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

[摘要] ENGLISH ABSTRACT: The Cape Town metropolitan area has limited water supply due to rapid populationand urban growth. In many instances, surface water is the only source in water supplyschemes. There is a need for additional water supplies to supplement the existingwater sources. Groundwater systems can be used as primary or supplemental watersupply 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 TableMountain Group (TMG) with the assistance of remote sensing and GeographicalInformation System (GIS). Previous hydrogeological studies have found the TMG tobe the second largest hydrogeological unit in South Africa with extensively fracturedand 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 toidentify lineaments. The identified lineaments were overlaid with vegetation,drainage patterns, faults and fractures digitized from 1:250 000 geological maps andborehole yields to show areas with promising groundwater resources.The results did not show correlation between vegetation and lineaments. Most of thelineaments intersected drainage lines at some points, and a few were parallel to thedrainage lines. Forty five percent of the digitized faults and fractures overlap with theLandsat lineament. The most dominating lineaments are oriented in a NW-SEdirection. High yielding boreholes with average yield of about 12 lIs were foundwithin the distance of 150m from the lineaments. The lineaments were furtheranalysed to locate areas that could be suitable for groundwater exploration. Theseareas were identified using Landsat lineaments, boreholes and a Digital ElevationModel (DEM). The results showed that the most favourable lineaments andgeological features were oriented in a 135-180° and 0-45° direction and areas withslopes of less than 40% were found to be suitable for drilling boreholes.The amount of available groundwater within the TMG was also investigated bylooking at both volume of recharge and amount that could be held in storage. Rainfalldata was used to estimate recharge. Groundwater recharge was calculated to be 5% ofthe total precipitation that falls on this area. Based on the average rainfall of 600mmper annum, the results show that TMG has an average recharge value of 30mm perannum. The total recharge for the area covered by TMG, which has an area of1336km2, is 160 million nr'. Geological profiles and cross sections were drawn todetermine the storage capacity of the TMG, which was estimated to be 525 millionm.3According 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 willincrease from 243 million m3 in 1990 to 560 million m3 in 2020. The estimatedvolume of water that can be stored within the TMG can meet the current demand forthe next 10 years and supplement the existing surface water sources. Groundwatervulnerability of the TMG to contamination was assessed and mapped by using theDRASTIC index. The results demonstrate that the TMG area is at low risk tocontamination.

[发布机构] Stellenbosch University