Investigating the role of vadose zone breathing in vapor intrusion from contaminated groundwater

The fluctuation of the groundwater table can cause upward and downward advection of soil gas within the vadose zone, just like breathing. In this study, we developed a three-dimensional numerical model and used it to investigate the role of vadose zone breathing in vapor intrusion, through which subsurface volatile chemicals migrate into the concerned building at contaminated sites. The developed model was first applied to a sand tank experiment, followed by examining the influences of fluctuation amplitude and period of water table, soil textures, and groundwater level depth. Our results suggest that the indoor pollutant concentration can be increased by about three times with the oscillatory water table of 0.4 m amplitude and 4 d period. Within one cycle of vadose zone breathing, the indoor pollutant concentration is found to vary by about 7 orders of magnitude. The results also show that, compared to the groundwater level depth, the soil texture plays a significant role in determining vapor intrusion risks. Specifically, when soil particles increase from 0.25 mm to 0.44 mm, the indoor pollutant concentration tends to increase and becomes more sensitive to groundwater table fluctuation.