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WATER RESOURCES RESEARCH, VOL. 43, W07440, doi:10.1029/2005WR004841, 2007

Modeling the movement of a pH perturbation and its impact on adsorbed zinc and phosphate in a wastewater-contaminated aquifer

D. B. Kent

U.S. Geological Survey, Menlo Park, California, USA


J. A. Wilkie

ENVIRON International Corporation, Chicago, Illinois, USA


J. A. Davis

U.S. Geological Survey, Menlo Park, California, USA


Abstract

Chemical conditions were perturbed in an aquifer with an ambient pH of 5.9 and wastewater-derived adsorbed zinc (Zn) and phosphate (P) contamination by injecting a pulse of amended groundwater. The injected groundwater had low concentrations of dissolved Zn and P, a pH value of 4.5 resulting from equilibration with carbon dioxide gas, and added potassium bromide (KBr). Downgradient of the injection, breakthrough of nonreactive Br and total dissolved carbonate concentrations in excess of ambient values (excess TCO2) were accompanied by a decrease in pH values and over twentyfold increases in dissolved Zn concentrations above preinjection values. Peak concentrations of Br and excess TCO2 were followed by slow increases in pH values accompanied by significant increases in dissolved P above preinjection concentrations. The injected tracers mobilized a significant mass of wastewater-derived Zn. Reactive transport simulations incorporating surface complexation models for adsorption of Zn, P, hydrogen ions, and major cations onto the aquifer sediments, calibrated using laboratory experimental data, captured most of the important trends observed during the experiment. These include increases in Zn concentrations in response to the pH perturbation, perturbations in major cation concentrations, attenuation of the pH perturbation with transport distance, and increases in alkalinity with transport distance. Observed desorption of P in response to chemical perturbations was not predicted, possibly because of a disparity between the range of chemical conditions in the calibration data set and those encountered during the field experiment. Zinc and P desorbed rapidly in response to changing chemical conditions despite decades of contact with the sediments. Surface complexation models with relatively few parameters in the form of logK values and site concentrations show considerable promise for describing the influence of variable chemistry on the transport of adsorbing contaminants.

Received 27 December 2005; accepted 19 February 2007; published 24 July 2007.

Keywords: sorption; mobility; metal ion.

Index Terms: 1832 Hydrology: Groundwater transport; 1847 Hydrology: Modeling; 1009 Geochemistry: Geochemical modeling (3610, 8410); 0412 Biogeosciences: Biogeochemical kinetics and reaction modeling (0414, 0793, 1615, 4805, 4912).

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Citation: Kent, D. B., J. A. Wilkie, and J. A. Davis (2007), Modeling the movement of a pH perturbation and its impact on adsorbed zinc and phosphate in a wastewater-contaminated aquifer, Water Resour. Res., 43, W07440, doi:10.1029/2005WR004841.