UPTAKE OF PHOSPHATE BY IRON HYDROXIDES DURING SEEPAGE IN RELATION TO DEVELOPMENT OF GROUNDWATER COMPOSITION IN COASTAL AREAS

Seepage of o-PO4-rich groundwater may give rise to surface water eutrophication. Hydrogeochemical modeling based on groundwater quality data from the coastal lowland of The Netherlands was performed to relate in a general way the fixation of PO4 at the oxic/anoxic interface in the surface water sediment to the development of the composition of anoxic groundwater that seeps through these sediments. Solid-solution aqueous-solution modeling with the end members ferrihydrite and strengite shows that pH and the aqueous Fe(II):PO4 ratio determine the uptake of PO4 in iron hydroxides following the oxidation of Fe(II) during seepage. The groundwater composition, including the Fe(II):PO4 ratio, in coastal areas is largely controlled by cation-exchange processes following displacement of the fresh/saline interface together with mineral control for carbonates and vivianite and, eventually, reduction of seawater SO4. Combining the processes for the two environments leads to the conclusion that potential eutrophication is related to different stages that develop during these displacements.