Investigating the Role of Soil Texture in Petroleum Vapor Intrusion

In this work we investigate the role of soil texture in petroleum vapor intrusion (PVI) by performing numerical modeling, analytical calculations, and statistical analysis of the USEPA's PVI database. Numerical simulations were conducted for three kinds of soil (sand, sandy loam, and clay), and the results indicate that the maximum attenuations of vapor concentrations from source to indoor air were observed when the clay soil is below the building. In the anaerobic zone, the normalized soil gas concentration profiles were observed to be similar and independent of soil type, whereas in the aerobic zone, a more significant attenuation was observed in finer grained soils. Such a finding is consistent with the statistical results of the USEPA's PVI database, which indicate that in the near-source zone, the soil gas concentration in coarse-grained soil tends to be lower than that in fine-grained soil, possibly caused by a weaker source due to mass loss by volatilization, whereas at a distance away from the source, the measured soil gas concentrations in fine-grained soils become much lower because of aerobic biodegradation with a shorter diffusive reaction length. Thus, 3 and 5 m are proposed as soil-type-dependent vertical screening distances for fine and coarse-grained soils, respectively. It should be noted that the validity of these screening distances is examined only for relatively homogeneous soils, and they may not be applicable for cases involving layered soil systems, where the availability of O in the subfoundation should be evaluated with subslab or multidepth samples to confirm the presence of aerobic biodegradation.