Formatted Title
Derivation of Site-Specific Soil Standards Reflective of Mass Loading from PFAS Source Areas
Background/Objectives
The overall objective of this study is to develop a framework for evaluating site-specific soil-to-groundwater leaching and mass loading to facilitate source strength ranking and prioritization. The study results will assess processes influencing migration and fluxes of source zone-derived poly and perfluoroalkyl substances (PFAS) and the applicability of current vadose zone models for quantitative prediction of PFAS flux. Twenty-two (22) lysimeters were installed in four source areas located at a remote Department of Defense installation to evaluate PFAS contaminant migration from validated source areas to the underlying unconfined groundwater aquifer. Specific objectives of this study include: 1) generate data to support development of a regulatory-accepted methodology to calculate an actionable site-specific leaching value for PFAS, and 2) develop a defensible methodology to prevent unnecessary remedial action or cleanup of sites that do not pose a risk to human health or the environment for environmental restoration as well as MILCON projects. The approach will employ tiered levels of model parameterization and model sophistication in comparison to observed PFAS porewater concentrations to determine if source behavior can be modeled with sufficient accuracy to predict groundwater concentrations and whether simpler models can achieve the desired accuracy required for decision-making in comparison to the results generated from the more complex models. The ultimate goal of the project is to establish a site-specific approach for identifying relevant sources and determining associated source strength (mass discharge over time) that can be applied to a range of conditions and define a framework for source prioritization based on source strength.
Approach/Activities
A network of lysimeter monitoring locations equipped with soil moisture probes was established to evaluate PFAS contaminant migration from source areas to underlying groundwater and to determine if there was a measurable increase in soil moisture associated with event-driven precipitation events. Soil samples co-located with lysimeters were collected for soil characterization and PFAS analysis. We will demonstrate the PFAS-LEACH decision-support modeling platform at the field-scale by using it to simulate site-specific attenuation and mass loading. PFAS-LEACH is based on the standard United States Environmental Protection Agency (US EPA) soil screening limit framework but considers PFAS-specific retention properties. In particular, the PFAS-LEACH models take into account PFAS adsorption at the air-water interface and the unique soil properties that influence PFAS retention within the vadose zone on a site-specific basis. The approach will reconcile datasets including soil moisture, porewater, and source area data to identify the primary parameters governing PFAS mass loading from the vadose zone most critical to achieve optimal model performance and defensibility. The performance of the models to simulate leaching and mass discharge will be verified against measured porewater concentrations. The intent is to verify the capability of the models to match observed groundwater concentrations and determine the data density required to parameterize the different model tiers of PFAS-LEACH (from the most comprehensive 3D model to the simplest 1D model) reliably and accurately.
Results/Lessons Learned
This study employs source-specific models to provide a framework by which to estimate mass loading and a scientifically robust basis for site prioritization under actual field conditions. We will compare the results of porewater sampling obtained from multiple events to the simulated leaching results and determine whether accurate results can be obtained using the simplest model or if more sophisticated tiers are required. We will evaluate source strength in terms of the magnitude and persistence of mass loading. The results of this work will establish a mass-loading based prioritization approach for PFAS impacted source areas optimizing resource allocation and expediting citing locations suitable for military construction (MILCON) activities.