| Summary: | Thesis (MSc)--Stellenbosch University, 2003. === ENGLISH ABSTRACT: The Cape Town metropolitan area has limited water supply due to rapid population
and urban growth. In many instances, surface water is the only source in water supply
schemes. There is a need for additional water supplies to supplement the existing
water sources. Groundwater systems can be used as primary or supplemental water
supply sources especially in areas where there is high demand for water resources.
The aim of this study is to evaluate the groundwater potential within the Table
Mountain Group (TMG) with the assistance of remote sensing and Geographical
Information System (GIS). Previous hydrogeological studies have found the TMG to
be the second largest hydrogeological unit in South Africa with extensively fractured
and multi-porous rock. The study area is 5660 km2 with TMG covering 1336 km".
In this study a Landsat Enhanced Thematic Mapper Plus (ETM+) image was used to
identify lineaments. The identified lineaments were overlaid with vegetation,
drainage patterns, faults and fractures digitized from 1:250 000 geological maps and
borehole yields to show areas with promising groundwater resources.
The results did not show correlation between vegetation and lineaments. Most of the
lineaments intersected drainage lines at some points, and a few were parallel to the
drainage lines. Forty five percent of the digitized faults and fractures overlap with the
Landsat lineament. The most dominating lineaments are oriented in a NW-SE
direction. High yielding boreholes with average yield of about 12 lIs were found
within the distance of 150m from the lineaments. The lineaments were further
analysed to locate areas that could be suitable for groundwater exploration. These
areas were identified using Landsat lineaments, boreholes and a Digital Elevation
Model (DEM). The results showed that the most favourable lineaments and
geological features were oriented in a 135-180° and 0-45° direction and areas with
slopes of less than 40% were found to be suitable for drilling boreholes.
The amount of available groundwater within the TMG was also investigated by
looking at both volume of recharge and amount that could be held in storage. Rainfall
data was used to estimate recharge. Groundwater recharge was calculated to be 5% of
the total precipitation that falls on this area. Based on the average rainfall of 600mm
per annum, the results show that TMG has an average recharge value of 30mm per
annum. The total recharge for the area covered by TMG, which has an area of
1336km2
, is 160 million nr'. Geological profiles and cross sections were drawn to
determine the storage capacity of the TMG, which was estimated to be 525 million
m.3
According to a study done by the Department of Water Affairs and Forestry (DW AF)
in 1996, the anticipated water demand in the Cape Town metropolitan area will
increase from 243 million m3 in 1990 to 560 million m3 in 2020. The estimated
volume of water that can be stored within the TMG can meet the current demand for
the next 10 years and supplement the existing surface water sources. Groundwater
vulnerability of the TMG to contamination was assessed and mapped by using the
DRASTIC index. The results demonstrate that the TMG area is at low risk to
contamination. === AFRIKAANSE OPSOMMING: 'n Vinnig groeiende bevolking en stedelike uitbreiding plaas toenemende druk op
Kaapstad se water voorraad. Addisionele waterbronne sal benodig word om bestaande
bronne aan te vul. Oppervlakwater is in die meeste gevalle die enigste waterbron,
maar grondwater het die potensiaal om te dien as 'n primêre of aanvullende
voorsieningsbron, veral in areas waar groot water tekorte bestaan.
Die doel van hierdie studie is om die grondwaterpotensiaal van gesteentes van die
Tafelberg Groep (TBG) te evalueer deur van afstandswaarneming en geografiese
inligtingstelsels gebruik te maak. Geohidrologiese studies het getoon dat die TBG
gesteentes met sy veelvuldige nate en breuksones, die tweede grootste
geohidrologiese eenheid in Suid Afrika is. Die studiegebied beslaan 5660 km",
waarvan 1336 km2 deur Tafelberg Sandsteen beslaan word.
Vir hierdie studie is 'n "Landsat Enhanced Thematic Mapper Plus (ETM+)" beeld
gebruik in die identifisering van breuksones (lineamente). Verdere analises is
uitgevoer om areas geskik vir grondwater ontginning te identifiseer. Geïdentifiseerde
verskuiwings op Landsat beelde is met plantegroei, dreinerings patrone en bekende
verskuiwings en fraktuur sones vanaf gelogiese kaarte vergelyk in 'n poging om areas
met belowende grondwaterbronne uit te wys. Bekende boorgat posisies en lewerings
volumes was 'n primêre databron vir die berekening van groundwater reserves.
Die studie het egter geen korrelasie tussen plantegroei en die voorkoms van
lineamente gevind nie. Die riviere in die studiegebied word op verskeie plekke deur
verskuiwings gekruis. Slegs 'n paar van die verskuiwings lê parallel met die
dreinering. Daar is gevind dat vyf-en-veertig persent van bekende verskuiwings en
fraktuursones met die geïdentifiseerd op Landsat beelde oorvleuel. Die mees
prominente lineamente het 'n NW-SO oriëntasie. Boorgate met lewerings van
gemiddeld 12 lis is binne 'n 150m afstand van die verskuiwings gevind. Die
verskuiwings is ook geanaliseer om die mees produktiewe areas vir grondwater
ontginning te identifiseer. Landsat beelde, boorgate en 'n Digitale Elevasie Model
(DEM) is gebruik om moontlike boorposisies te identifiseer. Die mees produktiewe
verskuiwings en geologiese verskynsels het 'n N 135-180W en NO-450 oriëntasie,
terwyl areas met 'n helling < 40% vir die boor van boorgate geskik is.
Berekeninge oor die hoeveelheid water wat binne die TBG gesteentes beskikbaar is, is
gemaak deur die hoeveelheid aanvulling en stoorkapasiteit van die TBG gesteentes te
beraam. Grondwater aanvulling, soos bereken vanaf reënval data, is 5% van die totale
presipitasie van 'n gegewe area. Met 'n gemiddelde jaarlikse reënval van 600mm in
die studie gebied is die TBG se jaarlikse aanvulling ongeveer 30mm. Daar word
beraam dat die totale aanvulling in die 1336km2 TBG area 160-miljoen m3 per jaar is.
Geologiese profiele en dwarsnitte is gemaak om die stoorvermoë van die TBG te
bepaal, en is beraam op 525-miljoen rrr'.
'n 1996 navorsing studie deur die Departement van Waterwese en Bosbou toon dat
waterverbruik in die Kaapse Metropolitaanse gebied sal toeneem vanaf die 1990 vlak
van 243-miljoen m3 tot 560 miljoen m3 teen 2020. Die berekende volume water wat
binne die TBG gestoor word, kan die water aanvraag oor die volgende 10 jaar
bevredig en as aanvulling dien vir oppervlak waterbronne. Die kwesbaarheid van die
TBG akwifer vir besoedeling is met behulp van die DRASTIC indeks geëvalueer en
gekarteer. Die resultate toon dat die TBG 'n lae risiko vir besoedeling het.
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