Modelling PFAS transport in unsaturated soil using LEACHM and implications for source zone remediation.

Author: Colin McKay

McKay, Colin, 2024 Modelling PFAS transport in unsaturated soil using LEACHM and implications for source zone remediation., Flinders University, College of Science and Engineering

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The properties of per- and polyfluoroalkyl substances (PFAS), and environmental factors affecting their fate and transport in groundwater have been extensively researched. However, relatively few studies have focused on migration of PFAS in the unsaturated zone. Understanding of PFAS behaviour in the vadose zone is particularly important for remediation planning at source sites, as the bulk of PFAS mass is often present in shallow horizons. This study was conducted to assess the relative effects of climate and soil type on the vertical migration of PFAS compounds PFOS, PFHxS and PFOA across a range of published soil sorption coefficient (Kd) values. The study also simulated PFAS flux following application of various management options to assess their efficacy over time compared with a “do nothing” approach. The model outputs reinforce that soil type, rainfall and sorption properties govern the PFAS flux beneath source zones. Both stabilisation and capping, as remediation technologies, were effective at reducing the rate of vertical flux to groundwater. PFAS stabilisation was found to be very effective in reducing flux, for the material that was treated. However, any residual PFAS within the affected soil profile below the treated material was shown to continue to sustain vertical PFAS mass transfer into the future. The relative effectiveness of partial profile stabilisation appeared to decrease with increasing sand content, further exacerbated in soils of high sorption capacity. In contrast, clay capping, controlling infiltration, was effective from the surface to depth, and in sandier soils, was more effective than partial profile stabilisation alone. Generally, a combination of partial profile stabilisation and clay capping was of little additional benefit compared to providing the cap alone.

Keywords: Per- and polyfluoroalkyl substance (PFAS), numerical modelling, unsaturated zone, source zone remediation, stabilisation, capping

Subject: Environmental Science thesis

Thesis type: Masters
Completed: 2024
School: College of Science and Engineering
Supervisor: Ilka Wallis