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.

The quantitative understanding of the transport behavior of volatile organic compounds (VOCs) in near-surface soils is highly important in light of the potential impacts of soil VOC emissions on the air quality and climate. Previous studies have suggested that temperature changes affect the transport behavior; however, the effects are not well understood. Indeed, much larger changes in the VOC flux under in situ dynamic temperatures than those expected from the temperature dependence of the diffusion coefficients of VOCs in the air have been suggested but rarely investigated experimentally. Here, we present the results of a set of experiments on the upward vertical vapor-phase diffusive transport of benzene and trichloroethylene (TCE) in sandy soils with water contents ranging from an air-dried value to 10 wt% during sinusoidal temperature variation between 20 and 30 °C. In all experiments, the flux from the soil surface was correlated with the temperature, as expected. However, the changes in flux under wet conditions were unexpectedly large and increased with increasing water content; they were also larger for TCE, the volatility of which depended more strongly on the temperature. Additionally, the larger flux changes were accompanied by a recently discovered water-induced inverse correlation between temperature and flux into the overlying soil. These results demonstrated that the flux changes of VOCs under dynamic temperatures could be increased by volatilization-dissolution interactions of VOCs with water. Future extensive studies on this newly discovered phenomenon would contribute to a better understanding of the impacts of soil VOC emissions on the air quality and climate.

<p>In recent years, exposure to volatile chemical substances (VCSs) from contaminated soil has become a serious problem so it has become increasingly important to study the transport phenomena of VCSs. In this study we focused on the transport phenomena of VCSs at the boundary layer between the soil surface and the atmosphere, and defined it as volatilization flux, which express the amount of volatilized substances per unit volume per unit time. In order to estimate the phenomena of mercury transport in unsaturated soil and mercury released from soil to the atmosphere, it is necessary to consider in detail the spatiotemporal fluctuations of factors that affect the volatilization of mercury and the physical transport phenomena in soil.</p><p>The present study developed a model for predicting the volatilization flux from the unsaturated soil contaminated by VCSs. The model considers a series of phenomena under the unsaturated condition such as gas-liquid two-phase flow consisting of convection and diffusion. The effects of various transport phenomena on the surface soil on changes in the magnitude of this flux due to variations in meteorological factors such as temperature and soil moisture content were quantitatively evaluated. This developed prediction model can be utilized to estimate dynamic variations in the flux under real-environmental conditions.</p>

Understanding the diffusive transport behavior of volatile organic compounds (VOCs) in near-surface soils is important because soil VOC emissions affect atmospheric conditions and climate. Previous studies have suggested that temperature changes affect the transport behavior; however, the effect of these changes are poorly understood. Indeed, under dynamic temperature conditions, the change in VOC flux is much larger than that expected from the temperature dependency of the diffusion coefficient of VOCs in air. However, the mechanism is not well understood, although water in soil has been considered to play an important role. Here, we present the results of experiments for the upward vertical vapor-phase diffusive transport of two VOCs (benzene and tetrachloroethylene) in sandy soil under sinusoidal temperature variations of 20–30 °C, as well as its numerical representation. The results clarify that the unexpectedly large changes in emission flux can occur as a result of changes in the VOC concentration gradient due to VOC release (volatilization) from/trapping (dissolution) into water, and that such flux changes may occur in various environments. This study suggests the importance of a global evaluation of soil VOC emissions by continuous measurements in various soil environments and/or predictions through numerical simulations with thorough consideration of the role of water in dynamic soil environments.

<p>With the entry into force of the Minamata Convention from 2017, anthropogenic mercury emissions into the atmosphere have been regulated, and therefore global mercury management has become an important issue. It should be noted that the amounts of anthropogenic mercury emissions and natural emissions from terrestrial sources such as soil and vegetation are almost the same based on the Global Mercury Assessment Model by UNEP (2013). Previous studies have clarified that various environmental factors such as temperature, soil porosity and water content, and pH of the water influence mercury volatilization flux from soils. In order to understand and predict transport phenomena of mercury in soils including the emission to the atmosphere, it is necessary to consider in detail not only static factors such as soil porosity but also dynamic factors such as temperature including their spatial variations.</p><p>In this study, continuous measurements of mercury volatilization flux were conducted for dynamic temperature, different soil water contents and pH-dependent dissolved mercury species. The results showed that the flux values under dynamic temperature were different from those under static temperature even at the same temperature. Additionally, changes in the flux under dynamic temperature depended on the soil water content. We have found that it is difficult to predict mercury volatilization flux under dynamic condition based on the knowledges obtained under static condition probably due to large influence of water vapor generation under dynamic temperature. It is therefore necessary to understand advection and diffusion in soils in the presence of volatilization and condensation of water and dissolved mercury for better understanding mercury flux emission from the soils.</p>