環境

[3K0508-11-01] Vapor phase diffusive transport of benzene in Toyoura sand under dynamic temperature conditions

○Asma Akter Parlin¹, Mizuki Yamada¹, Kyohei Tsuchida¹, Kengo Nakamura¹, Noriaki Watanabe¹, Takeshi Komai¹ (1. Tohoku university)

The chairman: Atsushi IIZUKA (Tohoku University)

Keywords:Vapor phase diffusive transport, Benzene, Toyoura sand, Dynamic temperature condition

Volatile organic compounds (VOCs) in contaminated soils have been investigated under a variety of environments to describe its transport behaviors and resultant impacts for vapor intrusion (VI) into buildings. One of the keys to assessing VI pathways from subsurface sources to the interior of the building is to investigate the vapor phase diffusive transport of VOCs in near-surface soils under dynamic temperature environments. Previous studies have suggested significant influences of temperature variations on the transport behaviors. However, the influences and its mechanisms have not been clear because of encounters with unexpected behaviors of VOCs. For instance, temperature increase does not always result in an increase in VOC diffusion flux despite that Fickian diffusion flux positively correlates with temperature in a static temperature environment. The present study has therefore explored vapor phase diffusive transport of VOCs in subsurface soils under dynamic temperature environments by laboratory experiments. A set of experiments of vertical and upward vapor phase diffusive transport of benzene in unsaturated Toyoura sand have been conducted under changing temperature conditions from 20℃ to 30℃. As a result, at all water contents tested (0, 5 and 10 wt%), the outlet or top flux changes of benzene seemed to positively correlate with temperature changes, whereas the inlet or bottom flux changes seemed to have negative temperature dependence. This result is consistent with previously reported in-situ monitoring on changes in trichloroethylene flux in near-surface soils, in which shallower and deeper flux changes respectively correlates positively and negatively with soil temperature changes. The present results have suggested that temperature increase may cause smaller vertical concentration gradient, i.e. diffusion flux, of VOCs at deeper parts due to faster VOCs desorption for initially lower concentration parts, while temperature increase at shallower parts may not cause such smaller concentration gradient because lower concentration is always kept at the ground surface.