Practical Framework for Using <scp>HYDRUS</scp> to Estimate <scp>PFAS</scp> Mass Discharge from the Vadose Zone to Groundwater

Abstract Accurately quantifying per‐ and polyfluoroalkyl substances (PFAS) mass discharge from the vadose zone to groundwater is critical for effective site characterization, risk assessment, and remediation planning. However, few standardized approaches exist for translating site‐specific PFAS data into actionable mass discharge estimates. This paper presents a practical modeling framework that uses HYDRUS, a widely validated vadose‐zone flow and transport code, to simulate PFAS leaching and estimate mass discharge to groundwater. The framework outlines step‐by‐step procedures for model selection, domain design, parameterization, and boundary condition specification, with emphasis on key PFAS processes such as tension‐driven flow, nonlinear kinetic solid‐phase sorption, and air–water interfacial partitioning. Guidance is provided on incorporating site‐specific data—such as soil hydraulic properties, PFAS concentrations in biosolids or soils, and climate‐driven infiltration dynamics—while addressing uncertainty through sensitivity analyses. Application of the framework demonstrates how HYDRUS can bridge laboratory, field, and regulatory needs by offering defensible, mechanistic predictions of PFAS flux to groundwater. The approach is adaptable across diverse site conditions, from agricultural fields impacted by biosolids land application to fire‐training areas affected by aqueous film‐forming foams (AFFF). By providing a transparent and reproducible methodology, this framework supports practitioners and regulators in improving conceptual site models, prioritizing monitoring strategies, and evaluating remediation performance. Ultimately, the framework advances the integration of vadose‐zone modeling into groundwater management and decision‐making for PFAS‐impacted sites.