HR: 1340h
AN: H23E-1473    [Abstracts]
TI: Influence of Initial Solute Concentration Distribution (Aging) Within a Diffusion Region on Desorption Rates and Solute Availability.
AU: Ball, W P
EM: bball@jhu.edu
AF: Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218 United States
AU: * Haws, N W
EM: nhaws@jhu.edu
AF: Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218 United States
AB: Low-permeability diffusion zones reduce the availability of a contaminant for remediation and can function as long-term sources of contamination. Understanding the mass transfer phenomenon from these regions is complicated in part by unknown solute distributions. When mass transfer models are calibrated to such systems, the fitted parameter values are often interpreted as solely reflecting soil properties and system heterogeneities and with assumptions of either (1) single representative average concentrations in the immobile zones (for kinetic rate models) or (2) the immobile zone geometry and initial contaminant distributions within the immobile zone (for diffusive models). However, because the actual geometry and initial solute concentration distribution influences the mass transfer dynamics of the contaminant, model calibrations will only provide "apparent" parameters that are applicable only for the specific case studied. For a selected batch system (believed to be representative of conditions in the Borden, Ontario aquifer) model simulations show that fitted first-order rate coefficients were insensitive to initial solute concentration distributions; whereas, the fraction of slower desorption sites increased with an initial solute distribution characteristic of an "aged" scenario. Additional simulations of solute elution from a hypothetical soil column containing the same media and assuming alternative conditions with respect to the initial state of phenanthrene sorption show that while aging serves to reduce the desorptive mass flux at the early stages of flushing, aged sites actually have a greater desorptive mass flux than "freshly" contaminated media at later stages. This occurs because concentration gradients within the diffusion zone of the "freshly" contaminated media drives solute both out of and further into the diffusion zone, thus more rapidly dissipating the desorptive mass flux. Because the initial solute concentration profile in the diffusion zones of real systems can rarely, if ever, be characterized, calibrated models should be regarded as case and time specific, and contaminated site management should use adaptive approaches.
DE: 1832 Groundwater transport
DE: 1899 General or miscellaneous
SC: Hydrology [H]
MN: Fall Meeting 2005