Ammonium perchlorate (NH₄ClO₄) has been used for several decades in the United States as an oxidant in solid propellants and explosives. It is the primary oxidant used in many rocket motors and boosters, and various perchlorate salts (e.g., ammonium, potassium and magnesium perchlorate) are also used in flares, fireworks matches, and air bags. The primary sources of soil and groundwater contamination with perchlorate, however, are related to its production for aerospace and military applications, testing of rockets and munitions, and the periodic removal and replacement of solid fuels in rockets. The handling of perchlorates by manufacturers and the rocket propellant industry has led to widespread contamination of surface water and groundwater, and the high solubility of perchlorate, coupled with its chemical stability in water, has led to expansive plumes and both human health and environmental concerns.

The objective of this project was to develop a biological treatment technology for the in situ remediation of perchlorate in groundwater.

Aquifer and surface soil samples from perchlorate-contaminated sites with various geochemical characteristics and contaminant concentrations were obtained for use in enrichment, microcosm, and column studies. A microbial consortia and individual bacterial isolates capable of perchlorate degradation were also obtained, and conditions required for in situ biostimulation of perchlorate degradation were identified. The most effective treatments for perchlorate degradation in the microcosm studies were further tested using pilot-scale flow through model aquifers. Biodegradation and reactive transport modeling also were performed.

The laboratory results from this project suggest that in situ biodegradation of ammonium perchlorate through electron donor addition is likely to be a viable remediation approach at numerous Department of Defense sites. The next step in transitioning this technology is to perform one or more field demonstrations. Site studies revealed that naturally occurring perchlorate-degrading bacteria were present in groundwater at the Indian Head Division Naval Surface Warfare Center (IHDIV) and that those organisms could be stimulated to degrade perchlorate, from more than 50mg/L to below detection, using lactate as an electron donor. It was also found that the pH of the aquifer must be buffered to achieve optimal perchlorate biodegradation. As a result of these findings, a field demonstration of in situ perchlorate treatment was performed at IHDIV on a shallow narrow plume of perchlorate-contaminated groundwater. Another field demonstration was conducted under ESTCP project ER-200224 to show that electron donor addition can be used to efficiently and cost-effectively treat perchlorate in subsurface groundwater to below the minimum detection limit.

This research provided information on (1) the potential for successful perchlorate remediation at subsurface sites by addition of electron donors (i.e., biostimulation); (2) the most effective electron donors to use in biostimulation efforts and the expected concentrations and remediation kinetics achievable with these donors; (3) the probable influence of alternate electron acceptors and environmental variables on perchlorate reduction; and (4) the potential to treat column co-contaminants with perchlorate. These data provide the fundamental knowledge required for the design and implementation of pilot-scale and full-scale remediation efforts at perchlorate-contaminated sites.