Groundwater contamination related to the production, handling and use of solid rocket propellants such as ammonium perchlorate has been identified as a widespread problem at rocket manufacturing and testing facilities nationwide. The costs for remediation of perchlorateimpacted groundwater are expected to be in the billions of dollars, which may jeopardize major Department of Defense (DoD) and propulsion contractor production programs. Few cost-effective technologies exist for the treatment of perchlorate-contaminated groundwater. Of the technologies being developed, in situ bioremediation is among the most promising because it has the potential to destroy perchlorate in place, rather than transferring it to another waste stream requiring costly treatment or disposal.
The research program included laboratory studies and a small field demonstration to (1) evaluate the ubiquity of perchlorate-degrading bacteria in groundwater at DoD and related facilities, (2) assess the applicability of in situ bioremediation in varying geochemical environments, and (3) generate preliminary field data for technology scale-up and demonstration/validation through a follow-on program.
Perchlorate biodegradation results from microbially-mediated redox reactions, whereby perchlorate serves as an electron acceptor and is reduced via chlorate to chlorite. Chlorite then undergoes a biologically mediated dismutation reaction, releasing chloride and oxygen. A variety of electron donors have been used to stimulate perchlorate reduction, including alcohols, organic acids, edible oils and some sugars. The key to successfully implementing in situ bioremediation for perchlorate appears to be the addition of appropriate carbon substrates to reduce competing electron acceptors present in the groundwater and to promote the perchlorate reduction reaction.
This study demonstrated that perchlorate-reducing bacteria are ubiquitous and that perchlorate concentrations could be reduced from as high as 660 mg/L to less than the provisional action level of 0.018 mg/L within weeks to months at the varying sites. The results of the field pilot test clearly demonstrated that the approach was capable of jointly biodegrading perchlorate, nitrate, and trichloroethene (TCE) to environmentally acceptable end products. Perchlorate concentrations declined quickly from the steady state influent of 8 mg/L to less than the practical quantitation limit (PQL) of 0.004 mg/L within several weeks of electron donor (ethanol) addition. Similarly, nitrate concentrations were reduced from an average influent of 24 mg/L to less than the PQL of 0.5 mg/L within weeks of electron donor addition. Concurrent with perchlorate reduction, TCE at 2 mg/L was completely dechlorinated to ethene within 35 feet from the electron donor delivery well. This project was completed in FY 2001.
Current approaches for the remediation of perchlorate-impacted groundwater typically involve long-term pump-and-treat solutions. Comparison of cost estimates for in situ versus ex situ treatment for perchlorate plumes at several rocket facilities suggests that in situ bioremediation may cost between 50 to 75 percent of ex situ treatment costs. Given the number of perchlorate-impacted DoD and related contractor sites that may require groundwater remediation in the coming years, in situ bioremediation may represent a cost savings in the $100Ms. The ability to jointly treat perchlorate and common co-contaminants such as nitrate and chlorinated solvents via in situ bioremediation will also increase cost savings to DoD.