Assessing the effect of grain-scale sorption rate limitations on the fate of hydrophobic organic groundwater pollutants
2012
Werner, David | Karapanagioti, Hrissi K. | Sabatini, David A.
Subsurface pollutant transport models accounting for sorption rate limitations are computationally more demanding than those assuming local sorption equilibrium. We combine batch and column tests with modeling for a comparative assessment of different sorption models. For the relatively hydrophobic compound naphthalene, a model assuming local sorption equilibrium was unable to reproduce breakthrough curves in column studies with Canadian River Alluvium sediment which contains carbonaceous particles. Fully calibrated independent forward predictions of a first-order kinetic and two diffusion kinetic sorption models were in much better agreement with the experimental data. Predictions using a diffusion-limited kinetic sorption model assuming concentration-independent sorption coefficients performed equally well as a model using the Freundlich isotherm. Both diffusion-based kinetic sorption models were superior to the first-order rate approach. In the present study, the validity of the local sorption equilibrium assumption is discussed based on a Damköhler number and thus, the compound's sorption properties, the aquifer properties, and the scale of the process. Relatively high groundwater velocities in combination with a low sorption coefficient Kd and slow diffusion limited sorption kinetic rates are necessary conditions to justify the implementation of grain-scale sorption rate limitations in groundwater contaminant fate models. Such conditions exist when a low amount of carbonaceous particles is present in aquifers with high permeability.
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