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

Abstract

SummaryEfforts to reduce the flux of strontium-90 ( 90 Sr) to the Columbia River from past-practice liquid waste disposal sites have been underway since the early 1990s in the 100-N Area at the Hanford Site. Termination of all liquid discharges to the ground in 1993 was a major step toward meeting this goal. However, 90 Sr adsorbed on aquifer solids beneath the liquid waste disposal sites and extending beneath the near-shore riverbed remains a continuing source to groundwater and the Columbia River. Researchers realized from the onset that the initial pump-and-treat system was unlikely to be an effective long-term solution because of the geochemical characteristics of 90 Sr; subsequent performance monitoring has substantiated this theory. Accordingly, the first Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA) 5-year review re-emphasized the need to pursue alternative methods to reduce impacts on the Columbia River. 1Following an evaluation of potential 90 Sr treatment technologies and their applicability under 100-NR-2 hydrogeologic conditions, U.S. Department of Energy, Fluor Hanford, Inc. (FH), Pacific Northwest National Laboratory, and the Washington State Department of Ecology agreed the long-term strategy for groundwater remediation at the 100-N Area will include apatite sequestration as the primary treatment, followed by a secondary treatment⎯or polishing step⎯if necessary (most likely phytoremediation). Since then, the agencies have worked together to agree on which apatite-sequestration technology has the greatest chance of reducing 90 Sr flux to the Columbia River at a reasonable cost. In July 2005, aqueous injection, (i.e., the introduction of apatite-forming chemicals into the subsurface) was endorsed as the interim remedy and selected for field testing. Studies are in progress to assess the efficacy of in situ apatite formation by aqueous solution injection to address both the vadose zone and the shallow aquifer along the 91 m (300 ft) of shoreline where 90 Sr concentrations are highest. This report describes the field testing of the shallow aquifer treatment that was funded by FH.A low-concentration, apatite-forming solution was injected into the shallow aquifer in 10 injection wells during fiscal years 2006 and 2007, and performance monitoring is underway. The lowconcentration, apatite-forming solution consists primarily of calcium chloride, trisodium citrate, and sodium phosphate. The objective of the low-concentration Ca-citrate-PO 4 injections is to stabilize the 90 Sr in the aquifer at the test site, to be followed by high-concentration injections to provide for long-term 90 Sr treatment. Two pilot test sites at the east and west ends of the barrier, which are equipped with extensive monitoring well networks, were used for the initial injections to develop the injection design for the remaining portions of the barrier. Based on a comparison of hydraulic and transport response data at the two pilot test sites, it was determined the apparent permeability ...

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

“…Comparing test results from these two locations indicated that the permeability contrast between the Hanford and Ringold Formations was significantly less over the upstream-most one third of the barrier. Williams et al (2008) estimated hydraulic conductivity for the Hanford and Ringold Formations over the upstream portion of the barrier at 12 and 10 m/day (39 and 33 ft/day), respectively. By contrast, hydraulic conductivity for the Hanford and Ringold Formations over the downstream portion of the barrier was estimated at 29 and 9 m/day (95 and 30 ft/day), respectively.…”

“…Springs, seeps, and subsurface discharge along the riverbank are the primary pathway of 100-N Area groundwater contaminants to the Columbia River. Additional details about the extent of seasonal and daily changes in river stage at the site from PRD discharge are reported by Williams et al (2008). …”

“…Because of the short time between the installation of compliance, injection, and pilot-test monitoring wells at the 100-N Area apatite treatability test site and the Ca-citrate-PO 4 injections (started at the site in the spring of 2006), there were insufficient data from these wells to establish baseline conditions for 90 Sr. Therefore, baseline 90 Sr ranges were developed for the injection and compliance wells at the treatability test site based on gross beta concentrations from nearby aquifer tubes and limited pre-injection 90 Sr monitoring from the treatability-test wells (Williams et al 2008). These developed baseline ranges are shown on all post-treatment performance-assessment groundwater monitoring data plots for reference.…”

“…Results from these pilot tests were used to refine the injection design for the remaining injections wells used to emplace the 91-m (300-ft) -long PRB section. The pilot-scale testing activities were reported by Williams et al (2008).…”

“…The low-concentration Ca-citrate-PO 4 amendment formulation was designed to emplace a small amount of treatment capacity (i.e., apatite formation), while minimizing any short-term increase in 90 Sr concentration associated with injecting relatively high-ionic-strength solutions. A detailed description of the PRB-emplacement operations, including performanceassessment monitoring results through November 2007, is provided by Williams et al (2008).…”