One of the methods being considered at the Hanford site in the state of Washington for safely disposing of low-level radioactive wastes (LLW) is to encase the waste in concrete and entomb the packages in the Hanford vadose zone sediments. The current plan for waste isolation consists of stacking low-level waste packages on a trench floor, surrounding the stacks with reinforced steel, and encasing these packages with concrete. Any failure of the concrete encasement may result in water intrusion and consequent mobilization of radionuclides from the waste packages. The mobilized radionuclides may escape from the encased concrete by mass flow or diffusion and move into the surrounding subsurface sediments. It is therefore necessary to conduct an assessment of the performance of the concrete encasement structure and the surrounding soil’s ability to retard radionuclide migration. Because of their anionic nature in aqueous solutions, the radionuclides, 99Tc and 129I were identified as long-term dose contributors in LLW. The leachability or diffusion of these radionuclide species must be measured to assess the long-term performance of waste grouts when contacted with vadose zone pore water or groundwater. To supplement the previously obtained data, a set of experiments were conducted using 99Tc-spiked concrete (with 0 or 4% metallic Fe additions) in contact with Hanford soil at extremely low moisture content (~1% by mass). The 99Tc diffusion profiles in the soil half-cells were measured after a time lapse of ~1.9 yr and the diffusion coefficient was calculated to be ~1.0 × 10−9 cm2 s−1.