Modeling the influence of variable pH on the transport of zinc in a contaminated aquifer using semiempirical surface complexation models

Abstract

Abstract. Land disposal of sewage effluent resulted in contamination of a sand and gravel aquifer (Cape Cod, Massachusetts) with zinc (Zn). The distribution of Zn was controlled by pH-dependent adsorption; the Zn extended 15 rn into the 30-m-thick sewage plume within approximately 100 rn of the source but only 2-4 rn into the plume between 100 and 400 rn downgradient. A two-dimensional vertical cross section model coupling groundwater flow with solute transport and equilibrium adsorption is 'used to simulate the influence of pH on Zn transport. Adsorption is described using semiempirical surface complexation models (SCM) by writing chemical reactions between dissolved Zn and mineral surface sites. SCM parameters were determined in independent laboratory experiments. A 59-year simulation with a one-site SCM describes the influence of pH on Zn transport well, with greater mobility at the low pH values near the upper sewage plume boundary than at the higher pH values deeper in the sewage-contaminated zone. Simulation with a two-site SCM describes both the sharpness and approximate location of the leading edge of the Zn-contaminated region. Temporal variations in pH of incoming groundwater can result in large increases in Zn concentration and mobility. The influence of spatial and temporal variability in pH on adsorption and transport of Zn was accomplished much more easily with the semiempirical SCM approach than could be achieved with distribution coefficients or adsorption isotherms.

The influence of groundwater chemistry on arsenic concentrations and speciation in a quartz sand and gravel aquifer

“…This narrow range of values is consistent with the hypothesis that the As leached by the reductive extractions was primarily associated with coatings on sediment grains, which are ubiquitous and relatively uniformly distributed. 15,29 At the average solid/liquid ratio in the aquifer of 4145 g/l, 59 an As concentration in the coatings of 1 nmol/g corresponds to a concentration of 4.1 M. Concentrations of As͑V͒ observed in the suboxic zone and released from the sediments in the uncontaminated zone during the tracer test were 0.03-0.08 M, which corresponds to 0.7%-2% of the total As in the coatings. Some of the As leached by the reductive extractions may be occluded in Fe oxides or in some other form that is not readily desorbable.…”

“…58 The total concentration of adsorption sites was set equal to 4.77 mM ͑millimoles per liter͒, the estimated total concentration of adsorption sites in the aquifer. 59 For the purposes of these computations, the total As and total phosphate concentrations were set equal to 0.6 M and 3.1 mM, respectively, which yield a concentration of As͑V͒ equal to 0.06 M and dissolved phosphate concentration equal to 50 M at pH 6.1. These are typical values for the suboxic zone of the sewage plume ͑cf., Fig.…”

Comparing Approaches for Simulating the Reactive Transport of U(VI) in Ground Water

“…tailing) caused by the nonlinear equilibrium adsorption isotherms (Bethke and Brady 2000;Glynn 2003;Kohler et al 1996;Zhu 2003). The semi-mechanistic surface complexation approach provides an efficient alternative approach for simulating sorption onto complex soils and sediments while simultaneously considering variable chemical conditions in the ground water (Davis et al 1998Kent et al 2000Kent et al , 2007Kent et al , 2008. In this approach, the adsorbing surface is considered to possess surface functional groups that can form surface complexes analogous to the formation of aqueous complexes in solution.…”

Mathematical Formulation Requirements and Specifications for the Process Models

“…Postulating multiple site-types is also important for simulating peak tailing observed in experimental studies of U(VI) transport in columns (Kohler et al, 1996). Reactive transport simulations that use multisite adsorption models can also simulate significant peak tailing in field-scale simulations (Curtis et al, 2006;Kent et al, 2000Kent et al, , 2008Kent et al, , 2007.…”

“…In the GC approach, it is assumed that sorption can be described by mass laws written with "generic" surface functional groups, with the stoichiometry and formation constants for each mass law determined by fitting experimental data for the mineral assemblage as a whole (Bond et al, 2008;Davis et al, 2004;Hyun et al, 2009). The GC modeling approach has generally been applied using a non-electrostatic model (NEM), which considers surface equilibria strictly as chemical reactions without explicit correction for electrostatic attraction or repulsion Kent et al, 2000;Yabusaki et al, 2008). In an NEM, the apparent binding constants and stoichiometry of the mass action equations are derived by fitting the macroscopic dependence of adsorption as a function of pH (Davis et al, 1998).…”