The effect of salinization and freshening events in coastal aquifers on nutrient characteristics as deduced from field data

•We test the salinization and freshening effect on nutrients using field work.•Nitrification occurred at the interface, while denitrification occurred below it.•Phosphate increased in the saline groundwater below the interface.•Summer salinization caused to silica increase due to biogenic silica dissolution. The effect of seawater intrusion and freshening events in coastal aquifers on nutrient (dissolved inorganic nitrogen species, phosphate and silica) dynamics across the fresh–saline groundwater interface (FSI) were quantified using field data. Seasonal vertical profiles revealed a clear transition between nutrient species across the FSI, which is also an oxycline. In view of the results of our experimental simulations, it is clear that the major process controlling the nutrient dynamics at the FSI, besides the mixing that takes place between the two different water bodies, is the seasonal variation between seawater intrusion (salinization) in summer and flushing of the aquifer (freshening) in winter. Aquifer salinization during the summer shifts the FSI and the anaerobic depth-location upwards and leads to the enrichment of NH4+, PO43− and DSi (dissolved silica) in the saline groundwater. NH4+ and PO43− are enriched due to ion exchange, and DSi is enriched either by ion exchange (as PO43−) or as a result of dissolution of biogenic silica. Denitrification occurs at the base of the FSI, as indicated by the slight NO3− depletion and the enrichment in δ15N of NO3−. Aquifer freshening during the winter shifts the FSI downward and the water becomes suboxic with the influence of the oxic fresh groundwater. This leads to nitrification of the NH4+, enrichment of NO2− and depletion of 15N in the residual NO3− in the FSI. These cyclic processes generate a certain depletion of N and enrichment of P in the saline groundwater. Circulation of the saline groundwater below the FSI back to the sea can serve as a partial counterbalance to the high anthropogenic load of N impacting the coastal groundwater system.