This paper presents our findings on the effect of adding biopolymers to locally available soil to form low-cost alternative landfill liner materials. Hydraulic conductivity is the key parameter for assessing the suitability of any geomaterial as a landfill liner. In view of this, the hydraulic conductivity of biopolymer-modified soil was determined at various dosages. Two biopolymers - xanthan gum and guar gum - were used in the experimental work at dosages of 0.5%, 1%, 2%, and 4% by weight of dry soil mass. The results indicated that the hydraulic conductivity values decreased with an increase in biopolymer content and consolidation pressure (σcp). The influence of xanthan gum content (G1) and guar gum content (G2) on the soil hydraulic conductivity is represented by nonlinear regression models based on experimental data obtained for σcps of 50, 100, 200, 400, and 800 kPa. The performance of the soils was assessed using reliability-based design optimization methodology. The optimal mixtures of cohesive soils blended with a range of G1 and G2 were evaluated considering the variability associated with the dosages of G1 and G2, and the σcp. © 2022 American Society of Civil Engineers.