Geochemical Data Package for the 2005 Hanford Integrated Disposal Facility Performance Assessment

摘要

CH2M HILL Hanford Group, Inc. (CH2M HILL) is designing and assessing the performance of an integrated disposal facility (IDF) to receive low-level waste (LLW), mixed low-level waste (MLLW), immobilized low-activity waste (ILAW), and failed or decommissioned melters. The CH2M HILL project to assess the performance of this disposal facility is the Hanford IDF Performance Assessment (PA) activity. The goal of the Hanford IDF PA activity 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 vadose zone to groundwater where contaminants may be re-introduced to receptors via drinking water wells or mixing in the Columbia River. Pacific Northwest National Laboratory (PNNL) assists CH2M HILL in their performance assessment activities. One of the PNNL tasks is to provide estimates of the geochemical properties of the materials comprising the IDF, 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. The Kd values and solution concentration limits for each contaminant are direct inputs to subsurface flow and transport codes used to predict the performance of the IDF system. In addition to the best-estimate Kd values, a reasonable conservative value and a range are provided. The data package does not list estimates for the range in solubility limits or their uncertainty. However, the data package does provide different values for both the Kd values and solution concentration limits for different spatial zones in the IDF system and does supply time-varying Kd values for the cement solidified waste. The Kd values and solution concentration limits presented for each contaminant were previously presented in a report prepared by Kaplan and Serne (2000) for the 2001 ILAW PA, and have been updated to include applicable data from investigations completed since the issuance of that report and improvements in our understanding of the geochemistry specific to Hanford. A discussion is also included of the evolution of the Kd values recommended from the original 1999 ILAW PA through the 2001 ILAW and 2003 Supplement PAs to the current values to be used for the 2005 IDF PA for the key contaminants of concern: Cr(VI), nitrate, 129I, 79Se, 99Tc, and U(VI). This discussion provides the rationale for why certain Kd have changed with time.