Physicochemical quality and irrigation suitability of groundwater resources in the Feija aquifer (southeastern Morocco)

Abstract

Groundwater is the primary water resource in the arid Feija plain (Zagora, Morocco), but its quality is increasingly threatened by climate change and intensive agriculture. This study assessed the physicochemical quality and irrigation suitability of the Feija aquifer. Over three years (2022–2024), 96 groundwater samples were collected from eight wells (P1–P8) in all four seasons to assess spatial and temporal variability. Physicochemical parameters, including pH, electrical conductivity (EC), and major ions, were analysed. The results indicate high mineralisation, with a mean EC of 2934.14 µS∙cm⁻¹ and significant spatial heterogeneity. Correlation analysis and principal component analysis (PCA) identified two dominant processes: a salinity gradient driven by natural water–rock interactions and evaporative concentration, and a pollution gradient linked to diffuse agricultural inputs (nitrates and bicarbonates). Seasonal box plots identify summer as a critical period, with nitrate concentrations frequently exceeding the 50 mg∙dm⁻³ drinking-water threshold and EC values peaking in autumn. High sodium and chloride concentrations indicate a sodium–chloride hydrochemical facies, with agricultural intensification contributing to groundwater degradation. Irrigation-suitability indices calculated from the mean ionic composition (sodium adsorption ratio, SAR ≈ 3.4; permeability index, PI ≈ 67%) indicate a high to very high salinity hazard (Richards’ USSL classes C3–C4, based on measured EC), a low sodium hazard (class S1) and a permissible to good permeability class (Doneen class II). Salinity, rather than sodicity, is therefore the main constraint on irrigation use.