In Situ Colloidal Activated Carbon for <scp>PFAS</scp> Remediation in Groundwater: Principles, Performance Expectations, and Lessons from the First Decade

Abstract Injectable colloidal activated carbon (CAC) has been commercially used to remediate per and polyfluoroalkyl substances (PFAS) in groundwater since 2016. The basis of the technology is enhanced PFAS retention within the aquifer from engineered increases in aquifer‐matrix sorptivity (“PFAS enhanced retention”), with re‐equilibration from the dissolved to the sorbed phase reducing contaminant mobility and exposure potential. Emplacement of 0.1 to 0.2% CAC can increase PFAS retention in aquifer solids by orders of magnitude relative to natural sorption alone. We synthesize a decade of publications and field experience to define practical design and performance expectations for CAC‐based in situ PFAS remediation. Governing principles and key features of CAC reagents are outlined, showing how transport and emplacement behavior, enhanced sorptivity, and competitive PFAS/co‐contaminant sorption control treatment longevity and spatial performance. Field data from 37 CAC barriers show PFAS concentration reductions of up to five orders of magnitude, with many post‐treatment concentrations at or below reporting limits, while reactive‐transport modeling under representative loading and competitive‐sorption conditions indicates that well‐designed CAC barriers can sustain protective performance over multidecadal to century‐scale times. We argue that retention‐in‐place via CAC can achieve comparable risk reduction to active removal at lower lifecycle cost and liability, and outline conditions where integration with PFAS monitored retention may be advantageous. Practitioner experience highlights that accommodating matrix mass, competitive interactions, and flux delineation in design, together with optimized monitoring well placement, is critical to achieving and interpreting CAC performance.