EVALUATION OF AMENDMENTS FOR MENDING THE INSITU REDOX MANIPULATION (ISRM) BARRIER
In May of 2004, the U.S. Department of Energy (DOE) Richland and Fluor Hanford requested technical assistance from DOE Headquarters EM-23 to provide a team of technical experts to evaluate likely chemical/biological amendments for mending the In Situ Redox Manipulation (ISRM) Barrier in the 100-D Area of the Hanford Site. This request was a follow-on to an earlier request for assistance regarding the cause of chromium (Cr) breakthrough and recommendations for mending the barrier (March 2004 workshop). This report provides written documentation of the team's findings and recommendations. In 1995, a plume of dissolved hexavalent chromium [Cr(VI)] was discovered along the Columbia River shoreline and in the 100-D Area. Between 1999 and 2003, a reactive barrier using the ISRM technology, was installed at a distance of 680 meters along the river to reduce the Cr(VI) in the groundwater. The ISRM technology creates a treatment zone within the aquifer by injection of sodium dithionite, a strong reducing agent that scavenges dissolved oxygen (DO) from the aquifer and reduces ferric iron [Fe(III)], related metals, and oxy-ions. Bench-scale and field-scale treatability tests were conducted to demonstrate proof-of principle and to estimate barrier longevity, calculated to be in excess of twenty years. However, several years after initial and secondary treatment, groundwater in approximately 17 wells has been found to contain elevated Cr concentrations. The March 2004 technical assistance team (TAT) identified potential causes of Cr breakthrough as likely related to physical and chemical heterogeneity within the aquifer (including loss of reductive capacity within preferential flow paths) and the presence of other oxidants (DO and nitrate) significantly affecting the reductive capacity of the treated aquifer. These aquifer characteristics may limit the ability of alternative amendments to extend the reducing capacity of the barrier. A 2001 Bechtel Hanford report and evaluation of the ISRM performance data and barrier longevity assessment corroborate the observations and findings of the March 2004 TAT. The March 2004 TAT recommended the collection of new aquifer characterization data in combination with the interpretation of existing data to develop a conceptual model of aquifer heterogeneity to enable design of the most appropriate barrier mending system. The current TAT was convened to examine the most promising amendment that could be applied to mend the ISRM barrier. The Technical Assistance Team (TAT) performed the following activities: (1) Evaluate the most appropriate single or combination of chemical/biological amendments suitable for increasing the reductive capacity of the ISRM barrier; (2) Evaluate the most practicable means of introducing chemical/biological amendments in the target zones along the current BRM barrier; (3) Provide recommendations for laboratory treatability-testing protocol development to evaluate the type and delivery mechanisms of amendments in the current ISRM barrier location. Sections of this report present analyses and recommendations of potential amendments and delivery options to improve performance of the ISRM barrier. The report covers the spectrum of passive barrier mending to chemical and biological amendments that have been shown to perform more efficiently in more active remedial design approaches. Because DOE/RL is considering significant aquifer characterization studies as additional time and cost investment to mending the barrier, the TAT strongly recommends that DOE/RL conduct cost-benefit analyses of alternative designs to mend the barrier. In this way, the value and extent of characterization studies, compared to passive amendment delivery, compared to engineering redesign, can be quantitatively estimated for decision-making purposes.
- Research Organization:
- Hanford Site (HNF), Richland, WA (United States)
- Sponsoring Organization:
- USDOE - Office of Environmental Management (EM)
- DOE Contract Number:
- DE-AC06-96RL13200
- OSTI ID:
- 876697
- Report Number(s):
- WMP-28124 Rev 0; TRN: US200606%%608
- Country of Publication:
- United States
- Language:
- English
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