Rate‐limited mass transfer and transport of organic solutes in porous media that contain immobile immiscible organic liquid

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Abstract
A general model is presented that simulates the effect of an immobile immiscible organic liquid, as well as rate‐limited solid phase sorption, on the transport of dissolved organic contaminants in heterogeneous porous media. Specific models are developed for four cases (1) immobile immiscible liquid that resides in nonadvective domains of a heterogeneous porous medium; (2) immobile immiscible liquid that resides in the advective domain; (3) homogeneous porous media wherein mass transfer within the immiscible liquid, as well as across the liquid‐water interface, is rate limited; (4) homogeneous porous media with a single resistance to liquid‐liquid mass transfer. The performance of the homogeneous‐based, single‐resistance model was evaluated by attempting to predict a break‐through curve reported in the literature. Based on the successful prediction of the data, where values for all parameters were obtained independent of the data being simulated, it appears that the conceptual basis of the model is valid. Through a series of simulations, the effect of the immiscible liquid on retention was shown to be great, even at relatively low levels of saturation. Release of solute from residual saturation located within regions of relatively low hydraulic conductivity can be significantly rate limited, even when mass transfer between immiscible liquid and water is rapid. This has ramifications concerning the efficacy of remediation systems based on imposed hydraulic gradients (e.g., pump‐and‐treat). The model should prove useful in further investigations of the effect of immobile immiscible organic liquids on the retention and transport of organic solutes in porous media.