Investigating subirrigation, seasonal variability and hydrogeological control in a salinized shallow aquifer-aquitard system

This study investigates the seasonal hydrogeochemical evolution and persistent hydrogeological controls in a salinized shallow aquifer-aquitard system located in a reclaimed coastal lowland. The study focuses on two adjacent agricultural field plots with contrasting hydrostratigraphic settings. Major ions, stable isotopes, and redox-sensitive species were integrated to identify the processes controlling groundwater composition and their seasonal evolution. The key findings include: (i) groundwater salinization primarily reflects inherited paleo-marine salinity preserved within low-permeability Holocene sediments; (ii) subirrigation produces a temporary and spatially variable dilution of groundwater, without substantially altering the composition of the saline groundwater end member or causing systematic mobilization of dissolved trace elements, indicating that the effects of subirrigation are largely transient; (iii) the deep saline groundwater component is controlled by processes operating over timescales much longer than the seasonal recharge cycle. Persistent reducing conditions within the peat-rich groundwater domain promote ammonium accumulation, while sulphur isotope compositions suggest differences in sulphur cycling between the two groundwater domains, reflecting differences in groundwater evolution under reducing conditions. Methane isotope data are consistent with microbially produced methane in the deeper peat-rich sequence. The results show that the hydrogeochemistry of the aquifer-aquitard system is governed by the interaction between inherited salinity, peat-rich sediments and redox-driven biogeochemical processes. These processes must be considered, for example, when attempting to mitigate the salinization of this aquifer-aquitard system through subirrigation practices; moreover, the findings here identified can be transferred to similar alluvial deltaic environments around the world.