Groundwater vulnerability assessment using GIS-based DRASTIC models in shallow aquifer of Gabes North (South East Tunisia)

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Aquifer vulnerability has been assessed in the northern part of Jeffarra Basin (Southern Tunisia) using the DRASTIC models, based on a geographic information sys- tem. Two vulnerability maps have been developed in this study: the pesticide DRASTIC vulnerability map and the generic DRASTIC vulnerability map. These maps show that the study area is more sensitive to pesticide contamination. About 23 % of this area is under high to very high vulnera- bility to pesticide contamination, while only 9 % under high vulnerability to generic pollution. In the both cases, the Oued El Maleh Basin and the Oued El Mazra Basin in the coastal plain are the most vulnerable zones. The middle part of the study area is dominated by moderate vulnerability classes, while the southwest part displayed "low" vulnerability clas- ses. The superposition of the DRASTIC maps with land use map shows that most agricultural areas are located in the high to very high vulnerability zones, which imposes a high risk of contamination by pesticides in these areas. Depth to water parameter inflicted the largest impact on the generic vulnera- bility index followed by the impact of the vadose zone, net recharge, and topography. The pesticide vulnerability index is more sensitive to the removal of the net recharge parameter followed by the depth to water, soil media, and topography. Compared with other studies, the sensitivity analyses show thatinthecaseofporousaquifers,theaquifermediaparameter is always the layer which compels less variation of the generic DRASTIC index.