Assessment of urban groundwater vulnerability in arid areas: Case of Sidi Bouzid aquifer (central Tunisia)

Rapid urbanization has several adverse repercussions on the hydrological cycle due to great demands of land and water for housing, transport, health, agriculture, education etc. This paper investigates the impact of the urbanization on the groundwater regime in Sidi Bouzid, an old city located in Central Tunisia under a semi-arid climate where groundwater is the sole source for water needs of the city and the surrounding farming areas. The groundwater vulnerability has been evaluated using the DRASTIC model under a GIS environment. This well-known model is based on the combination of seven parameters, Depth to water, net Recharge, Aquifer media, Soil media, Topography, Impact of vadose zone, and hydraulic Conductivity, that are considered as directly controlling the vulnerability of groundwater to potential risks. However, the original model does not consider the effect of urban land uses (urban sprawl and density) on groundwater so a modified DRASTICU model, that includes Urban land use as an eighth parameter has been applied. The net recharge assessment was also modified to include the contribution of leakages from water supply network and sewage networks to the aquifer recharge. On the other hand, soil classes of the aquifer and the vadose zone that are not included in the original listing of the model have been discussed and rated. Sensitivity analysis has been carried out to evaluate the effect of each individual parameter on the vulnerability zoning and to validate the use of each one. The validation of the DRASTICU model was done through the comparison between groundwater vulnerability index and nitrate concentrations. The vulnerability map shows that all the groundwater underlying the city is exposed to pollution risks with vulnerability index varying between extreme in the centre of the city (38%), very high in eastern parts (33.6%) and high (28.4%) elsewhere.