Development of a Model for Predicting the Volatilization Flux from Unsaturated Soil Contaminated by Volatile Chemical Substances

Abstract

This work developed a model for predicting the volatilization flux from the unsaturated soil contaminated by volatile chemical substances (VCSs) such as mercury and benzene. The model considers a series of phenomena under the unsaturated condition such as multi-phase flow consisting of a non-aqueous phase liquid, water, and gases together with the permeation of rainfall into the surface soil, the volatilization/condensation of VCSs, and the adsorption/desorption of VCSs. On this basis, this work clarified a mechanism for the generation of a volatilization flux at the ground surface. In addition, the effects of various transport phenomena in the surface soil on the magnitude and seasonal changes in this flux due to variations in weather factors such as rainfall level, temperature, and air pressure were quantitatively evaluated. This newly developed prediction model can be utilized to estimate dynamic variations in the flux under real-environmental conditions.

Role of water in unexpectedly large changes in emission flux of volatile organic compounds in soils under dynamic temperature conditions

“…Based on the mechanisms or role of water in the enhanced emission flux changes, such changes may occur in any soil environment where water exists, with some site-specific characteristics. Consequently, continuous measurements of soil VOC emissions for various soil environments, and/or predictions of emissions through numerical simulations considering the role of water in dynamic soil environments 32 , are essential for a global assessment of the impacts of soil VOC emissions on atmospheric conditions and climate.…”

“…The effective diffusion coefficient in the present dynamic temperature experiments can be assumed constant because substantial changes in the diffusion coefficient in air and amount of liquid water are unlikely due to small changes at low temperatures (20‒30 °C). Here, a numerical model developed recently by the authors 32 was used to conduct the simulation for benzene (as a representative VOC), for which the governing equations are: and where ϕ is the porosity (volume fraction); S g and S w (= 1 − S g ) are the gas and water saturations (volume fraction); x g , k and x w , k are the mole fractions of component k in the gas and liquid water phases (gas phase component: benzene, water, and air; liquid water phase component: benzene and water), respectively; ρ g and ρ w are the molar densities (mol/m 3 ) of the gas and liquid water phases, respectively; and R vwc , k is the volatilization/condensation or volatilization/dissolution rate for component k in the liquid water phase (mol/m 3 /s). Here, R vwc , k for volatilization and condensation (or dissolution) are represented as: and where k v , k is the empirical rate constant (1/Pa/s) for component k (1 × 10 –3 for benzene, 1 × 10 –7 for water); P sat , k is the temperature-dependent saturated vapor pressure (Pa) for component k ; and P is the pressure of the system (Pa).…”

“…Here, R vwc , k for volatilization and condensation (or dissolution) are represented as: and where k v , k is the empirical rate constant (1/Pa/s) for component k (1 × 10 –3 for benzene, 1 × 10 –7 for water); P sat , k is the temperature-dependent saturated vapor pressure (Pa) for component k ; and P is the pressure of the system (Pa). The values of the rate constants were determined experimentally in a previous study 32 . Although the values in the previous and present experiments may not be identical, it is reasonable to assume that the values of rate constants have the same order of magnitude in both studies.…”

“…Next, we provide the results of two experiments using benzene and PCE at a dynamic temperature (25 ± 5 °C) to clarify the concentration changes at multiple depths and the resultant concentration gradient, phase change of water, and their relationships to changes in VOC emission flux. Finally, a plausible mechanism of the role of water in large emission flux changes is provided based on the results of the dynamic temperature experiments and a complementary numerical analysis with a model developed recently by the authors 32 . The mechanism suggests the importance of a global assessment of VOC emissions by continuous measurements of soil VOC emissions of various soil environments and/or predictions of emissions through numerical simulations with thorough consideration of the role of water in dynamic soil environments.…”

Numerical Analysis on the Effect of Soil Properties on the Generation of Volatilization Flux from Unsaturated Soil Contaminated by Volatile Chemical Substances

“…In our previous study [16], a numerical model for the prediction of volatilization flux was developed considering a series of physical phenomena, as shown in Figure 1. In our previous paper, the generation mechanisms of volatilization flux targeting benzene as a model substance for VCSs were revealed using the developed model.…”