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

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⁺, PO₄^3- and DSi (dissolved silica) in the saline groundwater. NH₄⁺ and PO₄^3- are enriched due to ion exchange, and DSi is enriched either by ion exchange (as PO₄^3-) 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.