Use of Electrical Resistivity Tomography to Study the Impact of Long-Term Application of Sewage Sludge: Case Study of Oued Souhil Experimental Station, Nabeul (Tunisia)

Abstract

Soil salinization following repetitive applications of sewage sludge (S.S.) is a major environmental impediment with severe adverse impacts on agricultural productivity and sustainability, particularly in arid and semiarid regions. Electrical resistivity tomography (ERT) can be easily and rapidly assessed in the field for soil salinity investigations. Accordingly, ERT was conducted on sandy soil (soil S) in a semiarid Region in Tunisia (Oued Souhil, Nabeul) after repetitive S.S. application at the equivalent rates: 0, 40, 80, and 120 t/ha. The study aimed to monitor topsoil and subsoil salinization following the sludge trial. As such, the ERT survey was measured across each field plot of 4 m2 using the Lund System of ABEM with SAS4000 Resistivimeter. Wenner array was applied using 16 electrodes and 0.2 m electrode spacing, which corresponds to approximately 0.6 m depth of investigation. Laboratory analyses were conducted on soil samples taken in the middle of the treated plots at four depths (0–20 cm), (20–40 cm), and (40–60 cm) for a comparison with the ERT survey result. ERT panels showed a general decrease in soil resistivity below (40 cm) with extremely low resistivity in each plot’s corner, indicating a vertical leaching and a lateral migration of conductive materials in all the treatments. However, the laboratory analysis did not confirm these results, showing the decrease in soil electrical conductivity (E.C.) with soil depth in the amended soils. Soil parameters (E.C., O.M.) decreased drastically from the last amendment (in 2017), reaching lower levels than the experimental soil (before sludge trial). This fact could be attributed to the heavy rainfall recorded in Nabeul in September 2018 that caused soil leaching throughout the soil profile, as shown by the ERT panels.