Concrete Impacted by PFAS at Fire Training Areas

Introduction

Per- and polyfluoroalkyl substances (PFAS) can readily penetrate concrete pads at fire training areas. Concrete can act as a long-term source of contamination (Baduel, 2015, Dauchy, 2019, Thai, 2022, Phong, 2023, Williams 2023). These sites were subject to historic and repeated application of Aqueous Film Forming Foam (AFFF). PFAS can partition into concrete which is relatively porous and also self-assemble on surfaces. Understanding the surface and vertical distribution of PFAS in AFFF-impacted concrete is important for assessing the risk and informing potential remediation strategies.

PFAS comprise a very broad group of over seven million persistent synthetic chemicals (Schymanski, 2023). Our understanding of the presence, toxicity and potential effects of this group of contaminants continues to advance. Focus has been on soil and groundwater impacts in source zones. By contrast, much less attention has been paid to concrete as a secondary source of PFAS contamination (Douglas 2023). This remains a key source of uncertainty in Conceptual Site Models (CSM).

AFFF in the environment typically forms complex mixtures of many individual PFAS which is influenced by the use of different foam formulations over time (Barzen-Hanson, 2017). Interactions with constituents within the concrete and the presence of co-contaminants also introduces further complications in assessing PFAS fate and transport (Williams, 2023).

Site Investigation

Cores of concrete were collected at locations of previous AFFF application. Representative samples were collected from 0-30 mm by drilling the surface. Cores of 100 mm diameter and up to 300 mm length were cut and crushed at discrete intervals to determine the depth of PFAS penetration within the concrete. The drilling technique, sampling methodology and laboratory preparation procedures minimised the potential for cross-contamination. The sub-samples were extracted with subsequent analysis carried out by Liquid Chromatography coupled with tandem Mass Spectrometry (LC-MS/MS).

Vertical Profile and PFAS Composition

The highest concentrations of PFAS were found to be 6:2 fluorotelomer sulfonate (6:2 FTS), perfluorohexanoic acid (PFHxA) and perfluorohexane sulfonate (PFHxS) and perfluorooctane sulfonic acid (PFOS). Further information is provided in Figure 1. The vertical profiles of PFAS within the different concrete cores are similar despite the anticipated heterogeneous nature of the concrete matrix. The majority of the PFAS mass was detected in the upper sections of the core. Much reduced PFAS concentrations were determined at depth.

 
Figure 1 PFAS composition in slabs

Conclusions

Consideration of concrete as a potential secondary source of contamination is required when developing a reliable and robust CSM for sites with fire training areas or those that have experienced AFFF discharge to ground. The need for appropriate sampling and analytical methods is a prerequisite to assess risks. This study highlighted the need to target near surface contamination. 6:2 FTS, PFHxA, PFHxS and PFOS were determined at highest concentrations. Initial leaching testing were carried out. Further investigation is required to determine the potential for mass release during rainfall events.

Past research has highlighted:

• The possibility of a wicking effect where PFAS in the sub-surface can be brought to the surface for release during the next surface water run-off event (Phong 2022).
• Advective flow of water, diffusion and back-diffusion and/or air/water partitioning and sorption/desorption may be important transport mechanisms.

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References