Combining graphical, indexing, and unsupervised machine learning methods for the assessment of groundwater quality in the Nubian Aquifer, Shendi Sub-Basin, Sudan

Groundwater quality is crucial for sustaining life and supporting socioeconomic activities in arid and semi-arid regions. This study investigates the hydrogeochemical characteristics, spatial distribution, and factors influencing groundwater quality in the Shendi area, Sudan using graphical, unsupervised machine learning, and indexing techniques. These include self-organizing maps (SOM), principal component analysis (PCA), and groundwater quality index (GWQI). The groundwater in the study area is predominantly neutral to slightly alkaline conditions with total dissolved solids (TDS) ranging between 219.3 and 2118 mg/L. Hydrochemical facies is dominated by Ca-Mg-HCO₃ and Ca-Cl types, with localized occurrences of Na-Cl and Na-HCO₃ facies. Rock-water interactions including cation exchange, carbonate weathering, and silicate dissolution were identified as key geochemical reactions shaping groundwater composition. Saturation indices suggested active dissolution of gypsum and halite, while oversaturation of calcite and dolomite points to precipitation processes. SOM clustering classified groundwater into three groups based on salinity. The samples with high salinity are associated with mineralized Precambrian formations and localized contamination near the Nile River, while low-TDS samples reflect dilution processes in recharge zones. PCA identified mineral dissolution, agricultural activities, and nitrogen cycling as primary factors influencing groundwater quality. GWQI analysis classified the samples into four quality categories including excellent, good, poor, and very poor. While most samples fall under the excellent category, hotspots of poor and very poor water quality are evident, driven by agricultural runoff and salinization processes. The recent analysis highlighted the need for targeted management strategies, such as controlling pollution sources and regulating agricultural practices to safeguard groundwater resources in the Shendi area.