Recently, in-situ injections of polymer-stabilized colloidal activated carbon (CAC) have shown successful immobilization of per/polyfluoroalkyl substances from groundwater. Performance of an in-situ CAC barrier will depend on its subsurface distribution, governed partly by its colloidal properties/stability. Ours is the first study to provide key electrostatic properties of CAC and investigate the effects of aqueous chemistry on its ζ-potential, aggregation, and sedimentation kinetics. In this study, disparity between point of zero charge and isoelectric point of CAC suggest that protonation-deprotonation may not be its only surface charging mechanisms. The single and combined effects of pH, cations, and organic matter (bovine serum albumin (BSA), humic acid (HA)) observed in aggregation/sedimentation studies highlighted Ca²⁺ as a key factor in determining the CAC destabilization in aqueous environments. However, high Na⁺ concentrations reduced the effect of Ca²⁺, suggesting that high salinity environments might be favourable for CAC transport. Ca²⁺ showed ability to induce bridging of CAC particles through formation of chain-like CAC homo-aggregates and/or CAC-HA hetero-aggregates, seen in TEM images, indicating Ca²⁺-specific cation bridging as the main destabilizing mechanism for CAC. The sustained stability of CAC under aqueous conditions, where ζ-potential values (-30 to +30 mV) predicted aggregation/sedimentation, demonstrated that stabilization/destabilization mechanisms other than electrostatic forces were also present. Steric and/or Lewis acid base repulsion may be the main stabilizing forces, as indicated by reduction in CAC particle size prior to aggregation/sedimentation. Overall, this study highlights aqueous geochemical conditions that are critical for predicting subsurface CAC transport and spatial distribution.