Interfacial sequestration and transport antagonism: The CAH–PFAS co-contamination conundrum in groundwater remediation

The widespread co-occurrence of per- and polyfluoroalkyl substances (PFAS) and chlorinated aliphatic hydrocarbons (CAHs) in groundwater represents a major remediation challenge, driven by overlapping industrial legacies, strong concentration asymmetry, and fundamentally different transport and treatment behaviors. This review synthesizes mechanistic evidence from field studies, laboratory investigations, and technological assessments to examine the coupled transport processes and treatment-stage conflicts that define CAH–PFAS co-contamination. We demonstrate that interfacial sequestration of PFAS at CAHs dense non-aqueous phase liquid (DNAPL)-water interfaces creates persistent secondary sources, while their surfactant properties mobilize DNAPLs within high-concentration source zones, potentially altering initial source architecture. At the plume scale, contrasting physicochemical properties and aquifer heterogeneity drive hydraulic and geochemical decoupling, such that CAH distributions often provide a poor proxy for PFAS occurrence. At the treatment stage, remediation antagonism emerges because the presence, behavior, or required treatment conditions of one contaminant class diminish the effectiveness or compatibility of remedial actions directed at the other. This antagonism is expressed through incompatible redox requirements, microbial inhibition of CAH dechlorination by PFAS, and sorption/process conflicts that destabilize engineered treatment systems. Consequently, conventional single-technology approaches are structurally inadequate for co-contaminant sites. This review concludes that effective management of CAH–PFAS co-contamination requires integrated conceptual site models, contaminant-specific characterization, and staged treatment strategies that account for plume decoupling, long-term rebound, and the limitations of sequestration-based remedies.