Discharge of treated wastewater: hydrodynamic and hydrogeochemical impacts on the Kairouan plain aquifer (Central Tunisia)

In Tunisia, the discharge of treated wastewater onto land area has become a common practice, especially in arid and semi-arid regions where there are no permanent rivers to receive discharge. Treated wastewater constitutes a potential anthropogenic source of aquifer recharge as is the case for the alluvial aquifer downstream of the Kairouan plain. This alluvial aquifer receives annually an average volume of 4.9 hm³ of treated wastewater from the Dhraa Tammar wastewater treatment plant, which is discharged over a land area of 182 ha. This research presents the first piezometric and hydrogeochemical characterization of Kairouan plain shallow groundwater and the assessment of treated wastewater impacts on groundwater piezometric levels, salinity and hydrogeochemical composition and microbiological contents of groundwater. To achieve these objectives, a piezometric surveys of 15 shallow wells was undertaken. Additionally, groundwater and treated wastewater samples were collected in February and April 2017 for chemical and microbiological analysis. The piezometric study shows that treated wastewater discharge induces a local increase of the water table level materialized by the appearance of a piezometric dome (a height estimated at 1.5 m) in the discharge site during a period of 4 years (2014–2017). Shallow groundwater downstream of Kairouan plain shows a naturally high mineralization state (a salinity more than 20 g L⁻¹) caused by natural mineralization process. A considerable reduction of groundwater salinity (between 3 and 10 g L⁻¹) is observed in the discharge site due to treated wastewater infiltration. Discharge of treated wastewater has caused an increase of groundwater bicarbonate concentration due to leaching of organic matter and local microbiological groundwater contamination (250–450 fecal germs per 100 ml) in the discharge site.