Intrinsic Remediation Research Articles

Microbial assembly and co-occurrence network in an aquifer under press perturbation

PurposeThousands of aquifers worldwide have been polluted by leachate from landfills and many more remained threatened. Microbial communities from these environments play a crucial role in mediating biodegradation and maintaining the biogeochemical cycles, but their co-occurrence and assembly mechanism have not been investigated.MethodHere, we coupled network analysis with multivariate statistics to assess the relative importance of deterministic versus stochastic microbial assembly in an aquifer undergoing intrinsic remediation, using 16S metabarcoding data generated through Illumina MiSeq sequencing of the archaeal/bacterial V3–V4 hypervariable region.ResultsResults show that both the aquifer-wide and localised community co-occurrences deviate from expectations under null models, indicating the predominance of deterministic processes in shaping the microbial communities. Further, the amount of variation in the microbial community explained by the measured environmental variables was 55.3%, which illustrates the importance of causal factors in forming the structure of microbial communities in the aquifer. Based on the network topology, several putative keystone taxa were identified which varied remarkably among the wells in terms of their number and composition. They included Nitrospira, Nitrosomonadaceae, Patulibacter, Legionella, uncharacterised Chloroflexi, Vicinamibacteriales, Neisseriaceae, Gemmatimonadaceae, and Steroidobacteraceae. The putative keystone taxa may be providing crucial functions in the aquifer ranging from nitrogen cycling by Nitrospira, Nitrosomonadaceae, and Steroidobacteraceae, to phosphorous bioaccumulation by Gemmatimonadaceae.ConclusionCollectively, the findings provide answers to fundamental ecological questions which improve our understanding of the microbial ecology of landfill leachate plumes, an ecosystem that remains understudied.

Spatiotemporal and seasonal dynamics in the microbial communities of a landfill-leachate contaminated aquifer.

ABSTRACTThe microbiome of an aquifer contaminated by landfill leachate and undergoing intrinsic remediation was characterised using 16S rRNA metabarcoding. The archaeal/bacterial V3–V4 hypervariable region of the 16S rRNA gene was sequenced using Illumina MiSeq, and multivariate statistics were applied to make inferences. Results indicate that the aquifer recharge and aquifer sediment samples harbour different microbial communities compared to the groundwater samples. While Proteobacteria dominated both the recharge and groundwater samples, Acidobacteria dominated the aquifer sediment. The most abundant genera detected from the contaminated aquifer were Polynucleobacter, Rhodoferax, Pedobacter, Brevundimonas, Pseudomonas, Undibacterium, Sulfurifustis, Janthinobacterium, Rhodanobacter, Methylobacter and Aquabacterium. The result also shows that the microbial communities of the groundwater varied spatially, seasonally and interannually, although the interannual variation was significant for only one of the wells. Variation partitioning analysis indicates that water chemistry and well distance are intercorrelated and they jointly accounted for most of the variation in microbial composition. This implies that the species composition and water chemistry characteristics have a similar spatial structuring, presumably caused by the landfill leachate plume. The study improves our understanding of the dynamics in subsurface microbial communities in space and time.

Characterisation of the bacterial microbiota of a landfill-contaminated confined aquifer undergoing intrinsic remediation

Literature on microbiomes of landfill leachate-contaminated aquifers is scarce despite groundwater contaminations from landfills being common globally. In this study, a combination of microbiological techniques was applied to groundwater samples from an aquifer contaminated by a municipal landfill and undergoing intrinsic remediation. Groundwater samples were obtained from three multilevel sampling wells placed along the groundwater flow path in the contaminated aquifer, and additionally from a background well located in a nearby uncontaminated aquifer. The samples were subjected to chemical analysis, microbial culturing and characterisation, cell counting by fluorescence microscopy, and 16S rRNA metabarcoding. Good concordance was realised with the results from the different microbiological techniques. Samples from the uncontaminated aquifer had both lower cell density and lower microbial diversity compared to samples from the contaminated aquifer. Among the wells located in the contaminated aquifer, microbial diversity increased between the well closest to the landfill and the intermediate well but was lower at the most distant well. The majority of the cultured microbes represent taxa frequently recovered from contaminated environments, with 47% belonging to taxa with previously documented bioremediation potential. Multivariate redundancy analysis showed that the microbial composition was most similar in wells located closer to the landfill, although beta diversity analysis indicated a significant difference in microbial composition across the wells. Taken together with the results of cell counting, culture, and metabarcoding, these findings illustrate the effect of landfill leachates on microbial communities.

Microbial Communities in Sediments of Lagos Lagoon, Nigeria: Elucidation of Community Structure and Potential Impacts of Contamination by Municipal and Industrial Wastes.

Estuarine sediments are significant repositories of anthropogenic contaminants, and thus knowledge of the impacts of pollution upon microbial communities in these environments is important to understand potential effects on estuaries as a whole. The Lagos lagoon (Nigeria) is one of Africa’s largest estuarine ecosystems, and is impacted by hydrocarbon pollutants and other industrial and municipal wastes. The goal of this study was to elucidate microbial community structure in Lagos lagoon sediments to identify groups that may be adversely affected by pollution, and those that may serve as degraders of environmental contaminants, especially polycyclic aromatic hydrocarbons (PAHs). Sediment samples were collected from sites that ranged in types and levels of anthropogenic impacts. The sediments were characterized for a range of physicochemical properties, and microbial community structure was determined by Illumina sequencing of the 16S rRNA genes. Microbial diversity (species richness and evenness) in the Apapa and Eledu sediments was reduced compared to that of the Ofin site, and communities of both of the former two were dominated by a single operational taxonomic unit (OTU) assigned to the family Helicobacteraceae (Epsilonproteobacteria). In the Ofin community, Epsilonproteobacteria were minor constituents, while the major groups were Cyanobacteria, Bacteroidetes, and Firmicutes, which were all minor in the Apapa and Eledu sediments. Sediment oxygen demand (SOD), a broad indicator of contamination, was identified by multivariate analyses as strongly correlated with variation in alpha diversity. Environmental variables that explained beta diversity patterns included SOD, as well as levels of naphthalene, acenaphthylene, cobalt, cadmium, total organic matter, or nitrate. Of 582 OTU identified, abundance of 167 was significantly correlated (false discovery rate q≤ 0.05) to environmental variables. The largest group of OTU correlated with PAH levels were PAH/hydrocarbon-degrading genera of the Oceanospirillales order (Gammaproteobacteria), which were most abundant in the hydrocarbon-contaminated Apapa sediment. Similar Oceanospirillales taxa are responsive to marine oil spills and thus may present a unifying theme in marine microbiology as bacteria adapted for degradation of high hydrocarbon loads, and may represent a potential means for intrinsic remediation in the case of the Lagos lagoon sediments.

Phytoassessment of an Enhanced Naturally Attenuated Oil-Polluted Soil after Exposure to Various Concentrations of Sodium Bicarbonate Solution

The present study investigated the effects of sodium bicarbonate-induced soft water on the intrinsic qualities of a bioremediated petroleum hydrocarbon-polluted soil. this was also subjected to phytoassessment using African yam bean (Sphenostylis stenocarpa). Into already perforated buckets were measured 5kg sun-dried top soil (0-10cm). Waste engine oil (WEO) was added to soil and mixed thoroughly to obtain similar concentrations of 5 % w/w oil in soil. The entire setup was divided into 5 treatment sets, depending on the concentration of NaHCO3 solution used (0, 25, 50, 100 and 200mg/l). Each set was wetted daily with 200ml of different solutions for 3 months. Results showed significant reductions in heavy metal concentrations particularly in the 200mg/l of NaHCO3-wetted soils. Copper reduced to a range of 4.84 - 6.08mg/kg in the NaHCO3-wetted soil, compared to 9.32mg/kg in the control. However, total polyaromatic hydrocarbon (PAH) content of soil was lowest when the soil was wetted with 25mg/l NaHCO3 (96.43mg/kg), indicating a 90.51% efficiency, compared to the control with an efficiency of 79.15%. There were significant reductions in pollution indices (contamination factor, hazard quotient, toxicity equivalency factors, and probable effect concentration) used in the study; an indication that water softness positively impacted on the intrinsic remediation of the waste engine oil-polluted soil. Results of phytoassessment showed the improved plant growth and yield response in the oil-polluted soils upon addition of lower concentrations of NaHCO3 in soil. Grain yield per plant in the oil-polluted soil was 8.33g, compared to 29.29g in the control and 25.87g when oil-polluted soil was wetted with 25mg/l solution.

Field metabolomics and laboratory assessments of anaerobic intrinsic bioremediation of hydrocarbons at a petroleum‐contaminated site

SummaryField metabolomics and laboratory assays were used to assess the in situ anaerobic attenuation of hydrocarbons in a contaminated aquifer underlying a former refinery. Benzene, ethylbenzene, 2‐methylnaphthalene, 1,2,4‐ and 1,3,5‐trimethylbenzene were targeted as contaminants of greatest regulatory concern (COC) whose intrinsic remediation has been previously reported. Metabolite profiles associated with anaerobic hydrocarbon decay revealed the microbial utilization of alkylbenzenes, including the trimethylbenzene COC, PAHs and several n‐alkanes in the contaminated portions of the aquifer. Anaerobic biodegradation experiments designed to mimic in situ conditions showed no loss of exogenously amended COC; however, a substantive rate of endogenous electron acceptor reduction was measured (55 ± 8 µM SO4 day−1). An assessment of hydrocarbon loss in laboratory experiments relative to a conserved internal marker revealed that non‐COC hydrocarbons were being metabolized. Purge and trap analysis of laboratory assays showed a substantial loss of toluene, m‐ and o‐xylene, as well as several alkanes (C6–C12). Multiple lines of evidence suggest that benzene is persistent under the prevailing site anaerobic conditions. We could find no in situ benzene intermediates (phenol or benzoate), the parent molecule proved recalcitrant in laboratory assays and low copy numbers of Desulfobacterium were found, a genus previously implicated in anaerobic benzene biodegradation. This study also showed that there was a reasonable correlation between field and laboratory findings, although with notable exception. Thus, while the intrinsic anaerobic bioremediation was clearly evident at the site, non‐COC hydrocarbons were preferentially metabolized, even though there was ample literature precedence for the biodegradation of the target molecules.

Guest Editorial: Contaminant Source Zones: Remediation or Perpetual Stewardship?

Guest Editorial: Contaminant Source Zones: Remediation or Perpetual Stewardship?

Sorbent Wicking Device for Sampling Hydrophobic Organic Compounds in Unsaturated Soil Pore Water. II: Chemical Capture, Recovery, and Analysis

Experiments investigated the chemical capture and quantitative recovery of chlorinated hydrophobic organic compounds (HOCs) sampled from the pore-water of unsaturated soils with a new in situ sorbent wicking device. The sampling device is comprised of an annular porous stainless-steel interface encasing granular-activated carbon and a fiberglass wick. Laboratory column studies using Ottawa sand spiked with tri- and tetrachlorobenzenes evaluated the accuracy of the sampling device in assessing average pore-water concentrations. The findings from these experiments showed that the sorbent-wick sampler provided reliable and repeatable estimates of the pore-water concentrations of the chlorobenzenes in the unsaturated soil columns at pore-water concentrations as low as several parts per billion. Field trials were conducted in a land biotreatment unit containing industrial soil/sludge materials to assess the availability of trace levels of aqueous-phase polychlorinated biphenyls (PCBs). The land treatment unit had undergone 5 years of intrinsic remediation and the test results indicated that the samplers were able to replicate the aqueous-phase PCB homolog pattern measured in ex situ aqueous extraction tests. Orthosubstituted PCBs were dominant in sorbent-wick samples and aqueous extracts; these PCBs appear relatively recalcitrant in land biotreatment units. The sorbent-wicking device offers the ability to obtain time-averaged mass and volumetric flux rates of HOCs in land treatment units.

Intrinsic remediation of trichloroethene driven by tetraalkoxysilanes as co‐contaminants: Results of microcosm and field studies

AbstractBiological reduction of trichloroethene (TCE), driven by the transformation products of tetraalkoxysilanes, was investigated in seasonal field monitorings and anaerobic groundwater microcosms. Under anaerobic conditions, tetraalkoxysilanes such as tetrabutoxysilane (TBOS) and tetrakis(2‐ethylbutoxy)silane (TKEBS) are abiotically hydrolyzed to their corresponding alcohols, 1‐butanol and 2‐ethylbutanol, respectively, and silicic acid. These alcohols are then fermented by subsurface microorganisms to their corresponding acids and hydrogen, which likely serves as the ultimate electron donor facilitating reductive dechlorination of TCE. At Site 300, Lawrence Livermore National Laboratory (LLNL), California, tetraalkoxysilanes are present along with TCE as subsurface contaminants. Intrinsic transformation of TCE to cis‐dichloroethene (c‐DCE) was observed at this site, and this was promoted by the transformation products of these tetraalkoxysilanes. Efficient transformation of high concentrations of TCE (150–300 mg/l aqueous concentration) was observed in anaerobic microcosms constructed with groundwater from this site. The lack of transformation of c‐DCE to vinyl chloride (VC) and ethene in the microcosms is consistent with studies of the microbial community in the site groundwater reported by Lowe et al. (2002). © 2003 Wiley Periodicals, Inc.

Laboratory evidence of MTBE biodegradation in Borden aquifer material

Mainly due to intrinsic biodegradation, monitored natural attenuation can be an effective and inexpensive remediation strategy at petroleum release sites. However, gasoline additives such as methyl tert-butyl ether (MTBE) can jeopardize this strategy because these compounds often degrade, if at all, at a slower rate than the collectively benzene, toluene, ethylbenzene and the xylene (BTEX) compounds. Investigation of whether a compound degrades under certain conditions, and at what rate, is therefore important to the assessment of the intrinsic remediation potential of aquifers. A natural gradient experiment with dissolved MTBE-containing gasoline in the shallow, aerobic sand aquifer at Canadian Forces Base (CFB) Borden (Ontario, Canada) from 1988 to 1996 suggested that biodegradation was the main cause of attenuation for MTBE within the aquifer. This laboratory study demonstrates biologically catalyzed MTBE degradation in Borden aquifer-like environments, and so supports the idea that attenuation due to biodegradation may have occurred in the natural gradient experiment. In an experiment with batch microcosms of aquifer material, three of the microcosms ultimately degraded MTBE to below detection, although this required more than 189 days (or >300 days in one case). Failure to detect the daughter product tert-butyl alcohol (TBA) in the field and the batch experiments could be because TBA was more readily degradable than MTBE under Borden conditions.

Natural Attenuation: How Is Mother Nature Aiding Us to Cleanse Our Environmental Sins?

For a long time, mankind has ~purposely or negligibly! forgotten that we are not alone on this planet, and the ecosystem and its plentiful ~but not unlimited! resources do not exclusively belong to us ~either as a species or as a generation!. Although we may have the ‘‘divine’’ entitlement ~and obligation! to utilize, develop and indeed enjoy these resources for our well-being and prosperity, we have no right whatsoever to alter, abuse, damage, and/or over-exploit such wealth at the expense of the rest of earth’s fellow residents or future human generations. Even if we try, with all of our technological advancement and economic affluence, to remedy our old/new environmental mistakes, we will not be successful without the sustainable aid of nature itself. Nature has the strength to rectify assaults, assimilate alien substances and phenomena, attenuate toxins, mitigate injuries, and purify itself. This compassionate and intrinsic healing power of nature however is not infinite. Natural attenuation ~occasionally designated as intrinsic remediation! is an inherent natural cleanup process achieved when integrated assimilation mechanisms of self-purification are instinctively triggered ~or activated! at a contaminated site. The merits of natural attenuation are often recognized not only by bringing about diminishing ~or neutralization! of contaminants, but also by the nonintrusive nature of its processes. Natural attenuation mechanisms contain a variety of physical, chemical, and/or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, bioavailability, volume, or concentration of contaminants in soil and groundwater ~and other media!. Such mechanisms nevertheless can be classified as either destructive or nondestructive. The destructive mechanisms ~mainly biological and chemical! include biodegradation, abiotic oxidation, transformation, hydrolysis, UV destruction ~in surface media!, stabilization and mineralization. The nondestructive mechanisms ~mostly physical! include sorption, dilution, dispersion, and volatilization. The term ‘‘monitored natural attenuation’’ was denoted by the United States Environmental Protection Agency ~USEPA! to indicate: ‘‘the reliance on natural processes to achieve site-specific objectives within a time frame that is reasonable compared to that offered by other more active methods.’’ The USEPA ‘‘does not view Monitored Natural Attenuation to be a no-action or walkaway approach, but rather considers it to be an alternative means of achieving remediation objectives.’’ Accordingly, the successful implementation of natural attenuation entails the need for systematic characterization, monitoring, analyses, risk assessment, and demonstration protocol, to illustrate that degradation and cleanup of contaminants ~and therefore restoration of the site! are occurring at a satisfactory rate to protect human health and the environment. The most important among natural attenuation processes is

The role of microbial populations in the containment of aromatic hydrocarbons in the subsurface.

A survey of soil gases associated with gasoline stations on the Swan Coastal Plain of Western Australia has shown that 20% leak detectable amounts of petroleum. The fates of volatile hydrocarbons in the vadose zone at one contaminated site, and dissolved hydrocarbons in groundwater at another site were followed in a number of studies which are herein reviewed. Geochemical evidence from a plume of hydrocarbon-contaminated groundwater has shown that sulfate reduction rapidly developed as the terminal electron accepting process. Toluene degradation but not benzene degradation was linked to sulfate reduction. The sulfate-reducing bacteria isolated from the plume represented a new species, Desulfosporosinus meridiei. Strains of the species do not mineralise 14C-toluene in pure culture. The addition of large numbers of cells and sulfate to microcosms did stimulate toluene mineralisation but not benzene mineralisation. Attempts to follow populations of sulfate-reducing bacteria by phospholipid signatures, or Desulfosporosinus meridiei by FISH in the plume were unsuccessful, but fluorescently-labeled polyclonal antibodies were successfully used. In the vadose zone at a different site, volatile hydrocarbons were consumed in the top 0.5 m of the soil profile. The fastest measured rate of mineralisation of 14C-benzene in soils collected from the most active zone (6.5 mg kg(-1) day(-1)) could account for the majority of the flux of hydrocarbon vapourtowards the surface. The studies concluded that intrinsic remediation by subsurface microbial populations in groundwater on the Swan Coastal Plain can control transport of aromatic hydrocarbon contamination, except for the transport of benzene in groundwater. In the vadose zone, intrinsic remediation by the microbial populations in the soil profile can contain the transport of aromatic hydrocarbons, provided the physical transport of gases, in particular oxygen from the atmosphere, is not impeded by structures.

Indirect Immunofluorescence and FISH for Enumerating Contaminated Site Sulfate-Reducers

A new species of sulfate-reducer, Desulfosporosinus meridiei, was recently isolated from gasoline-contaminated anaerobic groundwater in which degradation of toluene and other hydrocarbons occurred. Ground-water from inside (three sites) and outside (one site) the contaminant plume was probed with specific polyclonal antibodies raised against two strains of D. meridiei (strains T2 and S6). Molecular 16S rRNA probes designed to hybridize with cells of D. meridiei were also used. Cell counts using antistrain T2 antibodies (specific for all strains of D. meridiei and two strains of D. orientis) were similar (103 cells/mL) both inside and outside the plume as were total DAPI counts (106 cells/mL). The numbers of cells stained with antibodies specific for Group B strains of D. meridiei varied between locations. The molecular probes DSP477A and DSP477B were designed for, and were effective on, pure cultures of D. meridiei and were able to distinguish this species from Desulfovibrio desulfuricans in hybridization experiments. No cells were seen to hybridize with probe DSP477B in groundwater samples. Cell counts in groundwater using the universal eubacterial probe, EUB338, were only 8% to 29% of DAPI counts. Fluorescence intensity was poor and auto fluorescence of particles made counting difficult. This study showed that molecular probing using techniques commonly employed in many laboratories was of little use for evaluating microbial populations in this groundwater. Polyclonal antibodies were considerably more useful for identifying populations of specific cells. The lack of difference in cell numbers between contaminated and nearby uncontami-nated groundwater suggests that cell counts will not always be useful as indicators of intrinsic remediation.

ABSTRACT: Characterizing the Intrinsic Remediation of MTBE at Field Sites in Texas

ABSTRACT: Characterizing the Intrinsic Remediation of MTBE at Field Sites in Texas

Factors influencing rates and products in the transformation of trichloroethylene by iron sulfide and iron metal.

Batch experiments were performed to assess (i) the influence of pH, solution amendments, and mineral aging on the rates and products of trichloroethylene (TCE) transformation by iron sulfide (FeS) and (ii) the influence of pretreatment of iron metal with NaHS on TCE transformation rates. The relative rates of FeS-mediated transformation of TCE to different products were quantified by branching ratios. Both pseudo-first-order rate constants and branching ratios for TCE transformation by FeS were significantly influenced by pH, possibly due to a decrease in the reduction potential of reactive surface species with increasing pH. Neither Mn2+, expected to adsorb to FeS surface S atoms, nor 2,2'-bipyridine, expected to adsorb to surface Fe atoms, significantly influenced rate constants or branching ratios. FeS that had been aged at 76 degrees C for 3 days was completely unreactive with respect to TCE over 6.5 months, yet this aged FeS transformed hexachloroethane to tetrachloroethylene with a rate constant only slightly lower than that for nonaged FeS. This finding suggests that the oxidation state of iron sulfide minerals in the environment will strongly influence the potential for intrinsic remediation of pollutants such as TCE. Treatment of iron metal with bisulfide significantly increased the pseudo-first-order rate constant for TCE transformation at pH 8.3. This effect was attributed to formation of a reactive FeS coating or precipitate on the iron surface.

Phytoremediation potential of willow trees for aquifers contaminated with ethanol-blended gasoline

Ethanol-blended gasoline has been used in Brazil for 20 years and, probably, is going to be more widely used in North America due to the MtBE environmental effects on groundwater. The potential impacts caused by the presence of ethanol in UST spills are related to the co-solvency effect and the preferential degradation of ethanol over the BTEX compounds. These interactions may increase the length of dissolved hydrocarbon plumes and the costs associated with site remediation. This study investigates the advantages of phytoremediation to overcome the problems associated with the presence of ethanol in groundwater contaminanted with gasoline–ethanol mixtures. Experiments were performed under lab conditions with cuttings of Willow tree ( Salix babylonica) cultivated hydroponically. Results showed that the cuttings were able to reduce ethanol and benzene concentrations by more than 99% in less than a week. The uptake of both contaminants was confirmed by blank controls and was significantly related to cuttings transpiration capacity. Sorption onto roots biomass also markedly affected the behavior of contaminants in solution. Experiments to evaluate plants’ toxicity to ethanol indicated that plants were only affected when aqueous ethanol concentration reached 2000 mg l −1. Results suggest that phytoremediation can be a good complement to intrinsic remediation in shallow aquifer sites contaminated with ethanol-blended gasoline spills.

Redox: fundamentals, processes, and applications

Redox Potential Measurements in Natural Waters: Significance: Concepts and Problems. - Technique of Measurement, Electrode Processes and Electrode Treatment. - Characterisation of the Redox State of Aqueous Systems: Towards a Problem-Oriented Approach. - Comparison of Different Methods for Redox Potential Determination in Natural Waters. - A Novel Approach to the Presentation of p/pH-Diagrams. - The Couple As(V) - As(III) as a Redox Indicator. - In Situ Long-Term-Measurement of Redox Potential in Redoximorphic Soils. - Redox Measurements as a Qualitative Indicator of Spatial and Temporal Variability of Redox State in a Sandy Forest Soil. - Implementation of Redox Reactions in Groundwater Models. - Variance of the Redox Potential Value in Two Anoxic Groundwater Systems. - Redox Fronts in Aquifer Systems and Parameters Controlling their Dimensions. - Redox Processes Active in Denitrification. - Measurement of Redox Potentials at the Test Site 'Insel Hengsen'. - Redox Reactions, Multi-Component Stability Diagrams and Isotopic Investigations in Sulfur- and Iron-Dominated Groundwater Systems. - Redox Buffer Capacity Concept as a Tool for the Assessment of Long-Term Effects in Natural Attenuation / Intrinsic Remediation. - Redox Zones in the Plume of a Previously Operating Gas Plant. - Degradation of Organic Groundwater Contaminants: Redox Processes and EH-Values. - Microbial Metabolism of Iron Species in Freshwater Lake Sediments. - Redox Measurements in Marine Sediments. - Subject Index.

Fate of Hydrocarbon Contaminants In Natural Wetlands

Abstract Conventional clean-up approaches to contaminated wetlands are aggressive and expensive, and may harm the wetland ecosystem. Current findings suggest that wetlands may be capable of attenuating contamination through natural processes including sorption, biodegradation, and volatilization. The objective of the study was to conduct a field evaluation of the role of these attenuation mechanisms at four contaminated natural wetlands in Alberta. A second focus of the study was to evaluate contaminant impact on the wetlands vegetation. The results indicate that contaminant attenuation in wetlands are influenced by several parameters, including depth of impact, contaminant composition, presence of peat, flow rates, and seasonal effects. Overall, natural processes appear to be completely attenuating hydrocarbon contamination at two of the four wetlands study sites. At the other two sites, minor concentrations were reaching downgradient receptors. Impact on vegetation was observed at three of the four sites, primarily through mortality of black spruce. The impact on shallow vegetation was generally negligible. Introduction Wetland contamination by hydrocarbons as a result of upstream oil and gas activity is well known, both in Alberta and worldwide. Conventional clean-up approaches (e.g., excavation, trenching, and burning) are aggressive and expensive, and may harm the wetland ecosystem. A more passive, longer term solution may be to focus on containing or removing the source and allow natural processes to remediate the remaining contamination. This approach, known as intrinsic remediation or natural attenuation, has been shown to be effective in various scenarios, from fuel-contaminated groundwater at Air Force Bases to oil spill-impacted shorelines. There has been little investigative work on the subject of intrinsic remediation in hydrocarbon-impacted wetlands. Current findings suggest that wetlands may be capable of retarding and potentially depleting contaminant plumes through natural remediation processes including: sorption, biodegradation, and volatilization. Wetlands are typically water-saturated environments with a high percentage of peat. Peat is an organic-rich material containing partially to highly decomposed plant fibres, roots, and mosses(1). Wetlands containing peat have several unique attributes, which enhance contaminant attenuation in the following ways:Sorption: Sorption may be described as the partitioning of contaminants from the liquid phase to the soil matrix. The peat matrix is highly porous (up to 70%) and has been found to be capable of absorbing up to eight times its weight in oil(2). Peat contains approximately 40% organic carbon by weight. Laboratory testing completed by Tinh et al.(3) indicates that peat's adsorptive capacity is between 30 to 50% of granular activated carbon. Peat is therefore an excellent filter for organic contaminants.Biodegradation: Microorganisms indigenous to the environment are capable of biodegrading most organic contaminants found in petroleum fuels, both aerobically and under anerobic conditions. Due to their high organic content and biodiversity, wetlands exhibit microbial activity exceeding most natural environments(4). The enhanced microbial activity results in greater potential for biodegradation, whereby microbes stimulate the breakdown of organics such as hydrocarbons to form CO2, water, and biomass.Volatilization: Volatilization describes the partitioning of hydrocarbons from the soil phase, dissolved phase, and/or liquid phase into the vapour phase.

Soil clean-up: lessons to remember

In the past, various attempts have been made to alter the microbial composition of the soil. Moreover, considerable effort has been devoted to develop mechanisms to control the soil (micro)biological processes. Currently, a number of novel and powerful strategies are proposed to optimise the microbiota in order to achieve soil clean-up. Nevertheless, in practice the most relevant approach is the so-called intrinsic remediation, which often corresponds with minimal intervention. It is important for the scientist, the regulator and the practitioner to carefully weigh the social consequences of treatment such as `Isolating–controlling–monitoring' versus an approach based on `Binding and immobilization of the pollutants'.

Natural Recovery Reduces Impact of the 1970 Arrow Oil Spill

ABSTRACT In 1970 the tanker Arrow ran aground releasing 2,000 m3 of Bunker C crude oil along 300 km of Nova Scotia's coastline, of which only 10% was subjected to cleanup, the rest was left to degrade naturally. Sediment and interstitial water collected in 1993 and 1997 from Black Duck Cove in Chedabucto Bay, a representative untreated site, showed that the remaining residual oil has undergone substantial biodegradation. The environmental significance of this intrinsic remediation process was assessed with a battery of microscale biotests: CYP1A and mixed function oxygenase induction in winter flounder, Amphipod Survival, Echinoid Fertilization, Grass Shrimp Embryo-Larval Toxicity, Microtox® Solid-Phase and 100% Tests. While much oil remains in the sediment (426–12,744 ppm), results of the biotests show that it is of low toxicity and habitat recovery is evident from the level of benthic diversity.