Modeling of multicomponent transport in groundwater and its application to chromium system

Date of Completion

January 1996

Keywords

Hydrology|Engineering, Civil|Engineering, Environmental

Degree

Ph.D.

Abstract

The distribution of chromium in groundwater are determined by two processes: solute transport and chemical reactions. The transport of aqueous species is governed by a set of partial differential equations (PDEs), and the speciation of chemical species is governed by a set of nonlinear algebraic equations (AEs).^ An efficient 1-D multicomponent transport model was developed using the direct substitution approach (DSA). The model incorporates aqueous complexation, surface complexation and precipitation/dissolution. PDEs governing transport processes were discretized and solved together with non-linear AEs for chemical reactions using the Newton-Raphson method. The Jacobian matrix was obtained analytically. The DSA model is about 100% more efficient than a model based on the sequential substitution approach (SIA) commonly used in practice. A 3-D multicomponent transport model was developed based on the 1-D model. The slow process of chromium migration into the soil matrix was modeled by a first order mass transfer process.^ Groundwater flow is the driving force for chemical transport. A 3-D groundwater flow model was developed based on a multigrid finite difference method. The model is capable of simulating domains with irregular boundaries as well as fluctuating groundwater table, i.e., the cells are allowed to dry out and to be re-watered. The model saves about 30% in CPU time compared with MODFLOW.^ Chromium contamination found at the National Chromium Inc. site, Putnam, Connecticut, was modeled. The groundwater flow at the site was calibrated, and simulated water table agreed well with the observed well water elevations. The chromium adsorption data from the isotherm studies were also modeled. Information from both isotherm study and sequential extraction study was used to simulate the site. A calibration framework was developed to assess heavy metal contamination sites of this kind where long term chromium contamination occurs. The hindcast technique was used for calibration, where most weight was given to chromium data on soil collected at the two multilevel nests since it represents the accumulative effect of chromium contamination over time. The simulated soil chromium concentrations were with the range of the observed chromium data at the two multilevel nests. A pump-and-treat remediation alternative was recommended. ^

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