PREPARING INPUT DATA FOR A NATIONAL-SCALE GROUNDWATER VULNERABILITY MAP OF SOUTHERN AFRICA

Groundwater quality is affected by virtually every activity of society , thereby making groundwater protection complicated and of national im portance. However, protection of the groundwater will always be cheape r and less protracted than restoring an already polluted aquifer. In c ompiling a national-scale groundwater vulnerability map of Southern Af rica, it was decided to use the widely known DRASTIC methodology that includes the following components: the Depth to groundwater the Rechar ge due to rainfall the Aquifer media the Soil media the Topography the Impact of the vadose zone the Conductivity (hydraulic). This methodol ogy is well suited to gridded information sets of factors influencing groundwater vulnerability. The fact that the major strength of a grid cell-based model lies in its analytical capabilities, makes the ARC/IN FO geographic information system (GIS) the ideal choice for manipulati on and displaying the data surrounding the DRASTIC model. This paper o utlines the techniques used in compiling the data sets for those facto rs that influence the susceptibility of groundwater to contamination o ver Southern Africa and the techniques involved in manipulating and di splaying these data in a GIS. The different techniques employed in the gathering and calculation of the different information sets required by the DRASTIC model to describe the groundwater vulnerability are pre sented in detail. The final output, which is in the form of a colour p aper map, will be useful in presenting the concept of groundwater vuln erability and groundwater protection to the layman.