Effects of flow bypassing and nonuniform NAPL distribution on the mass transfer characteristics of NAPL dissolution

Abstract

Abstract. Ground water concentrations of nonaqueous phase liquid (NAPL) are usually less than their aqueous solubility; this can be attributed to irregular NAPL distribution, nonuniform flow patterns, dilution effects, and rate-limited mass transfer between the NAPL and aqueous phases. This paper uses two-domain and parallel column models to demonstrate the effects of nonuniform NAPL distribution and flow bypassing on apparent mass transfer kinetics for NAPL dissolution. The hypothesis of this study is that much of the apparent nonequilibrium dissolution can be explained by the effects of heterogeneity in NAPL distribution and in porous media properties. Models incorporating two-domain concepts to represent heterogeneity are able to reproduce the timescale dependency of mass transfer coefficients which has often been observed but is inconsistent with mass transfer theory. Parallel column models, with equilibrium partitioning assumed in each column, are able to reproduce the concentration drop-off and tailing observed in published column studies of NAPL dissolution. The parallel column model is also able to reproduce the experimentally observed phenomenon of mass transfer zones which lengthen with time and distance traveled. The results of this study support the hypothesis that much of the apparent nonequilibrium mass transfer kinetics of NAPL dissolution can be described by heterogeneity in NAPL distribution and in porous media properties.

Interphase mass transfer between fluids in subsurface formations: A review

“…The derived partial differential equations for mobile and immobile regions were solved using a Laplace transform approach. Soerens et al [216] also divided the porous media into two domains at the meso-scale namely: a NAPL region which represents flow paths that are in contact with NAPL and a clean region which represents flow paths that bypass the NAPL. Unlike the Hatfield model, Soerens et al [216] assumed flow through the NAPL region.…”

“…Soerens et al [216] also divided the porous media into two domains at the meso-scale namely: a NAPL region which represents flow paths that are in contact with NAPL and a clean region which represents flow paths that bypass the NAPL. Unlike the Hatfield model, Soerens et al [216] assumed flow through the NAPL region. The total effluent concentration was defined as the sum of the concentration of the NAPL region multiplied by the fraction of flow through that region and the concentration of the clean region multiplied by the fraction of flow through that region.…”

Computational simulation for the morphological evolution of nonaqueous phase liquid dissolution fronts in two-dimensional fluid-saturated porous media

“…Due to their low solubility and relatively high toxicity, NAPLs consisting of immiscible fluids and organic species cannot be adequately removed from the subsurface by means of traditional pump-and-treat methods [18,24]. To develop effective and efficient remediation techniques for NAPL-contaminated sites, a considerable amount of laboratory experiments have been conducted to understand the potential physical and chemical processes controlling the movement of the NAPL in fluidsaturated porous media.…”

Dependence of lumped mass transfer coefficient on scale and reactions kinetics for biologically enhanced NAPL dissolution

“…In order to understand the impacts and the risks of NAPL contamination to the environment and public in general, the dissolution behavior of NAPL residues and pools have been extensively studied [21]. Research efforts have also been focused on quantifying the dissolution processes [2,3,13,15,16,21,24,27,32,33,37,40,44,50] and possible remediation technologies [14,17,18,20,23,25,26,28,36,38,39,41,46,49].…”