Geochemical Data Package for the Hanford Immobilized Low-Activity Tank Waste Performance Assessment (ILAWPA)

Lockheed Martin Hanford Company (LMHC) is designing and assessing the performance of disposal facilities to receive radioactive wastes that are stored in single- and double-shell tanks at the Hanford Site. The preferred method of disposing of the portion that is classified as low-activity waste is to vitrify the liquid/slurry and place the solid product in near-surface, shallow-land burial facilities. The LMHC project to assess the performance of these disposal facilities is the Hanford Immobilized Low-Activity Tank Waste (ILAW) Performance Assessment (PA) activity. The goal of this project is to provide a reasonable expectation that the disposal of the waste is protective of the general public, groundwater resources, air resources, surface-water resources, and inadvertent intruders. Achieving this goal will require prediction of contaminant migration from the facilities. This migration is expected to occur primarily via the movement of water through the facilities, and the consequent transport of dissolved contaminants in the porewater of the vadose zone. Pacific Northwest National Laboratory (PNNL) assists LMHC in their performance assessment activities. One of the PNNL tasks is to provide estimates of the geochemical properties of the materials comprising the disposal facility, the disturbed region around the facility, and the physically undisturbed sediments below the facility (including the vadose zone sediments and the aquifer sediments in the upper unconfined aquifer). The geochemical properties are expressed as parameters that quantify the adsorption of contaminants and the solubility constraints that might apply for those contaminants that may exceed solubility constraints. The common parameters used to quantify adsorption and solubility are the distribution coefficient (Kd) and the thermodynamic solubility product (Ksp), respectively. In this data package, we approximate the solubility of contaminants using a more simplified construct, called the solution concentration limit, a constant value. In future geochemical data packages, we will determine whether a more rigorous measure of solubility is necessary or warranted based on the dose predictions emanating from the ILAW 2001 PA and reviewers' comments. The Kds and solution concentration limits for each contaminant are direct inputs to subsurface flow and transport codes used to predict the performance of the ILAW system. In addition to the best-estimate Kds, a reasonable conservative value and a range are provided. We assume that Kd values are log normally distributed over the cited ranges. Currently, we do not give estimates for the range in solubility limits or their uncertainty. However, we supply different values for both the Kds and solution concentration limits for different spatial zones in the ILAW system and supply time-varying Kds for the concrete zone, should the final repository design include concrete vaults or cement amendments to buffer the system pH. If warranted, more technically rigorous information on uncertainty in the geochemical parameter values and estimates of the changes in parameter values over time will be supplied in future data packages