- Title
- Multi-dimensional vapour modelling for assessing vapour risk at hydrocarbon contaminated sites
- Creator
- Unnithan, Aravind
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2023
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Over the past century, volatile organic compounds (VOCs) have been widely used in many industries, releasing copious amounts of organic wastes intentionally or accidentally into the subsurface. These VOCs from the subsurface source migrate upwards vertically to the surface through the vadose zone soil and penetrate overlying buildings, a process that is known as vapour intrusion (VI). Volatile hydrocarbons are predominant environmental contaminants and are associated with the health hazards from inhaling potentially toxic and carcinogenic contaminants. Over the past three decades, significant research and regulatory guidelines have been developed for on-site investigations and health risk assessment from VI exposure pathways. Understanding subsurface conditions, environmental factors, and building conditions is critical for an accurate VI pathway assessment. While direct measurements of indoor air concentration are more effective in quantifying health risks associated with vapour intrusion, a wide-scale monitoring system might not be possible in all scenarios due to concentration fluctuations with time, background sources, and access restrictions. Therefore, mathematical models referred to as vapour intrusion models (VIMs) can be applied as a decision support tool to predict indoor air vapour contaminant concentration. Most VIMs use conceptual site models (CSMs) where vapours originate from a subsurface source and diffuse vertically and/or laterally through a vadose soil zone and eventually migrate into overlying buildings through foundation cracks. The existing VIMs do not consider CSMs with preferential pathways like utility line, high permeability zones of soil, rocks, etc., which can act as a path of least resistance for vapour transport resulting in under-predicting actual health risks. This thesis presents the development of a two-dimensional (2-D) VIM depicting lateral and vertical flux of chlorinated hydrocarbon vapours in the presence of highly permeable course grained soil layer such as gravel or crushed rocks that act as preferential pathway for vapour transport. The vadose zone considers steady state diffusion of vapour transport except in the highly permeable preferential pathway layer, which considers advection. Governing equations were formulated and solved using the central difference scheme of the finite difference method and coded in Python programming language for simulating vapour transport from the source to the building foundations through sub-surface soil. When compared to existing VIMs, the developed model returned promising results. The role of highly permeable preferential pathways in exacerbated VI risks was examined through simulations for a variety of hypothetical scenarios with and without preferential pathways. The results indicated an increase in indoor air concentration as high as 200% compared to a no preferential pathway scenario, when the vapour source is located close to the preferential pathway layer and decreased as the depth of source to preferential pathway increased. Furthermore, the effect of preferential pathway for long lateral distances was investigated and the impact of preferential pathway in VI was evident only until a few meters from the edge of the contaminant source. Consequently, preferential pathway seems to play no substantial role in increasing indoor air concentration as the source to building distance increases. The one-at-a-time (OAT) sensitivity analysis of the newly developed model demonstrates that the parameters governing vapour transport is highly sensitive to the type of vadose zone soil and the thickness of the highly permeable soil used for installing the utility lines. Low permeability soils like clay and sandy clay lead to lower indoor air contaminant concentration as opposed to sand, which causes a larger indoor air contaminant concentration due to its high permeability. Soil moisture and porosity play a vital role in vapour transport as they influence the effective vapour diffusivity in the soil, which is more pronounced in low permeability soils (less than 10-12 m2). The parameters governing vapour entry through building foundation such as floor thickness, crack width and building depressurisation were observed to be more sensitive to high permeability soils since the vapour flux through cracks is dominated by advection. Model validation was also conducted using published case study site investigation data obtained from Building 418 at Altus, Oklahoma, U.S.A., by comparing the measured indoor air concentration with the predicted concentration using the devised model. The model prediction without considering preferential pathway for the given data was found to be less than the measured indoor air concentration, indicating that highly permeable fill materials beneath ground surface at a depth of 0.3 m could have served as a preferential route for vapour migration. This strongly suggests that the presence of highly permeable soil layers in the subsurface tends to increase the indoor air concentration and thereby exacerbate VI risks. Further detailed laboratory investigations were conducted using two large 2-D soil columns simulating the presence of preferential pathway and control column (without preferential pathway). Benzene was introduced as vapour source and was monitored for 13 days from multiple sampling ports across the surface of the column. The results showed similar trends for both columns on Day 1, and a notable two-fold increase in contaminant concentration was observed in the sampling ports of the column with preferential pathway on Day 13. This could establish that the presence of a highly permeable granular layer as the preferential pathway in the subsurface can influence the distribution of contaminants across the vadose zone.
- Subject
- volatile organic compounds; vapour intrusion; vapour intrusion models; two-dimensional VIM; preferential pathways
- Identifier
- http://hdl.handle.net/1959.13/1477676
- Identifier
- uon:50016
- Rights
- Copyright 2023 Aravind Unnithan
- Language
- eng
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