Phosphate mobilization in coastal aquifers due to seawater intrusion: a model assessment
Spiteri, C.; Van Cappellen, P.; Regnier, P. ; Meile, C.; Slomp, C. P. (2008). Phosphate mobilization in coastal aquifers due to seawater intrusion: a model assessment, in: Groundwater quality: securing groundwater quality in urban and industrial environments. Selected and reviewed papers from GQ07, the Groundwater Quality Conference held in Fremantle, Australia, in December 2007. pp. 396-403 In: (2008). Groundwater quality: securing groundwater quality in urban and industrial environments. Selected and reviewed papers from GQ07, the Groundwater Quality Conference held in Fremantle, Australia, in December 2007. IAHS Publ. 324: Australia. ISBN 978-1-901502-79-4. 566 + x pp., more |
| Authors | | Top | - Spiteri, C.
- Van Cappellen, P.
- Regnier, P., more
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| Abstract | Non-conservative behaviour of dissolved inorganic phosphate (DIP) upon groundwater salinization is generally ascribed to desorption from iron (hydr)oxides. We assess this hypothesis by incorporating the reversible adsorption of phosphate onto a model oxide (goethite) in a two-dimensional, density-dependent groundwater flow model. The model is then applied to simulate the transient behaviour of DIP during seawater intrusion in a coastal freshwater aquifer. Two different approaches are used to simulate phosphate adsorption: (1) a surface complexation model (SCM), which accounts for the changes in aqueous and surface-bound speciation of phosphate with variable pH and salinity, and (2) a linear adsorption isotherm (Kd), in which the partition coefficient Kd is assumed to remain constant. In the latter case, a typical Kd value for freshwater aquifer conditions is imposed. The seawater intrusion scenario is implemented using a set of aquifer settings similar to the Henry saltwater intrusion benchmark problem. In addition, the evolution of the pH distribution is computed assuming conservative mixing of dissolved inorganic carbon and alkalinity between the groundwater and seawater. The results show that, as the saltwater wedge advances, DIP is released and progressively dispersed at the freshwater–seawater interface. The released DIP is flushed out with the discharging groundwater, giving rise to a final, steady-state DIP concentration distribution that follows a conservative mixing curve with respect to salinity. Use of a freshwater end-member Kd value overestimates the overall release of DIP. In general, however, the salinity-induced phosphate mobilization predicted by the SCM and K models is lower than that observed in coastal aquifers, implying that other factors besides pH and salinity gradients are likely to contribute to DIP release observed upon saltwater intrusion. |
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