Upward transport of trichloroethylene vapour in a soil column

Vapour phase migration of volatile organic compounds (VOCs) from contaminated groundwater or soil into overlying buildings, termed as vapour intrusion (VI), has received increased attention in many countries during the last two decades, because long-term indoor exposure even at relatively low vapour concentrations may cause human health problems. In particular, VI of Trichloroethylene (TCE) may also pose acute health risk to persons residing or working in the buildings (Hosangadi et al. 2017). Many mathematical models have been developed and used as a tool in conjunction with field investigation data, as part of a multiple-lines-of-evidence approach for evaluating VI pathways. However, these models so far lack validation (Yao et al. 2013), which limits the confidence of these models in prediction. For vapour phase migration originating from liquid, sorbed or dissolved VOCs in subsurface under natural conditions, gas diffusion and advection both can be important. Particularly, dense vapours or light hazardous gases mixed in the air may substantially alter the density of gas mixtures in the subsurface and induce density-driven transport. Vapour phase diffusion is usually described by Fick's law which has been adopted in most of the gas phase transport models. The dusty gas model (DGM) equations, however, are more complete and rigorous for multicomponent gas diffusion in porous systems. The purpose of this study is to investigate flux mechanisms controlling upward transport of a dense vapour in soils and to assess the predictability of gas phase transport models on this scenario. We conducted experiments to investigate vertically upward transport of TCE vapour in a soil column. Pressure difference between the column ends and TCE vapour concentration at the column ends were measured. Two gas phase transport models, Michigan Soil Vapour Extraction Remediation (MISER; Abriola et al. 1997) and DGM-based Gas Phase Transport (DGPT; Fen 2014), were applied to simulate the transport scenario of the experiment. MISER and DGPT are based on Fick's law and the DGM equations, respectively, for gas phase diffusion.