A rainfall event may produce a biased estimation of the indoor vapor intrusion risk through exterior soil-gas sampling

Abstract

Exterior (i.e., outside the building) soil-gas sampling has been often used to estimate the sub-slab soil-gas concentration and to assess the risk of indoor vapor intrusion. However, the effectiveness of this method can be influenced by rainfall events. In this study, a two-dimensional, water-air two-phase flow numerical model is developed to investigate the effect of rainfall infiltration on the temporal variation of the soil-gas concentration at different depths outside and beneath the building footprint. It is found that rainfall infiltration generally exerts the capping effect and the dilution effect, resulting the increase and decrease in the soil-gas concentration, respectively. Given the soil type of sandy loam, the variation of the shallow and mid-depth soil-gas concentration depends respectively on the rainfall intensity and the total amount of rainwater, while the deep soil-gas concentration is found insensitive to a rainfall event. Moreover, the response timescale of the soil-gas concentration to a rainfall event is larger for a soil type with lower permeability. Considering a rainfall event with the intensity of 4 mm/h and the duration of 24 h, the response timescales of the soil-gas concentration at a depth of 2 m below the ground surface are larger than 1 year for silt loam, sandy clay loam, and sandy loam, and about 5 and 2 months for loamy sand and sand, respectively, where evapotranspiration is not considered. For the soil type of sand, rainfall infiltration can yield a maximum underestimation error of 12%, when using exterior soil-gas sampling to estimate the sub-slab soil-gas concentration. For the relatively less permeable soil type, such as sandy loam, silt loam and sandy clay loam, by contrast, a maximum overestimation error of 8% is found. The results obtained in this study can offer important implications for practitioners to assess the indoor vapor intrusion risk through exterior soil-gas sampling under the rainfall effect.