Two-dimensional chlorinated vapour intrusion model involving advective transport of vapours with a highly permeable granular layer in the vadose zone serving as the preferential pathway.

Vapour intrusion (VI) is the process through which volatile organic compounds migrate from the subsurface source to the soil predominantly by diffusion, entering the overlying buildings through joints, cracks or other openings. This activity poses potentially serious health hazards for the occupants. Because of these health risks, recommendations for site closure are often made by quantifying the VI risks using mathematical models known as 'vapour intrusion models' (VIM). Most of these VIMs seem to overlook the role of preferred pathways like utility lines, high conductivity zones of soil or rocks, etc., which act as the path of least resistance for vapour transport thereby increasing vapour intrusion risks. This study presents a two-dimensional (2-D) chlorinated vapour intrusion (CVI) model which seeks to estimate the source-to-indoor air concentration attenuation. It takes into account the effects of a highly permeable utility line embedment as a preferential pathway. The transport of 2-D soil gas is described using the finite difference method where advection serves as the dominant transport mechanism in the preferential pathway layer, while diffusion applies to the rest of the vadose zone. The model returned results comparable with other models for the same input parameters, and was found to closely replicate the results of 3-D models. The simulations indicate that the presence of highly permeable utility line embedment and backfill layers do trigger a higher indoor air concentration compared to a no preferential pathway scenario.

Insights into vapour intrusion phenomena: Current outlook and preferential pathway scenario.

Vapour intrusion (VI) is the phenomenon by which volatile organic compounds (VOCs) migrate from the subsurface source through the soil and enter into the overlying buildings, affecting the indoor air quality and ultimately causing health hazards to the occupants. Health risk assessments associated with hydrocarbon contaminated sites and recommendations of site closure are often made by quantifying the VI risks using mathematical models known as 'vapour intrusion models' (VIM). In order to predict the health risk, various factors such as the lithological and geochemical conditions of the subsurface, environmental conditions, building operational conditions etc. are commonly evaluated using VIMs. Use of these models can overlook the role of preferential pathways like highly permeable subsurface layers and utility lines which act as the path of least resistance for vapour transport, which can increase the VI risks. The extensive networks of utility lines and sanitary sewer systems in urban areas can significantly exacerbate the uncertainty of VI investigations. The backfill materials like sand and gravel surrounding the utility lines can allow the vapours to easily pass through due to their high porosity as compared to natural formations. Hence, failure to understand the role of preferential pathways on the fate and transport of VOC in the vadose zone can result in more conservative predictions of indoor air vapour concentrations and wrong clean up approaches. This comprehensive review outlines the vapour transport mechanisms, factors influencing VI, VIMs and the role of preferential pathways in predicting indoor air vapour concentrations.