Natural Attenuation Potential Research Articles

Overload of dissolved organic matter (DOM) in riparian infiltration zone increasing the pollution risk of naphthalene, insight from the competitive inhibition of naphthalene biodegradation by DOM.

Riparian infiltration zones are crucial for maintaining water quality by reducing the aqueous concentrations of polycyclic aromatic hydrocarbons (PAHs) through adsorption and biodegradation within the aquatic ecosystem. Dissolved organic matter (DOM) are ubiquitous in riparian infiltration zones where they extensively engage in the adsorption and biodegradation of PAHs, thereby influencing PAHs natural attenuation potential within riparian infiltration zones. Few studies have explored the natural attenuation mechanisms of PAHs influenced by DOM in riparian infiltration zones. In this study, the natural attenuation mechanisms of naphthalene (a typical PAHs component), under the influence of DOM, were explored, based on a case riverside source area. Analysis of microbial community structures, and the electron acceptor (e.g., Fe(III), DO/NO3-, SO42-)/electron donor (naphthalene and DOM) concentration changes within the riparian infiltration zone revealed a competitive inhibition relationship between DOM and naphthalene during microbial metabolism. Biodegradation experiments showed that when the concentration of DOM is higher than 4.0 mg·L-1, it inhibits the biodegradation of naphthalene. DOM competitively inhibits the biodegradation of naphthalene through the following mechanisms: (i) triggering microbial antioxidative defense mechanisms, diminishing the available resources for microbial participation in naphthalene degradation; (ii) altering microbial community structure; (iii) modulating microbial EPS composition, reducing the efficiency of microorganisms in utilizing carbon sources; and (iv) inhibiting the expression levels of downstream genes involved in naphthalene degradation. The competitive inhibition constants of DOM with concentrations of 1.0, 2.0, 4.0, 8.0, and 16.0 mg·L-1 on naphthalene biodegradation are -2.0 × 10-3, -5.0 × 10-3,1.0 × 10-3, 4.0 × 10-4, and 1.0 × 10-4, respectively. These findings enhance understanding of PAHs attenuation in riparian infiltration zone, providing a basis for assessing and managing PAHs pollution risks during riparian extraction.

Natural Attenuation Potential of Vinyl Chloride and Butyl Acrylate Released in the East Palestine, Ohio Train Derailment Accident.

The East Palestine, Ohio train derailment released toxic vinyl chloride (VC) and butyl acrylate (BA), which entered the watershed. Streambed sediment, surface water, and private well water samples were collected 128 and 276 days postaccident to assess the natural attenuation potential of VC and BA by quantifying biodegradation biomarker genes and conducting microcosm treatability studies. qPCR detected the aerobic VC degradation biomarkers etnC in ∼40% and etnE in ∼27% of sediments collected in both sampling campaigns in abundances reaching 105 gene copies g-1. The 16S rRNA genes of organohalide-respiring Dehalococcoides and Dehalogenimonas were, respectively, detected in 50 and 64% of sediment samples collected 128 days postaccident and in 63 and 88% of sediment samples collected 276 days postaccident, in abundances reaching 107 cells g-1. Elevated detection frequencies of VC degradation biomarker genes were measured immediately downstream of the accident site (i.e., Sulphur Run). Aerobic VC degradation occurred in all sediment microcosms and coincided with increases of etnC/etnE genes and Mycobacterium, a genus comprising aerobic VC degraders. The conversion of VC to ethene and an increased abundance of VC reductive dechlorination biomarker genes were observed in microcosms established with sediments collected from Sulphur Run. All anoxic microcosms rapidly degraded BA to innocuous products with intermediate formation of n-butanol and acrylate. The results indicate that microbiomes in the East Palestine watershed have natural attenuation capacity for VC and BA. Recommendations are made to improve first-response actions in future contaminant release accidents of this magnitude.

Sorption and attenuation of petroleum VOCs in five unsaturated soils: Microcosms and column experiments

The fate of volatile organic compounds (VOC) vapors in the unsaturated zone is the basis for evaluating the natural attenuation potential and vapor intrusion risk. Microcosm and column experiments were conducted to study the effects chemical speciation and soil types/properties on the fate of petroleum VOCs in unsaturated zone. The biodegradation and total attenuation rates of the seven VOCs obtained by microcosm experiments in black soil and yellow earth were also generally higher than those in floodplain soil, lateritic red earth, and quartz sand. The VOC vapors in floodplain soil, lateritic red earth, and quartz sand showed slow total attenuation rates (<0.3 d−1). N-pentane, methylcyclopentane, and methylcyclohexane showed lower biodegradation rates than octane and three monoaromatic hydrocarbons. Volatilization into the atmosphere and biodegradation are two important natural attenuation paths for VOCs in unsaturated soil columns. The volatilization loss fractions of different volatile hydrocarbons in all five unsaturated soils were generally in the order: n-pentane (93.5%–97.8%) > methylcyclopentane (77.2%–85.5%) > methylcyclohexane (53.5%–69.2%) > benzene (17.1%–73.3%) > toluene (0–45.7%) > octane (1.9%–34.2%) > m-xylene (0–5.7%). The fractions by volatilization into the atmosphere of all seven hydrocarbons in quartz sand, lateritic red earth, and floodplain soil were close and higher compared to the yellow earth and black soil. Overall, this study illustrated the important roles chemical speciation and soil properties in determining the vapor-phase transport and natural attenuation of VOCs in the unsaturated zone.

Measuring the Fate and Natural Attenuation Potential of a Viscous Marine Fuel on an Artificial Beach Mesocosm

Shoreline oiling poses a risk to coastal ecosystems and resources. Understanding the natural attenuation potential and impact of different sediment types is important for choosing appropriate intervention strategies and priority areas following a spill. Simulated IFO-40 oil spills on artificial beach mesocosms were carried out using different sediment types: sandy beach and sandy tidal flat, under low energy tidal cycles over a 5-day period. Chemical and biological analysis of leachate and sediment was conducted to understand the movement of oil through these mesocosms. Rapid oil movement from the oil slick to the surface sediment layer was observed in the sandy beach enclosures, while slower oil movement was observed in the sandy tidal flat enclosures. Increased hydrocarbon dissolution was observed in the sandy beach enclosures, marked by higher concentrations of low molecular weight n-Alkanes (C12 − 15) and naphthalenes (C0 − 3) in sandy beach leachate compared to sandy tidal flat samples. Despite the increase in hydrocarbons, there were no major shifts in microbial communities observed in the leachate and sediment compartments for either sediment type. Both prokaryote and microeukaryote communities differed between the two sediment types, with little overlap between dominant sequences. Our results indicate that limited oil penetration occurs within sandy tidal flat shorelines resulting in oil accumulation suggesting that sorbent or vacuuming could be used as emergency response to minimize the environmental and ecological impacts of spilled oil.

Residual hydrocarbons in long-term contaminated soils: implications to risk-based management

Petroleum hydrocarbon (PHC) contamination is a widespread and severe environmental issue affecting many countries’ resource sectors. PHCs are mixtures of hydrocarbon compounds with varying molar masses that naturally attenuate at different rates. Lighter fractions attenuate first, followed by medium-molar-mass constituents, while larger molecules remain for longer periods. This results in significant regulatory challenges concerning residual hydrocarbons in long-term contaminated soils. This study examined the potential risks associated with residual PHC and its implications for risk-based management of heavily contaminated soils (23,000–26,000 mg PHC/kg). Ecotoxicological properties, such as seedling emergence and growth of two native plant species—small Flinders grass (Iseilema membranaceum) and ruby saltbush (Enchylaena tomentosa)—and earthworm survival tests in PHC-contaminated soils, were assessed. Additionally, the effects of aging on the attenuation of PHC in contaminated soils were evaluated. Toxicity responses of plant growth parameters were determined as no-observed-effect concentrations: 75%–100% for seedling emergence, < 25%–75% for plant shoot height, and 75%–100% for earthworm survival. After 42 weeks of aging, the total PHC levels in weathered soils decreased by 14% to 30% and by 67% in diesel-spiked soil due to natural attenuation. Dehydrogenase enzyme activity in soils increased during the initial aging period. Furthermore, a clear shift of bacterial communities was observed in the soils following aging, including enrichment of PHC-resistant and -utilizing bacteria—for example, Nocardia sp. This study underscores the potential of natural attenuation for eco-friendly and cost-effective soil management, underlining that its success depends on site-specific factors like water content and nutrient availability. Therefore, we recommend detailed soil assessments to evaluate these conditions prior to adopting a risk-based management approach.

Characteristics of groundwater microbial communities and the correlation with the environmental factors in a decommissioned acid in-situ uranium mine.

Microorganisms play an important role in the bioremediation process for the decommissioned acid in-situ leaching uranium mine. It is crucial to understand the original microbial community characteristics before the in-situ bioremediation. However, there are limited studies on the groundwater microbial characteristics in the decommissioned acid in-situ uranium mine. To this end, we collected groundwater samples, including the groundwater that originally residual in the borehole (RW) and the aquifer water (AW), from a decommissioned acid in-situ uranium mine in the southern margin of Ili Basin in Xinjiang, China. The occurrence characteristics of the groundwater microbial communities and their correlation with environmental factors were systematically studied based on the high throughput 16S rRNA gene sequencing data and geochemical data. Results found that the AW samples had higher alpha- and beta- diversity than the RW samples. The relative abundance of Sporosarcina, Sulfobacillus, Pedobacter and Pseudomonas were significantly different in the AW and RW samples, which had significant correlation with pH, metals, and sulfate, etc. A series of reducing microorganisms were discovered, such as sulfate reduction (e.g., Desulfosporosinus) and metal reduction (e.g., Arthrobacter, Bacillus, Clostridium, Pseudomonas, and Rhodanobacter), which have the potential to attenuate sulfate and uranium in groundwater. In addition, we found that pH and redox potential (Eh) were the dominant environmental factors affecting the microbial composition. This study extends our knowledge of microbial community structure changes in the decommissioned acid in-situ uranium mine and has positive implications for assessing the potential of natural attenuation and bioremediation strategies.

A Computational Framework for Design and Optimization of Risk-Based Soil and Groundwater Remediation Strategies

Soil and groundwater systems have natural attenuation potential to degrade or detoxify contaminants due to biogeochemical processes. However, such potential is rarely incorporated into active remediation strategies, leading to over-remediation at many remediation sites. Here, we propose a framework for designing and searching optimal remediation strategies that fully consider the combined effects of active remediation strategies and natural attenuation potentials. The framework integrates machine-learning and process-based models for expediting the optimization process with its applicability demonstrated at a field site contaminated with arsenic (As). The process-based model was employed in the framework to simulate the evolution of As concentrations by integrating geochemical and biogeochemical processes in soil and groundwater systems under various scenarios of remedial activities. The simulation results of As concentration evolution, remedial activities, and associated remediation costs were used to train a machine learning model, random forest regression, with a goal to establish a relationship between the remediation inputs, outcomes, and associated cost. The relationship was then used to search for optimal (low cost) remedial strategies that meet remediation constraints. The strategy was successfully applied at the field site, and the framework provides an effective way to search for optimal remediation strategies at other remediation sites.

Coupled effect of porous network and water content on the natural attenuation of diesel in unsaturated soils.

The natural attenuation potential of a vadose zone against diesel is critical for optimizing remedial actions and determining groundwater vulnerability to contamination. Here, diesel attenuation in unsaturated soils was systematically examined to develop a qualitative relationship between physical soil properties and the natural attenuation capacity of a vadose zone against diesel. The uniformity coefficient (Cu) and water saturation (Sw, %) were considered as the proxies reflecting the degree of effects by porous network and water content in different soils, respectively. These, in turn, are related to the primary diesel attenuation mechanisms of volatilization and biodegradation. The volatilization of diesel was inversely proportional to Cu and Sw, which could be attributed to effective pore channels facilitating gas transport. Conversely, biodegradation was highly proportional to Cu under unsaturated conditions (Sw=35-71%), owing to nutrients typically associated with fine soil particles. The microbial community in unsaturated soils was affected by Sw rather than Cu. The overall diesel attenuation including volatilization and biodegradation was optimized at Sw=35% for all tested soils.

Soil-pH and cement influence the weathering kinetics of chrysotile asbestos in soils and its hydroxyl radical yield

Chrysotile asbestos is a toxic and carcinogenic mineral that has been used in a variety of industrial and consumer applications. Much of the fiber- and cement-containing asbestos waste has ended up in terrestrial environments. Chrysotile weathering in soils and the potential for natural attenuation have, however, hardly been examined yet. Here we explored how soil properties influence the dissolution rate of chrysotile, the release of the carcinogenic metals chromium and nickel, and the hydroxyl radical (HO•) generation by chrysotile fibers. Chrysotile dissolution rates in soil suspensions decreased with increasing soil-pH and were lower than reported rates in soil-free systems. Dissolved organic carbon did not markedly accelerate dissolution at circumneutral pH, whereas cement mixed with soil inhibited dissolution because of its alkalinity. The HO•-yield of incubated fibers in non-amended soils eventually decreased by 60–75%. The decline was fastest in an acidic podzol soil, yet was followed by a small rebound. Cement amendment induced the largest HO•-yield reduction (∼90%), presumably due to surface coating of the fibers. Overall, this work demonstrates that the potential for natural attenuation of chrysotile asbestos in soils critically depends on soil chemical parameters and the presence of cement in association with the fibers.

Modelling the attenuation of flowback chemicals for a soil-groundwater pathway from a hypothetical spill accident

Flowback water from shale gas operations contains formation-derived compounds, including trace metals, radionuclides, and organics. While accidental releases from storage tanks with flowback water are low-probability events if multiple containment barriers are put in place, they cannot be entirely excluded. Here the natural attenuation potential of deep unsaturated zones and groundwater was explored using predictive modelling involving a hypothetical leak from a storage tank. Actual chemical concentrations from flowback water at two shale gas wells with contrasting salinity (12,300 and 105,000 ppm TDS) in the Beetaloo Sub-basin (Northern Territory, Australia) served as input to the one-dimensional HYDRUS model for simulating chemical transport through the unsaturated zone, with groundwater at 50 and 100 m depth, respectively. Subsequent chemical transport in groundwater involved the use of a three-dimensional analytical transport model. For a total of 63 chemicals the long-term attenuation from dilution and dispersion in unsaturated sediments and groundwater was calculated. Predicted environmental concentrations for aquatic receptors were compared with no-effect levels of individual chemicals to derive risk quotients (RQ) and identify chemicals of no concern to ecosystem health (i.e. RQ <1). Except for salinity and radium-228 in one of the two wells, RQ < 1 for all other chemicals. The initial approach considered testing of toxicity to individual chemicals only. When direct toxicity assessments (DTAs) were used to account for effects of chemical mixtures, the required DTA-derived safe dilution factor for 95% species protection was 1.8 to 2.5 times higher than the dilution factor accounting for dispersion and dilution only. Accounting for biodegradation, sorption and radioactive decay decreased chemical concentrations in unsaturated sediments to safe levels using the DTA for all chemicals. The study highlighted the importance of incorporating DTA in chemical risk assessments involving complex chemical mixtures. Improved understanding of fate and transport of flowback chemicals will help effectively manage water-quality risks associated with shale gas extraction.

Distribution of a Sulfolane-Metabolizing Rhodoferax sp. Throughout a Contaminated Subarctic Aquifer and Two Groundwater Treatment Systems.

An extensive plume of the emerging contaminant sulfolane has been found emanating from a refinery in Interior Alaska, raising questions about the microbial potential for natural attenuation and bioremediation in this subarctic aquifer. Previously, an aerobic sulfolane-assimilating Rhodoferax sp. was identified from the aquifer using stable isotope probing. Here, we assessed the distribution of known sulfolane-assimilating bacteria throughout the contaminated subarctic aquifer using 16S-rRNA-amplicon analyses of ~100 samples collected from groundwater monitoring wells and two groundwater treatment systems. One treatment system was an in situ air sparging system where air was injected directly into the aquifer. The other was an ex situ granular activated carbon (GAC) filtration system for the treatment of private well water. We found that the sulfolane-assimilating Rhodoferax sp. was present throughout the aquifer but was significantly more abundant in groundwater associated with the air sparge system. The reduction of sulfolane concentrations combined with the apparent enrichment of sulfolane degraders in the air sparging zone suggests that the addition of oxygen facilitated sulfolane biodegradation. To investigate other environmental controls on Rhodoferax populations, we also examined correlations between groundwater geochemical parameters and the relative abundance of the Rhodoferax sp. and found only manganese to be significantly positively correlated. The sulfolane-assimilating Rhodoferax sp. was not a major component of the GAC filtration system, suggesting that biodegradation is not an important contributor to sulfolane removal in these systems. We conclude that air sparging is a promising approach for enhancing the abundance and activity of aerobic sulfolane-degraders like Rhodoferax to locally stimulate sulfolane biodegradation in situ.

Surfactant-enhanced treatment of oil-contaminated Arctic tundra soil: Ecotoxicological assessment

During 2018–2019 summer expeditions to the Yamal peninsula, we collected tundra gley soil samples at the oil-contaminated sites. The contamination level was determined by fluorimetry and was ranged from 30 to 107 g/kg. The soil quality was evaluated by its enzymatic activity; the low catalase activity of the soils confirmed the presumed slow soil natural attenuation and the need for land remediation. The oil-contained and background samples were ex-situ treated with the aqueous solutions of non-ionic BOK-6 and anionic AddiMax PV01 synthetic surfactants. The treatment efficiency reached 71 ± 2% and 97 ± 3% in the experimental design with BOK-6 and AddiMax PV01, respectively. We derived the ecotoxicity endpoints for the surfactant solutions, using infusoria Paramecium caudatum Ehrenberg and higher plants Raphanus sativus L. and Avena sativa L. The protozoan mortality was studied and resulted in LC 50 − 24 values << 0.1 mg/L and 339 mg/L for BOK-6 and AddiMax PV01, respectively. Both surfactant solutions and surfactant residues in soil showed the inhibition potential towards higher plants. BOK-6, with a strong relation to its chemical and ecotoxic profile, completely inhibited seed germination in the treated soil. The less ecotoxic surfactant AddiMax PV01 inhibited plant growth by 38% compared to control. The treatment increased ecotoxicity and dehydrogenase activity and further lowered the catalase activity of the background soil, and, therefore, the surfactant residues migrating to the non-contaminated territories should be controlled. By this study, we show the efficiency of the surfactant-enhanced remediation of arctic soils, but the associated environmental risks must be addressed. • Ex-situ treatment of oil-contaminated tundra gley soil reached 69%–99% efficiency. • Enzymes activity indicated slow natural attenuation potential of arctic soil. • Residues of toxic surfactant could cause greater ecotoxic effect than contamination. • Surfactants residues inhibited biological activity of non-contaminated arctic soil.

Thermally-induced changes in tropical soils properties and potential implications to sequential nature-based solutions

Some remediation techniques, such as thermal remediation, can significantly change the soil properties. These changes can be beneficial or detrimental to the sequential application of Nature-based Solutions. This work evaluated the effects of thermal remediation on the properties of two tropical soils (Technosol and Oxisol), and discuss how these changes might impact both biotic and abiotic degradation processes. Bench tests using disturbed samples were performed under oxic and anoxic conditions, whereas 3D physical models were used to simulate the heat distribution along undisturbed samples. The changes in soils texture, density, hydraulic conductivity, iron concentration, mineralogy and microbiota were evaluated. The properties of Oxisol were more affected than those of Technosol due to the higher levels in Fe(III), organic carbon and finer texture. When heated in the range of 120 to 300 °C under oxic and anoxic conditions, the Fe(II) content and the magnetism intensity increased in Oxisol, probably due to the formation of magnetite. Under oxic conditions, the burning of Oxisol organic matter promoted an anoxic atmosphere, favoring the formation of Fe(II). However, the continuous increase of the temperature (>300 °C) lead to the decrease of Fe(II) due to the transformation of magnetite to maghemite, and then to hematite. The heating process also promoted some minerals decomposition and cementation of the clay fraction, increasing the soil texture. Bacterial populations were impacted, but showed ability to recover at 60 °C. However, above 100 °C no culturable cells were recovered and at temperatures above 270 °C soil sterilization occurred. The changes observed, especially in Oxisol samples, indicated that mild heating (between 120 and 240 °C), in turn, can increase the potential for abiotic degradation of some contaminants, such as chlorinated solvents. Therefore, heating conditions up to 240 °C during thermal remediation can be defined as to promote beneficial changes in soil properties, increasing its potential for natural attenuation by abiotic processes even when the microbiota is affected, and improving its sustainability.

Evaluation of natural attenuation-potential and biogeochemical analysis in nitrate contaminated bedrock aquifers by carbon source injection.

In this study, the natural attenuation potential and biogeochemical analysis of nitrate contaminated bedrock aquifers by injection of carbon sources was evaluated. The denitrification capacity was assessed by injecting different carbon sources (succinate, acetate, fumarate) into the groundwater. Acetate was identified as the optimum source of electron donors for microbial metabolic processes, as it improved the effect of nitrate removal and microbial activity in the groundwater. In addition, when acetate was injected with a C/N ratio=2.1:1, the ratio of denitrifying bacteria was the greatest (C/N 2.1 (2.1%)>C/N 4.2 (1.9%)>C/N 7.0 (0.9%)>control (0.7%)). Reflecting the geochemical characteristics of the bedrock aquifer environment, acetate was injected into groundwater at the research site to activate biological heterotrophic denitrification. As a result, the nitrate reduction rate was 0.377g-N/day (YP-3), while the rate in groundwater unaffected by acetate was significantly lower, at 0.028g-N/day (YP-4) over the same reaction time. In particular, the ratio of Dechloromonas denitrificans sp., which is a representative denitrification bacteria involved in anaerobic reduction of nitrate, increased (before injection: 0.0089%, after injection: 1.3067%). Expression of the nosZ gene, which is involved in the denitrification pathway (N2O→N2), increased from 4.82 Log (gene copies L-1) to 9.71 Log (gene copies L-1). Together, these results demonstrate that denitrification in bedrock aquifers can be activated by injection of carbon sources and identified the genetic reason for that denitrification.

1,2-DCA Natural Attenuation Evaluation in Groundwater: Insight by Dual Isotope 13C/37Cl and Molecular Analysis Approach

Natural attenuation (NA) processes represent a valuable option in groundwater remediation. At a heavily 1,2-dichloroethane (1,2-DCA) contaminated site, Compound-Specific Isotope Analysis (CSIA) in combination with Biological Molecular Tools (BMTs) were implemented as a rigorous characterization approach to evaluate the occurrence of Natural Attenuation in the proximity of the source area. By the use of microcosm experiments, the potential for natural and enhanced biodegradation under anaerobic conditions was documented, following the dichloroelimination pathway. Enrichment factors of −9.1‰ and −11.3‰ were obtained for 13C while Geobacter spp. and reductive dehalogenase genes (rdhs) were identified as main site-specific biomarkers. At pilot scale, enrichments of 13.5‰ and 6.3‰ for δ13C and δ37Cl, respectively, high levels of reductive dehalogenase (rdh group VI) along with the dominance of Geobacter spp. indicated the occurrence of significant dichloroelimination processes in groundwater under anaerobic conditions. By using the site-specific enrichment factors, degradation extents over approximately 70–80% were estimated, highlighting the relevant potential of NA in 1,2-DCA degradation in the vicinity of the source area at the site. The proposed fine-tuned protocol, including CSIA and BMTs, is proven to be effective as a groundwater remediation strategy, properly assessing and monitoring NA at site scale.

Anaerobic benzene mineralization by natural microbial communities from Niger Delta

The Niger Delta is one of the most damaged ecosystems in the world, mainly due to petroleum contamination by oil exploration accidents. We investigated the natural attenuation potential of Niger Delta subsurface sediment samples for anaerobic hydrocarbon degradation using benzene as a model compound under iron-reducing, sulfate-reducing, and methanogenic conditions. Benzene was slowly mineralized under methanogenic and iron-reducing conditions using nitrilotriacetic acid (NTA)-Fe(III), or poorly crystalline Fe(III) oxyhydroxides as electron acceptors, analyzed by measurement of 13CO2 produced from added 13C-labelled benzene. Highest mineralization rates were observed in microcosms amended with Fe(III) oxyhydroxides. The microbial communities of benzene-mineralizing enrichment cultures were characterized by next-generation sequencing of the genes coding for 16S rRNA and methyl coenzyme M reductase A (mcrA). Abundant phylotypes were affiliated to Betaproteobacteriales, Ignavibacteriales, Desulfuromonadales, and Methanosarcinales of the genera Methanosarcina and Methanothrix, illustrating that the enriched benzene-mineralizing communities were diverse and may contain more than a single benzene degrader. The diversity of the microbial communities was furthermore confirmed by scanning helium-ion microscopy which revealed the presence of various rod-shaped as well as filamentous microbial morphotypes.

Assessment of Biodegradation in Natural Attenuation Process of Chlorinated Hydrocarbons Contaminated Site: An Anaerobic Microcosm Study

ABSTRACT In this study, the anaerobic microcosm systems under in-situ conditions were constructed using the soil and the groundwater collected at a former pesticide manufacturing site. The in-situ natural attenuation potential was evaluated and its relationships with the geochemistry conditions and the microbial communities were carefully explored. The site was heavily contaminated with chlorinated hydrocarbons (CHCs) including perchloroethylene (PCE) and 1,2-dichloroethane (1,2-DCA). The 0 ~ 20 mV redox conditions measured in both the heterogeneous soil and the groundwater favored a series of CHCs biodegradation correlated processes. The significant contributions of biodegradation processes to the attenuation of CHCs were confirmed by comparing the PCE and 1,2-DCA degradations as accompanied with Cl− accumulation rates between the unsterilized microcosms and the sterilized controls. The 16S rRNA gene based denaturing gradient gel electrophoresis (DGGE) analysis revealed the distinct shifts of microbial community compositions in the soil and the groundwater microcosm systems during the incubation. The Shannon-Winner index analysis further suggested the decrease and the increase of microbial diversities in the soil and the groundwater samples, respectively. The dominant species mainly associated with phylum Proteobacteria, Firmicutes, and Actinobacteria. The recovered genera Geobacter, Streptomyces, and Acetoanaerobium associated 16S rRNA gene sequences confirmed the in-situ biological dechlorination potentials of CHCs. In summary, this study revealed high biological attenuation potentials with the suitable redox conditions for the studied actual CHCs contaminated site and indicated the high probability of the in-situ natural attenuation approach for CHCs contaminated site remediation.

Redox characteristics of humins and their coupling with potential PCB dechlorinators in southern Yellow Sea sediments

Natural attenuation of polychlorinated biphenyls (PCBs) by indigenous bacteria is an effective remediation strategy for polluted marine sediments. This study investigated the relationships between PCB concentrations in sediment pore water, humin electron transfer capacity, and potential PCB dechlorinators at eight sediment sampling sites in the southern Yellow Sea, China, with differential PCB contamination. Station A2 showed the highest PCB concentration (453.16 ng L−1 for seven indicator PCBs), especially of less chlorinated PCB congeners (≤5 Cl atoms), humin redox activity, and Dehalococcoides abundance (p < 0.05). Statistical analyses revealed a highly positive correlation between Dehalococcoides abundance and PCB concentration (r = 0.836, p < 0.05) and the electron shuttling ability of humins (r = 0.952, p < 0.01), whereas this was not observed for total bacteria and other potential PCB dechlorinators, e.g., Dehalobacter and Dehalogenimonas. Based on these results, Dehalococcoides might play an important role in the in situ reductive dechlorination of PCBs involving humins in marine sediments, and the natural microbial PCB attenuation capacity at station A2 was high. Chemical characterizations, electrochemical properties, and Fourier transform infrared analysis suggested that humins at station A2 had the highest electron transfer capacity. Furthermore, quinones are likely to be the functional groups that shuttle electrons during PCB dechlorination. Overall, this study provides a useful foundation for evaluating the natural microbial attenuation potential and fates of PCBs in marine sediments and for determining the role of humins as redox mediators in in situ PCB dechlorination by putative indigenous dechlorinators.

In situ biodegradation, photooxidation and dissolution of petroleum compounds in Arctic seawater and sea ice

In pristine sea ice-covered Arctic waters the potential of natural attenuation of oil spills has yet to be uncovered, but increasing shipping and oil exploitation may bring along unprecedented risks of oil spills.We deployed adsorbents coated with thin oil films for up to 2.5 month in ice-covered seawater and sea ice in Godthaab Fjord, SW Greenland, to simulate and investigate in situ biodegradation and photooxidation of dispersed oil.GC-MS-based chemometric methods for oil fingerprinting were used to identify characteristic signatures for dissolution, biodegradation and photooxidation. In sub-zero temperature seawater, fast degradation of n-alkanes was observed with estimated half-life times of ∼7 days. PCR amplicon sequencing and qPCR quantification of bacterial genes showed that a biofilm with a diverse microbial community colonised the oil films, yet a population related to the psychrophilic hydrocarbonoclastic gammaproteobacterium Oleispira antarctica seemed to play a key role in n-alkane degradation. Although Oleispira populations were also present in sea ice, we found that biofilms in sea ice had 25 to 100 times lower bacterial densities than in seawater, which explained the non-detectable n-alkane degradation in sea ice. Fingerprinting revealed that photooxidation, but not biodegradation, transformed polycyclic aromatic compounds through 50 cm-thick sea ice and in the upper water column with removal rates up to ∼1% per day.Overall, our results showed a fast biodegradation of n-alkanes in sea ice-covered seawater, but suggested that oils spills will expose the Arctic ecosystem to bio-recalcitrant PACs over prolonged periods of time.

Microbial communities associated with uranium in-situ recovery mining process are related to acid mine drainage assemblages

A large fraction (47%) of the world's uranium is mined by a technique called “In Situ Recovery” (ISR). This mining technique involves the injection of a leaching fluid (acidic or alkaline) into a uranium-bearing aquifer and the pumping of the resulting solution through cation exchange columns for the recovery of dissolved uranium. The present study reports the in-depth alterations brought to autochthonous microbial communities during acidic ISR activities. Water samples were collected from a uranium roll-front deposit that is part of an ISR mine in operation (Tortkuduk, Kazakhstan). Water samples were obtained at a depth of ca 500 m below ground level from several zones of the Uyuk aquifer following the natural redox zonation inherited from the roll front deposit, including the native mineralized orebody and both upstream and downstream adjacent locations. Samples were collected equally from both the entrance and the exit of the uranium concentration plant. Next-generation sequencing data showed that the redox gradient shaped the community structures, within the anaerobic, reduced, and oligotrophic habitats of the native aquifer zones. Acid injection induced drastic changes in the structures of these communities, with a large decrease in both cell numbers and diversity. Communities present in the acidified (pH values < 2) mining areas exhibited similarities to those present in acid mine drainage, with the dominance of Sulfobacillus sp., Leptospirillum sp. and Acidithiobacillus sp., as well as the archaean Ferroplasma sp. Communities located up- and downstream of the mineralized zone under ISR and affected by acidic fluids were blended with additional facultative anaerobic and acidophilic microorganisms. These mixed biomes may be suitable communities for the natural attenuation of ISR mining-affected subsurface through the reduction of metals and sulfate. Assessing the effect of acidification on the microbial community is critical to evaluating the potential for natural attenuation or active bioremediation strategies.