Assessment of groundwater risk using intrinsic vulnerability and hazard mapping: Application to Souassi aquifer, Tunisian Sahel

Special attention has been paid to risk assessment in water resource management in arid and semi-arid regions like the Souassi aquifer, Tunisia. Risk assessment, using vulnerability and hazard mapping, is considered as a fundamental aspect of sustainable groundwater management. To determine the degree of risk affecting the study area, an attempt has been made to combine hydro-geologic parameters using the DRASTIC method and the hazard assessment by taking the product of the weighted hazard value (H_I), the ranking factor (Q_N) and the reduction factor (R_f).All parameters used in this risk assessment were prepared, classified, weighted and integrated in a GIS environment. Data treatment shows that large areas in the Souassi aquifer can be classified as high or very high risk areas corresponding to pollution sources located in high vulnerability zones. The areas with low and very low risk are situated in the south, which could, consequently, be interesting for future development and long term planning of protective measures. Sensitivity analyses indicated that the removal of groundwater depth, net recharge and aquifer media parameters from the DRASTIC index, causes large variation in vulnerability assessment. Moreover, hydraulic conductivity and topography were found to be more effective in assessing aquifer vulnerability. Therefore, they should have higher weights than those assumed by the DRASTIC standard method, and contrary to the impact of the vadose zone parameter. The validity of the DRASTIC and the risk methods, verified by comparing the distribution of nitrates in the groundwater and the different vulnerability classes, shows a high similarity.