Factors controlling BTEX and chlorinated solvents plume length under natural attenuation conditions

Natural attenuation is presently used at numerous sites where groundwater is contaminated. In order to simulate this attenuation, reactive transport models are often used but they are quite complex and depend on both physical and chemical conditions in the aquifer. As complex numerical models cannot be used to study all possible cases, we develop here analytical solutions to draw general conclusions. Our strategy, called MIKSS (Mixed Instantaneous and Kinetics Superposition Sequence), allows the calculation of the concentrations of all reacting substances in a plume. It is an extension of the superimposition principle that is able to treat the case of joint kinetics and instantaneous reactions. The basic equations have been extended to treat different reactions that occur in the plume core and at its fringe. At first we consider one organic substance degraded under all oxidising conditions (toluene for instance). For this problem the size of the plume depends on the reduced source width and on the ratio of the organic substance concentration to the sum of the electron acceptors' concentrations. For several BTEX substances having different degradation behaviour the formulation is similar, but leads to quite different plume lengths for each substance. Contrary to the case of one substance, the plumes can be quite long and may not satisfy the target risk level. For chlorinated solvents we developed a specific approach to take under consideration all reactions and particularly the competition for hydrogen. A formula is given to assess the size of the plume core, i.e. the zone with highly reducing conditions. The factors influencing the core length are the same as for BTEX (source width, dispersivity, organic carbon content). The size of the TCE plume is calculated from the plume core length and the kinetic constant of TCE degradation. Using assumptions of degradation constants for DCE and VC it is also possible to calculate the longitudinal concentration profile of these substances. The degradation of moderately substituted solvents under oxic conditions reduces the size of their plumes but under these conditions TCE becomes the major threat. Among the conditions studied in this paper, very few chlorinated solvents sites can lead to a negligible risk at an acceptable distance from the source.     

Sorption of chlorinated solvents and degradation products on natural clayey tills

The sorption of chlorinated solvents and degradation products on seven natural clayey till samples from three contaminated sites was investigated by laboratory batch experiments in order to obtain reliable sorption coefficients (K_d values). The sorption isotherms for all compounds were nearly linear, but fitted by Freundlich isotherms slightly better over the entire concentration range. For chloroethylenes, tetrachloroethylene (PCE) was most strongly sorbed to the clayey till samples (K_d = 0.84-2.45 L kg⁻¹), followed by trichloroethylene (TCE, K_d = 0.62-0.96 L kg⁻¹), cis-dichloroethylene (cis-DCE, K_d = 0.17-0.82 L kg⁻¹) and vinyl chloride (VC, K_d = 0.12-0.36 L kg⁻¹). For chloroethanes, 1,1,1-trichloroethane (1,1,1-TCA) was most strongly sorbed (K_d = 0.2-0.45 L kg⁻¹), followed by 1,1-dichloroethane (1,1-DCA, K_d = 0.16-0.24 L kg⁻¹) and chloroethane (CA, K_d = 0.12-0.18 L kg⁻¹). This is consistent with the order of hydrophobicity of the compounds. The octanol-water coefficient (log K_ow) correlated slightly better with log K_d values than log K_oc values indicating that the K_d values may be independent of the actual organic carbon content (f_oc). The estimated log K_oc or log K_d for chlorinated solvents and degradation products determined by regression of data in this study were significantly higher than values determined by previously published empirical relationships. The site specific K_d values as well as the new empirical relationship compared well with calculations on water and soil core concentration for cis-DCE and VC from the Rugårdsvej site. In conclusion, this study with a wide range of chlorinated ethenes and ethanes - in line with previous studies on PCE and TCE - suggest that sorption in clayey tills could be higher than typically expected.     

糖脂类生物表面活性剂去除污染含水层中石油烃的研究

研究了糖脂类生物表面活性剂对石油烃的增溶,并将其用于污染含水层中石油烃的去除。结果表明:石油烃溶解度随着糖脂类生物表面活性剂浓度的增大而增大,糖脂类生物表面活性剂质量浓度为1 200mg/L时,石油烃溶解度达10 077.7mg/L;界面张力随着糖脂类生物表面活性剂浓度的增加而减小,糖脂类生物表面活性剂质量浓度为1mg/L时,界面张力为34.3mN/m,糖脂类生物表面活性剂质量浓度为800mg/L时,界面张力为5.2 mN/m。采用糖脂类生物表面活性剂对污染含水层进行清洗处理,在固液比(质量体积比)为1g∶2mL的体系中,糖脂类生物表面活性剂质量浓度为3 000mg/L,120r/min、10℃下振荡12h,石油烃去除率达70.82%。污染含水层柱冲洗结果表明,糖脂类生物表面活性剂质量浓度分别为1 200、3 000mg/L时,10倍孔隙体积的表面活性剂冲洗后,分别从污染砂样中去除41.81%和63.30%的石油烃。     

Henry's law constants of chlorinated solvents at elevated temperatures

Henry’s law constants for 12 chlorinated volatile organic compounds (CVOCs) were measured as a function of temperature ranging from 8 to 93 °C, using the modified equilibrium partitioning in closed system (EPICS) method. The chlorinated compounds include tetrachloroethylene, trichloroethylene, cis-1,2-dichloroethylene, vinyl chloride, 1,1,1-trichloroethane, 1,1-dichloroethane, 1,2-dichloroethane, chloroethane, carbon tetrachloride, chloroform, dichloromethane, and chloromethane. The variation in Henry’s constants for these compounds as a function of temperature ranged from around 3-fold (chloroethane) to 30-fold (1,2-dichloroethane). Aqueous solubilities of the pure compounds were measured over the temperature range of 8-75 °C. The temperature dependence of Henry’s constant was predicted using the ratio of pure vapor pressure to aqueous solubility, both of which are functions of temperature. The calculated Henry’s constants are in a reasonable agreement with the measured results. With the improved data on Henry’s law constants at high temperatures measured in this study, it will be possible to more accurately model subsurface remediation processes that operate near the boiling point of water.     

采油废水中多环芳烃的生态风险评价

先利用C-18固相萃取小柱富集大港油田港东联合处理站污水处理站的采油废水中16种多环芳烃(PAHs,即萘、苊烯、苊、芴、菲、蒽、荧蒽、芘、、苯并[a]蒽、苯并[b]荧蒽、苯并[k]荧蒽、苯并[a]芘、茚并[1,2,3-cd]芘、二苯并[a,h]蒽和苯并[g,h,i]苝),再用气相色谱/质谱(GC/MS)分析测定其浓度,以评价PAHs的去除率和生态风险。结果表明:(1)采油废水经处理后,COD、石油类去除率分别达到82.27%、91.06%;外排水COD、石油类达到《污水综合排放标准》(GB 8978—1996)一级标准要求,优于中国采油废水处理的一般水平。(2)采油废水主要以2、3环的PAHs为主,约占总量的93%以上。(3)苯并[a]芘超过《地表水环境质量标准》(GB 3838—2002)中限值。(4)处理前的采油废水中蒽、菲和苯并[a]芘具有一定的生态风险;处理后的外排水中萘、蒽、菲、荧蒽、苯并[a]芘的暴露浓度(PEC)/预测无效应浓度(PNEC)均小于1,目前尚未对环境造成威胁。但是8种PAHs(苊烯和苯并类PAHs除外)总和表现出较大的毒性,需要引起重视。     

Use of tree rings to investigate the onset of contamination of a shallow aquifer by chlorinated hydrocarbons

Oaks (Quercus velutina Lam.) growing over a shallow aquifer contaminated by chlorinated hydrocarbons were studied to determine if it was possible to estimate the approximate year that contamination began. The annual rings of some trees downgradient from the contaminant release site contained elevated concentrations of chloride possibly derived from dechlorination of contaminants. Additionally, a radial-growth decline began in these trees at approximately the same time that chloride became elevated. Growth did not decline in trees that contained smaller concentrations of chloride. The source of elevated chloride and the corresponding reductions in tree growth could not be explained by factors other than contamination. On the basis of tree-ring evidence alone, the release occurred in the late 1960s or early 1970s. Contaminant release at a second location apparently occurred in the mid- to late 1970s, suggesting that the area was used for disposal for at least 5 years and possibly longer.     

Anaerobic transformations and bioremediation of chlorinated solvents

Chlorinated aliphatic compounds, notably the chlorinated solvents, are common contaminants in soil and groundwater at hazardous waste sites. While these compounds are often recalcitrant, under favorable conditions they can be transformed and degraded through microbially mediated processes. There is great interest in understanding the transformations that are observed at contaminated sites and in manipulating these systems to achieve remediation. An important class of transformations occurs in anaerobic environments. Many of the transformations are reductive, and many yield useful energy to specific anaerobic bacteria. They include reductive dechlorination, dehydrochlorination and dichloroelemination. Of these, reductive dechlorination is often a growth-supporting reaction, while the others may be abiological or catalyzed by biological molecules. The reactions may result in chlorinated products, but there are often reaction sequences leading to completely dechlorinated products. The behavior of carbon tetrachloride (CT), 1,1,2,2-tetrachloroethane (TeCA) and the chloroethenes, perchloroethylene (PCE) and trichloroethylene (TCE), illustrate the range of anaerobic transformations that are possible, as well as the limited transformation that often is seen in the environment. CT undergoes reductive and substitutive reactions that are catalyzed by biological molecules but do not support bacterial growth. The anaerobic degradation of TeCA, which is a major contaminant at a site near Tacoma, WA, USA, provides examples of each type of transformation, and the products formed are consistent with the chlorinated compounds that are found in groundwater extraction wells. A laboratory study, using anaerobic sludge that had been fed chlorinated compounds, a cell-free extract from the sludge, and killed controls, showed that TeCA was transformed to four products and that these were further transformed, suggesting that it might be possible to degrade TeCA to innocuous products. Reductive dechlorination of PCE and TCE has been studied in many laboratories. Studies with mixed anaerobic consortia and with several dehalogenating bacteria, including strain 195 (. Isolation of a bacterium that reductively dechlorinates tetrachloroethane to ethane. Science 276, 1568-1571) that transforms PCE to ethene, have demonstrated that reductive dechlorination supports growth of the novel bacteria that carry out the reactions. Hydrogen has been shown to be an electron donor for the bacterial dehalogenation reactions, and kinetic and thermodynamic considerations indicate that dehalogenators can compete in some, but not all, anaerobic environments, pointing to applications of in situ bioremediation and to its limitations. Selected field studies of anaerobic transformations help delineate the applications of this type of bioremediation.     

Quantification of biotransformation of chlorinated hydrocarbons in a biostimulation study: added value via stable carbon isotope analysis

Stable carbon isotope analysis of chlorinated aliphatic compounds was performed at an in situ biostimulation pilot test area (PTA) at a site where 1,2-dichloroethane (1,2-DCA) and trichloroethene (TCE) were present in groundwater. Chlorinated products of TCE reductive dechlorination (cis-dichloroethene (cDCE) and vinyl chloride (VC)) were present at concentrations of 17.5 to 126.4 micromol/L. Ethene, a potential degradation product of both 1,2-DCA dihaloelimination and TCE reductive dechlorination was also present in the PTA. Emulsified soybean oil and lactate were added as electron donors to stimulate anaerobic dechlorination in the PTA. Stable carbon isotope analysis provided evidence that dechlorination was occurring in the PTA during biostimulation, and a means of monitoring changes in dechlorination efficiency over the 183 day monitoring period. Stable carbon isotope analysis was also used to determine if ethene production in the PTA was due to dechlorination of TCE, 1,2-DCA, or both. Fractionation factors (alpha) were determined in the laboratory during anaerobic biotransformation of 1,2-DCA via a dihaloelimination reaction in four separate enrichment cultures. These alpha values (as well as the previously published ranges of alpha for the dechlorination of TCE, cDCE and 1,2-DCA) were used, along with isotopic values measured during the pilot test, to derive quantitative estimates of biotransformation during the pilot test. Dechlorination was found to account for 10.7 to 35.9%, 21.9 to 74.9%, and 54.4 to 67.8% of 1,2-DCA, TCE and cDCE concentration loss respectively in the PTA. Stable carbon isotope analysis indicates that dechlorination of 1,2-DCA, TCE and cDCE were all significant processes during the pilot test, while ethene production during the pilot test was dominated by 1,2-DCA dihaloelimination. This study demonstrates how stable carbon isotope analysis can provide more conservative estimates of the extent of biotransformation than do conventional protocols. In addition, in a complex mixed plume such as this, compound specific isotope analysis is shown to be one of the few methods available for clarifying dominant biotransformation pathways where breakdown products are non-exclusive (i.e. ethene).     

Effectiveness of zerovalent iron and nickel catalysts for degrading chlorinated solvents and n-nitrosodimethylamine in natural groundwater.

Laboratory batch experiments were performed to evaluate the effectiveness of nickel catalysts, nanoscale zerovalent iron (nZVI) doped with palladium, and microscale ZVI for treatment of tetrachloroethene, trichloroethene, and n-nitrosodimethylamine (NDMA) in soil-groundwater slurries. Results indicated that the presence of NDMA inhibited degradation of chlorinated solvents. Although both the nickel catalyst and nZVI were able to degrade NDMA in deionized water, neither of these metals was effective at degrading NDMA in the soil-groundwater system evaluated in this study. The effectiveness of the nickel catalyst and nZVI, with respect to treatment of the chlorinated solvents, also appeared to be highly dependent on the groundwater geochemical conditions. Overall results of this study suggest that the degradation mechanisms involving nickel catalysts and nZVI in natural soil/groundwater systems are not well-understood, and addition of metal catalysts to ZVI may not necessarily enhance observed degradation rates in natural systems.     

Comparative study of microbial dechlorination of chlorinated ethenes in an aquifer and a clayey aquitard

In order to determine whether natural attenuation of chlorinated ethenes by microbial activity occurs in aquitards, sediments at a site contaminated with tetrachloroethene were vertically studied by drilling. The distribution of microbes (Dehalococcoides group and anaerobic hydrogen producers) and the ability of the sediments to sustain microbial dechlorination were determined in an aquitard as well as in an aquifer. Close-spaced sampling revealed the existence of large populations of Dehalococcoides and H(2)-producing bacteria, especially in the organic-rich clayey aquitard rather than in the aquifer. The vinyl chloride reductase gene was also detected in the clay layer. Furthermore, incubation experiments indicated that the clay sediment could sustain transformations of tetrachloroethene at least to vinyl chloride. In contrast, no significant transformation was observed in the aquifer sand. Our results indicate that dechlorination of tetrachloroethene by bacteria can take place in an organic-rich clayey aquitard, and that organic-rich clay may also be important in the natural attenuation in an adjacent aquifer, possibly supplying a carbon source or an electron donor.     

Aliphatic and polycyclic aromatic hydrocarbons in the surface sediments from the Eastern Aegean: assessment and source recognition of petroleum hydrocarbons

Materials and methods  

Aliphatic and polycyclic aromatic hydrocarbons (PAHs) were determined in surficial sediments from the Aegean Sea in the Eastern Mediterranean in 2008.     

Assessment of biogeochemical natural attenuation and treatment of chlorinated solvents, Altus Air Force Base, Altus, Oklahoma

Biogeochemical reductive dechlorination (BiRD) is a newly recognized method for the remediation or natural attenuation of chlorinated solvents. Chlorinated solvents are rapidly treated by abiotic reaction with reduced mineral iron sulfides. Iron sulfides are formed by naturally occurring sulfate-reducing bacteria when sufficient SO(4)(2-) and organic carbon are present or supplied to sediments containing mineral iron. An example of site characterization focusing on BiRD is presented focusing on mineral phases. Methods demonstrated here may be employed at other sites to evaluate naturally occurring BiRD or to evaluate an engineered BiRD remediation. A field investigation was performed at a TCE contaminated site at Altus AFB with naturally high concentrations of SO(4)(2-) and Fe(III) minerals and where an accidental fuel spill provided organic carbon. In the area of this fuel spill significant mineral iron sulfides were found, sulfate was almost completely removed, and TCE was absent. Only small amounts of daughter products were found, further indicating that the BiRD pathway was operative. Mass balance data indicates all of the remaining TCE (182 kg) could be treated by the remaining FeS (66.5 kg) in the upper aquifer; however, the FeS was not co-located with TCE to enable complete reaction. Laboratory microcosm tests with FeS amended and FeS-rich sediment from Altus AFB also suggest that BiRD is capable of destroying TCE. The results suggest that an engineered BiRD treatment is possible for this site.     

Multi-component reactive transport modeling of natural attenuation of an acid groundwater plume at a uranium mill tailings site

Natural attenuation of an acidic plume in the aquifer underneath a uranium mill tailings pond in Wyoming, USA was simulated using the multi-component reactive transport code PHREEQC. A one-dimensional model was constructed for the site and the model included advective-dispersive transport, aqueous speciation of 11 components, and precipitation-dissolution of six minerals. Transport simulation was performed for a reclamation scenario in which the source of acidic seepage will be terminated after 5 years and the plume will then be flushed by uncontaminated upgradient groundwater. Simulations show that successive pH buffer reactions with calcite, Al(OH)3(a), and Fe(OH)3(a) create distinct geochemical zones and most reactions occur at the boundaries of geochemical zones. The complex interplay of physical transport processes and chemical reactions produce multiple concentration waves. For SO4(2-) transport, the concentration waves are related to advection-dispersion, and gypsum precipitation and dissolution. Wave speeds from numerical simulations compare well to an analytical solution for wave propagation.     

An equilibrium analysis of some chlorinated hydrocarbons in stoichiometric to fuel-rich post-flame combustion environments

The equilibrium composition of product gases from the combustion of chlorinated hydrocarbons (CHC) has been studied for varying ratios of C, H, Cl, and O under stoichiometric to fuel-rich conditions. An interactive, PC-compatible FORTRAN program, STANJAN, was utilized in conjunction with thermochemical data sources to calculate equilibrium compositions of gas mixtures as a function of temperature. The predicted results, when judiciously interpreted, assist in the understanding of the potential for formation of residual organic substance emissions (ROSEs) in post-flame environments of an incinerator. Arguments are presented for the potential formation of chlorinated species, which are stable at ambient temperature, if locally fuel-rich mixtures penetrate into the lower temperature zones of an incinerator. ROSEs that have been observed in field tests of incinerators burning chlorinated compounds are predicted to form under the fuel-rich condition. Furthermore, when the equivalence ratio is greater than unity by even a slight amount, the degree of chlorination of product gases increases markedly when the Cl/H ratio also exceeds unity. In that case, time, temperature, turbulence and an overall fuel-lean stoichiometry may not be sufficient to guarantee elimination of measurable levels of chlorinated products. Possible implications of the equilibrium calculations are discussed. Further systematic studies with additional CHCs, nitrogen-, sulfur-, and heavy metal-bearing compounds are recommended as well as continued efforts to carry out kinetic studies.     

Methane oxidation in a crude oil contaminated aquifer: Delineation of aerobic reactions at the plume fringes

High resolution direct-push profiling over short vertical distances was used to investigate CH(4) attenuation in a petroleum contaminated aquifer near Bemidji, Minnesota. The contaminant plume was delineated using dissolved gases, redox sensitive components, major ions, carbon isotope ratios in CH(4) and CO(2), and the presence of methanotrophic bacteria. Sharp redox gradients were observed near the water table. Shifts in δ(13)C(CH4) from an average of -57.6‰ (±1.7‰) in the methanogenic zone to -39.6‰ (±8.7‰) at 105m downgradient, strongly suggest CH(4) attenuation through microbially mediated degradation. In the downgradient zone the aerobic/anaerobic transition is up to 0.5m below the water table suggesting that transport of O(2) across the water table is leading to aerobic degradation of CH(4) at this interface. Dissolved N(2) concentrations that exceeded those expected for water in equilibrium with the atmosphere indicated bubble entrapment followed by preferential stripping of O(2) through aerobic degradation of CH(4) or other hydrocarbons. Multivariate and cluster analysis were used to distinguish between areas of significant bubble entrapment and areas where other processes such as the infiltration of O(2) rich recharge water were important O(2) transport mechanisms.     

Desorption of chlorinated hydrocarbons from spiked and anthropogenically contaminated sediments

The desorption of 20 chlorinated organics from sediments has been studied using a nitrogen purge/Tenax trap system for separating the “dissolved” and “sorbed” fractions in sediment/water slurries. The desorption partition coefficient, K_D, was found to decrease with increasing temperature and suspended sediment concentration. While some differences in K_D and desorption rates were observed for the study chemicals, considering their wide range of physical/chemical properties such as K_OW, these changes were small. Desorption half-lives averaged about 60d at 4°C, 40d at 20°C and 10d at 40°C under continuous gaseous purging. Estimates of the loadings of chemicals via desorption from bottom sediments in Lake Ontario are compared to loadings of these chemicals to the lake from the Niagara River.     

Natural attenuation of chlorinated solvents at Area 6, Dover Air Force Base: groundwater biogeochemistry

Monitored natural attenuation (MNA) has recently emerged as a viable groundwater remediation technology in the United States. Area 6 at Dover Air Force Base (Dover, DE) was chosen as a test site to examine the potential for MNA of tetrachloroethene (PCE) and trichloroethene (TCE) in groundwater and aquifer sediments. A "lines of evidence" approach was used to document the occurrence of natural attenuation. Chlorinated hydrocarbon and biogeochemical data were used to develop a site-specific conceptual model where both anaerobic and aerobic biological processes are responsible for the destruction of PCE, TCE, and daughter metabolites. An examination of groundwater biogeochemical data showed a region of depleted dissolved oxygen with elevated dissolved methane and hydrogen concentrations. Reductive dechlorination likely dominated in the anaerobic portion of the aquifer where PCE and TCE levels were observed to decrease with a simultaneous increase in cis-1,2-dichloroethene (cis-DCE), vinyl chloride (VC), ethene, and dissolved chloride. Near the anaerobic/ aerobic interface, concentrations of cis-DCE and VC decreased to below detection limits, presumably due to aerobic biotransformation processes. Therefore, the contaminant and daughter product plumes present at the site appear to have been naturally atteuated by a combination of active anaerobic and aerobic biotransformation processes.     

Extraction of chlorinated aliphatic hydrocarbons from groundwater at micromolar concentrations for isotopic analysis of chlorine.

A method is described for near-quantitative extraction of micromolar concentrations of chlorinated aliphatic hydrocarbons (CAHs) from water for determination of chlorine (Cl) isotope ratios. A low pressure, carrier-gas procedure of extraction was proven to be applicable to CH2Cl2, CCl4, C2H2Cl2, and C2HCl3. The pH of the water was adjusted with NaOH to prevent extraction of CO2 from air and/or dissolved inorganic carbonate species. Recoveries of CAH samples (approximately 15 mumol), added to and extracted from approximately 340 ml of water, averaged approximately 96%. Average changes in the delta 37Cl values of the CAHs, attributable to the extraction process, were -0.01 +/- 0.06@1000. Significant isotopic fractionation of Cl was measured during partial extraction of C2CHCl3 from water, indicating that near-quantitative extraction is required for reliable stable Cl isotope analysis of CAHs. This method is also suitable for the extraction of dissolved CAH for gas chromatography-combustion-isotope ratio mass spectrometric measurements of hydrogen and carbon.     

Anaerobic biotransformation of explosives in aquifer slurries amended with ethanol and propylene glycol

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), and 2,4,6-trinitrotoluene (TNT) are explosives that are frequently found as environmental contaminants on military installations. Hydrogen has been shown to support the anaerobic transformation of these explosives. We investigated ethanol and propylene glycol as electron donors for providing syntrophically produced H2 for stimulating the anaerobic biodegradation of explosives in contaminated soil. The study was conducted using anoxic microcosms constructed with slurries of the contaminated soil and groundwater. The addition of 5mM ethanol and propylene glycol enhanced the biodegradation of RDX and HMX relative to the control bottles. Ethanol was depleted within about 20 days, resulting in the transient formation of hydrogen, acetate, and methane. The hydrogen headspace concentration increased from 8 ppm to 1838 ppm before decreasing to background concentrations. Propylene glycol was completely degraded after 15 days, forming hydrogen, propionate, and acetate as end-products. The hydrogen headspace concentrations increased from 56 ppm to 628 ppm before decreasing to background concentrations. No methane formation was observed during the incubation period of 48 days. Our findings indicate the addition of ethanol and propylene to the aquifer slurries increased the hydrogen concentrations and enhanced the biotransformation of RDX and HMX in the explosive-contaminated soil.     

Assessment of the natural attenuation of chlorinated ethenes in an anaerobic contaminated aquifer in the Bitterfeld/Wolfen area using stable isotope techniques, microcosm studies and molecular biomarkers

The in situ degradation of chlorinated ethenes was assessed in an anaerobic aquifer using stable isotope fractionation approaches, microcosm studies and taxon specific detection of specific dehalogenating groups of bacteria. The aquifer in the Bitterfeld/Wolfen region in Germany contained all chlorinated ethenes, benzene and toluene as contaminants. The concentrations and isotope composition of the chlorinated ethenes indicated biodegradation of the contaminants. Microcosm studies confirmed the presence of in situ microbial communities capable of the complete dechlorination of tetrachloroethene. Taxon specific investigation of the microbial communities indicated the presence of various potential dechlorinating organisms including Dehalococcoides, Desulfuromonas, Desulfitobacterium and Dehalobacter. The integrated approach, using metabolite spectra, molecular marker analysis and isotope studies, provided several lines of evidence for natural attenuation of the chlorinated ethenes.