Potential risk of soil irrigation with treated wastewater over 40 years: a field experiment under semi-arid conditions in northeastern Tunisia

In Tunisia, water scarcity is only adding pressure on water demand in agriculture. In the context of sustainable development goals, Tunisia has been reusing treated wastewater (TWW) as a renewable and inexpensive source for soil fertigation and groundwater (GW) recharge. However, major risks can be expected when the irrigation water is of poor quality. This study aims for evaluating the potential risk of TWW and GW irrigation on soil parameters. Accordingly, we evaluated the suitability of water quality through the analysis of major and minor cations and anions, metallic trace elements (MTEs), and the sodium hazard by using the sodium adsorption ratio (SAR) and the soluble sodium percentage (SSP). The risk of soil sodicity was further assessed by SAR and the exchangeable sodium percentage (ESP). The degree of soil pollution caused by MTEs accumulation was evaluated using geoaccumulation index (Igeo) and pollution load index (PLI). Soil maps were generated using inverse spline interpolation in ArcGIS software. The results show that both water samples (i.e., TWW and GW) are suitable for soil irrigation in terms of salinity (electrical conductivity<7000 mu S/cm) and sodicity (SAR<10.00; SSP<60.00%). However, the contents of PO43-, Cu2+, and Cd2+ exceed the maximum threshold values set by the national and other standards. Concerning the soil samples, the average levels of SAR and ESP are within the standards (SAR<13.00; ESP<15.00%). On the other hand, PLI results reveal moderate pollution in the plot irrigated with TWW and no to moderate pollution in the plot irrigated with GW Igeo results indicate that Cu2+ is the metallic trace element (MTE) with the highest risk of soil pollution in both plots (Igeo>5.00), followed by Ni2+ and Pb2+. Nevertheless, Cd2+ presents the lowest risk of soil pollution (Igeo<0.00). Statistical data indicates that Ca2+, Na+, Ni2+, and Pb2+ are highly distributed in both plots (coefficient of variation>50.0%). This study shows that the use of imagery tools, such as ArcGIS, can provide important information for evaluating the current status of soil fertility or pollution and for better managing soil irrigation with TWW.