Monami Kondo

Mercury (Hg) is a widespread pollutant that poses significant risks to both ecosystems and human health. While previous studies have evaluated Hg concentrations in soils, few have measured Hg emissions in areas with high Hg concentrations or related these measurements to estimates of exposure via air inhalation based on Hg flux values. This study addresses this gap by proposing the use of gaseous elemental Hg (GEM) flux measurements for exposure risk assessments, with a particular focus on the importance of inhalation exposure, which has implications for future risk management strategies. Sensitivity analysis of the site-specific exposure and risk estimation model revealed that the fraction of land area in surface water had a significant impact on the estimated exposure through air inhalation. Additionally, the analysis showed that the GEM flux measured in situ in a high-Hg concentration area of Itomuka was not directly proportional to the Hg concentration, although the risk estimation model treated it as a function of concentration. Furthermore, we found that the flux values are influenced by both the fraction of land area in surface water and Hg concentration, indicating that this new flux-based indicator should be integrated into exposure risk assessments.

Global mercury (Hg) management is an important issue for regulators and regional stakeholders, especially since the entry into force of the Minamata Convention in 2017. Although Hg has been used widely in pharmaceutical and industrial products, some forms of Hg present adverse risks for humans and ecosystems and, thus, Hg is considered both an air and soil pollutant. The UNEP Global Mercury Assessment Model (2013) clearly stated that it is important to understand the mechanisms of Hg release from soils because the amount emitted could be comparable to that released by human activities and from rivers and seas. Zero-valent gaseous elemental mercury (GEM) is the main component of gaseous Hg in the atmosphere and the most mobile species. To reduce the risk of human exposure to Hg, the mechanisms of GEM transport in the environment should be investigated and understood better.A number of past studies have aimed to determine the relationship between GEM fluxes from soils and various environmental factors, both through laboratory experiments and field observations. These suggest that higher GEM fluxes are associated with higher soil and air temperature [1-3], increased solar irradiance [4,5], and with higher soil moisture [6,7]. However, it is not known which factors drive GEM flux and how to control conditions so as to suppress Hg emission from soil. The aim of this study was to investigate the main environmental factors influencing Hg release from soil, taking into account factors previously identified but not comprehensively interpreted.Rigorous monitoring data was collected at two sites, tree-covered and shaded high-humidity forested areas (Site 1) and an open artificial soil environment with no trees (Site 2) in Miyagi Prefecture, Japan. This paper discusses the sites and methods used as well as the in depth analysis carried out. In the case of Site 2, the PCA and FA results have shown that atmospheric pressure, solar irradiance, and soil moisture are the primary factors driving GEM flux. In contrast, for Site 1, the analysis indicated that GEM fluxes were driven by as a cohesive group of factors rather than sequentially acting parameters. The results also suggest that it may be possible to estimate Hg emissions from soil by observing the magnitude of primary causality which could be useful for Hg management for the protection of human health and to minimise adverse ecological risk. [1] Siegel, S.M., Siegel, B.Z. (1988) Water, Air, & Soil Pollution 40, 443–448. https://doi.org/10.1007/BF00163747[2] Gustin, M.S., Jaffe, D. (2010) Environmental Science & Technology, 44, 7, 2222–2227. https://doi.org/10.1021/es902736k[3] Lin, C.J., Gustin, M.S., Singhasuk, P., et al. (2010) Environmental Science & Technology, 44, 22, 8522-8528. https://doi.org/10.1021/es1021735[4] Floreani, F., Acquavita, A., Petranich, E., Covelli,S. (2019) Science of The Total Environment, Volume 668, 925-935. https://doi.org/10.1016/j.scitotenv.2019.03.012[5] Viktor V. Kalinchuk, Evgeny A. Lopatnikov, Anatoliy S. Astakhov, et al. (2021) Science of The Total Environment, Volume 753, 142003. https://doi.org/10.1016/j.scitotenv.2020.142003[6] Lindberg, S. E., Zhang, H., Gustin, M., et al. (1999) Journal of geophysical research, 104 (D17), 21879– 21888. https://doi.org/10.1029/1999JD900202[7] Gustin, M.S., Stamenkovic, J. (2005) Biogeochemistry 76, 215–232. https://doi.org/10.1007/s10533-005-4566-8

For the assessment of human health risks from soil contaminated by volatile chemical substances (VCSs), it is important to quantitatively estimate the volatilization fluxes that occur at the ground surface due to the upward transport of VCSs component in unsaturated soil. The model constructed by the authors is based on the quantitative evaluation of advection-dispersion behavior associated with volatilization of VCSs and allows detailed consideration of soil properties and the boundary conditions between the atmosphere-ground surface and unsaturated soil-aquifer compared to existing models. This study focuses on the evaluation of the effect of soil properties on the generation of volatilization flux through numerical analyses by changing the permeability characteristics of surface soil depending on the difference in soil particle size, porosity, and distribution coefficient between the water- and soil phases, targeting benzene as a model substance of VCSs. A series of calculated results can be classified into cases dominated by either an increase of volatilization flux or transport to aquifer, depending on soil properties, indicating the necessity of appropriate countermeasures for remediation and risk assessment. For the reduction of health risks derived from the generation of volatilization flux, removal of contaminants existing in the surface soil, including the ground surface, is essential. However, it is necessary to prevent the spread of contamination into the aquifer when the contaminants have high mobility in surface soil.

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.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.

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.

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.