Field demonstration of in situ perchlorate bioremediation in groundwater

Abstract

A wide variety of microorganisms are capable of utilizing perchlorate (C104-) as a terminal electron acceptor while growing on one of several different organic or inorganic electron donors1 (see also Chapters 12 and 13 in this volume). These bacteria reduce perchlorate initially to chlorate (C103-) and then to chlorite (CIO2-),2,3 The chlorite is subsequently disproportionated into chloride (C1-) and oxygen (02). 4Thus, the biodegradation process results in a detoxification of the perchlorate molecule. Biological reactors have been successfully applied at both pilot-scale and full-scale to treat perchlorate-contaminated groundwater. 5-7 Full-scale fluidized bed bioreactors are presently operating at five locations, removing perchlorate from more than 30 million liters of groundwater per day. Moreover, two full-scale suspended growth bioreactors have been constructed to treat high concentrations of perchlorate in military and industrial wastewaters. 5These ex situ systems have proven to be both reliable and economical. Moreover, biological treatment currently represents the only approach to both remove perchlorate from water and convert it to nonhazardous products (i.e., chloride and oxygen) in a single step. The successful application of ex situ biological treatment systems for perchlorate, as well as the isolation and characterization of numerous pure cultures of perchlorate-degrading bacteria from natural environments, has prompted significant research concerning the potential for in situ perchlorate treatment through electron donor amendment to soils and groundwater. Current laboratory data suggest that perchlorate-reducing bacteria (PRB) are indigenous to many soils and groundwaters 8-10. Studies also reveal that these microorganisms can be stimulated to metabolize perchlorate from concentrations as high as several hundred parts-per-million to below current analytical reporting limits (4 μg/L) by the addition of one of several different electron donors, such as acetate, lactate, citrate, ethanol, vegetable oil, or hydrogen gas. 7,8-12 The wide distribution of PRB combined with their ability to metabolize perchlorate using common substrates suggests that in situ bioremediation may be a viable approach for treatment of perchloratecontaminated groundwater. However, field data examining in situ perchlorate treatment in subsurface aquifers are currently very limited. This chapter describes the results from a pilot-scale field evaluation of in situ perchlorate bioremediation. The field demonstration was performed at the Indian Head Division, Naval Surface Warfare Center (IHDIV) in Indian Head, Maryland. A groundwater recirculation design was tested to distribute electron donor (lactate) to indigenous PRB in a shallow aquifer at the site. Two field plots were installed, each consisting of two extraction wells, two injection wells, and nine groundwater monitoring wells. In one plot (test plot), extracted groundwater was amended with lactate and buffer and then re-injected into the aquifer. In the second plot (control plot), the groundwater was extracted and re-injected without substrate or buffer amendment. Perchlorate levels in the test plot declined by > 95 % in 8 of 9 monitoring wells during the demonstration from an initial average value of 171 mg/L. Five wells reached perchlorate levels < 1 mg/L and two wells were below the reporting level of 5 μg/L. Nitrate concentrations were also appreciably reduced throughout the test plot, with 7 of the 9 wells showing nondetectable levels within 7 weeks. Conversely, there was no significant reduction in either perchlorate or nitrate within the control plot.Perchlorate levels at the beginning of the demonstration averaged 127 mg/L, and these values were 1 18 mg/L after the 5-month study was complete. The data from this demonstration indicate that in situ perchlorate bioremediation can be a viable approach for treatment of perchlorate in aquifers containing localized, high concentrations of the contaminant. © 2006 Springer Science+Business Media, Inc.