Trapping and treating Cr(VI) is of great importance to environmental applications. In this work, the natural polymer scleroglucan and the reducing agent sodium thiosulfate were used to produce a water-based polymeric fluid to simultaneously trap and treat Cr(VI) in the subsurface. The thiosulfate reduced toxic Cr(VI) to less toxic Cr(III), as was apparent from fluid discoloration from bright orange to dark brown. Additionally, the viscosity of scleroglucan fluids containing sodium thiosulfate was low before mixing with Cr(VI) (150 mPa·s with 1 wt% scleroglucan in water at 23 °C), suggesting that the fluids can be effectively pumped in the subsurface. As sorption of scleroglucan onto geological substrates can affect its transport in geological media, sorption was measured using a Quartz-Crystal Microbalance with Dissipation Monitoring (QCM-D). Sorption of scleroglucan onto silica (used as a model geological substrate) was promoted in 100 mM CaCl2 at pH = 7 and in deionised water at pH = 4, whereas it was hindered by humic acids (HA), which are usually naturally present in groundwater. Upon contact with Cr(VI) and sodium thiosulfate, scleroglucan chains crosslinked and the fluids gelled, acquiring high viscosity (approximately 2000 mPa·s) in water, in salt solutions (100 mM KCl or 100 mM CaCl2) and in the presence of HA. Gelled scleroglucan was also characterized by high shear viscoelastic moduli and became plastic at high compressional loads (70 N to 100 N, depending on the water chemistry). Gelation is due to the crosslinking of scleroglucan by Cr(III), following the reduction of Cr(VI) by sodium thiosulfate. Scleroglucan fluids containing sodium thiosulfate may potentially form a barrier around the Cr(VI) contaminated zone, preventing its migration during treatment.