Water-induced inverse correlation between temperature and flux changes in vertical vapor-phase diffusive transport of volatile organic compounds in near-surface soil environments

Abstract

Volatile organic compounds (VOCs) in contaminated soils have been investigated in near-surface environments to describe their transport behaviors and the resultant impacts of vapor intrusion into buildings. Prior studies have suggested that temperature changes significantly influence such transport behaviors in near-surface soils; however, the nature of these influences and their mechanisms have remained unclear. This is because an inverse correlation between the in situ temperature and VOC flux changes has been suggested but has not been experimentally investigated or demonstrated. Herein, we show the results of a set of experiments on the vertical and upward vapor-phase diffusive transport of benzene in sandy soils for different sand grain size and water content combinations under sinusoidal temperature changes between 20 and 30◦ C. Under all experimental conditions, changes in the flux from the soil surface correlated with temperature changes, whereas changes in the flux into the overlying soil showed inverse correlations. Concurrent monitoring of the relative humidity revealed that an inverse correlation was exhibited in response to the condensation and volatilization of water. Moreover, the intensity of the inverse correlation was independent of grain size but was larger at higher water contents. Overall, water in soil may induce an inverse correlation.