SUMMARY CONCLUSIONS FOR THE PILOT IN-SITU CHROMIUM REDUCTION TEST AT RIVERBANK ARMY AMMUNITIONS PLANT

Ridley, M

Lawrence Livermore National Laboratory

关键词: By-Products;    Leaching;    Sulfates;    Monitoring;    Sediments;   

DOI  :  10.2172/920478
RP-ID  :  UCRL-TR-231326
RP-ID  :  W-7405-ENG-48
RP-ID  :  920478

美国|英语

来源: UNT Digital Library

【 摘 要 】

A treatability study was conducted at Riverbank Army Ammunition Plant's (RBAAP) Site 17, to evaluate the effectiveness of a permeable reactive barrier (PRB) for the treatment of hexavalent chromium (Cr{sup 6+}). The chromium contamination at Site 17 is hydrologically isolated and unsuitable for standard extraction and treatment (pump and treat). The majority of the chromium contamination at Site 17 is trapped within the fine grain sediments of a clay/slit zone (45 to 63). The PRB was established above and adjacent to the contaminated zone at Site 17 to reduce the hexavalent chromium as it leaches out of the contaminated clay/silt zone separating the A zone from the A zone. Site 17 and the monitoring network are described in the In-Situ Chromium Reduction Treatability Study Work Plan (CH2MHILL, January 2004). The PRB was created by reducing naturally occurring Fe{sup 3+} to Fe{sup 2+} with the injection of a buffered sodium dithionite solution into subsurface chromium source area. The Cr{sup 6+} leaching out of the contaminated clay/silt zone and migrating through the PRB is reduced by Fe{sup 2+} to Cr{sup 3+} and immobilized (Amonette, et al., 1994). The sodium dithionite will also reduce accessible Cr{sup 6+}, however the long-term reductant is the Fe{sup 2+}. Bench scale tests (Appendix A) were conducted to assess the quantity and availability of the naturally occurring iron at Site 17, the ability of the sodium dithionite to reduce the hexavalent chromium and Fe within the sediments, and the by-products produced during the treatment. Appendix A, provides a detailed description of the laboratory treatability tests, and provides background information on the technologies considered as possible treatment options for Site 17. Following the sodium dithionite treatment, groundwater/treatment solution was extracted to remove treatment by-products (sulfate, manganese, and iron). The following sections briefly discuss the current treatment status, future recommendations for Site 17, and future recommendations for the application of sodium dithionite at additional sites. At the completion of the treatability test, none of the wells at Site 17 had detectable hexavalent chromium, but the sulfate, iron, and manganese concentrations were detected and exceeded the CA secondary drinking water standards. The extraction done after the injection of the sodium dithionite solution to remove the sulfate, manganese, and iron has to a large extent negated the effectiveness of the iron reduction. Riverbank's local groundwater is naturally high in dissolved oxygen (concentration range at Site 17: 1.8 to 6.0 mg/l) and moving this type of groundwater through the reduced zone caused oxidation of the Fe2+ within the treatment zone, followed by a new release of hexavalent chromium detected in one of the treatment wells. Additional extraction at Site 17 will continue to degrade the PRB, threatening to release additional Cr{sup 6+} into the groundwater. Sulfate and manganese only exceed the CA secondary drinking water standards in the area immediately surrounding the PRB. It is unlikely that these contaminants will threaten any water supply wells in the area. The chromium concentrations are increasing in IW-17. The current concentration is still only a third of the original concentration. It might be worth investigating some of the new zero valent iron treatments, such as nanoscale zero-valent iron (NZVI) to replace the in-situ naturally occurring iron, which may no longer be available for reduction. The NZVI has been successfully tested at NASA in Florida (O'Hara, 2006), and demonstrated no release of metals from the natural sediments. This might also be a viable option for other sites at RBAAP.