An analytical solution for vertical transport of volatile chemicals in the vadose zone

An analytical solution is presented for one-dimensional vertical transport of volatile chemicals through the vadose zone to groundwater. The solution accounts for the important transport mechanisms of the steady advection of water and gas, diffusion and dispersion in water and gas, as well as adsorption, and first-order degradation. By assuming a linear, equilibrium partitioning between water, gas and the adsorbed chemical phases, the dependent variable in the mathematical model becomes the total resident concentration. The general solution was derived for cases having a constant initial total concentration over a discrete depth interval and a zero initial total concentration elsewhere. A zero concentration gradient is assumed at the groundwater table. Examples are given to demonstrate the application of the new solution for calculating the case of a non-uniform initial source concentration, and estimating the transport of chemicals to the groundwater and the atmosphere. The solution was also used to verify a numerical code called VLEACH. We discovered an error in VLEACH, and found that the new solution agreed very well with the numerical results by corrected VLEACH. A simplified solution to predict the migration of volatile organic chemical due to the gas density effect has shown that a high source concentration may lead to significant downward advective gas-phase transport in a soil with a high air-permeability.     

One-dimensional simulation of solute transfer in saturated–unsaturated porous media using the discontinuous finite elements method

A one-dimensional transport model for simulating water flow and solute transport in homogeneous–heterogeneous, saturated–unsaturated porous media is presented. The model is composed of a combination of accurate numerical algorithms for solving the nonlinear Richard's and advection–dispersion equations (ADE). The mixed form of Richard's equation is solved using a standard finite element method (FEM) with primary variable switching. The transport equation is solved using operator splitting, with the discontinuous finite element method (DFE) for discretization of the advective term. A slope limiting procedure for DFE avoids numerical instabilities but creates very limited numerical dispersion for high Peclet numbers. An implicit finite differences scheme (FD) is used for the dispersive term.The unsaturated flow and transport model (Wamos-T) is applied to a variety of rigorous problems including transient flow, heterogeneous medium and abrupt variations of velocity in magnitude and direction due to time-varying boundary conditions. It produces accurate and mass-conservative solutions for a very large range of grid Peclet numbers. The Wamos-T model is a good and robust alternative for the simulation of mass transport in unsaturated domain.     

Evaluating the sensitivity of a subsurface multicomponent reactive transport model with respect to transport and reaction parameters.

The input variables for a numerical model of reactive solute transport in groundwater include both transport parameters, such as hydraulic conductivity and infiltration, and reaction parameters that describe the important chemical and biological processes in the system. These parameters are subject to uncertainty due to measurement error and due to the spatial variability of properties in the subsurface environment. This paper compares the relative effects of uncertainty in the transport and reaction parameters on the results of a solute transport model. This question is addressed by comparing the magnitudes of the local sensitivity coefficients for transport and reaction parameters. General sensitivity equations are presented for transport parameters, reaction parameters, and the initial (background) concentrations in the problem domain. Parameter sensitivity coefficients are then calculated for an example problem in which uranium(VI) hydrolysis species are transported through a two-dimensional domain with a spatially variable pattern of surface complexation sites. In this example, the reaction model includes equilibrium speciation reactions and mass transfer-limited non-electrostatic surface complexation reactions. The set of parameters to which the model is most sensitive includes the initial concentration of one of the surface sites, the formation constant (Kf) of one of the surface complexes and the hydraulic conductivity within the reactive zone. For this example problem, the sensitivity analysis demonstrates that transport and reaction parameters are equally important in terms of how their variability affects the model results.     

One-dimensional simulation of solute transfer in saturated-unsaturated porous media using the discontinuous finite elements method

A one-dimensional transport model for simulating water flow and solute transport in homogeneous-heterogeneous, saturated-unsaturated porous media is presented. The model is composed of a combination of accurate numerical algorithms for solving the nonlinear Richard's and advection-dispersion equations (ADE). The mixed form of Richard's equation is solved using a standard finite element method (FEM) with primary variable switching. The transport equation is solved using operator splitting, with the discontinuous finite element method (DFE) for discretization of the advective term. A slope limiting procedure for DFE avoids numerical instabilities but creates very limited numerical dispersion for high Peclet numbers. An implicit finite differences scheme (FD) is used for the dispersive term. The unsaturated flow and transport model (Wamos-T) is applied to a variety of rigorous problems including transient flow, heterogeneous medium and abrupt variations of velocity in magnitude and direction due to time-varying boundary conditions. It produces accurate and mass-conservative solutions for a very large range of grid Peclet numbers. The Wamos-T model is a good and robust alternative for the simulation of mass transport in unsaturated domain.     

Oxygenated gasoline release in the unsaturated zone, Part 2: Downgradient transport of ethanol and hydrocarbons

In the event of a gasoline spill containing oxygenated compounds such as ethanol and MTBE, it is important to consider the impacts these compounds might have on subsurface contamination. One of the main concerns commonly associated with ethanol is that it might decrease the biodegradation of aromatic hydrocarbon compounds, leading to an increase in the hydrocarbon dissolved plume lengths. The first part of this study (Part 1) showed that when gasoline containing ethanol infiltrates the unsaturated zone, ethanol is likely to partition to and be retained in the unsaturated zone pore water. In this study (Part 2), a controlled field test is combined with a two-dimensional laboratory test and three-dimensional numerical modelling to investigate how ethanol retention in the unsaturated zone affects the downgradient behaviour of ethanol and aromatic hydrocarbon compounds. Ethanol transport downgradient was extremely limited. The appearance of ethanol in downgradient wells was delayed and the concentrations were lower than would be expected based on equilibrium dissolution. Oscillations in the water table resulted in minor flushing of ethanol, but its effect could still be perceived as an increase in the groundwater concentrations downgradient from the source zone. Ethanol partitioning to the unsaturated zone pore water reduced its mass fraction within the NAPL thus reducing its anticipated impact on the fate of the hydrocarbon compounds. A conceptual numerical simulation indicated that the potential ethanol-induced increase in benzene plume length after 20 years could decrease from 136% to 40% when ethanol retention in the unsaturated zone is considered.     

Design and laboratory testing of a chamber device to measure total flux of volatile organic compounds from the unsaturated zone under natural conditions

To determine if an aquifer contaminated with volatile organic compounds (VOCs) has potential for natural remediation, all natural processes affecting the fate and transport of VOCs in the subsurface must be identified and quantified. This research addresses the quantification of air-phase volatile organic compounds (VOCs) leaving the unsaturated zone soil gas and entering the atmosphere-including the additional flux provided by advective soil-gas movement induced by barometric pumping. A simple and easy-to-use device for measuring VOC flux under natural conditions is presented. The vertical flux chamber (VFC) was designed using numerical simulations and evaluated in the laboratory. Mass-balance numerical simulations based on continuously stirred tank reactor equations (CSTR) provided information on flux measurement performance of several sampling configurations with the final chamber configuration measuring greater than 96% of model-simulated fluxes. A laboratory device was constructed to evaluate the flux chamber under both diffusion-only and advection-plus-diffusion transport conditions. The flux chamber measured an average of 82% of 15 diffusion-only fluxes and an average of 95% of 15 additional advection-plus-diffusion flux experiments. The vertical flux chamber has the capability of providing reliable measurement of VOC flux from the unsaturated zone under both diffusion and advection transport conditions.     

Modeling colloid-facilitated transport of multi-species contaminants in unsaturated porous media

Colloid-facilitated transport has been recognized as a potentially important and overlooked contaminant transport process. In particular, it has been observed that conventional two phase sorption models are often unable to explain transport of highly sorbing compounds in the subsurface appropriately in the presence of colloids. In this study a one-dimensional model for colloid-facilitated transport of chemicals in unsaturated porous media is developed. The model has parts for simulating coupled flow, and colloid transport and dissolved and colloidal contaminant transport. Richards' equation is solved to model unsaturated flow, and the effect of colloid entrapment and release on porosity and hydraulic conductivity of the porous media is incorporated into the model. Both random sequential adsorption and Langmuir approaches have been implemented in the model in order to incorporate the effect of surface jamming. The concept of entrapment of colloids into the air-water interface is used for taking into account the effect of retardation caused due to existence of the air phase. A non-equilibrium sorption approach with options of linear and Langmuir sorption assumptions are implemented that can represent the competition and site saturation effects on sorption of multiple compounds both to the solid matrix and to the colloidal particles. Several demonstration calculations are performed and the conditions in which the non-equilibrium model can be approximated by an equilibrium model are also studied.     

Preliminary 3-D site-scale studies of radioactive colloid transport in the unsaturated zone at Yucca Mountain,Nevada

The U.S. Department of Energy (DOE) is actively investigating the technical feasibility of permanent disposal of high-level nuclear waste in a repository to be situated in the unsaturated zone (UZ) at Yucca Mountain (YM), Nevada. In this study we investigate, by means of numerical simulation, the transport of radioactive colloids under ambient conditions from the potential repository horizon to the water table. The site hydrology and the effects of the spatial distribution of hydraulic and transport properties in the Yucca Mountain subsurface are considered. The study of migration and retardation of colloids accounts for the complex processes in the unsaturated zone of Yucca Mountain, and includes advection, diffusion, hydrodynamic dispersion, kinetic colloid filtration, colloid straining, and radioactive decay. The results of the study indicate that the most important factors affecting colloid transport are the subsurface geology and site hydrology, i.e., the presence of faults (they dominate and control transport), fractures (the main migration pathways), and the relative distribution of zeolitic and vitric tuffs. The transport of colloids is strongly influenced by their size (as it affects diffusion into the matrix, straining at hydrogeologic unit interfaces, and transport velocity) and by the parameters of the kinetic-filtration model used for the simulations. Arrival times at the water table decrease with an increasing colloid size because of smaller diffusion, increased straining, and higher transport velocities. The importance of diffusion as a retardation mechanism increases with a decreasing colloid size, but appears to be minimal in large colloids.     

Sorption of isoxaflutole diketonitrile degradate (DKN) and dicamba in unsaturated soil

When analyzing the sorption characteristics of weakly sorbing or labile pesticides, batch methods tend to yield a high margin of error attributable to errors in concentration measurement and to degradation, respectively. This study employs a recently developed unsaturated transient flow method to determine the sorption of isoxaflutole's herbicidally active diketonitrile degradate (DKN) and dicamba. A 20-cm acrylic column was packed with soils with varied texture that had been uniformly treated with 14C-labeled chemical.The antecedent solution herbicide in equilibrium with sorbed phase herbicide was displaced by herbicide-free water, which was infiltrated into the column. Sorption coefficients, Kd, were obtained from a plot of total herbicide concentration in the soil versus water content in the region where the antecedent solution accumulated. DKN Kd values were approximately 2-3 times (average Kd = 0.71 L kg-1) greater using the unsaturated transient flow method as compared to the batch equilibration method in clay loam (Kd = 0.33 L kg-1), but similar for the two methods in sand (0.12 vs 0.09 L kg-1) soils. Dicamba Kd values were 3 times greater using the unsaturated transient flow method as compared to the batch equilibration method in the clay loam soil (0.38 vs 0.13 L kg-1), however, the Kd values were the same for the two methods in the sand (approximately 0.06 L kg-1). This demonstrates that to determine sorption coefficients for labile hydrophilic pesticides, an unsaturated transient flow method may be a suitable alternative to the batch method. In fact, it may be better in cases where transport models have overpredicted herbicide leaching when batch sorption coefficients have been used.     

Abiotic behavior of entrapped petroleum products in the subsurface during leaching

Petroleum products are generally volatile hydrocarbon mixtures. These mixtures may contaminate land surfaces, the unsaturated zone and ground waters at numerous sites and thus represent a long-term source of environmental subsurface pollution. Based on laboratory and field evidence obtained by our research groups and others we emphasize in the present review paper the effect of leaching on the abiotic processes controlling the fate of volatile petroleum hydrocarbon mixtures (VPHMs) in the soil environment. The modification of petroleum hydrocarbon mixtures due to changes in the soil water content is considered the result of a “leaching phenomenon”. The experimental evidence of VPHM behavior in the porous media is linked to four major processes: retention, redistribution and attenuation in the subsurface, and dissolution in the soil water.

Once VPHMs reach the soil surface, their residual concentration and composition is influenced by the amount and quality of the leaching water. The transport and natural attenuation of the VPHMs in the unsaturated zone is affected by the amount and rate of leaching. Since VPHM are a mixture of volatile and non-volatile hydrocarbons whose components differ by several orders of magnitude in their vapor pressure and water solubility, their fates in the soil environment under leaching will be diverse too. This will influence the temporal concentrations of the VPHM components and their distributions with depth, as vapors, solutes, or water-immiscible solutions. Wetting the soil before or after the VPHMs reach it, differentially affects the abiotic processes governing petroleum products behavior into the porous media.     

A sensitivity analysis of adsorption and degradation parameters in the modeling of pesticide transport in soils

The increasing use of deterministic models in predicting the movement of pesticides in soils, has focused attention on the evaluation of major parameters which represent attenuation factors of organics in the subsurface. These parameters are the degradation rate constant and the adsorption constant for the pesticide. In view of the large in situ variability of these parameters and of the difficulty in obtaining accurate field data, there is a high degree of uncertainty associated with the results obtained from deterministic models. A sensitivity analysis is performed here to quantify the impact of such variation in each of these input parameters on the output results of an unsaturated zone transport model (PRZM). Results show that variations in these parameters about their respective mean values greatly affect the predicted concentration distributions, obtained after three years, of the pesticide aldicarb in all the soil profile. A 15–22% variation in the degradation constant, or a 24% variation in the adsorption constant, lead to a 100% uncertainty in the various simulation results defined as the cumulative quantity of aldicarb or the dissolved aldicarb concentration leached below the root zone (or the unsaturated zone) of the soil. Such a deterministic model presents a high degree of sensitivity to these input parameters. Accurate field data are then needed to obtain reliable model results in predicting pesticide movement inthe unsaturated zone.     

Influence of contaminated particles on the bioaccumulation of hydrophobic organic micropollutants in fish

The influence of contaminated particles on the bioconcentration of hydrophobic chemicals by fish is dependent on the hydrophobicity of the chemicals. This has been shown for polychlorinated benzenes and biphenyls (ranging over three orders of magnitude in octan-1-ol/water partition coefficient) which are sorbed on very low organic carbon content particles. For chemicals with low to moderate hydrophobicity, the amount of the chemical which is sorbed, relative to the amount which is dissolved, determines the influence that contaminated particles have on the uptake of the chemicals by fish. In this present experiment, for lower chlorinated benzenes and biphenyls, the amount dissolved in water are high compared with the amounts which are present in the sorbed state, and the influence of contaminated particles on their uptake by fish is negligible. For more hydrophobic chemicals, which have lower aqueous solubilities, such as penta and hexachlorobenzene, and tri and tetrachlorobiphenyls, contaminated particles can have a much greater influence on the uptake by fish. If the number of contaminated particles is sufficiently high, the low aqueous solubilities, in combination with relatively high rates of desorption or dissolution, enable the particles to act as a source of the hydrophobic chemicals. For extremely hydrophobic chemicals, the rates of dissolution or desorption determine the rates of uptake of the chemical by the fish. Hence, during relatively short periods of exposure, there is no influence of contaminated particles on the bioaccumulation.     

Field investigation into unsaturated flow and transport in a fault: model analyses

Results of a fault test performed in the unsaturated zone of Yucca Mountain, Nevada, were analyzed using a three-dimensional numerical model. The fault was explicitly represented as a discrete feature and the surrounding rock was treated as a dual-continuum (fracture-matrix) system. Model calibration against seepage and water-travel-velocity data suggests that lithophysal cavities connected to fractures can considerably enhance the effective fracture porosity and therefore retard water flow in fractures. Comparisons between simulation results and tracer concentration data also indicate that matrix diffusion is an important mechanism for solute transport in unsaturated fractured rock. We found that an increased fault-matrix and fracture-matrix interface areas were needed to match the observed tracer data, which is consistent with previous studies. The study results suggest that the current site-scale model for the unsaturated zone of Yucca Mountain may underestimate radionuclide transport time within the unsaturated zone, because an increased fracture-matrix interface area and the increased effective fracture porosity arising from lithophysal cavities are not considered in the current site-scale model.     

A reaction-based paradigm to model reactive chemical transport in groundwater with general kinetic and equilibrium reactions

This paper presents a reaction-based water quality transport model in subsurface flow systems. Transport of chemical species with a variety of chemical and physical processes is mathematically described by M partial differential equations (PDEs). Decomposition via Gauss-Jordan column reduction of the reaction network transforms M species reactive transport equations into two sets of equations: a set of thermodynamic equilibrium equations representing N(E) equilibrium reactions and a set of reactive transport equations of M-N(E) kinetic-variables involving no equilibrium reactions (a kinetic-variable is a linear combination of species). The elimination of equilibrium reactions from reactive transport equations allows robust and efficient numerical integration. The model solves the PDEs of kinetic-variables rather than individual chemical species, which reduces the number of reactive transport equations and simplifies the reaction terms in the equations. A variety of numerical methods are investigated for solving the coupled transport and reaction equations. Simulation comparisons with exact solutions were performed to verify numerical accuracy and assess the effectiveness of various numerical strategies to deal with different application circumstances. Two validation examples involving simulations of uranium transport in soil columns are presented to evaluate the ability of the model to simulate reactive transport with complex reaction networks involving both kinetic and equilibrium reactions.     

Review of unsaturated-zone transport and attenuation of volatile organic compound (VOC) plumes leached from shallow source zones

Reliable prediction of the unsaturated zone transport and attenuation of dissolved-phase VOC (volatile organic compound) plumes leached from shallow source zones is a complex, multi-process, environmental problem. It is an important problem as sources, which include solid-waste landfills, aqueous-phase liquid discharge lagoons and NAPL releases partially penetrating the unsaturated zone, may persist for decades. Natural attenuation processes operating in the unsaturated zone that, uniquely for VOCs includes volatilisation, may, however, serve to protect underlying groundwater and potentially reduce the need for expensive remedial actions. Review of the literature indicates that only a few studies have focused upon the overall leached VOC source and plume scenario as a whole. These are mostly modelling studies that often involve high strength, non-aqueous phase liquid (NAPL) sources for which density-induced and diffusive vapour transport is significant. Occasional dissolved-phase aromatic hydrocarbon controlled infiltration field studies also exist. Despite this lack of focus on the overall problem, a wide range of process-based unsaturated zone - VOC research has been conducted that may be collated to build good conceptual model understanding of the scenario, particularly for the much studied aromatic hydrocarbons and chlorinated aliphatic hydrocarbons (CAHs). In general, the former group is likely to be attenuated in the unsaturated zone due to their ready aerobic biodegradation, albeit with rate variability across the literature, whereas the fate of the latter is far less likely to be dominated by a single mechanism and dependent upon the relative importance of the various attenuation processes within individual site - VOC scenarios. Analytical and numerical modelling tools permit effective process representation of the whole scenario, albeit with potential for inclusion of additional processes - e.g., multi-mechanistic sorption phase partitioning, and provide good opportunity for further sensitivity analysis and development to practitioner use. There remains a significant need to obtain intermediate laboratory-scale and particularly field-scale (actual site and controlled release) datasets that address the scenario as a whole and permit validation of the available models. Integrated assessment of the range of simultaneous processes that combine to influence leached plume generation, transport and attenuation in the unsaturated zone is required. Component process research needs are required across the problem scenario and include: the simultaneous volatilisation and dissolution of source zones; development of appropriate field-scale dispersion estimates for the unsaturated zone; assessment of transient VOC exchanges between aqueous, vapour and sorbed phases and their influence upon plume attenuation; development of improved field methods to recognise and quantify biodegradation of CAHs; establishment of the influence of co-contaminants; and, finally, translation of research findings into more robust practitioner practice.     

A large-scale experiment on mass transfer of trichloroethylene from the unsaturated zone of a sandy aquifer to its interfaces

A large-scale experiment was conducted to investigate the transport of trichloroethylene (TCE) vapors in the unsaturated zone and to determine the mass transfer to the groundwater and the atmosphere. The experiment involved injection of 5 1 of TCE in the unsaturated zone under controlled conditions, with multidepth sampling of gas and water through the unsaturated zone and across the capillary zone into underlying groundwater. The mass transfer of TCE vapors from the vadose zone to the atmosphere was quantified using a vertical flux chamber. A special soil water sampler was used to monitor transport across the capillary fringe. Experimental data indicated that TCE in the unsaturated zone was mainly transported to the atmosphere and this exchange reduced significantly the potential for groundwater pollution. The maximum measured TCE flux to the atmosphere was about 3 g/m(2)/day. Observed and calculated fluxes based on vertical TCE vapor concentration gradients and Fick's law were in good agreement. This confirms that TCE vapor transport under the experimental conditions was governed essentially by molecular diffusion. TCE vapors also caused a lower, but significant contamination of the underlying groundwater by dispersion across the capillary fringe with a corresponding maximum flux of about 0.1 g/m(2)/day. This mass transfer to groundwater is partly uncertain due to an inadvertent entry of some nonaqueous phase liquid (NAPL) from the source area into the saturated zone. Application of an analytical solution to estimate the TCE flux from the unsaturated zone to the groundwater indicated that this phenomenon is not only influenced by molecular diffusion but also by vertical dispersion. The mass balance indicates that, under the given experimental conditions (e.g. proximity of the source emplacement relative to the soil surface, relatively high permeable porous medium), nearly 95% of the initial TCE mass was transferred to the atmosphere.     

A high-resolution non-invasive approach to quantify oxygen transport across the capillary fringe and within the underlying groundwater

Oxygen transport across the capillary fringe is relevant for many biogeochemical processes. We present a non-invasive technique, based on optode technology, to measure high-resolution concentration profiles of oxygen across the unsaturated/saturated interface. By conducting a series of quasi two-dimensional flow-through laboratory experiments, we show that vertical hydrodynamic dispersion in the water-saturated part of the capillary fringe is the process limiting the mass transfer of oxygen. A number of experimental conditions were tested in order to investigate the influence of grain size and horizontal flow velocity on transverse vertical dispersion in the capillary fringe. In the same setup, analogous experiments were simultaneously carried out in the fully water-saturated zone, therefore allowing a direct comparison with oxygen transfer across the capillary fringe. The outcomes of the experiments under various conditions show that oxygen transport in the two zones of interest (i.e., the unsaturated/saturated interface and the saturated zone) is characterized by very similar transverse dispersion coefficients. An influence of the capillary fringe morphology on oxygen transport has not been observed. These results may be explained by the narrow grain size distribution used in the experiments, leading to a steep decline in water saturation at the unsaturated/saturated interface and to the absence of trapped gas in this transition zone. We also modeled flow (applying the van Genuchten and the Brooks-Corey relationships) and two-dimensional transport across the capillary fringe, obtaining simulated profiles of equivalent aqueous oxygen concentration that were in good agreement with the observations.     

Characterization of flow and transport processes within the unsaturated zone of Yucca Mountain, Nevada, under current and future climates

This paper presents a large-scale modeling study characterizing fluid flow and tracer transport in the unsaturated zone of Yucca Mountain, Nevada, a potential repository site for storing high-level radioactive waste. The study has been conducted using a three-dimensional numerical model, which incorporates a wide variety of field data and takes into account the coupled processes of flow and transport in the highly heterogeneous, unsaturated fractured porous rock. The modeling approach is based on a dual-continuum formulation of coupled multiphase fluid and tracer transport through fractured porous rock. Various scenarios of current and future climate conditions and their effects on the unsaturated zone are evaluated to aid in the assessment of the proposed repository's system performance using different conceptual models. These models are calibrated against field-measured data. Model-predicted flow and transport processes under current and future climates are discussed.     

Dimension reduction and source identification for multispecies groundwater contamination

Assessment of chemical contamination at large industrial complexes with long and sometimes unknown histories of operation represents a challenging environmental problem. The spatial and temporal complexity of the contaminant may be due to changes in production processes, differences in the chemical transport, and the physical heterogeneity of the soil and aquifer materials. Traditional mapping techniques are of limited value for sites where dozens of chemicals with diverse transport characteristics may be scattered over large spatial areas without documentation of disposal histories. In this context, a site with a long and largely undocumented disposal history of shallow groundwater contamination is examined using principal component analysis (PCA). The dominant chemical groups and chemical "modes" at the site were identified. PCA results indicate that five primary and three transition chemical groups can be identified in the space of the first three eigenvectors of the correlation matrix, which account for 61% of the total variance of the data. These groups represent a significant reduction in the dimension of the original data (116 chemicals). It is shown that each group represents a class of chemicals with similar chemo-dynamic properties and/or environmental response. Finally, the groups are mapped back onto the site map to infer delineation of contaminant source areas for each class of compounds. The approach serves as a preliminary step in subsurface characterization, and a data reduction strategy for source identification, subsurface modeling and remediation planning.     

One-at-a-time sensitivity analysis of pollutant loadings to subsurface properties for the assessment of soil and groundwater pollution potential

Chemical leak was numerically simulated for four chemical substances: benzene (light non-aqueous phase liquid (NAPL)), tetrachloroethylene (dense NAPL), phenol (soluble in water), and pentachlorophenol (white crystalline solid) in a hypothetical subsurface leak situation using a multiphase compositional transport model. One metric ton of chemical substances was assumed to leak at a point 3.51 m above the water table in a homogeneous unconfined aquifer which had the depth to water table of 7.135 m, the hydraulic gradient of 0.00097, the recharge rate of 0.7 mm/day, and the permeability of 2.92?×?10^?10 m². For comparison, surface spill scenarios, which had a long pathway from source to the water table, were simulated. Using the model results, point-source pollutant loadings to soil and groundwater were calculated by multiplying mass, impact area, and duration above and below the water table respectively. Their sensitivity to subsurface properties (depth to water table, recharge rate, porosity, organic carbon content, decay rate, hydraulic gradient, capillary pressure, relative permeability, permeability) was analyzed, with changing each parameter within acceptable ranges. The study result showed that the pollutant loading to groundwater was more sensitive to the subsurface properties than the pollutant loading to soil. Decay rate, groundwater depth, hydraulic gradient and porosity were influential to pollutant loadings. The impact of influential parameters on pollutant loadings was nonlinear. The dominant subsurface properties of pollution loadings (e.g., decay rate, groundwater depth, hydraulic gradient, and porosity for groundwater) also affect the vulnerability, and the subsurface pollutant loadings defined in this study are dependent on chemical properties as well, which indicates that the influential hydrogeological and physicochemical parameters to pollutant loadings can be used for pollution potential assessment. The contribution of this work is the suggestion that the sensitivity of pollutant loadings can be used for pollution potential assessment. Soil and groundwater pollution potential of chemicals are discussed altogether for leak scenarios. A physics-based model is used to understand the impact of subsurface properties on the fate and transport of chemicals above and below the water table, and consequently their impact on the pollutant loading to soil and groundwater.