Formatted Title
Validation of a Cell-Based Screening Model for Assessing PFAS Mass Flux from the Vadose Zone to Groundwater
Background/Objectives
Assessing the rate of mass flux from the vadose zone to underlying groundwater has received renewed attention due to per- and polyfluorinated alkyl substance (PFAS) contamination. PFAS are recalcitrant and bioaccumulative chemicals with a high toxicity. The most common exposure pathways for PFAS in groundwater are intentional application at the land surface, unintentional spills, and aerosol deposition. In these cases, PFAS must migrate through the vadose zone before contaminating the underlying aquifer. Because vadose zone transport of PFAS is transient and highly complex, many contaminated sites do not have sufficient data or budget to justify a comprehensive modelling effort. Screening models have been developed as an alternative to comprehensive modelling efforts for PFAS and other contaminants when a model-based risk assessment is needed, but comprehensive modelling is not justifiable or possible.
Approach/Activities
Screening models and cell‑based models both have applications for PFAS contamination in the environment. But to date, no screening model has been optimized for simplicity and the ability to readily assimilate the large amounts of transient data needed for vadose zone simulations. Similarly, no cell-based model has attempted to account for changes in unsaturated hydraulic conductivity due to wetting and drying of the media. Column studies were conducted over the course of 8 weeks which simulated rainfall and drying events. The concentration of PFAS leached from AFFF-impacted soils was measured. Both the water flux and PFAS concentration in the leachate were measured throughout the course of the experiment.
Mass Balance Validation: A simplified mass balance-based approach for assessing PFAS partitioning combined the leaching of PFAS is simulated using equilibrium measured experimentally and obtained from literature. The leaching of PFAS was then simulated for two AFFF impacted sites using the mass balance from Equation 1.
Equation 1
A 1-D HYDRUS model was calibrated to the water flux term and forward simulations were conducted using the same partitioning parameters as those for used for the mass balance model. The results of each equilibrium model are compared against leaching data to assess the mass-balance models performance against a well validated numerical code.
Water Flux Validation: The mass balance model uses Richard’s equation with unit gradient conditions for assessing hydraulic leaching. The Van Genuchten water retention model is used with parameters estimated the Rosetta pedotransfer function software for Richard’s equation simulations of water flux. A lysimeter study with monitored changes in recharge was used to validate the estimation of water flux in the cell-based model.
Results/Lessons Learned
The mass balance model simulations of the column studies had very similar results to those in HYDRUS. The small differences between the models are attributed to differences in water content estimation and air-water interfacial area estimation in the two models. Both performed very well at predicting the total mass flux from the columns over a period of 8 weeks. Water flux data from the field lysimeter studies was also well predicted by the hydraulic model with unit gradient conditions. These results combined suggest that the simplified, cell-based model presented herein is an effective alternative to comprehensive modelling for assessing PFAS mass flux from the vadose zone. This is advantageous as the model is easy to implement , significantly less complicated than HYDRUS modelling, and can readily assimilate the necessary agroclimatalogical data from open-source datasets.