Formatted Title
Numerical Simulations of Per- and Polyfluorinated Substances for Uncertainty and Sensitivity Development
Background/Objectives
Soil and groundwater contamination due to per- and polyfluorinated substances (PFAS) has emerged as a significant concern across many states and countries. While the environmental community continues to evaluate treatment mechanisms and clean-up methods, the need for evaluating fate and transport sensitivities continue to represent challenges with mathematical modeling techniques. The challenges with PFAS modeling involves a variety of PFAS specific properties, some yet to be understood (e.g., unknown precursors, variability in sorption, etc.). Therefore most current groundwater modeling efforts account for these unknowns through simplifying assumptions. While simplifying assumptions as an acceptable and standard methodology the results should be accompanied by uncertainty analyses and sensitivity testing. This study represents methods and practical use of open source mathematical tools for improved uncertainty analyses and sensitivity testing as related to PFAS modeling. Furthermore the results provide remedial practitioners and technical teams a quantitative representation of PFAS variability as related to the physical environment.
Approach/Activities
The approach for this study uses synthetic and/or publicly available data to generate a mathematical model for uncertainty evaluations and sensitivity testing. Through parameter perturbation of soil properties, chemical properties and conceptual model alterations a suite of plume transport rate and distances were calculated. This suite of results was then analyzed for standard statistical metrics and visualizations to provide a comprehensive review of the potential variability which may be encountered at a PFAS site. Furthermore, the results will demonstrate variability of sensitive parameters such that other sites can be evaluated and reviewed in context of focus areas, data collection, and prioritization of investigations and remedial actions. For this study standard groundwater modeling tools were used, such as MODFLOW-USG, PEST++, Groundwater Vistas, and Flopy.
Results/Lessons Learned
The preliminary results indicate that significant variability of plume travel time and distance potentially exists. The travel time and distance are dependent on parameters which are typically more understood in “classical” groundwater modeling. For example, the octanal-water coefficient (Kow) of more typical volatile organic compounds (such as chlorinated solvents) are fairly well understood through chemical property testing. Traditionally, this results in the fraction of organic content (Foc) being less understood and more sensitive as related to retardation. However, for PFAS, the expected range of sorption properties (i.e., sorption to organics and ionic exchange) is typically unknown and can be rather large. Therefore the retardation parameter for PFAs becomes significantly more sensitive in terms of transport, without consideration of variability other parameters (hydraulic conductivity, gradient, recharge, etc.). Initial testing from this study indicate that if FOC is approximately 0.005 and Koc is allowed to change by 2 orders of magnitude plume travel distances may vary by a difference of 150%. Meanwhile, if Foc is assumed to be 0.001 and Koc is allowed to change by 2 orders of magnitude the plume travel distances may vary by a factor of 25%. These results give indication of the level of importance the sorption plays in PFAs transport. When variability of all parameters are accounted for, the practitioner can become overwhelmed with uncertainty. Extended results from this study will include uncertainty and sensitivity analyses to guide practitioners in overcoming challenges in prediction for ill-posed problems such as multi-parameter systems with difficult to constrain parameter ranges.