Extent of immobilisation of phosphate during aeration of nutrient-rich, anoxic groundwater

Nutrient-rich exfiltrating groundwater may impose a heavy phosphate load on surface water systems. However, iron oxides that bind PO 4 precipitate fast upon oxygenation at neutral pH and PO 4 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 PO 4 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 PO 4 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 PO 4 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 PO 4 precipitated upon oxygenation and CO 2 degassing. A predictive multiple regression model with two primary variables that reflect the driving forces for PO 4 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 PO 4 molar ratio and the saturation state for hydroxyapatite after the CO 2 degassing of groundwater. The model is useful for calculating the PO 4 load of surface water from exfiltration groundwater, taking into account fast immobilisation (