Perchlorate is an inorganic anion and a primary ingredient in solid rocket propellant. It exhibits high solubility and mobility in water and has been identified in groundwater at numerous sites across the United States. The objective of this project was to demonstrate the efficacy of enhanced in situ bioremediation (EISB) for perchlorate-impacted groundwater and compare active and semi-passive approaches to generate design and cost information that Department of Defense (DoD) project managers would require to design and implement EISB at perchlorate-impacted sites. The electron donor delivery methodologies evaluated included: (1) an active in situ biobarrier where groundwater was captured and amended with an optimized concentration of a soluble electron donor; and (2) a semi-passive approach involving rapid batch injection of soluble electron donors via injection wells installed across a section of the perchlorate plume.

The active EISB demonstration test area was located within an Aerojet facility on the western boundary of the original Inactive Rancho Cordova Test Site (IRCTS) in California. This treatment approach involves ongoing groundwater recirculation and delivery of an electron donor to create a biologically active zone (BAZ) or biobarrier across a perchlorate plume, for the purposes of promoting perchlorate biodegradation and controlling plume migration. The active EISB test consisted of an active biobarrier, whereby groundwater containing perchlorate was extracted from the shallow aquifer, amended with a carbon-based electron donor (ethanol), and recharged to the shallow aquifer to promote in situ biodegradation of the perchlorate.

The semi-passive EISB demonstration was conducted at the Longhorn Army Ammunition Plant in Texas. This treatment approach involves periodic (e.g., two or three times per year) delivery of electron donor to create a biologically active zone for the purposes of promoting perchlorate biodegradation either as a biobarrier across a plume or for treatment of other target treatment zones. The semi-passive biobarrier involves the use of extraction and injection wells to add and mix the electron donors in the subsurface. Once electron donor is delivered, recirculation is shut off, and the electron donor promotes in situ biological treatment of the perchlorate.

Active EISB

The active biobarrier provided containment and treatment of a 600-foot wide section of the perchlorate and trichloroethene (TCE) plume in the shallow aquifer along the site using two groundwater extraction wells and a single groundwater recharge (electron donor delivery) well. The demonstration results showed that indigenous bacteria were capable of biodegrading perchlorate using ethanol as an electron donor. Perchlorate concentrations as high as 4,300 μg/L were reduced to less than the practical quantitation limit of 4 μg/L within 50 feet of the electron donor delivery/recharge well. The perchlorate biodegradation half-life was estimated to be approximately 1 day, consistent with perchlorate biodegradation half-lives reported for other sites. TCE dechlorination also was observed at the downgradient monitoring well following bioaugmentation of the shallow aquifer with dehalorespiring (TCE-degrading) bacteria at the recharge well.

With respect to operations, the test demonstrated that the active biobarrier approach is capable of providing effective containment and treatment of impacted groundwater. System operation time was very high (greater than 98%), with system shutdowns primarily related to the two bioaugmentation events. Injecting electron donor over a 1 hour period of time every 24 hours followed by injection of chlorine dioxide was effective in controlling biofouling in the electron donor delivery/recharge well over a sustained period (more than 6 months). 

Semi-Passive EISB

The demonstration results from the semi-passive approach showed that significant reductions in perchlorate concentrations can be achieved using EISB for perchlorate. At the end of the demonstration, perchlorate concentrations were reduced from levels over 800 μg/L to less than 4 μg/L in ten of the thirteen shallow wells within and downgradient of the biobarrier, and the concentrations in the other wells ranged from 7 to 10 μg/L. The average concentration of perchlorate in shallow wells within and downgradient of the biobarrier following the final addition of electron donor was 3.4 μg/L. After the last injection of electron donor, the concentrations of iron, manganese, and arsenic in groundwater samples increased within the area of the biobarrier relative to the upgradient concentrations, but the concentrations in wells downgradient of the biobarrier declined significantly.

The implementation of EISB in most jurisdictions requires a groundwater reinjection permit. The cost of treating groundwater using EISB is much less than it is for conventional pump-and-treat approaches. Potential concerns limiting the use of EISB include: (1) the effectiveness of the technology in reducing concentrations of target compounds below appropriate criteria, and (2) potential negative impacts of excess electron donor on water quality downgradient of the treatment zone. Design issues to be considered include treatment of sites with low hydraulic conductivity, significant variations in hydraulic conductivity, high concentrations of competing electron acceptors, and high concentrations of naturally occurring metals in the subsurface soil.