Citation:

Huang, J. Analytical Model for Volatile Organic Compound Transport in the Coupled Vadose Zone‐Groundwater System. Journal of Hydrologic Engineering . American Society of Civil Engineers (ASCE), Reston, VA, 26(1):1-14, (2021). https://doi.org/10.1061/(ASCE)HE.1943-5584.0002029

Impact/Purpose:

The development of dissolved phase volatile organic compound (VOC) plumes in the vadose zone and water table aquifer is one of the most common subsurface contamination scenario. The VOC may present as vapors in the soil gas, as solutes dissolved in pore water and absorbed on the soil particles, or as non-aqueous phase liquids (NAPLs) trapped in the pore system. As the VOC sources occur at the land surface or within the vadose zone, the contaminants are transported from the unsaturated zone to an underlying water table aquifer resulting in groundwater contamination. Conversely, as the VOC sources are trapped in the saturated zone, the contaminants may readily volatilize out of the flowing water into the vadose zone where vapors could further migrate to the ground surface resulting in vapor intrusion. This study developed a model used to simulate the fate and transport of volatile organic compounds in a tightly coupled dual layer system. A model solution for the mathematical equations that describe advection-dispersion-adsorption of a dissolved contaminant in a horizontal unconfined aquifer originating from a NAPL source zone, coupled to a model that describes effective diffusion in the vadose zone along with the concentration dependent mass flux to the ground surface was presented analytically. While an analytical solution requires certain simplifications that may make it unsuitable for specific field applications, such solutions have been found to be quite useful for the site environmental management plan aims at (1) analysis of solute transport behavior by means of model simulation, (2) verifying more complex numerical models, (3) efficiently and simply obtaining order-of-magnitude estimates, and (4) recognizing of hazardous waste site characterization.

Description:

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.

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