Title
Extent of immobilisation of phosphate during aeration of nutrient-rich, anoxic groundwater
Author
Griffioen, J.
Nederlands Instituut voor Toegepaste Geowetenschappen TNO
Publication year
2006
Abstract
Nutrient-rich exfiltrating groundwater may impose a heavy phosphate load on surface water systems. However, iron oxides that bind PO4 precipitate fast upon oxygenation at neutral pH and PO4 may also become bound in Ca precipitates following upon pH increase, so load estimates based on conservative behaviour during exfiltration will be overestimates. Aeration experiments using natural groundwater were performed to characterise the immobilisation of PO4 within one day after aeration started. Groundwaters having a wide variety in composition, were sampled in the coastal lowlands of the Western Netherlands. Three models were considered to describe the fast binding of PO4 by Fe oxide type phases that form upon the oxygenation of dissolved Fe(II), each based on a different concept. The concepts were surface complexation, solid-solution precipitation and two-mineral precipitation. When the experimental data were compared with model results, all three models were found to be inadequate. Frequently, more immobilisation of PO4 occurred than could be explained by binding to a Fe oxide type of phase alone. Uptake by Ca phosphates and/or Ca carbonates must additionally have played a role; alternatively, a non-ideal phase consisting of Ca, Fe and PO4 precipitated upon oxygenation and CO2 degassing. A predictive multiple regression model with two primary variables that reflect the driving forces for PO4 immobilisation was deduced that describes immobilisation of phosphate after aeration of anoxic groundwater. The two primary variables are the log value of the groundwater Fe to PO4 molar ratio and the saturation state for hydroxyapatite after the CO2 degassing of groundwater. The model is useful for calculating the PO 4 load of surface water from exfiltration groundwater, taking into account fast immobilisation (<1 day) during exfiltration. © 2005 Elsevier B.V. All rights reserved.
Subject
Geosciences
Geochemical modelling
Natural water geochemistry
Precipitation (chemical)
Aeration
Exfiltration
Geochemical modeling
Immobilisation
Groundwater
Calcium
Composition
Degassing
Groundwater
Iron oxides
Natural water geochemistry
Phosphates
Precipitation (chemical)
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DOI
https://doi.org/10.1016/j.jhydrol.2005.07.047
TNO identifier
239227
ISSN
0022-1694
Source
Journal of Hydrology, 320 (3-4), 359-369
Document type
article