One of the most common effective techniques for contaminant remediation in the vadose zone is to use a surface barrier to reduce or eliminate soil-water flow in order to reduce the contaminant flux to the underlying groundwater. Confirming the reduction of soil-water flow is challenging because of the difficulty of measuring soil-water flux directly. We propose a hydraulic-conductivity factor as an indicator to quantify the reduction of soil-water flow below a surface barrier. The factor can be calculated using the measured soil-water content or pressure. The pressure-based formula was applied to the interim surface barrier at the Hanford’s T Tank Farm.