Gas diffusion i urbane vadose zone: Eferkten af texture, kompaktering og struktur i forbindelse med risikovurdering
Translated title
Gas diffusivity in urban vadose zones: The effect of texture, compaction and structure and consequences for risk assessment
Author
Jensen, Maria Pilehave
Term
10. term
Publication year
2009
Abstract
Denne afhandling undersøger gasdiffusion i den urbane vadose zone med fokus på, hvordan jordens textur, kompaktering og struktur påvirker gasdiffusiviteten og dermed risikovurderinger for flygtig forurening. Der er udviklet og afprøvet nyt laboratorieudstyr til måling af diffusionskoefficienten i 100 cm3 jordprøver, som blev vurderet lettere at anvende og mere fleksibelt end tidligere opstillinger og potentielt anvendeligt til større prøver. Diffusiviteten (Dp/Do) blev målt på intakte og pakkede prøver med varierende textur, kompakteringsgrad og struktur (bl.a. aggregering, revner og steninhold), og resultaterne blev analyseret som Dp/Do versus luftfyldt porøsitet (ε). Den Water‑Induced Linear Reduction (WLR) model blev anvendt som reference for mellem‑kornede, relativt homogene jordtyper. Studiet viser, at især jordens struktur har stor betydning for gasdiffusionen; meget finskornede materialer, stærkt strukturerede jorde (aggregering og revner) og jorde med højt steninhold afviger markant og har lavere diffusivitet end forudsagt af WLR. Modellen overestimerede generelt Dp/Do i et omfang, der er foreneligt med konservative danske risikovurderinger, men den er ikke egnet til stærkt strukturerede, finskornede eller stenrige jorde. På den baggrund opstilles en konceptuel model baseret på WLR med forbehold for de nævnte forhold, som kan støtte mere robuste risikovurderinger i urbane områder.
This thesis examines gas diffusion in the urban vadose zone, focusing on how soil texture, compaction, and structure influence gas diffusivity and the implications for risk assessment of volatile contamination. New laboratory equipment was developed and tested to measure diffusion coefficients in 100 cm3 soil samples; it proved easier to use and more flexible than previous setups and potentially applicable to larger samples. Diffusivity (Dp/Do) was measured on intact and repacked samples spanning different textures, degrees of compaction, and structural features (including aggregation, fractures, and stone content), and results were analyzed as Dp/Do versus air‑filled porosity (ε). The Water‑Induced Linear Reduction (WLR) model was used as a reference for intermediate, relatively homogeneous single‑grained soils. Findings show that soil structure is particularly important: very fine textures, highly structured soils (aggregation and fractures), and soils with high stone content deviate substantially and exhibit lower diffusivity than predicted by WLR. The model generally overpredicted Dp/Do to a degree consistent with conservative Danish risk assessments, but it is not suitable for highly structured, fine‑textured, or stone‑rich soils. Accordingly, a conceptual model building on WLR with explicit allowances for these factors is proposed to support more robust urban risk assessments.
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