Spatial–temporal variation of surface water quality in the Ichkeul Lake and rivers ecosystem, northern Tunisia

This research aims to assess the spatial and temporal variations in surface water quality of the Ichkeul Lake and river ecosystems, in northern Tunisia. Surface water samples were collected during wet and dry seasons in December 2012, August 2013, and January and August 2014. The samples were analyzed for pH, water temperature, dissolved oxygen (O2), electrical conductivity (EC), salinity, major cations and anions, and nutrient elements using standard methods. Surface water was generally alkaline (average pH 8.27 in winter and 8.32 in summer). In the majority of the studied sites, we noted an increase of the pH value in winter. The highest average values of EC were recorded in the summer period; they ranged from 0.1 to 20.9 mS/cm in summer 2013, and from 0.7 to 21.8 mS/cm in summer 2014. Sodium was identified as the dominant cation; chloride was identified as the dominant anion in August 2014 with an average of 4230 mg L−1, which exceeded the Tunisian standard NT106.03 (1989) for the environmental protection (treated water for agricultural purposes) fixed at 2000 mg L−1. These concentrations are attributed to the leaching of the Triassic series which outcrop in most rivers (Morra, Douimis, Khloufi, Sejnane) as well as to the dissolution of halite NaCl and carbonate rocks (CaCO3, Ca3MgCO3) rich in chlorides. According to the main factor analysis results, chloride is the best indicator for monitoring water quality out of the other water quality parameters measured in this study. The results revealed a deteriorating water quality. The highest concentrations of chlorides are recorded at the level of the Tinja wadi, which permanently receives water supplies from the Bizerte lagoon. The concentrations in chlorides have a strong correlation with those of sodium. This can be explained by the abundance of salt rocks of the Triassic rich in halites which have particularly high degrees of solubility. This enrichment in NaCl is also due to contamination by Triassic outcrops.