Flow-interruption experiments to quantify trace elements leaching from a shallow saline aquitard

Salinity from reclaimed lands in coastal deltaic areas is of upcoming concern throughout the world due to the ongoing climatic change and the associated water resources overexploitation. Sub-irrigation in salinized deltaic soils can promote the transport of major and trace elements (TE) in groundwater. In this study, experiments on flow interruption in columns have been conducted for salinized peaty aquitard from the Po Delta using synthetic rainwater to replicate sub-irrigation practices and their effect on TE leaching. Non-equilibrium transport and cation exchange phenomena were assessed using a NaCl tracer test. This latter was also simulated via CXTFIT 2.0 to quantify sorption and physical non-equilibrium processes. Pre and post experiment sequential extraction allowed for the evaluation of geochemical alterations caused by the injection of the synthetic rainwater and the tracer solution into the reducing sediments. Results from the flow interruptions showed peaks of As, Mn, Pb, and Ni exceeding the World Health Organization standards for drinking water, indicating that irrigation periods are marked by the diffusion of trace elements hosted in exchange sites and solid phases. The employment of NaCl as a tracer resulted in a significant elution of trace elements due to Na⁺ exchange from clays and organic matter and retardation of Cl^- due to diffusion into the osmotic membranes of organic fragments. The partial oxidation of pyrite and arsenopyrite to Fe(III) oxides contributes to the prolonged release of TE. These results highlight how alternating sub-irrigation in a peaty reducing environment can trigger the mobilization of TE in deltaic contexts. The findings of this study are of relevance, as similar hydrogeological settings are widespread globally. Indeed, in real-world scenarios, such as agricultural systems with intermittent irrigation or aquifer restoration strategies, flow interruptions are commonly utilized worldwide.