Diffusive Transport of Volatile Pollutants in Nonaqueous (original) (raw)
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Water Resources Research, 1997
Modeling the diffusive transport of volatile organic contaminants (VOCs) has been previously described by lumping together vastly different soil regions (e.g., interaggregate and intra-aggregate regions) and assuming local equilibrium with linear contaminant phase distributions. This approach has sometimes failed to adequately describe diffusive transport. In this work, a diffusive transport model was developed that separately considered diffusion in the intra-aggregate and interaggregate regions and utilized nonlinear contaminant distributions among the phases. Input parameters were determined from independent sources, calculations, and measurements. No adjustments were made to input parameters. The model was compared to breakthrough and desorption data for two VOCs (toluene and trichloroethylene) on a silt loam soil at moistures from 1.6 to 14%. Breakthrough predictions were significantly better than those from a commonly used model. Desorption predictions were excellent over the first 3 orders of magnitude in contaminant flux after which they deviated by a factor of-3. Background Chlorinated aliphatic hydrocarbons (CAHs) and benzene, toluene, and the xylene isomers (BTX) are frequently encountered groundwater contaminants, originating from leakage of petroleum products and degreasing solvents from underground storage tanks and transfer pipes, accidental spills, and improper waste disposal. Both CAHs and BTX have significant vapor pressures and aqueous solubilities, allowing them to be transported through any fluid phase of the soil system. Diffusive volatile organic contaminant (VOC) transport in the vadose zone can be significantly greater than advective transport under certain conditions and is often rate limiting in contaminant removal efforts. Advection of contaminants in the vadose zone occurs in response to external influences, such as vapor extraction systems, displacement due to water infiltration, significant changes in barometric pressure, and gaseous density differences. These can be intermittent, whereas diffusion occurs continuously according to localized or extensive concentration gradients. Diffusive transport in the vadose zone can be dominant when no active treatment or infiltration is occurring, for example, in an open, unconfined landfill area [Karimi et al., 1987; Lin et al., 1996]. The gas-filled volume fraction of a soil has a significant effect on contaminant diffusion. The effective diffusion coefficient Def t for benzene diffusing through moist soils has been observed to vary with the gas-filled volume fraction of the soil [Karimi et al., 1987].Johnson and Perrott [1991] noted that Def t for methane diffusion through a nearly water-saturated soil (-90% of voids filled with water) was 30 times greater than if liquid phase diffusion were the only transport mechanism. Sorption to hydrated soil surfaces from the fluid phases strongly affects contaminant diffusion. Sorption to these hy-•Now at Merck and Co., Rahway, New Jersey.
Journal of Hydrology, 2003
Predicting the behavior of volatile organic compounds in soils or sediments is necessary for managing their use and designing appropriate remedial systems to eliminate potential threats to the environment, particularly the air and groundwater resources. In this effort, based on continuity of mass flux, we derive a mass flux boundary condition of the third type in terms of physically based mass transfer rate coefficients, describing the resistance to mass inflow of the soil-air interface, and obtain onedimensional analytical solutions for transport and degradation of volatile organic compounds in semi-infinite structured soils under steady, unsaturated flow conditions. The advective-dispersive mass balance formulation allows for mobile-immobile liquid phase and vapor diffusive mass transfer, with linear equilibrium adsorption and liquid-vapor phase partitioning in the dynamic and stagnant soil regions. The mass transfer rate coefficients of volatile organic chemicals across the soil-air interface are expressed in terms of solute properties and hydrodynamic characteristics of resistive soil and air-boundary layers. The solutions estimate solute vapor flux from soil surface and describe mobile-phase solute concentration as a function of depth in the soil and time. In particular, solutions were derived for: (1) zero-initial concentration in the soil profile subject to a continuous and pulsed source at the soil surface; and (2) depletion from the soil following an initially contaminated soil profile. Sensitivity analysis with respect to different dimensionless parameters is conducted and the effect on solute concentration and vapor flux of such parameters as volatilization mass transfer velocity relative to infiltration, soil Peclet number, biochemical decay, and diffusive mass transfer into the immobile phase, is plotted and the results are discussed. The mass transfer rate coefficients and the analytical solutions are applied to simulate transport of an example volatile organic compound in an aggregated soil. The simulated results indicate that macropore-aggregate vapor phase diffusion may profoundly impact transport of volatile compounds in aggregated soils.
Diffusion of Sorbing Organic Chemicals in the Liquid and Gaseous Phases of Repacked Soil
Soil Science Society of America Journal, 2001
sorption onto dry soil mineral surfaces are limited to very few different chemicals and in most cases only TCE Transport models for sorbing organic chemicals in soil require (e.g., Petersen et al., 1994, 1995, 1996a; Ong and Lion, accurate predictions of the diffusion and sorption processes in both the liquid and gaseous phases. In this study, the ability of recently-1991a,b; Shimuzu et al., 1992, 1994). Consequently, sevpresented diffusivity models in combination with equilibrium sorption eral predictive expressions to estimate hydrophobic admodels to predict the effective (i.e., including sorption effects) diffusorption coefficients from soil and chemical characterission coefficient, D eff , as a function of soil-water content, , is tested tics are available (e.g., Briggs, 1973; Karickhoff, 1981; for different sorbing organic chemicals in different soils. The water-Abdul et al., 1987), while it remains to be verified that induced linear reduction (WLR) gas diffusivity model, combined with recent predictive expressions for vapor sorption of TCE a two-component (hydrophobic and vapor) equilibrium sorption
Concentration-dependent kinetics of pollutant desorption from soils
Environmental Toxicology and Chemistry, 2002
Sorption-desorption kinetics play a major role in transport and bioavailability of pollutants in soils. Contaminant concentration is a potentially important factor controlling kinetics. A previous paper dealt with the effect of solute concentration on fractional uptake rates of phenanthrene and pyrene from a finite aqueous source. In this study we determined the effect of initial phenanthrene sorbed concentration (q 0 ) on the fractional mass desorption rates from each of six soils to a zero-concentration solution, approximated by including a polymer adsorbent (Tenax) as a third-phase sink. The soils were preequilibrated with phenanthrene for 180 d. Consistent with theory, the fractional desorption rates determined by empirical curve fitting increased with q 0 provided the isotherm was nonlinear. After 500 to 600 d of desorption at the steepest possible concentration gradient, all soils retained a highly resistant fraction, which ranged from 4 to 31% of q 0 , except for one soil at a high q 0 . The highly resistant fraction decreased with increasing q 0 for nonlinear isotherm cases, but increased with q 0 for linear or nearly linear isotherm cases. Application of a nonlinear diffusion model, the dual-mode diffusion model (DMDM), to the nonresistant fraction gave reasonably good fits. The DMDM attributes the increase with concentration of the apparent diffusivity to a decrease in the proportion of sorbate occupying immobile sites (holes) in soil organic matter. The concentration-dependent term in the expression for the apparent diffusivity correlated with either of two indices that reflect the linearity of the sorption isotherm. Bunker C oil present in one soil acted as a partition domain. The findings of this study are consistent with heterogeneous models of soil organic matter, and indicate that concentration effects should be taken into account whenever desorption rate is important.
Ecological Modelling, 2005
In this paper, lab tests coupled to a semi-pilot test section are used to derive data for the calibration of a numerical model. The paper is aimed at proposing a set of experiments, which can be used to calibrate a numerical model before using it on defined soils. The complexity of the phenomenon of transport of reactive pollutants in soil has to be faced in the most complete way. The different behaviour of soil after wet/dry cycles with respect to the fluidodynamic characteristics and the importance to consider the local biomass behaviour in case of organic contaminant has been underlined. An optimal approach has to take into account all the different components and here a simple series of experimental procedure is presented. The sensitivity analysis of the numerical model has shown that its results are not so much dependent on the classical numerical aspects (time or space increments) but mainly on a set of parameters related to soil structure which must then be derived through a good calibration. (P. Viotti). produced contamination of several aquifers all over the world. Contaminants dispersed at the land surface migrate through the vadose zone before being advected and dispersed by flowing groundwater. Solute transport in unsaturated soils has been the topic of experimental, theoretical research and studies above numerical simulation aspects . The behaviour and fate of chemicals in the vadose zone is complex , being a process occurring in conditions strongly influenced 0304-3800/$ -see front matter
Volatile Organic Vapor Diffusion and Adsorption in Soils
Journal of Environment Quality, 1994
Knowledge of the relationship between D~/D0 (ditfnsion coefficient in soil divided by diffusion coefficient in free air) and the volumetric soil-air content, e, is important when modeling gaseous movement of volatile organic compounds (VOCs) in soils. The effective dflfnsion (i.e., diffusion and retardation) of trichloroethylene (TCE), toluene and freon in Yolo silt loam (fine-silty, mixed, nonacid, thermic Typic Xerorthent) were measured in a two-chamber diffusion apparatus. The experiments were conducted on packed soil cores over a range of water contents. Vapor retardation factors were calculated from soil parameters and equilibrium partition coefficients. Partition coeificients were measured in batch experiments. It was found that for water contents higher than four molecular layers of water surface coverage, solid/vapor partition coefficients, KD', were consistent with values predicted by Henry's Law constants (KH), and aqueous/solid partition coefficients, Kv. For less than four molecular layers of water, sorption increased by orders of magnitude. The vapor retardation factors, along with the measured effective diffusion, allowed a calculation of diffusion coefficients (De) for the investigated species by using the analytical solution to diffusion in a two-chamber apparatus. Values of the ratio De/Do were generally higher than the values predicted by the Millington-Quirk equation, and lower than the values predicted by the Penman equation. Compared with the nonreactive tracer freon, De/Do values for TCE and toluene agreed very well for higher water contents. Values obtained for air -dry soil, however, were under-predicted. The experimental work for determination of the effective diffusion of reactive tracers can, therefore, for sufficiently high water contents be limited to the determination of Dp/D0-e relations for a nonreactive tracer and measurement of KD, KD ~ and Ks values for the reactive tracers.
Experimental and Mathematical Investigation of Time-Dependence of Contaminant Dispersivity in Soil
Advances in Environmental Technology (AET), 2018
Laboratory and field experiments have shown that dispersivity is one of the key parameters in contaminant transport in porous media and varies with elapsed time. This time-dependence can be shown using a time-variable dispersivity function. The advantage of this function as opposed to constant dispersivity is that it has at least two coefficients that increase the accuracy of the dispersivity prediction. In this study, longitudinal dispersivity values were obtained for the conservative NaCl solute transport in a laboratory porous medium saturated with tap water. The results showed that the longitudinal dispersivity initially increased with time (pre-asymptotic stage) and eventually reached a constant value (asymptotic stage). Four functions were used to investigate the time variations of dispersivity: linear, power, exponential and logarithmic. In general, because of the linear increase of dispersivity during a long time of transport, the linear function with R2=0.97 showed better time variations than the other three functions; the logarithmic function, having an asymptotic nature, predicted the asymptotic stage successfully (R2=0.95). The ratio of the longitudinal dispersivity to the medium length was not constant during the transport process and varied from 0.01 to 0.05 cm with elapsed time.
Transfer of Chemicals from Soil solution to Surface Runoff: A Diffusion‐based Soil Model
Soil Science Society of America Journal, 1988
A physically‐based diffusion and transport model is developed to describe chemical outflow concentrations during chemical removal from soil to overlying runoff water induced by continuous rainfall over the soil surface. In contrast to earlier models, movement from the soil to the runoff water is described as a liquid diffusion process to the surface, coupled to the runoff zone through a laminar boundary layer at the runoff interface with the soil surface. Within the soil, diffusion is moderated by equilibrium adsorption to solid surfaces characterized by a partition coefficient. The runoff concentration at the outlet is derived by treating the runoff zone as a wellmixed reactor, characterized by a residence time. The model was used to predict the results obtained in the experimental study of L.R. Ahuja and O.R. Lehman (1983) where infiltration was suppressed, with good agreement obtained between predicted and measured outflow concentrations when the model parameters were estimated i...
Computational Modelling of Movement of Water Soluble Pollutants Through Soil
African Journal of Applied Research (AJAR), 2016
Most heuristic studies of hydrogeology connected with the study of water flow pattern in the ground that uses the knowledge and continuous prediction of soil permeability changes and soil porosity have been very costly; yet, no concrete comprehensive concept and results have been realized. The need for cost effectiveness in the prediction of soluble pollutants' mobility and behavior in the soil is imperative in this computational modeling era where complex problems are numerically analyzed and simulated. This paper focuses on two dimensional modelling of water soluble pollutants through soil using computational fluid dynamics (CFD) approach and adapted Navier-Stokes equations for porous flow. The model is used to simulate flow of water soluble pollutants in the soil within the laminar flow regime and to examine the dispersion of water soluble pollutants through soil layers at various Reynolds numbers. A code was developed and used to simulate the model and simulated results validated qualitatively against experimental results. The dispersion pattern of the dye used was then physically examined at various times and the results compared. It was found that all the flow patterns of the experiments were comparable to the simulated results. The lessons learned from this study, recommendations and the potential contributions to future models in pollutant mobility and dispersion in the soil and groundwater are discussed herein.