Spatial variability of nitrate leaching and risk assessment of nitrate contamination in the Ghiss-Nekor alluvial aquifer system (Northeastern Morocco) through Disjunctive Kriging

Abstract

In this study, groundwater nitrate monitoring data from water supply and water management organizations were processed in a GIS environment to evaluate spatial patterns of nitrate occurrence and explore its correlation with the corresponding land uses in Ghiss-Nekor alluvial groundwater (northeastern Morocco). Land-use/Land Cover within the Ghiss-Nekor plain incorporated rangeland, built area, crop fields, water bodies, dense vegetation, and bare ground categories, encompassing respectively about 39.06%, 36.69%, 17.57%, 4.63%, 1.24%, and 0.81% of the total area. According to the World Health Organization (WHO), the selected thresholds are an acceptable concentration of nitrate for potable water (50 mg/L) and a human affected value (13 mg/L). Simple Kriging and Disjunctive Kriging (D.K.) geostatistical methods were compared with a deterministic interpolation method (GPI) to identify the convenient approach to predict nitrate concentrations at unsampled sites. D.K. provided more accurate predictive performance and highlighted that nitrate contamination is higher in wells in the southern half of the study area, where nitrate concentrations exceeded 50 mg/L in 29.2% (33,3 km²) of the study area. Areas occupying 21.28% of the total surface, with nitrate contents exceeding the human-affected value, are located near small industrial sites, residential septic tanks, and agricultural areas. No significant correlation was found between the nitrate concentrations and the depth of the wells. The probability of nitrate content exceeding the thresholds of 13 and 50 mg/L is respectively less than 0.5 in 61.35% (69.94 km²) and 77.63% (88.4 km²) of the total study area, whereas the maximum likelihood of nitrate contamination is found along uncontrolled urban discharges located within highly permeable deposits. The assessment also demonstrated that areas showing high predicted probabilities of nitrate contamination diminish with increasing threshold value. The present study shed light on the necessity of depicting the extent of polluted areas, particularly in vulnerable alluvial aquifers, where exposure to higher levels of drinking water contaminants threatens human health.