The overarching goal of this project is to demonstrate an advanced, integrated toolkit for measuring and predicting per- and polyfluoroalkyl substance (PFAS) leaching and chemical mass discharge in the vadose zone. This toolkit, referred to as PFAS-VZ-CMD (PFAS-vadose zone-chemical mass discharge), consists of three modules (see Figure 1). Module 1 produces direct, quantitative in situ measurements of PFAS mass discharge, whereas Modules 2 and 3 produce enhanced-accuracy estimates or predictions of mass discharge, respectively. The three modules are designed to provide maximum flexibility to support applications at a wide range of sites for multiple site-characterization, risk-assessment, and mitigation-remediation purposes. The performance of each of the components employed in the modules has been fully tested at the field and/or bench scales. The toolkit employs the porewater sampling devices (suction lysimeter) that have been or will be installed in vadose-zone sources at many PFAS-impacted Department of Defense (DoD) sites. The integrated toolkit combines the sampling lysimeter with additional components and in situ tests that provide direct measurements and enhanced-accuracy estimates of porewater flux. The combination thereby produces direct measurements of PFAS leaching and mass discharge. 

The project has the following five objectives, designed to encompass each of the components of the toolkit as shown in Figure 1:

• Objective 1 - Site Selection and Installation: Two sites that have been the subject of significant high-resolution characterization of PFAS concentrations in soil and the vadose zone will be selected for this project. The selected test sites will be highly instrumented with devices to produce independent measurements of porewater concentrations, porewater flux, and mass discharge. These data will be used for performance assessment.
• Objective 2 - Bench-scale Optimization Tests of the Modules: A highly instrumented three-dimensional test cell will be used to conduct testing to optimize the designs and operations of the module instrumentation to the conditions of the site.
• Objective 3 - Field-Scale Performance Testing of the Vadose Zone (VZ) Flux Meter (Module 1): Module 1 will undergo demonstration testing, with operation of the VZ flux meter producing direct measurements of porewater concentrations, porewater flux, and PFAS mass discharge. Performance assessment will be conducted by comparing these measurements to the independently measured data obtained from the field instrumentation installed for Objective 1.
• Objective 4 - Field-Scale Performance Testing of the Enhanced-Accuracy Flux Estimation System (Module 2): Module 2 will undergo demonstration testing, wherein in situ hydraulic tests are conducted using the porewater sampling lysimeter, and the generated data are used as input to a model to estimate porewater flux and PFAS mass discharge. Performance assessment will be conducted by comparing these estimates to the independently measured data obtained from the field instrumentation described for Objective 3. 
• Objective 5 - Performance Testing of the Mass Flux Prediction Model (Module 3). Module 3 will undergo demonstration testing, wherein in situ tracer tests are conducted using the porewater sampling lysimeter, and the generated data are used as input to PFAS-LEACH to predict porewater flux and PFAS mass discharge. The performance of the PFAS-LEACH model and Module 3 as a whole will be assessed by comparing simulated predictions to the high-resolution measurements produced with the field instrumentation described for Objective 3.

Figure 1. PFAS-VS-CMD Integrated Toolkit

The PFAS-VZ-CMD toolkit consists of three modules as described below and depicted in Figure 1. It can be applied at sites with existing porewater sampling lysimeter installations as well as sites where new installations are planned. A single module can be used if desired, or the modules can be used in combination. The selection of which module or set of modules to use is tailored to the conditions of the site and the objectives of the project. The PFAS-VZ-CMD toolkit will provide the following measurements and information:

• Direct measurements of PFAS porewater concentrations [Module 1, Module 2, Module 3]
• Direct measurements of porewater flux with high temporal resolution [Module 1]
• Direct estimates or predictions of porewater flux with high temporal resolution [Module 2, Module 3]
• Direct measurements of PFAS leaching and mass discharge from the vadose zone [Module 1]
• Accurate estimates or predictions of PFAS leaching and mass discharge [Module 2, Module 3]
• The impact of individual storm events or other recharge activities on PFAS leaching [Module 1, Module 3]
• Direct measurements of long-term water recharge to groundwater [Module 1]
• The impact of lysimeter sampling on changes in water content and PFAS distribution in soil [Module 1]
• The zone of influence of the sampling lysimeters [Module 1]
• In situ tests to characterize hydraulic properties of the soil [Module 1, Module 2, Module 3]
• In situ tracer tests to characterize retention properties of the soil [Module 1, Module 2, Module 3]
• Support for the application of mathematical models to predict leaching and mass discharge [Module 1, Module 2, Module 3]
• Support for determination of PFAS-relevant site-specific soil screening levels [Module 1, Module 2, Module 3]

The PFAS-VZ-CMD toolkit represents the first system designed specifically to directly measure PFAS leaching and mass discharge in source zones. It will also provide the capability to accurately predict PFAS leaching in the vadose zone and mass discharge to groundwater. The toolkit can also be used for in situ characterization of soil hydraulic properties and PFAS retention potential, which can increase the accuracy of mathematical modeling. Specifically, the toolkit can be used to develop site- and source-specific soil-to-groundwater leaching standards and soil-screening levels that will improve decision-making by improving the ranking and prioritizing of sites for remedial action. In addition, these quantitative measurements and predictions will improve the engineering design of remediation and mitigation efforts. It is anticipated that the toolkit will reduce risks while also reducing site-management costs for many DoD sites impacted by PFAS. It is expected that the PFAS-VZ-CMD toolkit can be applied to the hundreds of sites impacted by PFAS, as well as to many other PFAS-impacted sites. (Anticipated Project Completion - 2026)