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.

Role of water in unexpectedly large changes in emission flux of volatile organic compounds in soils under dynamic temperature conditions

“…Similar changes were observed in our previous studies using similar experimental systems, which involved concentration measurements only at two ports (i.e., P1 and P4 in the present study) using the same gas detectors 30 , 31 . Thus, the results of Run 1 confirmed the suitability of the experimental system, equipped with two additional measurement ports and gas detectors, to support the objective of the present study.…”

“…The few laboratory experiments that have considered dynamic conditions in near-surface soils did not cover the unexpected or counterintuitive effects of temperature on VOC flux 28 , 29 . In this context, our recent studies 30 , 31 revealed the inverse relationship between flux and temperature changes and enhanced flux changes. Moreover, enhanced flux changes were assessed based on experiments of upward vertical vapor-phase diffusive transport of benzene and TCE 30 , 31 .…”

“…In this context, our recent studies 30 , 31 revealed the inverse relationship between flux and temperature changes and enhanced flux changes. Moreover, enhanced flux changes were assessed based on experiments of upward vertical vapor-phase diffusive transport of benzene and TCE 30 , 31 . These experiments were conducted in finer and coarser grained sandy soils, with water contents ranging from air-dry to 10 wt% during sinusoidal temperature variation between 20 and 30 °C.…”

“…Our previous studies that used similar experimental systems reported that temperature changes at the center of the sand-packed column were delayed only by approximately 5 min when compared to those in the bottom and top spaces 31 . In these previous studies, concentrations were measured at two ports (i.e., P1 and P4 in the present study) using the same gas detectors 30 , 31 .…”

“…Therefore, in the present study we examined this hypothesis using a novel laboratory experiment and a complementary numerical simulation of upward vertical vapor-phase diffusive transport of VOCs in soil under dynamic temperature. We initially describe a new experimental method for exploring the vertical vapor-phase diffusive transport of VOCs in the soil, which has been developed based on the experimental methods in our previous studies 30 , 31 and allows continuous measurements of VOC concentrations at multiple depths and of relative humidity (RH) in a soil-packed column under dynamic temperature conditions. Thereafter, an experiment using benzene at a static temperature (25 °C) was conducted to verify that no fluctuations in concentration or RH occurred because of the multiple-depth concentration measurement by gas detectors equipped with a pump.…”