Assessment of groundwater vulnerability in a semi-arid basin: a comparative study of DRASTIC and SI methods (case study of Boulefreis Wadi watershed-northeast Algeria)

This study evaluates the vulnerability of groundwater in the Boulefreis Wadi catchment, located in the Constantinois highlands of north-eastern Algeria, and the basin covers a surface of 938 km2 and has a semi-arid climate. The catchment is known for its high agricultural productivity and extensive use of fertilizers, which poses a significant risk of nitrate pollution in groundwater. To assess this vulnerability, the DRASTIC [ (D) depth of water, (R) net recharge, (A) aquifer media, (S) soil media, (T) topography, (I) impact of the vadose zone, and (C) which refers to hydraulic conductivity] and Susceptibility Index (SI) methods were used, integrated with a Geographic Information System (GIS). The data used include the geological map, topography map, soil map, Digital Elevation Model (DEM), and land use map of Algeria. The map of the spatial distribution of nitrates NO3- at groundwater level, hydraulic conductivity, and net recharge was produced using spatial interpolation tools on ArcMAP 10.4 software. The DRASTIC vulnerability map displays four vulnerability classes, with highly vulnerable zones present in both parts of the catchment, covering 7.62% of the study area. However, the SI method shows the highest vulnerability values in the northeastern parts of the study area, specifically in the Remila and Timgad regions, where moderate to high and high classes cover 42.59%. After comparing the DRASTIC and SI vulnerability maps, it seems that the SI method is better suited for the Boulefreis Wadi basin. This is because the region is more vulnerable to groundwater pollution due to agricultural activities. The study reveals a significant risk of nitrate contamination in the central region of the study area, particularly in the Remila agricultural plain and surrounding areas. Nitrate concentrations in these areas range from 50 to 90 mg/l, which exceeds the WHO standard. The correlation coefficient (R2) was used to validate both methods, DRASTIC and SI, with NO3- concentration. The results showed R2 = 0.57327and R2 = 0.5704, respectively. Areas with high nitrate concentrations correspond well with those having high vulnerability indexes. However, the SI method is the most effective for assessing the risk of nitrate contamination in groundwater. This study's is limited by the use of 1/500,000 scale geological and topographic data, which may not be precise enough for detailed analysis. Future research should focus on incorporating more detailed data to enhance the accuracy and comprehensiveness of the groundwater vulnerability assessment.