Technology:
Biosparging
- Biosparging system includes two horizontal wells; injection pad - a compressor, a header for each well, NO2 cylinder and triethyl phosphate drum, and methane vents that discharge directly into the air
- Horizontal wells - installed to depth of 60 ft bgs; screened to length of 800 ft and 900 ft; 6-inch diameter outer casing of carbon steel with holes (0.17% open area); 4-inch inner HDPE casing with varied slit spacing to distribute injectate (0.28% open area); system operated on pulsed injection schedule
- Groundwater monitoring network includes 90 monitoring wells
- Initially, one well used to inject methane, air, and nutrients (nitrous oxide and triethyl phosphate) to stimulate the growth of methanotropic (methane oxidizing) organisms to complete the mineralization of TCE; second well used to inject air and nutrients to aerobically degrade and volatilize vinyl chloride
- Methane injection stopped in January 2001 after TCE concentrations decreased
- Air and nutrients continue to be injected in both wells; system operations to continue until cleanup goals are met
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Cleanup Authority:
RCRA Corrective Action
Contacts:
David Noffsinger
Westinghouse Savannah River Co.
Phone: (803) 952-7768
E-mail: d.noffsinger@srs.gov
Marianna DePratter
State Lead
RCRA Hydrogeology Section 1
Bureau of Land and Waste Management
South Carolina Department of Health and Environmental Control
Phone: (803) 896-4018
E-mail: DEPRATMP@dhec.sc.gov |
Contaminants:
Halogenated VOCs
- Primary contaminants of concern are TCE, cis-1,2-DCE, and vinyl chloride |
Waste Source:
Disposal of waste in unlined sanitary landfill |
Type/Quantity of Media Treated:
Groundwater
- The estimated volume of water that has moved through the treatment zone is 9.4 billion gallons
- Depth to groundwater - ranges from 30 to 60 ft bgs
- Contamination occurs in the uppermost hydrogeologic unit - Steel Pond Aquifer; water table/unconfined aquifer consisting of interbedded sands and clayey/silty sands |
Purpose/Significance of Application:
Biosparging, using horizontal wells, in conjunction with a cap, to treat chlorinated solvents in groundwater beneath a sanitary landfill |
Regulatory Requirements/Cleanup Goals:
- SRS negotiated with the state for Alternate Concentration Limit/Mixing Zone Concentration Limits (ACL/MZCLs)
- ACL/MZCLs include: TCE (21 µg/L), cis-1,2-DCE (287 µg/L), and vinyl chloride (12 µg/L) |
Results:
- As of 2001, the TCE plume had diminished and methane injection was stopped; DOE determined that TCE concentrations had decreased substantially and the results of numerical modeling predicted that further methane injection would not be beneficial
- As of FY2003, the maximum TCE concentrations ranged from not detected at wells in the interior of the landfill to a maximum of 8 ug/L at point of compliance wells upgradient of the treatment system; in the monitoring wells downgradient from the horizontal treatment wells, TCE was not detected at a quantifiable concentration (< 2 μg/L)
- Vinyl chloride concentrations have continued to decrease over the past year, with maximum concentrations during FY 2003 reaching 80 μg/L in an interior landfill monitoring well, and 14 μg/L in a point of compliance well at the base of the landfill (upgradient from the treatment system); vinyl chloride was not detected in wells downgradient from the treatment system
- Westinghouse Savannah River Company indicated that biosparging reduced concentrations in a well in the treatment zone by 99 percent for vinyl chloride and 75 percent for TCE. |
Cost Factors:
- The actual costs to date for the biosparging application are: installation of two horizontal injection wells – $1 million; construction of the injection pad/well piping – $750,000; operation of the biosparging system – $225,000/year; and cost of groundwater monitoring – $215,000/year |
Description:
The U.S. Department of Energy (DOE) Savannah River Site (SRS) is a 310 square-mile facility located near Aiken, South Carolina. From 1974 to 1994, a variety of wastes from SRS were disposed of in the unlined SRS Sanitary Landfill (SLF), which includes a main section (33 acres) and two expansion areas – a 22-acre southern expansion area and a 16-acre northern expansion area. In 1988, results of groundwater monitoring showed elevated levels of chlorinated solvents at the SLF , including TCE, 1,2-DCE, and vinyl chloride. In 1996, the South Carolina Department of Health and Environmental Control (SCDHEC) approved a closure plan for the SLF, which included the installation of a low-permeability geosynthetic cap (engineered RCRA cap). From 1996 to1997, the cap was installed over the main section and southern expansion area of the SLF, which were certified closed in October 1997. Installation of the cap minimized infiltration and produced anaerobic conditions in the subsurface, facilitating reductive dechlorination of TCE.
In 1999, two horizontal biosparging wells were installed with one well used to inject methane, air, and nutrients to stimulate the growth of methanotropic organisms to complete the mineralization of TCE, and the second well used to inject air and nutrients to promote the aerobic degradation and volatilization of vinyl chloride. In January 2001, methane injection was stopped as TCE concentrations had decreased substantially and the benefits of additional injections were determined to be limited. Air and nutrient injection is ongoing. As of 2003, biosparging reduced concentrations in a well in the treatment zone by 99 percent for vinyl chloride and 75 percent for TCE. SRS negotiated with the state for Alternate Concentration Limit/Mixing Zone Concentration Limits, and the system will continue to operate until these levels are met. According to the State, reducing conditions below the landfill helped degrade trichloroethene, but caused the vinyl chloride groundwater contaminant plume to increase. The current rate of growth of the vinyl chloride groundwater contaminant plume is insignificant. Future increases in the concentration of vinyl chloride in groundwater below the SLF are limited by the small mass of dissolved trichloroethene, its precursor, remaining and by the presence of the landfill cap, which prevents additional leaching of contamination from above. |
