Assessing the groundwater resources within the Table Mountain Group using remote sensing and geographic information system

Matoti, Andiswa (2003)

Thesis (MSc)--Stellenbosch University, 2003.

Thesis

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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