Role In Natural Attenuation Research Articles

Indigenous microbial populations of abandoned mining sites and their role in natural attenuation.

Environmental contamination by toxic effluents discharged by anthropogenic activities including the mining industries has increased extensively in the recent past. Microbial communities and their biofilms inhabiting these extreme habitats have developed different adaptive strategies in metabolizing and transforming the persistent pollutants. They also play a crucial role in natural attenuation of these abandoned mining sites and act as a major driver of many biogeochemical processes, which helps in ecological rehabilitation and is a viable approach for restoration of wide stretches of land. In this review, the types of mine wastes including the overburden and mine drainage and the types of microbial communities thriving in such environments were probed in detail. The types of biofilms formed along with their possible role in metal bioremediation were also reviewed. This review also provides an overview of the shift in microbial communities in natural reclamation process and also provides an insight into the restoration of the enzyme activities of the soils which may help in further revegetation of abundant mining areas in a sustainable manner. Moreover, the role of indigenous microbiota in bioremediation of heavy metals and their plant growth-promoting activity weres discussed to assess their role in phytoremedial processes.

Natural Biodegradation of Vinyl Chloride and cis-Dichloroethene in Aerobic and Suboxic Conditions.

Chlorinated ethene (CE) groundwater contamination is commonly treated through anaerobic biodegradation (i.e., reductive dechlorination) either as part of an engineered system or through natural attenuation. Aerobic biodegradation has also been recognized as a potentially significant pathway for the removal of the lower CEs cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC). However, the role of aerobic biodegradation under low oxygen conditions typical of contaminated groundwater is unclear. Bacteria capable of aerobic VC biodegradation appear to be common in the environment, while aerobic biodegradation of cDCE is less common and little is known regarding the organisms responsible. In this study, we investigate the role of aerobic cDCE and VC biodegradation in a mixed contaminant plume (including CEs, BTEX, and ketones) at Naval Air Station North Island, Installation Restoration Site 9. Sediment and groundwater collected from the plume source area, mid-plume, and shoreline were used to prepare microcosms under fully aerobic (8mg/L dissolved oxygen (DO)) and suboxic (< 1mg/L DO) conditions. In the shoreline microcosms, VC and cDCE were rapidly degraded under suboxic conditions (100% and 77% removal in < 62days). In the suboxic VC microcosms, biodegradation was associated with a > 5 order of magnitude increase in the abundance of functional gene etnE, part of the aerobic VC utilization pathway. VC and cDCE were degraded more slowly under fully aerobic conditions (74% and 30% removal) in 110days. High-throughput 16S rRNA and etnE sequencing suggest the presence of novel VC- and cDCE-degrading bacteria. These results suggest that natural aerobic biodegradation of cDCE and VC is occurring at the site and provide new evidence that low (< 1mg/L) DO levels play a significant role in natural attenuation of cDCE and VC.

Chlorinated ethene biodegradation and associated bacterial taxa in multi-polluted groundwater: Insights from biomolecular markers and stable isotope analysis

Chlorinated ethenes (CEs) are most problematic pollutants in groundwater. Dehalogenating bacteria, and in particular organohalide-respiring bacteria (OHRB), can transform PCE to ethene under anaerobic conditions, and thus contribute to bioremediation of contaminated sites. Current approaches to characterize in situ biodegradation of CEs include hydrochemical analyses, quantification of the abundance of key species (e.g. Dehalococcoides mccartyi) and dehalogenase genes (pceA, vcrA, bvcA and tceA) involved in different steps of organohalide respiration (OHR) by qPCR, and compound-specific isotope analysis (CSIA) of CEs. Here we combined these approaches with sequencing of 16S rRNA gene amplicons to consider both OHRB and bacterial taxa involved in CE transformation at a multi-contaminated site. Integrated analysis of hydrogeochemical characteristics, gene abundances and bacterial diversity shows that bacterial diversity and OHRB mainly correlated with hydrogeochemical conditions, suggesting that pollutant exposure acts as a central driver of bacterial diversity. CSIA, abundances of four reductive dehalogenase encoding genes and the prevalence of Dehalococcoides highlighted sustained PCE, DCE and VC degradation in several wells of the polluted plume. These results suggest that bacterial taxa associated with OHR play an essential role in natural attenuation of CEs, and that representatives of taxa including Dehalobacterium and Desulfosporosinus co-occur with Dehalococcoides. Overall, our study emphasizes the benefits of combining several approaches to evaluate the interplay between the dynamics of bacterial diversity in CE-polluted plumes and in situ degradation of CEs, and to contribute to a more robust assessment of natural attenuation at multi-polluted sites.

Numerical model of the behavior of chlorinated ethenes in a fractured, karstic limestone aquifer

Characterizing the transport and degradation of chlorinated ethenes in fractured aquifers, as well as the assessment of cleanup times, poses an extreme technical challenge. In the presented study, a method to analyze reactive transport and reductive dechlorination of chlorinated solvents in fractured aquifers is developed. A rough-walled parallel-plate model of nonlinear flow behavior is coupled with random-walk particle tracking, incorporating particle exchange between the mobile and stagnant zones, adsorption processes, and reductive dechlorination reaction pathways. The developed methodology, considering reductive dechlorination processes in a Lagrangian framework, is able to simulate the motion of particles affected by first-order network reactions, so that particles move according to their chemical state, affecting physical transport processes (advection, dispersion, mass-transfer exchange between mobile and stagnant zones). The developed model is applied to a case study of groundwater contamination in the industrial area of Bari and Modugno (Italy), where the limestone aquifer has a fractured, karstic nature. The steady-state distribution of the contamination by chlorinated ethenes from a source at a hot spot is obtained and compared with the observed scenario of contamination, in order to estimate the plausible transport and degradation processes and the mass loading at source. The study represents a valuable tool in deciding the role of natural attenuation as a treatment option, where the natural attenuation capacity of groundwater can be integrated with engineering methods in order to obtain site remediation.

Differential Protein Expression During Growth on Medium Versus Long-Chain Alkanes in the Obligate Marine Hydrocarbon-Degrading Bacterium Thalassolituus oleivorans MIL-1.

The marine obligate hydrocarbonoclastic bacterium Thalassolituus oleivorans MIL-1 metabolizes a broad range of aliphatic hydrocarbons almost exclusively as carbon and energy sources. We used LC-MS/MS shotgun proteomics to identify proteins involved in aerobic alkane degradation during growth on medium- (n-C14) or long-chain (n-C28) alkanes. During growth on n-C14, T. oleivorans expresses an alkane monooxygenase system involved in terminal oxidation including two alkane 1-monooxygenases, a ferredoxin, a ferredoxin reductase and an aldehyde dehydrogenase. In contrast, during growth on long-chain alkanes (n-C28), T. oleivorans may switch to a subterminal alkane oxidation pathway evidenced by significant upregulation of Baeyer-Villiger monooxygenase and an esterase, proteins catalyzing ketone and ester metabolism, respectively. The metabolite (primary alcohol) generated from terminal oxidation of an alkane was detected during growth on n-C14 but not on n-C28 also suggesting alternative metabolic pathways. Expression of both active and passive transport systems involved in uptake of long-chain alkanes was higher when compared to the non-hydrocarbon control, including a TonB-dependent receptor, a FadL homolog and a specialized porin. Also, an inner membrane transport protein involved in the export of an outer membrane protein was expressed. This study has demonstrated the substrate range of T. oleivorans is larger than previously reported with growth from n-C10 up to n-C32. It has also greatly enhanced our understanding of the fundamental physiology of T. oleivorans, a key bacterium that plays a significant role in natural attenuation of marine oil pollution, by identifying key enzymes expressed during the catabolism of n-alkanes.

Dissolved Mineral Ash Generated by Vegetation Fire Is Photoactive under the Solar Spectrum.

Vegetation fire generates vast amounts of mineral ash annually that can be readily mobilized by water or wind erosion. Little is known about the photoactivity of dissolved mineral ash in aquatic systems and its ability to mediate redox reactions of environmental pollutants. This study reports that dissolved mineral ash derived from pyrolysis of biomass is photoactive under simulated sunlight, generating reactive oxygen species. It can mediate the photoreduction of hexavalent chromium (Cr(VI)) in the presence of electron donors; for example, phenols and dissolved organic matter, at pH 4.7. The reaction kinetics followed the Langmuir-Hinshelwood model, suggesting a heterogeneous photocatalytic reaction. The enhancement of reduction efficiency was linearly correlated with the one-electron reduction potential of phenols. The synergy between dissolved mineral ash and phenols is attributed to the inhibition of electron-hole recombination. The reduction rate decreases with increasing solution pH, owing to the decreased reduction potential and surface adsorption of Cr(VI). The silicon and silicon carbide components are most likely responsible for the photocatalytic activity of dissolved mineral ash. Our results suggest that dissolved mineral ash is a natural photocatalyst that can mediate redox reactions of pollutants in sunlit aquatic systems, playing an overlooked role in natural attenuation and aquatic photochemistry.

Hydrocarbonoclastic Alcanivorax Isolates Exhibit Different Physiological and Expression Responses to n-dodecane.

Autochthonous microorganisms inhabiting hydrocarbon polluted marine environments play a fundamental role in natural attenuation and constitute promising resources for bioremediation approaches. Alcanivorax spp. members are ubiquitous in contaminated surface waters and are the first to flourish on a wide range of alkanes after an oil-spill. Following oil contamination, a transient community of different Alcanivorax spp. develop, but whether they use a similar physiological, cellular and transcriptomic response to hydrocarbon substrates is unknown. In order to identify which cellular mechanisms are implicated in alkane degradation, we investigated the response of two isolates belonging to different Alcanivorax species, A. dieselolei KS 293 and A. borkumensis SK2 growing on n-dodecane (C12) or on pyruvate. Both strains were equally able to grow on C12 but they activated different strategies to exploit it as carbon and energy source. The membrane morphology and hydrophobicity of SK2 changed remarkably, from neat and hydrophilic on pyruvate to indented and hydrophobic on C12, while no changes were observed in KS 293. In addition, SK2 accumulated a massive amount of intracellular grains when growing on pyruvate, which might constitute a carbon reservoir. Furthermore, SK2 significantly decreased medium surface tension with respect to KS 293 when growing on C12, as a putative result of higher production of biosurfactants. The transcriptomic responses of the two isolates were also highly different. KS 293 changes were relatively balanced when growing on C12 with respect to pyruvate, giving almost the same amount of upregulated (28%), downregulated (37%) and equally regulated (36%) genes, while SK2 transcription was upregulated for most of the genes (81%) when growing on pyruvate when compared to C12. While both strains, having similar genomic background in genes related to hydrocarbon metabolism, retained the same capability to grow on C12, they nevertheless presented very different physiological, cellular and transcriptomic landscapes.

Reductive transformation of hexabromocyclododecane (HBCD) by FeS

Both iron monosulfide (FeS) and brominated flame retardants (BFRs) are widely found at relatively high levels in anoxic sediments, but little is known about the reactions of FeS with BFRs. Prior studies showed that FeS was variously reactive with chlorinated organic pollutants in many anoxic environments. It is intuitive that FeS is also reactive with BFRs under anoxic conditions. This study was initiated to test such a hypothesis by quantifying the rates of reductive transformation of tetrabromobisphenol A (TBBPA), decabrominated diphenyl ether (decaBDE) and hexabromocyclododecane (HBCD) using synthetic FeS as the reactive agent. The results showed that over 90% of HBCD was transformed by FeS within 24 h, whereas both TBBPA and decaBDE were found nonreactive within 2 days. The transformation of HBCD followed a pseudo-first-order rate kinetic and the observed rate constants were dependent on the initial concentrations of FeS and HBCD. The transformation rates of β- and γ-HBCD were significantly faster than that of α-HBCD. Analysis of bromine ion and other transformation products suggested that sequential dibromoelimination to form 1,5,9-cyclododecatriene was likely to be a dominant pathway for the reductive transformation of HBCD by FeS. Surface characterization of FeS by XPS indicated that both Fe(II) and S(-II) on the FeS surface might have contributed considerably to the transformation of HBCD. These findings imply that FeS may play an important role in natural attenuation of HBCD and that it may be used as a reactive agent for treating HBCD-contaminated sediments.

Dynamics of communities of bacteria and ammonia-oxidizing microorganisms in response to simazine attenuation in agricultural soil

Autochthonous microbiota plays a crucial role in natural attenuation of s-triazine herbicides in agricultural soil. Soil microcosm study was carried out to investigate the shift in the structures of soil autochthonous microbial communities and the potential degraders associated with natural simazine attenuation. The relative abundance of soil autochthonous degraders and the structures of microbial communities were assessed using quantitative PCR (q-PCR) and terminal restriction fragment length polymorphism (TRFLP), respectively. Phylogenetic composition of bacterial community was also characterized using clone library analysis. Soil autochthonous microbiota could almost completely clean up simazine (100mgkg−1) in 10days after herbicide application, indicating a strong self-remediation potential of agricultural soil. A significant increase in the proportion of s-triazine-degrading atzC gene was found in 6days after simazine amendment. Simazine application could alter the community structures of total bacteria and ammonia-oxidizing archaea (AOA) and bacteria (AOB). AOA were more responsive to simazine application compared to AOB and bacteria. Actinobacteria, Alphaproteobacteria and Gammaproteobacteria were the dominant bacterial groups either at the initial stage after simazine amendment or at the end stage of herbicide biodegradation, but Actinobacteria predominated at the middle stage of biodegradation. Microorganisms from several bacterial genera might be involved in simazine biodegradation. This work could add some new insights on the bioremediation of herbicides contaminated agricultural soils.

Role of natural attenuation in modeling the leaching of contaminants in the risk analysis framework

Natural attenuation (NA) processes occurring in the subsurface can significantly affect the impact on groundwater from contamination sources located in the vadose zone, especially when mobile and readily biodegradable compounds, such as BTEX, are present. Besides, in the last decades several studies have shown natural attenuation to take place also for more persistent compounds, such as Polycyclic Aromatic Hydrocarbons (PAHs). Nevertheless, common risk analysis frameworks, based on the ASTM RBCA (Risk Based Corrective Action) approach, do not include NA pathways in the fate and transport models, thus possibly leading to an overestimation of the calculated risk. The aim of this study was to provide an insight on the relevance of the different key natural attenuation processes usually taking place in the subsurface and to highlight for which contamination scenarios their inclusion in the risk-analysis framework could provide a more realistic risk assessment. To this end, an analytical model accounting for source depletion and biodegradation, dispersion and diffusion during leaching was developed and applied to several contamination scenarios. These scenarios included contamination by BTEX, characterized by relatively high mobility and biodegradation rate, and PAHs, i.e. a more persistent class of compounds. The obtained results showed that BTEX are likely to be attenuated in the source zone due to their mobility and ready biodegradation (assuming biodegradation constant rates in the order of 0.01–1 d−1). Instead, attenuation along transport through the vadose zone was found to be less important, as the residence time of the contaminant in the unsaturated zone is often too low with respect to the time required to get a relevant biodegradation of BTEX. On the other hand, heavier compounds such as PAHs, were found to be attenuated during leaching since the residence time in the vadose zone can reach values up to thousands of years. In these cases, even with the relatively slow biodegradation rate of PAHs, in the order of 0.0001–0.001 d−1, attenuation can result significant. These conclusions were also confirmed by comparing the model results with experimental data collected at an hydrocarbon-contaminated site. The proposed model, that neglects the transport of NAPLs, could be easily included in the risk-analysis framework, allowing to get a more realistic assessment of risks, while keeping the intrinsic simplicity of the ASTM-RBCA approach.

Role of natural attenuation, phytoremediation and hybrid technologies in the remediation of a refinery soil with old/recent petroleum hydrocarbons contamination

Within a search for a biological remediation technology to remove petroleum hydrocarbons (PHC) from a contaminated soil from a refinery, the potential of monitored natural attenuation (MNA) was compared with the use of transplants of Cortaderia selloana both in the absence and in the presence of soil amendments. After 31 months of experiments, MNA was effective in removing most of the recent PHC contamination (50% of the initial total contamination) at 5–20 cm depth. The presence of weathered contamination explains the existence of an established community of PHC degraders, as can be inferred by the most probable number technique. C. selloana, in its turn, showed capacity to mobilize the most recalcitrant fraction of PHC to its roots, nevertheless masking its remediation capacity. The use of a hybrid technology (C. selloana together with treatments with a surfactant and a bioaugmentation product) improved the removal of PHC at 15–20 cm depth, the presence of C. selloana facilitating the migration of additives into the deeper layers of soil, which can be considered a secondary but positive role of the plant. In the surface soil layer, which was exposed to both microorganisms and the atmosphere, a further 20% of weathered PHC contamination disappeared (70% total removal) as a result of photo- and chemical degradation. Periodic revolving of the soil, like tillage, to expose all the contaminated soil to the atmosphere will therefore be a reliable option for reducing the contamination of the refinery soil if conditions (space and equipment) permit this operation.

Innovative mathematical modeling in environmental remediation

There are two different ways to model reactive transport: ad hoc and innovative reaction-based approaches. The former, such as the Kd simplification of adsorption, has been widely employed by practitioners, while the latter has been mainly used in scientific communities for elucidating mechanisms of biogeochemical transport processes. It is believed that innovative mechanistic-based models could serve as protocols for environmental remediation as well. This paper reviews the development of a mechanistically coupled fluid flow, thermal transport, hydrologic transport, and reactive biogeochemical model and example-applications to environmental remediation problems. Theoretical bases are sufficiently described. Four example problems previously carried out are used to demonstrate how numerical experimentation can be used to evaluate the feasibility of different remediation approaches. The first one involved the application of a 56-species uranium tailing problem to the Melton Branch Subwatershed at Oak Ridge National Laboratory (ORNL) using the parallel version of the model. Simulations were made to demonstrate the potential mobilization of uranium and other chelating agents in the proposed waste disposal site. The second problem simulated laboratory-scale system to investigate the role of natural attenuation in potential off-site migration of uranium from uranium mill tailings after restoration. It showed inadequacy of using a single Kd even for a homogeneous medium. The third example simulated laboratory experiments involving extremely high concentrations of uranium, technetium, aluminum, nitrate, and toxic metals (e.g., Ni, Cr, Co). The fourth example modeled microbially-mediated immobilization of uranium in an unconfined aquifer using acetate amendment in a field-scale experiment. The purposes of these modeling studies were to simulate various mechanisms of mobilization and immobilization of radioactive wastes and to illustrate how to apply reactive transport models for environmental remediation.

Diversity and Activity of PAH-Degrading Bacteria in the Phyllosphere of Ornamental Plants

Phyllosphere bacteria on ornamental plants were characterized based on their diversity and activity towards the removal of polycyclic aromatic hydrocarbons (PAHs), the major air pollutants in urban area. The amounts of PAH-degrading bacteria were about 1-10% of the total heterotrophic phyllosphere populations and consisted of diverse bacterial species such as Acinetobacter, Pseudomonas, Pseudoxanthomonas, Mycobacterium, and uncultured bacteria. Bacterial community structures analyzed by polymerase chain reaction-denaturing gradient gel electrophoresis from each plant species showed distinct band patterns. The uniqueness of these phyllosphere bacterial communities was partly due to the variation in leaf morphology and chemical properties of ornamental plants. The PAH degradation activity of these bacteria was monitored in gas-tight systems containing sterilized or unsterilized leaves. The results indicated that phyllosphere bacteria on unsterilized leaves were able to enhance the activity of leaves for phenanthrene removal. When compared between plant species, phenanthrene removal efficiency corresponded to the size of phenanthrene-degrading bacteria. In addition, phyllosphere bacteria on Wrightia religiosa were able to reduce other PAHs such as acenaphthylene, acenaphthene, and fluorine in 60-ml glass vials and in a 14-l glass chamber. Thus, phyllosphere bacteria on ornamental plants may play an important role in natural attenuation of airborne PAHs in urban areas.

PAH Biodegradative Genotypes in Lake Erie Sediments: Evidence for Broad Geographical Distribution of Pyrene-Degrading Mycobacteria

Despite a long history of anthropogenic contamination of Lake Erie sediments, little work has been done to understand the potential for PAH biodegradation by indigenous microbial communities. Pyrene-degrading Mycobacterium are prevalent in many polycyclic aromatic hydrocarbon (PAH)-contaminated freshwater sediments, and are of interest for their ability to degrade environmentally recalcitrant high molecular weight PAHs. This work tested the hypothesis that pyrene-degrading mycobacteria are prevalent in Lake Erie; an additional aim was to gain a baseline picture of the sediment microbial communities through sequencing a 16S rDNA clone library. Biodegradation potential of Lake Erie Mycobacterium populations was assessed through quantification of pyrene dioxygenase genes (nidA) and mycobacteria 16S rDNA genes using quantitative real time PCR. nidA was detected at all seven sampling sites across Lake Erie, with abundances ranging from 2.09 to 70.4 x 10(6) copies per gram sediment, with highest abundances at the most PAH-contaminated site (Cleveland Harbor). This is in contrastto naphthalene dioxygenase genes commonly used as biomarkers of PAH degradation: nahAc (from gamma-proteobacteria) was not detected anywhere, and nagAc (from beta-proteobacteria) was detected only in Cleveland Harbor, despite dominance by proteobacteria in Lake Erie sediment 16S rDNA clone libraries (>50% of clones). The prevalence of Mycobacterium nidA genotypes corroborated previous studies indicating that PAH-degrading mycobacteria have a cosmopolitan distribution and suggests they play an important but overlooked role in natural attenuation and cycling of PAHs in Lake Erie.

Evaluation of sulfate reduction at experimentally induced mixing interfaces using small-scale push–pull tests in an aquifer–wetland system

This paper presents small-scale push–pull tests designed to evaluate the kinetic controls on SO 4 2 - reduction in situ at mixing interfaces between a wetland and aquifer impacted by landfill leachate at the Norman Landfill research site, Norman, OK. Quantifying the rates of redox reactions initiated at interfaces is of great interest because interfaces have been shown to be zones of increased biogeochemical transformations and thus may play an important role in natural attenuation. To mimic the aquifer–wetland interface and evaluate reaction rates, SO 4 2 - -rich anaerobic aquifer water ( ∼ 100 mg / L SO 4 2 - ) was introduced into SO 4 2 - -depleted wetland porewater via push–pull tests. Results showed SO 4 2 - reduction was stimulated by the mixing of these waters and first-order rate coefficients were comparable to those measured in other push–pull studies. However, rate data were complex involving either multiple first-order rate coefficients or a more complex rate order. In addition, a lag phase was observed prior to SO 4 2 - reduction that persisted until the mixing interface between test solution and native water was recovered, irrespective of temporal and spatial constraints. The lag phase was not eliminated by the addition of electron donor (acetate) to the injected test solution. Subsequent push–pull tests designed to elucidate the nature of the lag phase support the importance of the mixing interface in controlling terminal electron accepting processes. These data suggest redox reactions may occur rapidly at the mixing interface between injected and native waters but not in the injected bulk water mass. Under these circumstances, push–pull test data should be evaluated to ensure the apparent rate is actually a function of time and that complexities in rate data be considered.

Relationship between geochemical parameters and the occurrence of Dehalococcoides DNA in contaminated aquifers

Strains of Dehalococcoides are the only microbes known that can completely dechlorinate PCE, TCE, cis‐DCE, and vinyl chloride to ethylene. Either naturally occurring strains or bioaugmentation cultures of Dehalococcoides are widely used for in situ bioremediation of contaminated groundwater. Naturally occurring strains have an important role in natural attenuation of PCE, TCE, cis‐DCE, and vinyl chloride in groundwater. This study evaluated the relationship between selected biogeochemical parameters and the presence of Dehalococcoides DNA in field‐scale plumes. A total of 81 monitoring wells were sampled from 15 groundwater plumes at 10 locations across the United States (one sample per monitoring well). The presence of Dehalococcoides DNA was determined with an assay based on the polymerase chain reaction (PCR) using DNA primers targeting the 16S rRNA gene of Dehalococcoides. The groundwater samples were also analyzed for concentrations of O2, NO3−1 plus NO2−1 − N, CH4, H2, Fe (II), SO4−2, TOC, Cl−1, and benzene, toluene, ethylbenzene, and xylene (BTEX) compounds and for alkalinity, ORP, electrical conductivity, pH, and temperature. Dehalococcoides DNA was unequivocally detected in 26 wells, most of which exhibited methanogenic conditions. A two‐sample Kolmogorov‐Smirnov test was used to compare the distribution of each parameter in water where Dehalococcoides DNA was present to the distribution where Dehalococcoides DNA was absent. The only parameters for which the distributions were different at 95% confidence were NO3−1 plus NO2−1 − N, CH4, and ORP. Using these three statistically significant geochemical parameters as descriptors, a predictive model for the presence of Dehalococcoides DNA was developed using logistic regression with a binary response. Under conditions where data of direct biochemical assay are not available a calculated probability could be used to properly calibrate computer models of natural attenuation.

Specific detection of Dehalococcoides species by fluorescence in situ hybridization with 16S rRNA-targeted oligonucleotide probes.

Dehalococcoides ethenogenes is the only known cultivated organism capable of complete dehalogenation of tetrachloroethene (PCE) to ethene. The prevalence of Dehalococcoides species in the environment and their association with complete dehalogenation of chloroethenes suggest that they play an important role in natural attenuation of chloroethenes and are promising candidates for engineered bioremediation of these contaminants. Both natural attenuation and bioremediation require reliable and sensitive methods to monitor the presence, distribution, and fate of the organisms of interest. Here we report the development of 16S rRNA-targeted oligonucleotide probes for Dehalococcoides species. The two designed probes together encompass 28 sequences of 16S rRNA genes retrieved from the public database. Except D. ethenogenes and CBDB1, all the others are environmental clones obtained from sites contaminated with chlorinated ethenes. They are all closely related and form a unique cluster of Dehalococcoides species. In situ hybridization of probe Dhe1259t with D. ethenogenes strain 195 and two enrichment cultures demonstrated the applicability of the probe to monitoring the abundance of active Dehalococcoides species in these enrichment samples.

Role of Natural Attenuation in Life Cycle of MTBE Plumes

The natural life cycle of a plume of methyl tert-butyl ether from a spill of gasoline is controlled by the rate of attenuation of the source (due to partitioning from the residual gasoline to the flow of groundwater) and the rate of attenuation in the plume (due to dispersion and natural biodegradation). Rates of attenuation were extracted for plumes in California, Florida, North Carolina, New York, and New Jersey. The maximum rate of attenuation of the source was 0.75 per year. The rates of attenuation in the plumes varied from 0.56 to 4.3 per year. In all cases, the rate of attenuation of the plume exceeded the rate of attenuation of the source. As these plumes progress through their life cycle, they should recede back toward their source.

The Effect of Groundwater Aeration on PCE Natural Attenuation Patterns

Attempts to remediate ground water contaminated with tetrachloroethylene at a Superfund site in Minnesota included the installation of a vacuum vaporizer well. Prior to the remedial system installation, the contaminant source half-life was approximately 0.3 years. Aquifer aeration by the vacuum vaporizer well unintentionally disrupted the ambient natural attenuation rate. Although the overall plume size did not increase, concentrations of the anaerobic breakdown products of tetrachloroethylene—trichloroethylene, cis-dichloroethylene, and vinyl chloride—all increased in downgradient monitoring wells after startup of the vacuum vaporizer well. At a well 360 feet downgradient of the source, trichloroethylene increased from concentrations below 10 µg/L to over 35 µg/L, while cis-dichloroethylene concentrations increased from 70 µg/L to 370 µg/L. Vinyl chloride, which was below detection limits at this location prior to operation of the vacuum vaporizor well, increased in concentration to 83 µg/L. Concentrations of these contaminants returned to pre-sparging levels after deactivation of the system, indicating that existing anaerobic natural attenuation processes play an important role in the remediation of ground water at this site. Investigations should routinely assess the role of natural attenuation in remediation before implementing engineered remedies that may disrupt existing beneficial attenuation processes.

Natural attenuation of a low mobility chlorinated insecticide in low‐level and high‐level contaminated soil: A feasibility study

AbstractEndosulfan is an economically important insecticide and widespread environmental pollutant, originating from a wide range of agricultural activities. The major implication from the feasibility study described was that endosulfan I can be remediated by natural attenuation processes in cotton‐farming soil, in which concentrations were relatively low, as well as heavily contaminated soil, from an agricultural chemical waste (evaporation) pit. Endosulfan I, the major isomer of endosulfan, was present in agricultural soils with low (2.2 mg/kg) and high (417 mg/kg) concentrations of technical‐grade endosulfan. The half‐lives of the major isomer of endosulfan were 94 and greater than 350 days in the low‐level (cotton farming soil) and high‐level (contaminated soil), respectively. Even under conditions of minimal intervention, as in the current study, endosulfan concentrations in contaminated soils can be substantially reduced. The nonbiological process of soil binding was predominantly responsible for the natural attenuation of endosulfan I in both soils. Low levels of mineralization of the chlorinated ring 14C‐labelled carbons were also reported, but mineralization did not play an important role in natural attenuation of endosulfan I in either soil studied.