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Comparison of Models for Describing the Transport of Dissolved Organic Carbon in Aquifer Columns

Environmental Sciences Division, Oak Ridge National Lab., P.O. Box 2008, Oak Ridge, TN 37831-6038

H. M. Selim

Louisiana State Univ., Baton Rouge, LA 70803

* Corresponding author.

ABSTRACT

Dissolved organic carbon (DOC) is a reactive constituent in aquifer and soil media and equilibrates between the mobile aqueous phase and the immobile solid phase. Since DOC is believed to accelerate the transport of associated contaminants, yet is known to interact with aquifer and soil material, our objective was to define and model those processes governing DOC adsorption to porous media that may affect the cotransport of contaminants. Column displacement experiments of DOC through aquifer sediments were modeled with various formulations of the convective-dispersive equation, which considered time-dependent adsorption reactions and linear or nonlinear adsorption processes. Batch equilibrium adsorption isotherm and kinetic studies for DOC interaction with the sediments were used to parameterize the transport models. The equilibrium DOC-adsorption isotherm was nonlinear and was adequately described by the Langmuir equation. The adsorption of DOC to the aquifer sediment was also time dependent and the batch results could be described by two simultaneous reaction rates for solution concentrations 10 mg DOC L^–1 and a single reaction rate for solution concentrations <10 mg DOC L^–1. Observed DOC breakthrough curves (BTCs) with influent concentrations 10 mg DOC L^–1, were adequately modeled as two-site, non-linear adsorption processes, with DOC interactions with both types of sites being time dependent. Batch adsorption and kinetic parameters were generally successful in describing DOC transport; however, the magnitude of the initial batch rate coefficient was significantly larger than that observed for the displacement experiments. The extended tailing of the observed DOC BTCs was influenced more by the slow, time-dependent adsorption of DOC during transport than to the non-linear features of the adsorption isotherms. Observed DOC BTCs with influent concentrations <10 mg DOC L^–1 did not exhibit extensive tailing and were modeled as a one-site, linear or nonlinear, time-dependent adsorption process.

Joint contribution from Oak Ridge National Lab. and Louisiana State Univ. F.M. Dunnivant was supported by an appointment to the U.S. Department of Energy Laboratory Cooperative Postgraduate Research Training Program administered by Oak Ridge Associated Universities. This research was funded by the Subsurface Science Program of the Ecological Research Division, Office of Health and Environmental Research, U.S. Dep. of Energy under Contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc. Publication number 3831.

Received for publication September 26, 1990.

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