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A 3D mathematical model that describes transport of volatile organic compounds in a coupled vadose-saturated zone system is proposed. The subsurface processes incorporated in the model include advection, dispersion, interphase mass transfer, and diffusive mass exchange between two horizontal porous media formations, as well as the time-dependent mass loading from a source zone. The analytical solutions are derived subject to the specific initial and boundary conditions. The solutions are evaluated by numerical Laplace inverse transform. The model solutions can be used to study the fate and transport in subsurface formations composed of a vadose zone and a water table aquifer, where the volatile organic compound is released from entrapped nonaqueous phase liquid in the vadose zone, or the dissolved volatile organic compound transports with groundwater accompanied by diffusive mass transfer into the overlying soil formations. Mass transfer between two layers is demonstrated to have back-diffusion characteristics, which results in secondary contamination and retains low levels of contaminant concentrations over a prolonged period of time. The model solutions are specifically useful in assessing the vapor intrusion process in a contaminated site where a vadose zone is underlain by a water table aquifer contaminated with volatile organic compounds. [ABSTRACT FROM AUTHOR]

Single-well push-pull tracer tests have been used to characterize the extent, fate, and transport of subsurface contamination. Analytical solutions provide one alternative for interpreting test results. In this work, an exact analytical solution to two-dimensional equations describing the governing processes acting on a dissolved compound during a modified push-pull test (advection, longitudinal and transverse dispersion, first-order decay, and rate-limited sorption/partitioning in steady, divergent, and convergent flow fields) is developed. The coupling of this solution with inverse modeling to estimate aquifer parameters provides an efficient methodology for subsurface characterization. Synthetic data for single-well push-pull tests are employed to demonstrate the utility of the solution for determining (1) estimates of aquifer longitudinal and transverse dispersivities, (2) sorption distribution coefficients and rate constants, and (3) non-aqueous phase liquid (NAPL) saturations. Employment of the solution to estimate NAPL saturations based on partitioning and non-partitioning tracers is designed to overcome limitations of previous efforts by including rate-limited mass transfer. This solution provides a new tool for use by practitioners when interpreting single-well push-pull test results. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the flux and separation factor for water/ethanol mixtures in PVA membrane system are detected and a modified solution-diffusion model is proposed to account for the mass transport of penetrants in the polymer membrane. In the model, the group contribution method (the UNIFAC-ZM model) is introduced to calculate the activity of penetrants in the polymer membrane, and a modified Fujita temperature-dependence diffusion coefficient equation is developed to describe the diffusion behavior of the penetrants. The aim of this work is to establish a predictive solution-diffusion model. As an exemplary application of the model, the permeation fluxes and separation factors of water/ethanol mixtures in PVA membrane system have been predicted and showed to be in good agreement with experimental values. [Copyright &y& Elsevier]

Abstract: The ability to quantify the mass flux of a groundwater contaminant that is leaching from a source area is critical to enable us to: (1) evaluate the risk posed by the contamination source and prioritize cleanup, (2) evaluate the effectiveness of source remediation technologies or natural attenuation processes, and (3) quantify a source term for use in models that may be applied to predict maximum contaminant concentrations in downstream wells. Recently, a number of new methods have been developed and subsequently applied to measure contaminant mass flux in groundwater in the field. However, none of these methods has been validated at larger than the laboratory-scale through a comparison of measured mass flux and a known flux that has been introduced into flowing groundwater. A couple of innovative flux measurement methods, the tandem circulation well (TCW) and modified integral pumping test (MIPT) methods, have recently been proposed. The TCW method can measure mass flux integrated over a large subsurface volume without extracting water. The TCW method may be implemented using two different techniques. One technique, the multi-dipole technique, is relatively simple and inexpensive, only requiring measurement of heads, while the second technique requires conducting a tracer test. The MIPT method is an easily implemented method of obtaining volume-integrated flux measurements. In the current study, flux measurements obtained using these two methods are compared with known mass fluxes in a three-dimensional, artificial aquifer. Experiments in the artificial aquifer show that the TCW multi-dipole and tracer test techniques accurately estimated flux, within 2% and 16%, respectively; although the good results obtained using the multi-dipole technique may be fortuitous. The MIPT method was not as accurate as the TCW method, underestimating flux by as much as 70%. MIPT method inaccuracies may be due to the fact that the method assumptions (two-dimensional steady groundwater flow to fully-screened wells) were not well-approximated. While fluxes measured using the MIPT method were consistently underestimated, the method''s simplicity and applicability to the field may compensate for the inaccuracies that were observed in this artificial aquifer test. [Copyright &y& Elsevier]