This project demonstrates the combined use of two innovative technologies: (1) bioremediation of perchlorate-contaminated groundwater using an electron donor, and (2) horizontal flow treatment wells (HFTW) to achieve in situ mixing of the electron donor with the perchlorate-contaminated water and deliver the electron donor to indigenous perchlorate-degrading bacteria.
- The HFTW system consisted of a pair of dual-screened treatment wells placed approximately 34 feet apart and perpendicular to groundwater flow in the contaminant plume. These wells worked in tandem to establish a groundwater recirculation zone in the subsurface.
- The first treatment well (HRTW-U) was operated in an upflow mode. This well pumped contaminated groundwater from a deep aquifer region, augmented the groundwater with the electron donor, and injected it into a shallower zone.
- The second treatment well (HFTW-D) operated in a downflow mode. This well pumped contaminated groundwater from the shallower aquifer region, augmented the groundwater with the electron donor, and injected it into the deeper zone.
- Citric acid was used as the electron donor and was mixed with contaminated groundwater in each well, creating an anaerobic, bioactive zone between and downgradient of the HFTWs during system operation.
- The system was operated under three different operational phases using varying amounts of citric acid as described below:
- Phase I: Daily, controlled addition of citric acid. The system was operated under a constant pumping scenario at 6 gallons per minute (gpm).
- Phase II: Large addition of citric acid weekly or twice-per-week. In addition, chlorine dioxide was added to each well on a daily basis (four to eight times per day) to prevent biofouling. The system was operated under a constant pumping scenario at 6 gpm.
- Phase III: HFTW treatment wells were operated on a 15-day cycle, consisting of 3 days of active mode operation(with pumping) followed by 12 days in a passive mode (without pumping). Citric acid was added to both HFTWs in three, 12-hour pulses during the active mode period.
Collaborative effort by Shaw Environmental, Inc. (Shaw), the U.S. Air Force Institute of Technology (AFIT), the University of New Mexico (UNM), and Aerojet General Corporation (Aerojet)
Shaw Environmental, Inc.
17 Princess Road
Lawrenceville, NJ 08648
Shaw Environmental, Inc.
111 West Pleasant Street, Suite 105
Milwaukee, WI 53212
Aerojet Project Coordinator
Aeroject General Corporation
P.O. box 13222
Sacramento, CA 95813-6000
Air Force Institute of Technology
2950 Hobson Way, Building 640
Wright-Patterson AFB, OH 45433
Environmental Restoration Program Manager
901 North Stuart Street, Suite 303
Arlington, VA 22203
Perchlorate and Trichloroethene (TCE)
During the 1950s, the Central Disposal Area (CDA) at the Aerojet site was used for open burning of waste propellant and solvents for disposal purposes. These activities resulted in the release of perchlorate and TCE to shallow and deep groundwater aquifers underneath the site.
Type/Quantity of Media Treated:
Groundwater (quantity not documented)
Purpose/Significance of Application:
The objectives of this project were to demonstrate that in situ bioremediation of perchlorate is feasible using an electron donor and that HFTW systems can achieve zones of influence and efficiencies that are sufficient to make the technology a viable, cost-effective option at many sites.
Specific objectives of each phase were as follows:
- Phase 1: Evaluate overall groundwater mixing and capture by the system and determine the extent of perchlorate and nitrate reduction possible without mobilizing significant quantities of Iron (Fe) and Manganese (Mn).
- Phase II: Treat perchlorate without promoting significant biofouling of the HFTW system.
- Phase III: Determine whether an "active-passive" system operation results in a consistent reduction in perchlorate concentrations and reduced system operation and maintenance (O&M) costs.
Regulatory Requirements/Cleanup Goals:
The primary performance objective of this demonstration was to achieve consistent reduction (99.8% reduction) in perchlorate concentrations to less than 4 micrograms per liter (µg/L). Other key performance objectives include minimal mobilizations of Fe and Mn; control of biofouling; and the reduction of TCE concentrations by 95%.
To evaluate the performance of the system, perchlorate and TCE concentrations were monitored in a series of 19 monitoring wells placed within the expected treatment zone of the HFTW system. Samples were collected during all three phases of the project. Results from groundwater collected from the monitoring wells indicated the following:
- Reduction of perchlorate levels by 96% in shallow wells and 88% in deep, downgradient wells. Very few wells achieved perchlorate concentrations below 4 µg/L.
- Reduction in TCE levels by 76% in shallow wells and 71% in four deep, downgradient wells.
- Minimal mobilization of Fe and Mn during Phase I; greater mobilization occurred during Phases II & III.
- Chlorine dioxide slowed but did not prevent biofouling in Phase I and II. Significant pressure increases were observed during both phases. The pressures did not increase enough during Phase III to impact system pumping or operation during the active operation mode.
- Consistent decline in perchlorate concentrations throughout the entire shallow aquifer during Phase II, confirming that the HFTW system operated well as a treatment technology in the shallow zone.
- Data from Phase III suggesting that perchlorate treatment can be achieved by using the HFTW system intermittently as a vehicle to mix the electron donor with the contaminated groundwater.
- Sampling results and operational data from Phase III suggesting that an active-passive approach may be the best overall operational strategy for an HFTW system to avoid issues associated with pressure increases from biofouling and to maintain consistent contaminant treatment.
The total cost for the two year field scale demonstration including site preparation, site characterization, system design and installation, and monitoring was approximately $1,023,900. A breakdown of these costs is provided below:
- Site selection and characterization: $195,400
- Treatability studies: $74,800
- System design and modeling: $208,100
- System installation (includes materials and equipment): $252,500
- System installation (includes labor and travel): $54,800
- O&M costs (includes sampling system equipment, labor, and lab fees): $238,300
Aerojet's 8500-acre Sacramento, CA facility has been manufacturing and testing rocket propulsion systems continuously since 1951. During the 1950s, the Central Disposal Area (CDA) was used for the open burning of waste propellant and solvents for disposal purposes. These activities resulted in release of perchlorate and VOCs (primarily TCE) into groundwater at the site.
The mixed TCE-perchlorate groundwater plume is thought to originate from the CDA and is approximately 5,800 feet long and 3,000 feet wide; the plume impacts multiple aquifer units to depths of 300 feet. Groundwater sampling results obtained before the demonstration study indicated that perchlorate and TCE concentrations increased with depth. The groundwater samples obtained from the upper water bearing zone (36 to 57 feet below ground surface (bgs) contained perchlorate concentrations ranging from 65 to 330 µg/L. The samples obtained from the lower water bearing zone (75 to 105 ft bgs) contained perchlorate concentrations ranging from 970 µg/L up to 3,920 µg/L.
A two year field scale study was conducted to demonstrate in situ bioremediation of perchlorate in a contaminated aquifer using electron donor addition to stimulate naturally occurring bacteria capable of perchlorate reduction. The demonstration site was located approximately 2,400 feet downgradient of the CDA. A groundwater recirculation system (HFTWs) was employed to distribute and mix an electron donor with perchlorate in the subsurface. The HTRW system consisted of one pair of treatment wells that amended contaminated groundwater with citric acid. Liquid chlorine dioxide solution was also added to prevent biofouling of the system. The HFTW system operated for three phases over a period of approximately 2 years from September 2004 until December 2006. The operational performance of the system was evaluated by measuring and comparing groundwater contaminant concentrations in system monitoring wells at the demonstration site. Sampling results indicated a reduction of perchlorate levels by 96% in shallow wells and 88% in deep, downgradient wells; however, the cleanup goal of 4 µg/L was not achieved in the majority of the shallow or deep aquifer wells. Based on operational data and post-treatment sampling results, the active/passive operating mode - coupled with large doses of electron donor and doses of chlorine dioxide - was recommended as a long-term operating strategy for this type of treatment system.