Vertebrae™ Segmented Horizontal Wells for Monitoring <scp>PFAS</scp> Contaminant Mass Flux and Discharge

Abstract Quantifying contaminant mass flux and plume mass discharge is essential for flux‐based management of per‐ and polyfluoroalkyl substances (PFAS), yet few monitoring technologies provide the spatial resolution, repeatability, and cost efficiency needed for long‐term application. Many PFAS plumes originating from discrete source areas exhibit high width‐to‐thickness ratios driven by recharge‐dominated transport, weak vertical dispersion, and hydraulic conductivity anisotropy, limiting the effectiveness of conventional vertical transects for control‐plane discharge assessment. This study evaluated the Vertebrae™ Well System (VWS), a segmented, nested horizontal multi‐screen technology, for quantifying groundwater flux and perfluorooctane sulfonate (PFOS) mass discharge under relatively homogeneous aquifer hydraulic conditions at the Grayling Army Airfield (GAAF) in northern Michigan, USA. Two 500‐feet VWS arrays were installed downgradient of a former aqueous film‐forming foam source area, oriented perpendicular to groundwater flow. Vertical placement was informed by prior high‐resolution characterization. Groundwater flux was quantified using multiple independent methods, including hydraulic profiling, tracer testing, active distributed temperature sensing, and grain‐size‐based permeability estimation, and integrated with PFOS concentrations across eight performance monitoring (PM) events. Geostatistical interpolation was used to support transect‐scale flux and discharge integration in an unconfined sandy aquifer. Results indicate consistent spatial patterns and reproducibility across methods, with cross‐method agreement within expected field‐scale variability, supporting the interpretation that segmented horizontal wells can capture subtle lateral flux variability within the groundwater table fluctuation zone and quantify plume‐scale PFOS mass discharge with high spatial and temporal resolution. Horizontal multi‐screen systems provided repeatable, spatially resolved estimates of groundwater flux and PFOS mass discharge when screen placement targeted dominant concentration and groundwater flow horizons. These findings highlight the utility of segmented horizontal wells for flux‐based remediation evaluation, regulatory decision‐making, and long‐term performance monitoring at PFAS‐impacted sites.