Abstract: Using low-carbon biopolymer xanthan gum (XG) as an amendment, two amendment construction methods are proposed for the in-situ silty cover soil: (1) a wet mixing process method, in which the XG is pre-hydrated into a solution and sprayed onto loosely placed silty clay, followed by infiltration and compaction to form an amended silty clay; and (2) a dry mixing process method, where the XG powder is directly mixed with the soil. The cover barrier consists of a protective layer and compacted soil layer. Four types of barrier configurations are constructed: XG-amended compacted soil, a composite layer of amended soil and XG-amended bentonite geosynthetic clay liner, a composite layer of unamended soil and geomembrane, and unamended compacted soil. Core samples analyses show that both unamended and amended compacted soil had similar volumetric water content, degree of compaction, and void ratio values. At degrees of saturation of 77%~90%, the gas permeability of amended soil significantly reduces from 8×10^-13~3.3×10^-12 m² for unamended soil to 4.7×10^-14~4.2×10^-13 m². The gas diffusion coefficient also decreases from 1.2×10^-8~6.9×10^-7 m²/s (unamended) and 4.9×10^-9~1.7×10^-8 m²/s (amended) representing about a fourfold reduction. The saturated hydraulic conductivity of unamended soil is 8.8×10^-8 m/s, whereas hydraulic conductivity of amended soils is below 1×10^-9 m/s. After 82 days of field monitoring, the volumetric water content of the amended CCL varies by less than 1% before and after rainfall, while the unamended soil shows a 33% relative variation. Moreover, the VOC concentration within the protection layer above the amended compacted soil remains below 0.2 mg/m³, which is one-tenth of the value observed above the unamended compacted soil (2 mg/m³), and just 1/60 of the (VOC) concentration measured in the contaminated soil layer (12.5 mg/m³), indicating superior gas and fluid isolation performance of the biopolymer-modified compacted barrier.