A new Method for determining Compound Specific Carbon Isotope of Chlorinated Solvents in Porewater

A new method for the extraction of chlorinated solvents (CSs) from porewater with dimethylacetamide (DMA) used as a solvent and the determination of δ13C by gas chromatography-isotope ratio mass spectrometry (GC-IRMS) with solid-phase microextraction (SPME) are presented. This method was used for the determination of δ13C of chloroethenes and chloromethanes. The extraction of the CSs from porewater with DMA led to a minimal loss of mass of solvent and chlorinated compounds. The accuracy of the method was verified with the analysis of the pure injected compounds using elemental analyser - isotope ratio mass spectrometry (EA-IRMS). It has been effectively applied in a study area in saturated soil samples of a pollutant source zone of perchloroethylene (PCE) and trichloroethylene (TCE). The limit of quantification of the new method was 0.034 µg/g for PCE and TCE for 10-20 g of soil sample. This new method allows for compound-specific isotope analysis (CSIA) of CSs in porewater, which can be beneficial in sites where the identification of contamination sources and the behaviour of the contaminants are not clear

Natural attenuation of pools and plumes of carbon tetrachloride and chloroform in the transition zone to bottom aquitards and the microorganisms involved in their degradation

In the transition zone between aquifers and aquitards, DNAPL pools of carbon tetrachloride and chloroform accumulate because of heterogeneity in this zone. Natural attenuation occurs at pools and plumes, indicating that remediation might be possible. The aims of the study were: i) to assess the role of heterogeneity in the natural attenuation of these compounds, ii) determine degradation processes within this zone, and iii) identify dechlorinating microorganisms. For this, groundwater concentrations, redox-sensitive parameters, CSIA isotopic and DGGE molecular techniques were used. The main findings at depth of the transition zone were: (1) the important key control played by heterogeneity on natural attenuation of contaminants. (2) Heterogeneity caused the highly anoxic environment and dominant sulfate-reducing conditions, which accounts for more efficient natural attenuation. (3) Heterogeneity also explains that the transition zone constitutes an ecotone. (4) The bacteria size exclusion is governed by the pore throat threshold and determines the penetration of dechlorinating microorganisms into the finest sediments, which is relevant, since it implies the need to verify whether microorganisms proposed for bioremediation can penetrate these materials. (5) Reductive dechlorination caused the natural attenuation of contaminants in groundwater and porewater of fine sediments. In the case of carbon tetrachloride, it was an abiotic process biogenically mediated by A. suillum, a bacterium capable of penetrating the finest sediments. In the case of chloroform, it was a biotic process performed by a Clostridiales bacterium, which is unable to penetrate the finest materials. (6) Both microorganisms have potential to be biostimulated to dechlorinate contaminants in the source and the plume in the transition zone. These outcomes are particularly relevant given the longevity of DNAPL sources and have considerable environmental implications as many supply wells in industrial areas exploit aquifers contaminated by chlorinated solvents emerging from DNAPL pools accumulated on the low-conductivity layers in transition zones

Combined use of ISCR and biostimulation techniques in incomplete processes of reductive dehalogenation of chlorinated solvents

Pools of chloroethenes are more recalcitrant in the transition zone between aquifers and basal aquitards than those elsewhere in the aquifer. Although biodegradation of chloroethenes occur in this zone, it is a slow process and a remediation strategy is needed. The aim of this study was to demonstrate that combined strategy of biostimulation and in situ chemical reduction (ISCR) is more efficient than the two separated strategies. Four different microcosm experiments with sediment and groundwater of a selected field site where an aged perchloroethene (PCE)-pool exists at the bottom of a transition zone, were designed under i) natural conditions, ii) biostimulation with lactic acid, iii) in situ chemical reduction (ISCR)with zero valent iron (ZVI) and under iv) a combined strategy with lactic acid and ZVI. Biotic and abiotic dehalogenation, terminal electron acceptor processes and evolution of microbial communities were investigated for each experiment. The main results where: i) limited reductive dehalogenation of PCE occurs under sulfate-reducing conditions; ii) biostimulation with lactic acid promotes a more pronounced reductive dehalogenation of PCE in comparison under natural conditions, but resulted in an accumulation of cis-dichloroethene (cDCE); iii) ISCR with zero-valent iron (ZVI) facilitates a sustained dehalogenation of PCE and its metabolites to non-halogenated products, however, the iv) combined strategy results in the fastest and sustained dehalogenation of PCE to non-halogenated products in comparison of all four set-ups. These findings suggest that biostimulation and ISCRwith ZVI are the most suitable strategy for a complete reductive dehalogenation of PCE-pools in the transition zone

The role of ecotones in the dehalogenation of chloroethenes in alluvial fan aquifers

The presence of ecotones in transition zones between geological strata (e.g. layers of gravel and sand interbedded with layers of silt in distal alluvial fan deposits) in aquifers plays a significant role in regulating the flux of matter and energy between compartments. Ecotones are characterised by steep physicochemical and biological gradients and considerable biological diversity. However, the link between organic pollutants and degradation potential in ecotones has scarcely been studied. The aim of this study is to relate the presence of ecotones with the dehalogenation of chloroethenes. A field site was selected where chloroethene contamination occurs in a granular aquifer with geological heterogeneities. The site is monitored by multilevel and conventional wells. Groundwater samples were analysed by chemical, isotopic, and molecular techniques. The main results were as follows: (1) two ecotones were characterised in the source area, one in the upper part of the aquifer and the second in the transition zone to the bottom aquitard, where the aged pool is located; (2) the ecotone located in the transition zone to the bottom aquitard has greater microbial diversity, due to higher geological heterogeneities; (3) both ecotones show the reductive dehalogenation of perchloroethylene and trichloroethylene; and (4) these ecotones are the main zones of the reductive dehalogenation of the pollutants, given the more reductive conditions at the centre of the plume. These findings suggest that ecotones are responsible for natural attenuation, where oxic conditions prevailed at the aquifer and bioremediation strategies could be applied more effectively in these zones to promote complete reductive dehalogenation

Biotic and abiotic reductive dechlorination of chloroethenes in aquitards

Chlorinated solvents occur as dense nonaqueous phase liquid (DNAPL) or as solutes when dissolved in water. They are present inmany pollution sites in urban and industrial areas. They are toxic, carcinogenic, and highly recalcitrant in aquifers and aquitards. Inthe latter case, they migrate by molecular diffusion into the matrix. When aquitards are fractured, chlorinated solvents also penetrateas a free phase through the fractures. The main objective of this study was to analyze the biogeochemical processes occurring insidethe matrix surrounding fractures and in the joint-points zones. The broader implications of this objective derive from the fact that,incomplete natural degradation of contaminants in aquitards generates accumulation of daughter products. This causes steepconcentration gradients and back-diffusion fluxes between aquitards and high hydraulic conductivity layers. This offers opportunitiesto develop remediation strategies based, for example, on the coupling of biotic and reactive abiotic processes. The main resultsshowed: 1) Degradation occurred especially in the matrix adjacent to the orthogonal network of fractures and textural heterogeneities,where texture contrasts favored microbial development because these zones constituted ecotones. 2) A dechlorinating bacterium notbelonging to the Dehalococcoides genus, namely Propionibacterium acnes, survived under the high concentrations of dissolvedperchloroethene (PCE) in contact with the PCE-DNAPL and was able to degrade it to trichloroethene (TCE). Dehalococcoides genus was able to conduct PCE reductive dechlorination at least up to cis-1,2-dichloroethene (cDCE), which shows again the potential of themedium to degrade chloroethenes in aquitards. 3) Degradation of PCE in the matrix resulted from the coupling of reactive abiotic andbiotic processes¿in the first case, promoted by Fe sorbed to iron oxides, and in the latter case, related to dechlorinatingmicroorganisms. The dechlorination resulting from these coupling processes is slow and limited by the need for an adequate supply ofelectron donors

Reductive dechlorination in recalcitrant sources of chloroethenes in the transition zone between aquifers and aquitards

In the transition zone between aquifers and basal aquitards, the perchloroethene pools at an early time in their evolution are more recalcitrant than those elsewhere in the aquifer. The aim of this study is to demonstrate that the biodegradation of chloroethenes from aged pools (i.e., pools after decades of continuous groundwater flushing and dissolution) of perchloroethene is favored in the transition zone. A field site was selected where an aged pool exists at the bottom of a transition zone. Two boreholes were drilled to obtain sediment and groundwater samples to perform chemical, isotopic, molecular, and clone library analyses and microcosm experiments. The main results were as follows: (i) the transition zone is characterized by a high microbial richness; (ii) reductively dechlorinating microorganisms are present and partial reductive dechlorination coexists with denitrification, Fe and Mn reduction, and sulfate reduction; (iii) reductively dechlorinating microorganisms were also present in the zone of the aged pool; (v) the high concentrations of perchloroethene in this zone resulted in a decrease in microbial richness; (vi) however, the presence of fermenting microorganisms supplying electrons for the reductively dechlorinating microorganisms prevented the reductive dechlorination to be inhibited. These findings suggest that biostimulation and/or bioaugmentation could be applied to promote complete reductive dechlorination and to enhance the dissolution of more nonaqueous phase liquids (DNAPL)

Identificació dels processos biogeoquímics que es donen en la zona de la font per al disseny d’estratègies de remediació en aqüífers contaminats per cloroetens 

[cat] El present treball de Tesi doctoral es centra en l'estudi d'episodis de contaminació afectats principalment per cloroetens, solvents organoclorats que formen un dels grups de DNAPL (dense non-aqueous phase liquids) quantitativament més importants. La baixa solubilitat dels DNAPL i la seva elevada persistència en les matrius ambientals, expliquen que en el nostre país, gran nombre d'episodis de contaminació tinguin el seu origen en èpoques passades, en què la falta d'una conscienciació ambiental i la inexistència d'una legislació apropiada varen possibilitar una mala praxis en la gestió d'aquests compostos. Alhora, les seves característiques intrínseques expliquen la llarga vida de les fonts i plomalls de contaminació associats que afecten tant els aqüífers com els sòls. La present Tesi Doctoral té com a principals objectius la caracterització dels processos biogeoquímics i les comunitats microbianes en la zona de la font que succeeixen en un context de ventalls al•uvials. A més, es caracteritzen les zones d'especial activitat microbiana, en especial els ecotons, i l'extensió dels processos biogeoquímics al llarg del plomall. Un altre dels objectius és la integració qualitativa i quantitativa dels elements que defineixen les diferents heterogeneïtats, amb la finalitat de comprendre la complexitat del medi, així com analitzar la viabilitat de l'aplicació d'estratègies de remediació integrals que permetin la descontaminació de la zona de la font. Els estudis portats a terme a nivell de camp per a la caracterització de les heterogeneïtats geològiques, geoquímiques i microbiològiques de la zona de la font de contaminació s'han fonamentat en la realització de sondatges d'investigació amb recuperació de testimoni continu. L'anàlisi integrada de les heterogeneïtats geològiques, hidrogeològiques, biogeoquímiques i microbiològiques dels materials del subsòl ha permès determinar com aquestes heterogeneïtats controlen la distribució dels cloroetens, els processos de transferència de massa i els processos de degradació, alhora que condicionen també la diversitat, el grau de desenvolupament i l'estructura de les comunitats microbianes. La caracterització dels processos de deshalogenació s'ha estudiat tant en la matriu sediment com en la matriu aigua, a partir de quatre aproximacions diferents: 1) la quantificació del percloroetilè (PCE) i els seus metabòlits; 2) la determinació de la composició isotòpica del PCE a partir de l'anàlisi isotòpica de compostos específics (Compound Specific Isotope Analysis, CSIA) tant en l'aigua intersticial com en l'aigua de l'aqüífer; 3) la identificació de processos d'acceptadors finals d'electrons (com ara desnitrificació, reducció del Mn i del Fe i sulfato-reducció); i 4) l'anàlisi de les comunitats microbianes. S’han estudiat quatre estratègies de remediació diferents en experiments de microcosmos en condicions anòxiques: l’atenuació natural, la bioestimulació amb àcid làctic, la reducció química in situ (ISCR) amb ferro zero valent (ZVI) i una combinació de les dues darreres estratègies. A més, s’ha determinat l’evolució dels processos biogeoquímics i de les comunitats microbianes per aquestes quatre estratègies en els sediments de la part superior de l’aqüífer (caracteritzada per una elevada conductivitat hidràulica i una heterogeneïtat baixa) i de la zona inferior de l’aqüífer. Aquesta zona inferior de l’aqüífer es caracteritza com a zona de transició, amb una baixa conductivitat hidràulica i una elevada heterogeneïtat. L’anàlisi integrada dels resultats obtinguts a partir de l’ús de tècniques de camp i laboratori ha permès descriure les comunitats microbianes i els processos biogeoquímics que inhibeixen o estimulen la deshalogenació del PCE i els seus metabòlits en la zona de la font de contaminació.[eng] The present PhD thesis focuses on the study of pollution episodes, mainly affected by chloroethenes, organochlorine solvents which form one of the groups of DNAPL (dense non-aqueous phase liquids) quantitatively most important. The low solubility of DNAPL and high persistence in environmental matrices, explain that in Catalonia, many episodes of pollution originating from the past, where a lack of environmental awareness and the absence of appropriate legislation enabled malpractice in the management of these compounds. Also, their intrinsic characteristics explain the long life of the sources and associated plumes affecting both groundwater and soil. This PhD thesis has as main objectives the characterization of the biogeochemical processes and microbial communities in the vicinity of the polluting source in a context of alluvial fans. In addition, areas of particular microbial activity, especially ecotones, and the extension of biogeochemical processes along the plume are characterized. Another objective is the qualitative and quantitative integration of the elements that define the various heterogeneities, in order to understand the complexity of the environment, and to analyze the feasibility of implementing comprehensive remediation strategies that allow decontamination area the source. The integrated analysis of geological, hydrogeological, biogeochemical and microbiological heterogeneities of the subsoil materials has allowed determining how these heterogeneities control the distribution of chloroethenes, processes of mass transfer and degradation processes, while also conditioning diversity, the degree of development and structure of microbial communities. Characterization of dehalogenation processes has been studied in the matrix sediment and matrix water from four different approaches: 1) quantification of perchlorethylene (PCE) and metabolites; 2) determination of the isotopic composition of PCE from compound specific isotope analysis (CSIA) in both porewater and groundwater; 3) the identification of terminal electron acceptor processes; and 4) the analysis of microbial communities

Nitrate removal by combining chemical and biostimulation approaches using micro-zero valent iron and lactic acid

The occurrence of nitrate is the most significant type of pollution affecting groundwater globally, being a major contributor to the poor condition of water bodies. This pollution is related to livestock-agricultural and urban activities, and the nitrate presence in drinking water has a clear impact on human health. For example, it causes the blue child syndrome. Moreover, the high nitrate content in aquifers and surface waters significantly affects aquatic ecosystems since it is responsible for the eutrophication of surface water bodies. A treatability test was performed in the laboratory to study the decrease of nitrate in the capture zone of water supply wells. For this purpose, two boreholes were drilled from which groundwater and sediments were collected to conduct the test. The goal was to demonstrate that nitrate in groundwater can be decreased much more efficiently using combined abiotic and biotic methods with micro-zero valent iron and biostimulation with lactic acid, respectively, than when both strategies are used separately. The broader implications of this goal derive from the fact that the separate use of these reagents decreases the efficiency of nitrate removal. Thus, while nitrate is removed using micro-valent iron, high concentrations of harmful ammonium are also generated. Furthermore, biostimulation alone leads to overgrowth of other microorganisms that do not result in denitrification, therefore complete denitrification requires more time to occur. In contrast, the combined strategy couples abiotic denitrification of nitrate with biostimulation of microorganisms capable of biotically transforming the abiotically generated harmful ammonium. The treatability test shows that the remediation strategy combining in situ chemical reduction using micro-zero valent iron and biostimulation with lactic acid could be a viable strategy for the creation of a reactive zone around supply wells located in regions where groundwater and porewater in low permeability layers are affected by diffuse nitrate contamination

Key factors controlling microbial distribution on a DNAPL source area

Chlorinated solvents are among the common groundwater contaminants that show high complexity in their distribution in the subsoil. Microorganisms play a vital role in the natural attenuation of chlorinated solvents. Thus far, how the in situ soil microbial community responds to chlorinated solvent contamination has remained unclear. In this study, the microbial community distribution within two boreholes located in the source area of perchloroethene (PCE) was investigated via terminal restriction fragment length polymorphism (T-RFLP) and clone library analysis. Microbial data were related to the lithological and geochemical data and the concentration and isotopic composition of chloroethenes to determine the key factors controlling the distribution of the microbial communities. The results indicated that Proteobacteria, Actinobacteria, and Firmicutes were the most abundant phylums in the sediment. The statistical correlation with the environmental data proved that fine granulometry, oxygen tolerance, terminal electron-acceptor processes, and toxicity control microbial structure. This study improves our understanding of how the microbial community in the subsoil responds to high concentrations of chlorinated solvents