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Field Study of In Situ Anaerobic Bioremediation of a Chlorinated Solvent Source Zone Federico Aulenta,† Andrea Canosa,† Michele Leccese,‡ Marco Petrangeli Papini,† Mauro Majone,*,† and Paolo Viotti‡ Department of Chemistry “Stanislao Cannizzaro”, UniVersity of Rome “La Sapienza”, P. le Aldo Moro 5, 00185 Roma, Italy, and Department of Hydraulics, Transportation, and Roads, UniVersity of Rome “La Sapienza”, Via Eudossiana 18, 00184 Roma, Italy
A field-scale in situ bioremediation test was conducted at a chlorinated solvent (mainly 1,1,2,2-tetrachloroethane) contaminated aquifer to evaluate the feasibility of source-zone treatment through anaerobic reductive dechlorination. Contaminated groundwater was extracted at the bottom of the testing zone, amended/ mixed with an electron donor (lactate) solution, and reinjected at the head of the testing zone. Both forced groundwater recirculation and lactate injection resulted in significant mobilization of chlorinated contaminants from the source zone, probably through enhanced dissolution of DNAPL (dense nonaqueous phase liquid) pools. The addition of lactate stimulated the establishment of anaerobic conditions in the aquifer and the onset of microbial reductive dechlorination processes. Dechlorination of 1,1,2,2-tetrachloroethane led to the formation of predominantly 1,1,2-trichloroethane and cis-dichloroethene. Noticeably, dechlorination occurred at aqueous 1,1,2,2-tetrachloroethane concentrations as high as 90 mg L-1, indicating than in situ bioremediation is a feasible source-treatment strategy for chlorinated solvents such as chloroethanes. Introduction Chlorinated aliphatic hydrocarbons (CAH), such as 1,1,2,2tetrachloroethane (1,1,2,2-TeCA), perchloroethylene (PCE), and trichloroethylene (TCE), are widely employed in industry as solvents, degreasing agents, and chemical feedstock. Careless storage, handling, and disposal practices have contributed to their status of most frequently encountered subsurface (soil and groundwater) contaminants. CAH are highly toxic, as well as known or suspected carcinogens; therefore, their presence in the environment poses important health risks and has prompted investigations concerning their fate in the subsurface. In subsurface environments, CAH are often present as dense nonaqueous phase liquids (DNAPLs), which slowly dissolve into the groundwater, serving as long-term sources of groundwater contamination.1,2 The time required for complete dissolution for a typical accumulation of the solvents can be hundreds of years under natural conditions.3 Since the conventional “pump and treat” approach of DNAPL source zones is only effective for containment or treatment of the dissolved contaminant plume,4-8 remedial methods based on the removal of contaminant mass from the source zone have to be also considered. Recent analytical and numerical modeling investigations suggested that even partial removal of contaminant mass from a source zone may result in significant (several orders of magnitude) reduction in posttreatment contaminant mass flux.1,6,9,10 A number of innovative technologies have then been recently developed to enhance contaminants removal or destruction from source zones. These technologies include the following: in situ chemical oxidation or reduction,11-16 alcohol and surfactant flushing,4,6,10,17 thermal treatment,18,19 and bioremediation via * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +39-06-49913646. Fax: +39-06490631. † Department of Chemistry “Stanislao Cannizzaro”, University of Rome “La Sapienza”. ‡ Department of Hydraulics, Transportation, and Roads, University of Rome “La Sapienza”.
anaerobic reductive dechlorination.20-23 Among these techniques, bioremediation is probably the least aggressive in terms of perturbation of aquifer composition and biocenosis, since it relies on the stimulation of the biodegradative activity of naturally occurring microorganisms. The ability of bioremediation to achieve substantial mass reduction has been given little attention in the past, mainly because high concentrations of chlorinated solvents occurring in the proximity of source zones were believed to be toxic to microorganisms. Moreover, the abilities of chlorinated solvent-degrading microorganisms to tolerate saturation concentrations of chlorinated solvents differ significantly.24,25 As an example, reductive dechlorination (RD) of PCE by Sulfurospirillum multiVorans and Dehalobacter restrictus is inhibited by PCE concentrations higher than 50 and 30 mg L-1, respectively. In contrast, Desulforomonas michiganensis, Desulfitobacterium strain Y5119, and Enterobacter agglomerans strain MS-124 are all reported to tolerate PCE concentrations close to saturation (∼150 mg L-1). Nielsen and Keasling26 enriched a microbial culture from a TCEcontaminated aquifer that could perform anaerobic degradation of saturated PCE at a faster rate than under subsaturating conditions. Moreover, recent field and laboratory studies have provided convincing evidence that bioremediation may result in enhanced DNAPL dissolution rates and reduced longevity of the chlorinated ethene components of DNAPLs.21-23 In microbial RD, chlorinated compounds are reduced and stepwise dechlorinated, serving as metabolic electron acceptors for energy respiration.27,28 Thus, microbial RD is a strict anaerobic process that requires the presence of an electron donor.29-33 Several different electron donors, including methanol, butyrate, lactate, and benzoate,34-36 have been shown to support microbial RD in both field and laboratory studies. Nevertheless, in most of the cases, the hydrogen produced during fermentation of organic compounds was the actual electron donor used for the RD.37 In contrast to chlorinated ethenes such as PCE and TCE, very few studies investigated the anaerobic
10.1021/ie070048m CCC: $37.00 © 2007 American Chemical Society Published on Web 05/18/2007
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degradation of 1,1,2,2-TeCA in contaminated groundwater, either as dissolved phase or dense nonaqueous phase liquids (DNAPLs). In this study, we investigated the possibility to stimulate in situ the reductive dechlorination of 1,1,2,2-TeCA, through the injection of lactate and nutrients, within the source zone of a contaminated aquifer. This bioremediation field study follows a laboratory-scale microcosm study that indicated the presence within the source zone of the aquifer of native dechlorinating populations able to fully transform 1,1,2,2-TeCA into harmless nonchlorinated products such as ethene and ethane.31 Experimental Section Site Description. The in situ bioremediation field study was conducted at a former industrial site, mainly involved in the production of synthetic dyes, located in Northern Italy. Subsurface contamination (mostly chlorinated solvents) probably resulted from leakage of an underground storage tank. Released solvents and other contaminants severely polluted both a shallow aquifer (from 3 to 10 m below ground surface) and a deeper aquifer (from 15 to 50 m below ground surface). The two aquifers were separated by a thin, discontinuous, low-permeability layer of clay. Around 20 years ago, a source-zone containment action was undertaken by local authorities. This action primarily consisted of the lateral isolation of a region of the shallow aquifer that was suspected to retain significant mass of contaminants, presumably as DNAPL. This zone is hereafter referred to as “source zone”. Groundwater analyses within the source zone, carried out over the last 10 years by local authorities, indicated the presence of nearly stable concentrations of 1,1,2,2-TeCA (∼10 mg L-1), PCE, and TCE (
