Formatted Title
Field Validation of a Modified HYDRUS Model for Simulating PFAS Leaching in the Vadose Zone
Background/Objectives
As a part of ongoing research, the HYDRUS unsaturated flow and transport model has been modified to simulate per- and polyfluoroalkyl substance (PFAS) transport in the vadose zone. While specific model modifications have been validated against laboratory transport data, model predictions have not yet been validated at the field scale. Therefore, this presentation will present results of the first field-scale evaluation of the modified HYDRUS model to simulate PFAS leaching from a aqueous film forming foam (AFFF)-impacted firefighter training area.
The primary goal of this research is to develop a methodology for modeling PFAS fate and transport in vadose zones, particularly for charactering the transport between the source zones and the groundwater table. Further, flow and transport modeling in the unsaturated zone at actual contaminated sites is notoriously complex, particularly for PFAS. Thus, our approach also endeavors to make the modeling process as practically approachable as feasible, invoking the principle of parsimony, where models should be “as simple as possible, but no simpler”, to meet site goals. Thus, our objectives also include evaluating the minimum data requirements for model input that results in acceptable and accurate model predictions. This modeling methodology will be described.
Approach/Activities
To meet these objectives, we are currently conducting a field-scale demonstration/validation project at a former fire-training area on an U.S. Air Force Base in Cheyenne, Wyoming. The use of available site data (i.e., weather data, site stratigraphy, hydraulic characterization, soil properties, depth to water table, concentration data, etc.) to develop a preliminary model with which to base our validation will be presented. The site was recently instrumented with soil matrix potential sensors and sampling lysimeters to collect temporal datasets of changes in soil moisture and PFAS porewater concentration over a two-year monitoring period. The two-step historical validation procedure used to calibrate and then validate both the flow and transport model will be described. Finally, we will discuss the results of laboratory work designed to parameterize key flow and transport processes for PFAS within the site vadose zone. These laboratory evaluations will form the basis for evaluating minimum data requirements to develop a unique flow and transport model for PFAS that best represents site conditions. Transport processes of interest include nonlinear moisture content dependent air-water interfacial adsorption, nonlinear rate limited sorption/desorption, wetting and drying hysteresis, and preferential flow effects.
Results/Lessons Learned
To date, we have completed initial site characterization activities to collect available site data and soil samples with which to develop a preliminary model for the site and establish model initial conditions. We have also completed the installation of the sampling/monitoring system. We have initiated our laboratory evaluation effort. At the time of the presentation, we will have completed our laboratory efforts and constructed our preliminary model for PFAS flow and transport. Although we are going into the winter season at the site, we may also have some field flow and transport data. However, the completion of the preliminary model alone will allow us to develop the proposed presentation topics.