الفهرس 
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Abstract
Surface and groundwater are the main sources of water supply in Libya. This study aims to assess the water quality index for drinking (WQI) and irrigation (IWQI) purposes. This was done by collecting 15 surfaces and 47 underground water samples in an area where lies in Al Jabal Al Akhdar region, northeast Libya during the wet and dry seasons of 2017 and 2018. For this purpose, water quality parameters such as water temperature (T), pH levels, EC, TDS, TH, TAK, major cations ( Ca2+, Mg2+, Na+, K+ and NH4+) and major anions (Cl-, PO43-, HCO3-, NO3-, SO42- and NO2-) were measured. Water quality indices including SAR, % Na, RSC, PI, SSP, MH, Kr and PS were also computed to categorize the water samples for different usages. In addition, main water types were classified based on Piper trilinear diagram.
The results revealed a basic pH level for surface water in all seasons with a maximum level of 7.88. In addition, it is also exhibited a basic pH level for groundwater in the wet season of 2017. Meanwhile, it is changed from acidic to basic in the dry season of 2017 with a maximum level of 8.1. This indicates high-level concentrations of bicarbonate in water samples as measured. It was observed that the ratio between TDS and EC increased with the decrease of pH levels, which indicates more releasing inorganic matter due to complex geomorphological constitutes of the study area.
Based on the U.S salinity, the Wilcox, and the Doneen classifications, the obtained results revealed that surface water is suitable for irrigation purposes. Meanwhile, the groundwater is classified between excellent and doubtful except for some samples that were classified as unfit for irrigation purposes east and west and west the study area. Furthermore and based on the Piper diagram, the surface, and groundwater types are classified as calcium sulfate (gypsum groundwater and mine drainage) and sodium chloride due to seawater intrusion east, along the Mediterranean shore and the west part of the study area. Meanwhile, the surface and groundwater types are classified as calcium bicarbonate (water passes through limestone or other calcium carbonate-containing minerals) in the middle part of the study area.
According to the computational method of water quality index for irrigation (IWQI) purposes and categorization of their water statuses, the surface and groundwater statuses were categorized as suitable for drinking and irrigation purposes expect for some samples east and west the study area and close to the Mediterranean coast of the study area due to the seawater. The revealed results of the water quality index for drinking and irrigation purposes agree with that concluded by US salinity, the Wilcox, and the Doneen diagram.