4.5 Article

Modelling the geochemical fate and transport of wastewater-derived phosphorus in contrasting groundwater systems

Journal

JOURNAL OF CONTAMINANT HYDROLOGY
Volume 92, Issue 1-2, Pages 87-108

Publisher

ELSEVIER SCIENCE BV
DOI: 10.1016/j.jconhyd.2007.01.002

Keywords

phosphorus; nitrogen; N : P ratio; groundwater; sorption capacity; reactive transport modelling

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A ID reactive transport model (RTM) is used to obtain a mechanistic understanding of the fate of phosphorus (P) in the saturated zone of two contrasting aquifer systems. We use the field data from two oxic, electron donor-poor, wastewater- impacted, sandy Canadian aquifers, (Cambridge and Muskoka sites) as an example of a calcareous and non-calcareous groundwater system, respectively, to validate our reaction network. After approximately 10 years of wastewater infiltration, P is effectively attenuated within the first 10 m downgradient of the source mainly through fast sorption onto calcite and Fe oxides. Slow, kinetic sorption contributes further to P removal, while precipitation of phosphate minerals (strengite, hydroxyapatite) is quantitatively unimportant in the saturated zone. Nitrogen (N) dynamics are also considered, but nitrate behaves essentially as a conservative tracer in both systems. The model-predicted advancement of the P plume upon continued wastewater discharge at the calcareous site is in line with field observations. Model results suggest that, upon removal of the wastewater source, the P plume at both sites will persist for at least 20 years, owing to desorption of P from aquifer solids and the slow rate of P mineral precipitation. Sensitivity analyses for the noncalcareous scenario (Muskoka) illustrate the importance of the sorption capacity of the aquifer solids for P in modulating groundwater N:P ratios in oxic groundwater. The model simulations predict the breakthrough of groundwater with high P concentrations and low N:P ratios after 17 years at 20 m from the source for an aquifer with low sorption capacity (< 0.02% w/w Fe(OH)(3)). In this type of system, denitrification plays a minor role in lowering the NT ratios because it is limited by the availability of labile dissolved organic matter. (c) 2007 Elsevier B.V. All rights reserved.

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