Mapping zone vulnerability to agricultural pollution with a DRASTIC model and Geographical Information System (GIS): The case of the Chaffar groundwater (South of Sfax, Tunisia)

Habib Smida; Chokri Abdellaoui; Moncef Zairi; Hamed Ben Dhia



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Mapping zone vulnerability to agricultural pollution with a DRASTIC model and Geographical Information System (GIS): The case of the Chaffar groundwater (South of Sfax, Tunisia)
Science et changements planétaires / Sécheresse. Volume 21, Number 2, 131-46, avril-mai-juin 2010, Article de recherche
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Author(s) : Habib Smida, Chokri Abdellaoui, Moncef Zairi, Hamed Ben Dhia
Summary : Vulnerability assessment to delineate areas which are more susceptible to contamination from anthropogenic sources has become an important element for sensible resource management and land use planning. Indeed, groundwater vulnerability maps have become a standard tool for protecting groundwater resources from pollution. They are especially valuable in the decision-making process related to land use planning. Land use planners have mostly modest experience and expertise at hand to decide which land uses and activities they can allow in certain areas without causing a negative impact on the quality of groundwater. Therefore, there is a need to ensure that the groundwater resources are of drinkable quality by protecting them against pollution sources such as agricultural pesticides and wastes. This contribution aims at estimating aquifer vulnerability by applying the DRASTIC model as well as utilizing sensitivity analyses to evaluate the relative importance of the model parameters for aquifer system vulnerability in the Chaffar area, located in the south of Sfax city, south-eastern Tunisia. This aquifer is formed by a multi-layered groundwater reservoir limited by the Mediterranean cost on its south-eastern border. The DRASTIC model has been applied with a Geographic Information System (GIS) tool to evaluate the vulnerability of this aquifer to pollution. Data of the study area which are related to seven critical factors of the applied model (Depth to water, net Recharge, Aquifer media, Soil media, Topography (slope), Impact of the vadose zone and hydraulic Conductivity (DRASTIC)), were collected from several sources and converted into thematic maps by the GIS system. For each critical parameter, weight and index were attributed according to its importance in the vulnerability model using the Theissen polygons method. By adding the product of weights and indexes of each parameter, we obtain a detailed hydrogeological vulnerability map. This map shows three classes of potential pollution of groundwater. Results indicate that 4% of the study area is highly vulnerable to pollution, 48% is moderately vulnerable and 48% has low vulnerability. The most vulnerable parts of the study area (inshore part) are those characterized by intense agricultural activity, where the nitrate concentrations in groundwater varies from 10 mgNO ₃-/L to 170 mgNO ₃-/L. Therefore, these agricultural areas were considered as the principal sources of nitrate contamination in Chaffar groundwater. It is however worth noting that contamination from urban sources is also possible. This source of contamination is essentially bound to the use of lost wells (old and no longer used wells) as places of discharge and of worn-out water storage (domestic) in the farming zones.
Keywords : cartography, geographic information system, groundwater, modeling, pollution, Tunisia