Thomas S. Soerens

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

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.

Cosolvency Effects on Sorption of a Semipolar, Ionogenic Compound (Rhodamine WT) with Subsurface Materials.

The sorption of rhodamine WT (RWT), a semipolar ionizable fluorescent dye, from methanoljnvater and acetone/water mixtures was investigated using batch and column studies with two natural subsurface media. The utilities of cosolvency, solubility, and sorption theories were evaluated. The effect of cosolvents on sorption of RWT showed significant deviations from the log-linear cosolvency model. The observed cosolvency effects on sorption are qualitatively consistent with that expected for semipolar solutes or for organic acids. However, the quantitative application of either the semipolar cosolvency model or the cosolvency model with speciation could not adequately describe the observed behavior. The magnitude of the cosolvency effect differed for the two subsurface media

Effect of dynamic competitive sorption on the transport of volatile organic chemicals through dry porous media

Dynamic (time-varying) competitive sorption, modulated through fluctuating relative humidity, is shown to significantly affect the diffusive flux of 1,2,4-trichlorobenzene through soil columns. The flux varied by a factor of up to 2 during a 24 hour cycle in the experimental system when subjected to humidity fluctuations of ±40%. This effect was observed despite conditions in which sufficient water coverage would have been present to significantly depress volatile organic chemical (VOC sorption). Many vadose zone chemical transport models do not incorporate vapor-solid sorption, which is known to be important in low-moisture soils. The models that do incorporate vapor sorption do not account for dynamic variations of the partitioning behavior associated with variable moisture content. We present and validate a mathematical model for VOC transport through soils that accounts for dynamic nonlinear competitive sorption on volatile species transport. Simulations are presented which show that the effect of a thin dry zone at the soil-air interface can result in large variations in the instantaneous chemical release rate. This implies that for some conditions in the field, VOC sampling should be conducted over longer periods to avoid bias due to short-term spikes in the chemical release rate.

Fate of Non-Aqueous Phase Liquids: Modeling of Surfactant Effects

When a sufficient volume of an organic liquid is released into the environment, it may persist in the environment in the form of a residual saturation in either the unsaturated or the saturated zones of the subsurface. In the case of organic liquids less dense than water (referred to as LNAPLs, Light Non-Aqueous Phase Liquids), if the spill is of sufficient volume to reach the water table (Figure 1), some fraction of the liquid will remain as a residual saturation in the pores of the soil in the unsaturated zone. The liquid that reaches the water table will form a liquid lens, with some depression of the water table. As the volume of the LNAPL lens decreases, the water table may return to its original height, resulting in the formation of a residual saturation of the LNAPL in the aquifer medium (Weber, et al., 1991; Johnson, et al., 1989). If the liquid is a Dense Non-Aqueous Phase Liquid (DNAPL)—a chlorinated hydrocarbon solvent such as trichloroethylene is an example—its fate in the soil above the water table is essentially the same as that of the LNAPL. Upon contact with the ground water, however, the chlorocarbon, being up to 60% more dense than water, can continue to move down through the aquifer, displacing the ground water (Figure 2). Within the aquifer, the chlorocarbon may also become trapped in pores of the aquifer medium by capillary forces. If the size of the spill is sufficient, pools of the chlorocarbon may collect in the aquifer on an impermeable layer. Though the chlorocarbon is immiscible with the ground water, it has sufficient solubility to contaminate the ground water to an unacceptable level. Chlorocarbon dissolved in the ground water will not only move away from the site of the residual saturation, but will also partition between the ground water and the solid phase of the aquifer as it moves away from the spill (Huling and Weaver, 1991). A residual saturation is particularly persistent in the environment because only a limited interfacial area may be exposed to the ground water; this has the effect of reducing the overall mass transfer rate away, from the site of the residual saturation which in turn increases the persistence of the contamination. In case of a residual saturation in a ground water environment, traditional pump-and-treat methods may require that many hundreds of pore volumes of water be circulated through the contaminated region before the residual saturation has been removed. This situation has led to an interest in the use of surfactants for enhancing the rate of removal of DNAPLs from a subsurface environment (Palmer and Fish, 1992).