A GIS-based approach for geospatial modeling of groundwater vulnerability and pollution risk mapping in Bou-Areg and Gareb aquifers, northeastern Morocco

Abstract

Groundwater resources are the main supply of freshwater for human activities. Nevertheless, during the last 50 years, groundwater has become very susceptible to chemical pollution due to human activities. The groundwater vulnerability assessment constitutes a worldwide recognized tool for water management and protection. In this study, the GIS-based DRASTIC and pollution risk models have been used to assess the intrinsic vulnerability and risk to pollution of the Gareb and Bou-Areg aquifers, the main irrigated areas in the northeast of Morocco, by analyzing available hydrogeological attributes. The seven hydrogeologic factors used to assess vulnerability were depth to aquifer, net recharge, aquifer media, soil media, topography, impact of the vadose zone, and hydraulic conductivity, while an eighth parameter has been added to assess the pollution risk which is the land use. The resultant vulnerability map reveals that about 0.06% of the study area is in low vulnerability zones, 83.68% is moderately vulnerable, and 16.26% is highly vulnerable to groundwater pollution. The results also reveal that groundwater is highly vulnerable in the Gareb aquifer and the coastal zone, where the water table is very low, the slope is gentle, and the geological formations are permeable. In addition, moderate to low vulnerability is found towards the west of the study area where the groundwater is located in deep aquifers. The groundwater pollution risk map is obtained by overlaying the land use with the DRASTIC vulnerability. The central and western parts of Bou-Areg as well as the south of Gareb are dominated by high and very high pollution risk classes, and present 43.07% of the study area, which is strongly influenced by urban areas, agricultural activities, and shallow groundwater systems. 30.11% of the surface is moderately vulnerable, mainly in areas where human activity is not widely observed, while the very low and low pollution risk classes represent a total of 26.82% of the total area. The mapping models were validated using nitrate concentration and electrical conductivity data in groundwater as an indicator of pollution. A positive correlation was observed when validating these models. The resultant groundwater vulnerability and pollution risk maps might provide an early warning to policy maker and manager to manage and avoid further stress on this invaluable resource.

Keywords: DRASTIC model; GIS; Groundwater vulnerability; Land use; Nitrate contamination; Pollution risk.