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SODIUM DITHIONITE INJECTIONS USED FOR CHROMIUM REDUCTION
Paul*, C J. SODIUM DITHIONITE INJECTIONS USED FOR CHROMIUM REDUCTION. U.S. Environmental Protection Agency, Washington, DC, 2002.
A field-scale pilot study was conducted in 1999 at the U.S. Coast Guard Support Center in Elizabeth City, NC, to evaluate the effectiveness of injecting sodium dithionite into the upper aquifer and lower vadose zone to create a permeable reactive barrier (PRB) system utilizing naturally occurring iron for hexavalent chromium (Cr(VI)) reduction. Within three days of chemical injection, Cr(VI) concentrations dropped to below the target cleanup levels. Based on these results, a full-scale treatment system employing sodium dithionite injections was implemented earlier this year.
Past operations of a chrome-plating shop at the Support Center created a contaminant plume with mixed organic and metal compounds, particularly Cr(VI). Due to indications that chromium in the soils would remain immobile and present no potential risk, initial cleanup efforts focused on treating the diffuse chromium plume. A PRB using zero-valent iron was installed at the site in 1996 and has since proved effective for remediating the mobile chromium plume. A water-main break in the immediate vicinity of the plating shop during 1994, however, caused an artificial rise in the water table directly beneath the shop. As a result, soluble Cr(VI) in concentrations reaching 28.0 mg/L had migrated into the ground water. Although concentrations decreased to approximately 4.4 mg/L by 1997, additional treatment was required to meet the maximum contaminant level of 0.1 mg/L for ground water.
Prior to the source control pilot study, steps were taken to re-characterize the site through the collection of soil cores from 32 locations within the plating shop. Preliminary screening was conducted using x-ray fluorescence and inductively coupled plasma spectroscopy to determine total chromium levels. Laboratory batch extractions were conducted to selectively remove Cr(VI) from contaminated soils in order to delineate the extent of the soluble, easily mobilized Cr(VI). Results indicated that the contaminant plume had migrated vertically through a series of sand and silt/clay loam layers into the ground water at 2 meters below ground surface and horizontally toward the Pasquotank River, which is located approximately 60 meters from the source area. These conditions provided a continual release of Cr(VI) into the mobile contaminant plume treated by the PRB.
Additional laboratory studies were conducted to better understand the potential for in situ reduction/oxidation (redox) manipulation (ISRM) and to evaluate sodium dithionite (Na2S2O4), l-ascorbic acid (C6H8O6), and free hydroxylamine (FH-50TM) as potential reductants. Results showed sodium dithionite to be the most effective at reducing chromium with the least adverse side effects. Injecting sodium dithionite into areas with high levels of naturally occurring iron (Fe) was found to create a spatially-fixed reducing zone analagous to a chemical PRB. Through such injection, Fe(III) is reduced to Fe(II) and redox-sensitive contaminants such as Cr(VI) are immobilized or precipitated as they migrate through the reducing zone.
In the field pilot, a solution of sodium dithionite (17 kg), potassium bicarbonate (19 kg), and bromide tracer (5.63 kg) in 1,874 liters of distilled and deionized water was injected into ground water at depths ranging from 1.2 to 2.4 meters below ground surface. The reductant was injected continuously for 38 hours at 1 liter/minute. To evaluate the use of ISRM in the capillary fringe areas, test data were collected from the 1-meter treatment area immediately surrounding the injection well.
Field results indicated that Cr(VI) concentrations dropped from pre-treatment levels of 2.2-5 mg/L to non-detect levels within 60 hours of injection and remained at virtually the same level throughout the remaining course of the pilot project's 48-week monitoring period (Figure 1). Data showed that by-products of the redox process (sulfate and dissolved iron) significantly increased during the initial 60-hour period but decreased to original levels approximately 32 weeks after the injection phase. Similarly, other process indicators such as pH and specific conductance initially rose but returned to or near original levels within approximately 48 weeks, which is typical of the redox process. Pumping and treatment of the injected material was not required because sodium dithionite is known to produce no adverse side effects or byproducts.
Costs for the pilot project totaled approximately $750,000, including expenses for soil excavation in the upper area of the treatment zone. Comparative analysis estimated that the addition of chemical injections for source control at the Support Center site helped to reduce the overall project (including mobile plume treatment through the PRB) costs by 40 percent and the cleanup time by 28 years. In comparison to conventional pump and treat methods, analysis estimated that this combined approach for source control and plume treatment achieved nearly an 80 percent reduction in costs. More information on the pilot study is available on the Internet at www.epa.gov/ORD/NRMRL or from Cynthia Paul (National Risk Management Research Laboratory) at 580-436-8556 or e-mail email@example.com.
Record Details:Record Type: DOCUMENT (NEWSLETTER)
Organization:U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
NATIONAL RISK MANAGEMENT RESEARCH LABORATORY
SUBSURFACE PROTECTION AND REMEDIATION DIVISION
SUBSURFACE REMEDIATION BRANCH