Spatial variation of groundwater vulnerability to nitrate pollution under excessive fertilization using index overlay method in central Tunisia (Sidi Bouzid basin)*

Abstract

AbstractOne of the major challenges in assessing groundwater vulnerability to pollution is the inadequate factors number and weight. Therefore, to carefully improve the assessment of groundwater vulnerability, a model independent of weight assignment errors was used. Moreover, the specific vulnerability index (SVI) of Sidi Bouzid North's groundwater was assessed in this study. Intrinsic vulnerability index (IVI) assessment was determined in the first step by the arithmetic mean calculation by the index overlay method (IOM) based on the D: Depth of aquifer; P: annual average Precipitation; L: Lithology of the vadose zone and S: percent of Slope (DPLS) model. Then SVI was assessed by linking new factors (LU and NO3−) to IVI. Consequently, 83 samples were analysed for NO3−, showing high values that exceed 50 mg l‾¹. The spatial distribution of IVI shows three vulnerability classes in the study area, namely low (8%), moderate (15%) and high (77%). The evaluation of SVI based on the risks associated with the NO3− and LU factors using GIS indicates that about 95% of the total study area is classified with high SVI levels. It displayed a good correlation with NO3− and provided a high discretization of the groundwater risk from natural and anthropogenic pollution. This alarming situation suggests the necessity to apply water‐saving irrigation action for adequate water management.