clés : Deep thermal groundwater, shallow groundwater, groundwater classification, hydrogeology, Table Mountain Group, Thermal springs, TopoDrive, Particle tracking

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