In Situ Bioremediation of Perchlorate in Vadose Zone Soil Using Gaseous Electron Donors

Dr. Patrick Evans | CDM Smith

ER-200511

Background

While extensive research and technology development on the treatment of perchlorate in water has been conducted, limited efforts have been focused on perchlorate in soil. Perchlorate contamination in soil is important because it can be a source of groundwater contamination. Currently, available technologies for the treatment of perchlorate in soil require excavation and are not always cost-effective or practical, particularly as the depth of contamination increases. In situ remediation of perchlorate in soil is an alternative, potentially more cost-effective solution.

Gaseous electron donor injection technology (GEDIT) (U.S. Patent No. 7,282,149 and patent pending) involves injection of gaseous electron donors (GED) into the soil with the purpose of promoting anaerobic biodegradation of perchlorate to water and chloride ion. This technology can be viewed as bioventing in reverse. Bioventing, a proven bioremediation technology for petroleum hydrocarbons, involves the injection of a gaseous electron acceptor (e.g., oxygen) into the vadose zone resulting in the biodegradation of an electron donor (e.g., hydrocarbons). In the present application, the electron acceptor and donor are reversed with the GED being injected in order to biodegrade the electron acceptor (i.e., perchlorate or nitrate). GEDIT is also applicable to treatment of other Department of Defense (DoD) and Department of Energy related contaminants such as hexavalent chromium, uranium, technetium, chlorinated volatile organic compounds (VOCs), and highly energetic compounds including trinitrotoluene (TNT), royal demolition explosive (RDX), and high melting explosive (HMX).

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Objectives of the Demonstration

The objective of this project was to demonstrate and validate GEDIT for treatment of perchlorate and nitrate in vadose zone soil. This project represents the first field demonstration of the technology. The demonstration yielded valuable engineering design information on GEDIT implementation. Quantitative performance objectives for the project included greater than 90% perchlorate and nitrate destruction within 12 months of operation and a radius of influence (ROI) of 10 ft or greater. Qualitative performance objectives regarding safety, regulatory acceptance, and ease of use were also included. All performance objectives were met or exceeded.

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Demonstration Results

The demonstration was conducted at the Boeing Inactive Rancho Cordova Test Site (IRCTS) Propellant Burn Area (PBA) in Rancho Cordova, California. The average percent perchlorate destruction was 93±9% within the targeted 10 ft ROI and the 10 to 40 ft below ground surface (bgs) depth interval and within 5 months of operation. The performance objective of 90% for perchlorate destruction was exceeded. Initial perchlorate concentrations within this ROI and depth ranged from 2,600 to 75,000 micrograms per kilogram (μg/kg). Final perchlorate concentrations ranged from less than 13 to 8,800 μg/kg. Seven final soil samples (i.e., six sample locations plus one duplicate) were non-detect (ND) for perchlorate (<13 to <15 μg/kg).

The average percent nitrate destruction was 94±9% within the targeted 10 ft ROI and the 10 to 50 ft bgs depth interval and within 5 months of operation. The performance objective of 90% for nitrate destruction was exceeded. When all data that comprised an ROI of 55 ft were considered, the average nitrate destruction was 90±14%. Initial concentrations of nitrate plus nitrite within the 10-ft target ROI ranged from 2.0 to 8.6 milligrams of nitrogen per kilogram (mg-N/kg). Final nitrate plus nitrite concentrations ranged from less than 0.054 to 2.9 mg-N/kg. Six final soil samples (i.e., five sample locations plus one duplicate) were ND for nitrate (<0.054 to <0.057 mg-N/kg).

ROI was used as a primary metric for implementability because it will determine the number of wells required to treat a given area. The ROI for perchlorate degradation was conservatively estimated to be 10 ft and likely to be 15 ft during the demonstration. The ROI for nitrate degradation was estimated to be at least 55 ft. The performance objective for implementability was an ROI of 10 ft. Therefore the performance objective was met.

Unit costs for various scenarios ranged from $21 to $87 per cubic yard (cy). These costs are considered to be conservative, and further optimization of the process is anticipated to decrease unit costs. An alternative approach to in situ treatment is excavation of vadose zone soil and ex situ bioremediation. This process includes soil excavation; rock screening and crushing; soil mixing with water, electron donor, and nutrients; storage in treatment cells during biodegradation; soil drying; and backfilling. Full-scale costs for this process were estimated to be about $35/ton or $45/cy.

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Implementation Issues

In addition to well spacing, regulatory acceptance, permitting, and safety are important implementation issues. Federal or state regulations driving site cleanup will drive the need for GEDIT. The primary application for GEDIT is anticipated to be treatment of contaminants such as perchlorate in soil for the purpose of groundwater protection. The feasibility study process will include evaluation of GEDIT compared to other alternatives such as pump-and-treat, liquid flushing, and excavation. Specific permits for GEDIT will be driven by local codes and will include drilling and well installation permits and hazardous materials storage permits. Other permits may be necessary and will be dependent on local codes.

Flammability is the primary end-user concern associated with GEDIT. As shown in this demonstration, this issue was easily managed and did not necessitate unusual efforts. The level of effort was similar to that for a construction site or remediation of a gasoline station site.

Engineering guidance for implementation of this technology is included in the Final Report.

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Points of Contact

Principal Investigator

Dr. Patrick Evans

CDM Smith

Phone: 206-351-0228

Program Manager

Environmental Restoration

SERDP and ESTCP

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