Formatted Title
Field Data Collection for Parameterization of a Modified HYDRUS Model for Simulating PFAS Leaching in the Vadose Zone
Background/Objectives
Per- and polyfluoroalkyl substance (PFAS) source zones have resulted from releases of PFAS to the ground surface at numerous sites such as firefighter training areas. These shallow source areas pose a risk to underlying groundwater, though current methods of quantifying the potential for downward migration are relatively limited. This presentation reviews the data collection approach in an ongoing project that is being used to to evaluate, demonstrate, and validate the use of the commercially available and widely used HYDRUS unsaturated flow and transport model for use at PFAS sites. The primary purpose 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. Flow and transport modeling in the unsaturated zone at contaminated sites is notoriously complex, particularly for PFAS, which has not yet been rigorously modeled. Our objectives include identifying critical input parameters and evaluating the minimum data requirements for model input to produce an acceptably calibrated and validated model.
Approach/Activities
The vadose zone below the PFAS source area will be instrumented with soil moisture probes and suction (sampling) lysimeters (for PFAS concentrations within infiltrating pore water) to collect a temporal data set with which to calibrate the model, and another data set to validate the model. Soil moisture probes and lysimeters will be installed at targeted depths using angled borings to prevent disturbing the vertical movement of water through the soil column. Geologic media samples will be collected from each distinct lithologic unit for analysis of PFAS concentrations, fraction organic carbon (foc) and model-specific parameters such as saturated and unsaturated hydraulic conductivity, soil-water retention curves, PFAS soil desorption rates, and hysteresis. We will also conduct a geophysical survey (electrical resistivity) to better understand site heterogeneities, which typically dominate contaminant transport in the vadose zone. Site-specific meteorological data such as precipitation, temperature, evaporation will be collected from an on-site weather station.
Results/Lessons Learned
Data from the project will produce a methodology for using and evaluating the performance of unsaturated zone models for PFAS transport from near-surface source zones to underlying groundwater. Specifically, the project will test the efficacy of the modified HYDRUS model, providing confidence in using this model at future sites. Preliminary data from this effort will inform the minimum data required to develop an acceptable PFAS transport model, which is highly valuable given the cost associated with detailed site characterization. This presentation will focus on data collection efforts associated with model parameterization including lessons learned from equipment installation and comparison of site-specific measurements from literature values.