Ion leaching and soil solution acidification in a vadose zone under soil treated with sewage sludge for agriculture

In this study, we performed monitoring of the soil solution (SS) over 10 years on a loamy/clayey-textured Dark Red Dystroferric Oxisol that received sewage sludge for agricultural purposes. The SS was obtained by lysimeters installed along the walls of a well at 1 m, 2 m, 3 m, 4 m and 5 m in depth. The major ions found in the SS were NO₃^-, SO₄^2-, Cl^-, Ca²⁺, Mg²⁺, Al³⁺, Pb²⁺, Cd²⁺ and Zn²⁺, and the pH level ranged from 4 to 6.5 along the profile. Throughout the first three years of monitoring, the pH to a 3-m depth became more acidic, and in the last year, this trend reached 5 m. At the 5-m depth, the pH decreased from 6.5 to 4.5 from the first to the last monitoring. The SS acidification was provoked by both nitrite oxidation and ion leaching. The leaching of H⁺ or the possible ion exchange/desorption of H⁺ due to the leached cations (Ca²⁺ and Mg²⁺) at the 4-m and 5-m depth caused the pH decrease. The ionic strength (IS) of the solution controlled the ion leaching. The sludge application increased the IS to 3 m, increasing the density of the soil charges and its ability to absorb ions. After the sludge application was completed, there was a decrease in IS of the SS as well as a decrease in ion absorption and retention abilities, which promoted leaching to greater depths. During the entire monitoring process, NO₃^-, Cd and Pb remained above the potability limit.