ASSESSMENT OF AQUIFER VULNERABILITY AND TRANSMISSIVITY IN SOUTHERN BAMBI-BUMBWI CORRIDOR-ZANZIBAR, TANZANIA

Abstract

Groundwater vulnerability assessment is an important aspect f'or development of groundwater protection strategies. This study deals with analysis of subsurface lithology and prevailing environmental conditions of the Zanzibar Island under which its groundwater may get contaminated. DRASTIC index approach integrated with GIS interface is used here in finding out the relative vulnerability of the oceanic island and risk zone of two important geological corridors namely (Bumbwi and Bambi-Upenja) of Zanzibar Island, Tanzania. Subsurface parameters: depth of water table, net recharge, aquifer media, soil media, topography, the impact of' vadose zone and hydraulic conductivity are used to evaluate underlying aquifer vulnerability in the form of DRASTIC index (1)1). To strengthen the data quality of crucial subsurface parameters like depth to water table, hydraulic conductivity, aquifer media and soil media and estimation of the transmissivity of the aquifer in the study area, vertical electrical soundings (VES) are used. A statistical analysis of' all these hydro-geological parameters are performed to check their importance in calculating the vulnerability map of the study area. The risk map is then prepared based on the computed vulnerability and land use patterns using Are GIS 10 and ILWIS 3.31 software. Modified DRASTIC index and groundwater risk mapping are performed subsequently based on the results of map removal and single parameter sensitivity analyses. Sensitivity analyses revealed that net recharge is the most sensitive parameter. Moreover, weight of net recharge, soil and topography found to be more effective in determining aquifer vulnerability than the weight assigned by the DRASTIC model. Results show that high vulnerability area and risk potential area covered 29.82% and 18.7%, respectively, before carrying out sensitivity analysis while covered 42.4% and 34% respectively of the study area after performing sensitivity analysis. This indicates the importance of carrying out sensitivity analysis is assessing vulnerability of groundwater using DRASTIC method.