Ionic composition, source and suitability indices of groundwater for irrigation uses in the Mediterranean region of Morocco

Groundwater has been the main source of irrigation in Morocco for many decades. However, irrigated agriculture is now constrained by groundwater depletion, severe soil salinity and alkalinity, among other factors. Irrigation practices with poor quality groundwater can limit crop choices, reduce agricultural yields and lead to long-term deterioration of soil quality and fertility. This study aims to identify the hydrogeochemical processes controlling groundwater composition in the Triffa plain study area and evaluates the effects of depth (shallow vs. deep) on its suitability for irrigation, in relation to dominant soil types, by adopting a comprehensive approach, combining the Irrigation Water Quality Index (IWQI), multiple traditional indices (e.g., SAR, SARadj, %Na, PS, PI, KR, SSP, MAR, RSBC, TH), USSL/Wilcox/Doneen diagrams, statistical analyses and GIS technique. The hydrogeochemical analysis reveals that groundwater in the Triffa plain is dominated by Cl⁻ and Na⁺ ions, with three main hydrochemical facies identified: Na⁺–Cl⁻, Ca²⁺–Mg²⁺–Cl⁻–SO₄²⁻, and Ca²⁺–Mg²⁺–Cl⁻ mixed type. The salinity and ionic composition of groundwater in the Triffa plain are primarily influenced by evaporite dissolution minerals, carbonate weathering and ion exchange processes, with no evidence of seawater intrusion, as demonstrated by the Cl⁻/Br⁻ ratio. Most groundwater samples were found very hard (TH > 300 mg/L) and display high ECw and TDS values, indicating poor suitability for irrigation, except dam water. IWQI results show that 88.88% of shallow and deep borehole groundwater falls within high to severe restriction use categories, which is confirmed by traditional indices and diagrams (USSL and Wilcox). Only 11.12% of samples were classified in the low to moderate restriction categories, especially those from dam water. Spatial analysis reveals that water quality deteriorates from the southern/southeastern zones to the central and northern regions of the plain, with the highest restrictions found in the northern regions, which present elevated values across different indices (e.g., SAR, %Na, KR), ECw, TDS, Na⁺, and Mg²⁺. To address salinity and sodicity risks, especially in Mollisols and Ultisols, this study advises soil-specific management, especially under high to severe restriction zones, with application of periodic leaching, very frequent irrigation, gypsum amendments, low-volume irrigation, and the use of polyacrylamides (PAMs) to mitigate salinity effects and preserve soil structure. Also, this study offers a promising and complete approach for assessing water irrigation quality in semi-arid areas for water management, soil quality, and sustainable agriculture.