Effects on the Groundwater and Drainage Water Under Irrigation by Treated Wastewater in the Cebala Borj-Touil Area, Tunisia

The Cebala Borj-Touil perimeter is the largest area irrigated by treated wastewater (TWW) in Tunisia. A sector of about 748 ha containing 13 secondary open drains and a main canal was selected to study the environmental impact of the reuse of TWW. The depth and the water quality of the shallow saline groundwater were controlled by 16 piezometers during two dates: at the end of irrigation cycle in September 2014 and before the irrigation cycle in March 2015. Results showed an increase of the groundwater by 1 m in rainy season in winter especially in the northeast and the southwest part of the sector and decrease in dry season in summer by 1.5 m. The average EC is between 12.2 and 14.6 dS/m. The soluble salts are dominated by sodium chloride. The contents of metallic trace elements (MTE) exceed in some sites the Tunisian standards for TWW discharge. Cd and Cr contents lie frequently above the norms, Co, Fe and Pb content only in some dates. This pollution is caused by the leaching of soils irrigated with TWW. The contamination with MTE also affects the drainage water. Samples taken several times from an open drain showed high amounts of Cd, Cr, Pb and Co. The contents reached 0.019 mg/l, 0.196 mg/l, 0.231 mg/l and 0.179 mg/l, respectively. The results prove that there is a risk of contamination of the marine ecosystem, where the drainage water finally flows.