How Macroporous Soil Heterogeneities Influence the Transport and Retention of PFASs in the Vadose Zone: A Controlled Laboratory Study

Per- and polyfluoroalkyl substances (PFASs) have received increasing attention in the last two decades due to the gathered knowledge about their risks to the environment and human health. The processes that contribute to the transport of PFASs contamination in the environment from the source to groundwater need to be better understood to implement effective mitigation strategies reducing the risk of the most common pathway of PFASs exposure to humans, drinking PFASs-contaminated tap water. Previous studies have already pointed out sorption of PFASs to soil surfaces as well as to the air-water interface (AWI) under unsaturated conditions. Additionally, it is known that physical heterogeneities, such as macropores in soils originating, for example, from earthworms or decayed roots, have an impact on the retention and transport of solutes. The relatively rapid preferential flow through the macropore channels interacts with the slow flow and diffusion in the soil matrix, affecting the chemical breakthrough. This influence of these macroscopic physical heterogeneities and the related hydrodynamics on the transport of PFASs in soil has not been fully elucidated, requiring controlled laboratory studies.

Our study aims to fill this scientific gap using column experiments where breakthrough curves (BTCs) from homogenously packed porous media are compared with those including artificial macropores. In preliminary experiments we were able to prove the primary hypothesis that the macropore configurations, defined by the diameter and length, affect the BTCs. It is expected that PFASs transport in sand under unsaturated experiences retardation caused by sorption only to the AWI, meaning that the sorption of PFASs is only controlled by the water saturation. Modeling the experimental BTCs helps to validate our conceptual model – derived from the column experiments - of the interactions of PFASs sorption and release from the double domain media (sand vs macropore).

This work presents the preliminary data and findings to test our hypothesis on the effect of macropore configuration on PFASs BTCs and provides a basis for further work with field-collected undisturbed soil containing macropores in a natural configuration.