Dynamic temperature effects on diffusive transport behaviors of VOC vapors in unsaturated soils

Abstract

<p>Investigation of the transport behaviors of volatile organic compounds (VOCs) in contaminated soils has previously been conducted in various environments. Accordingly, the present study focuses on specific phenomena in the near-surface soil environment where dynamic temperature affects the diffusive flux of VOC vapor phase as previous studies have suggested that temperature variations significantly influence such transport behaviors near-surface soils, but the nature of those influences and their mechanisms have remained unclear because of unexpected correlation of flux with the temperature that impacts on VOC vapor transport. More specifically, current practices report on a set of experiments into the vertical and upward vapor phase diffusive transport of benzene and trichloroethylene (TCE) in sandy soils that were conducted in soil column with water content conditions of up to 10 wt% and sinusoidal temperature conditions ranging from 20 to 30°C. This studies experimentally investigated that in all conditions tested, the top (outlet) flux change correlated positively with temperature change, while the bottom (inlet) flux change showed negative correlations. These results are consistent with previous observations showing that, at relatively deeper locations, there is little correlation between near-surface vertical VOC flux and soil temperature levels, and that VOC concentrations can be independent of the soil temperature at those locations. The present study's results highlighted for the first time that the negative correlation impact of temperature on VOC transport may occur frequently at deeper locations of subsurface soil. This occurs because the VOC concentration gradient is reduced by VOC desorption and the evaporation of water containing VOCs that accompany increasing temperature levels. However, our results also show that such mechanisms have a positive impact on VOC emissions from the upper part of subsurface soils to the atmosphere that can act as a low concertation boundary.</p>