100-NR-2 Apatite Treatability Test: High-Concentration Calcium-Citrate-Phosphate Solution Injection for In Situ Strontium-90 Immobilization

摘要

Following an evaluation of potential strontium-90 (90Sr) treatment technologies and their applicability under 100-NR-2 hydrogeologic conditions, the U.S. Department of Energy (DOE), Fluor Hanford, Inc. (now CH2M Hill Plateau Remediation Company (CHPRC)), Pacific Northwest National Laboratory, and the Washington State Department of Ecology agreed that the long-term strategy for groundwater remediation at the 100-N Area should include apatite as the primary treatment technology. This agreement was based on results from an evaluation of remedial alternatives that identified the apatite permeable reactive barrier (PRB) technology as the approach showing the greatest promise for reducing 90Sr flux to the Columbia River at a reasonable cost. This report documents work completed to date on developing a high-concentration amendment formulation and initial field-scale testing of this amendment solution. The general approach for developing an in situ remedial technology for sequestering 90Sr in groundwater through the formation of calcium-phosphate mineral phases (i.e., apatite) was documented in a project-specific treatability test plan that provides a detailed discussion of test objectives and outlines the technical approach for developing and deploying the technology. Activities completed to date in support of the 100-NR-2 apatite treatability test that have been reported in previous documents include (1) laboratory-scale studies, (2) pilot-scale field testing with a low-concentration solution, (3) initial treatment of a 91-m (300-ft) long PRB section with the low-concentration formulation, and (4) analysis of sediment samples collected following low-concentration treatment to determine whether any apatite had formed. A high-concentration amendment solution was formulated to maximize apatite formation within the targeted treatment zone while minimizing the short-term increases in 90Sr concentration associated with injecting high-ionic-strength solutions.