Colloidal silica (fumed silica) suspensions tested as carriers for remedial amendment delivery in subsurface contaminant remediation and as media for underground contamination containment. Rheological behavior of the silica suspensions under subsurface environment is not well documented while this knowledge is essential for the preparation and field injection of the suspensions. This contribution is focused on rheological characteristics of colloidal silica suspensions under various environmental conditions relevant to remedial amendment delivery for subsurface remediation. Influence of silica particle concentration, water source, ionic strength, pH, aging, amendment type and concentration, and subsurface sediment on rheological behavior of silica suspensions was evaluated. This work is the first attempt to relate the rheology of aqueous silica suspension to subsurface injection and amendment emplacement. Impact of remedial amendment and subsurface sediment is reported for the first time. Addition of potassium permanganate (KMnO4) amendment to aqueous silica suspensions increased viscosity; while presence of alcohol amendment in suspensions decreased their viscosity. All tested suspension formulations exhibited shear thinning before gelation. Addition of amendment to suspensions did not reduce the degree of shear thinning. Gelation rate of silica suspensions was increased with silica concentration and with addition of sediments. Rheological characteristics of shear thinning colloidal silica suspensions were compared to that of shear thinning polymer xanthan gum solutions, which was also applied for amendment delivery in subsurface remediation.