Abstract: Field tests were conducted to evaluate the performance of cover barrier for legacy organic-contaminated soil resulting from informal landfilling at a chemical plant site. Xanthan gum (XG), a low-carbon biopolymer, was used as a soil amendment to amend the existing on-site cover soil (silty clay) through two treatment methods: (1) a wet mixing method, in which the XG was 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 method, where the XG powder was directly mixed with the soil. The cover barrier consisted of a protective layer and compacted soil layer (gas/hydraulic barrier). Four types of barrier configurations were constructed: XG-amended compacted soil, a composite layer of amended soil and XG-amended geosynthetic clay liner (GCL), a composite layer of unamended soil and geomembrane, and unamended compacted soil. Core samples were collected from the compacted soil layer and protection layer. Laboratory tests were conducted to evaluate the homogeneity of the compacted soil, the distribution uniformity of XG, and the gas/hydraulic barrier performance of the compacted soil specimens. These tests included measurements of degree of compaction, moisture content, Atterberg limits, gas permeability, gas diffusion, and flexible-wall hydraulic conductivity tests. Results showed that both unamended and amended compacted soil had similar volumetric water content, degree of compaction, and void ratio values. At saturation levels of 77% - 90%, the gas permeability of amended compacted soil significantly reduced from 8×10^-13 to 3.3×10^-12 m² for unamended soils to 4.7×10^-14 to 4.2×10^-13 m²(approximately a tenfold decrease). The gas diffusion coefficient also decreased from 1.2×10^-8 to 6.9×10^-7 m²/s (unamended) to 4.9×10^-9 to 1.7×10^-8 m²/s (amended) representing about a fourfold reduction. The saturated hydraulic conductivity of unamended soil was 8.8×10⁻⁸ m/s, whereas amended soils exhibited hydraulic conductivity below 1×10^-9 m/s. After 82 days of field monitoring, the volumetric water content of the amended CCL varied by less than 1% before and after rainfall, while the unamended soil showed a 33% relative variation. Moreover, the VOC concentration within the protection layer above the amended compacted soil remained below 0.2 mg/m³, which is one-sixth 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.