Managing per- and polyfluoroalkyl substance (PFAS) sources to groundwater is a critical, emerging concern for the Department of Defense (DoD). One particular need is to understand the strength and long-term behavior of PFAS sources in the vadose zone. To enable the DoD to better understand PFAS sources and prioritize scarce remediation resources, the DoD needs better ways to determine the mass discharge (i.e., the mass flux) of key PFAS from vadose zone sources to groundwater. There is also a need to know if a particular vadose zone is conducive for transformation of PFAS precursors to the perfluoroalkyl acids that are the focus of many PFAS regulatory programs.

To meet these needs, the project team plans for a sensor-based alternative to complicated on-call field campaigns, where PFAS mass flux will be measured from a few key intermittent recharge events or from high-frequency routine sampling. With real time sensors and new process knowledge gained from this project, DoD remedial project managers and their consultants can remotely monitor PFAS vadose zone sources to determine recharge rates and know when to sample.

The primary objective of this project is to determine if a technology-driven solution to the problem, deployment of three-dimensional internet of things sensor arrays at PFAS vadose source zones, can: 1) solve “the recharge problem” when trying to calculate PFAS mass flux, and 2) enable intelligent, more efficient sampling of leachate to determine PFAS mass flux from the source vadose zone. Secondary objectives include measuring geochemical conditions, as measured by the sensor array. An indirect high-level goal is to provide enabling technology for the DoD to perform systematic, science-based portfolio reviews to determine which PFAS vadose source zones need to be remediated and which ones do not pose a significant threat to groundwater.

At two DoD PFAS vadose zone sources (one in an arid climate, one in a wet/humid climate), the project team plans to deploy and operate for two years the following sensors that collect data at 15 minute intervals: high density sensor array “sticks” measuring temperature, oxidation-reduction potential, and water levels; commercial Drain Gauge Lysimeters (DGL) equipped with a pressure sensor that directly measures the recharge collected by the lysimeters; soil moisture probes; soil tensiometers; and a weather station. In addition, an automatic sampler will be deployed to collect intra-recharge event PFAS samples to determine the best time to collect representative PFAS leachate samples (e.g., during dry periods versus first flush versus recharge flow-weighted averages) from the DGL. Finally, sensor data will drive five discrete manual PFAS leachate sampling events, where PFAS leachate samples will be collected from a novel water table sampler, shallow groundwater, and suction lysimeters.

This project will improve the approach to measure and estimate recharge and collect representative PFAS leachate samples at different types of sites at different locations. With these data, new guidance on how to directly measure the mass discharge (mass flux) of PFAS from vadose zone sources to groundwater will be available for the hundreds of PFAS sites around the world, with a recommended three-tiered system for simple, intermediate, and complex sites. In addition, a new technology to perform high-frequency sampling of geochemical conditions within a PFAS vadose zone source will be tested to shed light on the potential for aerobic transformation of precursors. (Anticipated Project Completion - 2025)