A capillary microporosity framework for modeling the fate and transport of PFAS in concrete

Concrete pads impacted by aqueous film forming foam (AFFF) represent long-term sources of per- and polyfluoroalkyl substances (PFAS) to stormwater runoff that require environmental impact assessment. However, existing assessment models have not reproduced field observed PFAS concentration profiles. Accordingly, a new mechanistic model for PFAS assessment (CAPCON) based on a capillary microporosity framework for PFAS transport within concrete is proposed. The proposed CAPCON model includes dual-domain diffusion within water-filled capillary pores together with episodic PFAS release to runoff driven by wetting and drying cycles based on site-specific climatic conditions. Comparison of CAPCON modeled versus measured PFAS concentration profiles from concrete cores recovered from two field sites with contrasting climatic and PFAS loading conditions resulted in good agreement (log-RMSEs of 0.03 to 0.36). The calibrated CAPCON model also captured the characteristic first-flush pattern observed in field-measured sheet-flow runoff tests, although first-flush magnitudes were overpredicted for three of the four site/compound combinations. Results indicate that climate influences PFAS transport in concrete. In wetter climates, deeper water penetration into concrete increases the extent of water-filled capillary pores, enabling faster diffusive transport through the capillary region and deeper PFAS penetration. In drier climates, diffusion is slower within unsaturated, lower-permeability regions of the concrete, resulting in greater near-surface accumulation. The proposed CAPCON framework provides a mechanistic basis for evaluating PFAS transport and release from concrete pads under variable climatic conditions and offers improved capability for assessing long-term environmental impacts of AFFF-impacted concrete pads.