Multi tracer test for the implementation of enhanced in-situ bioremediation at a BTEX-contaminated megasite

Evidence Based Estimation of Macrodispersivity for Groundwater Transport Applications

The scope of this work is to discuss the proper choice of macrodispersion coefficients in modeling contaminant transport through the advection dispersion equation (ADE). It is common to model solute concentrations in transport by groundwater with the aid of the ADE. Spreading is quantified by macrodispersivity coefficients, which are much larger than the laboratory observed pore-scale dispersivities. In the frame of stochastic theory, longitudinal macrodispersivity is related to the hydraulic conductivity spatial variability via its statistical moments (mean, variance, integral scales), which are generally determined by geostatistical analysis of field measurements. In many cases, especially for preliminary assessment of contaminant spreading, these data are not available and ad hoc values are adopted by practitioners. The present study aims at recommending dispersivity values based on a thorough analysis of tens of field experiments. Aquifers are classified as of weak, medium, and high heterogeneity and for each class a range of macrodispersivity values is recommended. Much less data are available for the transverse macrodispersivities, which are significantly smaller than the longitudinal one. Nevertheless, a few realistic values based on field data, are recommended for applications. Transport models using macrodispersivities can predict mean concentrations, different from the local ones. They can be used for estimation of robust measures, like plumes spatial moments, longitudinal mass distribution and breakthrough curves at control planes.

Soil and Groundwater Investigation for Sustainable Agricultural Development: A Case Study from Brunei Darussalam

Brunei has made significant progress in agricultural development over the past four decades. However, crop production, such as rice, is still insufficient to meet the demands of the growing population, mainly due to the scarcity of water resources, acidic soils, and advancing environmental deterioration. This study conducted soil and groundwater investigations at three selected rice cultivation sites in Brunei Darussalam. The geoelectrical resistivity method using the vertical electrical sounding (VES) technique delineated subsurface stratigraphy with resistivities ranging from 1 to 180 Ωm in the investigated area. A potential aquifer zone was inferred in one of the surveyed areas with resistivities ranging from 10 to 150 Ωm at depths of about 40 to 60 m below ground level. Groundwater levels were also shallower in irrigated sites compared to non-irrigated sites. Groundwater physicochemical properties revealed high alkalinity and mineralisation, as evident from high magnesium, bicarbonate, ammoniacal nitrogen, and iron, as well as pH values in the water samples, believed to be the effects of leaching of salt ions into the shallow aquifer through infiltration recharge. In addition, soil physicochemical properties showed that the soils near a pumping well were more acidic, had lower moisture, and total nutrients, likely due to the effects of groundwater pumping. Our results demonstrate that future irrigation activities need to consider local aquifer characteristics, groundwater flow paths, and nutrient transport to ensure sustainable development in agricultural areas.

Benzene degradation in contaminated aquifers: Enhancing natural attenuation by injecting nitrate

Natural attenuation processes depend on the availability of suitable electron acceptors. At the megasite Zeitz, concentrations of the main contaminant benzene were observed to increase constantly in the lower aquifer to levels of more than 2.5 mM. This was accompanied by decreasing concentrations of sulphate (SO₄^2-), which has been previously shown to be the main electron acceptor for benzene oxidation at this site, resulting in an electron acceptor-limited, sulphidic benzene plume. Therefore, a field experiment was conducted to stimulate benzene biodegradation by injecting nitrate (NO₃^-) into the sulphidic benzene plume aiming (i) to recycle sulphate by nitrate-dependent sulphide oxidation, and (ii) to serve as direct electron acceptor for benzene oxidation. Within 60 days, 6.74 tons sodium nitrate (NaNO₃) were injected into the lower aquifer, and the resulting biogeochemical effects within the benzene plume were monitored for more than one year by chemical and microbiological analyses of groundwater samples taken from various depths of ten monitoring wells located in three observation lines downstream of nitrate injection. Nitrate was microbiologically consumed, as shown by changes in δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- values, partial nitrite accumulation, and changing ratios of Na⁺/NO₃^-. Main electron donors for nitrate reduction were reduced sulphur compounds, verified by changing δ³⁴S-SO₄^2- and δ¹⁸O-SO₄^2- values, partially increasing sulphate concentrations, and strongly increasing abundances of typical sulphur-oxidizing, nitrate-reducing bacterial taxa within the nitrate plume. The general absent hydrogen isotope fractionation of benzene, also in the sulphidic, nitrate-free part of the plume, indicates that benzene was not biodegraded by sulphate-reducing consortia. However, detected small carbon isotope fractionation of benzene points to in situ benzene biodegradation processes in the plume, probably supported by nitrate. In conclusion, nitrate injection resulted in changing redox conditions and recycling of sulphate in the sulphidic, sulphate-depleted benzene plume due to microbial oxidation of reduced sulphur species, leading to presumably favored conditions for in situ benzene biodegradation.

Stable sulfur and oxygen isotope fractionation of anoxic sulfide oxidation by two different enzymatic pathways

The microbial oxidation of sulfide is a key reaction of the microbial sulfur cycle, recycling sulfur in its most reduced valence state back to more oxidized forms usable as electron acceptors. Under anoxic conditions, nitrate is a preferential electron acceptor for this process. Two enzymatic pathways have been proposed for sulfide oxidation under nitrate reducing conditions, the sulfide:quinone oxidoreductase (SQR) pathway and the Sox (sulfur oxidation) system. In experiments with the model strains Thiobacillus denitrificans and Sulfurimonas denitrificans, both pathways resulted in a similar small sulfur and oxygen isotope fractionation of -2.4 to -3.6‰ for (34)S and -2.4 to -3.4‰ for (18)O. A similar pattern was detected during the oxidation of sulfide in a column percolated with sulfidic, nitrate amended groundwater. In experiments with (18)O-labeled water, a strong oxygen isotope fractionation was observed for T. denitrificans and S. denitrificans, indicating a preferential incorporation of (18)O-depleted oxygen released as water by nitrate reduction to nitrogen. The study indicates that nitrate-dependent sulfide oxidation might be monitored in the environment by analysis of (18)O-depleted sulfate.

Phylogenetic and functional diversity within toluene-degrading, sulphate-reducing consortia enriched from a contaminated aquifer

Three toluene-degrading microbial consortia were enriched under sulphate-reducing conditions from different zones of a benzene, toluene, ethylbenzene and xylenes (BTEX) plume of two connected contaminated aquifers. Two cultures were obtained from a weakly contaminated zone of the lower aquifer, while one culture originated from the highly contaminated upper aquifer. We hypothesised that the different habitat characteristics are reflected by distinct degrader populations. Degradation of toluene with concomitant production of sulphide was demonstrated in laboratory microcosms and the enrichment cultures were phylogenetically characterised. The benzylsuccinate synthase alpha-subunit (bssA) marker gene, encoding the enzyme initiating anaerobic toluene degradation, was targeted to characterise the catabolic diversity within the enrichment cultures. It was shown that the hydrogeochemical parameters in the different zones of the plume determined the microbial composition of the enrichment cultures. Both enrichment cultures from the weakly contaminated zone were of a very similar composition, dominated by Deltaproteobacteria with the Desulfobulbaceae (a Desulfopila-related phylotype) as key players. Two different bssA sequence types were found, which were both affiliated to genes from sulphate-reducing Deltaproteobacteria. In contrast, the enrichment culture from the highly contaminated zone was dominated by Clostridia with a Desulfosporosinus-related phylotype as presumed key player. A distinct bssA sequence type with high similarity to other recently detected sequences from clostridial toluene degraders was dominant in this culture. This work contributes to our understanding of the niche partitioning between degrader populations in distinct compartments of BTEX-contaminated aquifers.

Delineation of subsurface hydrocarbon contamination at a former hydrogenation plant using spectral induced polarization imaging

Broadband spectral induced polarization (SIP) measurements were conducted at a former hydrogenation plant in Zeitz (NE Germany) to investigate the potential of SIP imaging to delineate areas with different BTEX (benzene, toluene, ethylbenzene, and xylene) concentrations. Conductivity images reveal a poor correlation with the distribution of contaminants; whereas phase images exhibit two main anomalies: low phase shift values (<5 mrad) for locations with high BTEX concentrations, including the occurrence of free-phase product (BTEX concentrations >1.7 g/l), and higher phase values for lower BTEX concentrations. Moreover, the spectral response of the areas with high BTEX concentration and free-phase products reveals a flattened spectrum in the low frequencies (<40 Hz), while areas with lower BTEX concentrations exhibit a response characterized by a frequency peak. The SIP response was modelled using a Debye decomposition to compute images of the median relaxation-time. Consistent with laboratory studies, we observed an increase in the relaxation-time associated with an increase in BTEX concentrations. Measurements were also collected in the time domain (TDIP), revealing imaging results consistent with those obtained for frequency domain (SIP) measurements. Results presented here demonstrate the potential of the SIP imaging method to discriminate source and plume of dissolved contaminants at BTEX contaminated sites.

Protein-SIP enables time-resolved analysis of the carbon flux in a sulfate-reducing, benzene-degrading microbial consortium

Benzene is a major contaminant in various environments, but the mechanisms behind its biodegradation under strictly anoxic conditions are not yet entirely clear. Here we analyzed a benzene-degrading, sulfate-reducing enrichment culture originating from a benzene-contaminated aquifer by a metagenome-based functional metaproteomic approach, using protein-based stable isotope probing (protein-SIP). The time-resolved, quantitative analysis of carbon fluxes within the community supplied with either (13)C-labeled benzene or (13)C-labeled carbonate yielded different functional groups of organisms, with their peptides showing specific time dependencies of (13)C relative isotope abundance indicating different carbon utilization. Through a detailed analysis of the mass spectrometric (MS) data, it was possible to quantify the utilization of the initial carbon source and the metabolic intermediates. The functional groups were affiliated to Clostridiales, Deltaproteobacteria and Bacteroidetes/Chlorobi. The Clostridiales-related organisms were involved in benzene degradation, putatively by fermentation, and additionally used significant amounts of carbonate as a carbon source. The other groups of organisms were found to perform diverse functions, with Deltaproteobacteria degrading fermentation products and Bacteroidetes/Chlorobi being putative scavengers feeding on dead cells. A functional classification of identified proteins supported this allocation and gave further insights into the metabolic pathways and the interactions between the community members. This example shows how protein-SIP can be applied to obtain temporal and phylogenetic information about functional interdependencies within microbial communities.

Integrating fluorescent dye flow-curve testing and acoustic Doppler velocimetry profiling for in situ hydraulic evaluation and improvement of clarifier performance

Enhancing the performance of clarifiers requires a thorough understanding of their hydraulics. Fluorescence spectroscopy and acoustic doppler velocimeter (ADV) profiling generally have been used separately to evaluate secondary settlers. We propose that simultaneous use of these techniques is needed to obtain a more reliable and useful evaluation. Experiments were performed on laboratory- and full-scale clarifiers. Factors affecting Fluorescein and Rhodamine 6G properties were identified. Underestimations up to 500% in fluorescence intensities may be derived from differential fluorescence quenching by oxygen. A careful control and interpretation of fluorescent dye experiments is needed to minimize artifacts in real settings. While flow-curve tests constructed under controlled conditions provided a more accurate overall quantitative estimation of the hydraulic performance, ADV velocity and turbulence profiling provided a detailed spatial understanding of flow patterns that was used to troubleshoot and fix the causes of hydraulic short-circuits.

Current approaches for the assessment of in situ biodegradation

Considering the high costs and technical difficulties associated with conventional remediation strategies, in situ biodegradation has become a promising approach for cleaning up contaminated aquifers. To verify if in situ biodegradation of organic contaminants is taking place at a contaminated site and to determine if these processes are efficient enough to replace conventional cleanup technologies, a comprehensive characterization of site-specific biodegradation processes is essential. In recent years, several strategies including geochemical analyses, microbial and molecular methods, tracer tests, metabolite analysis, compound-specific isotope analysis, and in situ microcosms have been developed to investigate the relevance of biodegradation processes for cleaning up contaminated aquifers. In this review, we outline current approaches for the assessment of in situ biodegradation and discuss their potential and limitations. We also discuss the benefits of research strategies combining complementary methods to gain a more comprehensive understanding of the complex hydrogeological and microbial interactions governing contaminant biodegradation in the field.

Improving protein extraction and separation methods for investigating the metaproteome of anaerobic benzene communities within sediments

BTEX compounds such as benzene are frequent soil and groundwater contaminants that are easily biodegraded under oxic conditions by bacteria. In contrast, benzene is rather recalcitrant under anaerobic conditions. The analysis of anoxic degradation is often hampered by difficult sampling conditions, limited amounts of biomass and interference of matrix compounds with proteomic approaches. In order to improve the procedure for protein extraction we established a scheme consisting of the following steps: dissociation of cells from lava granules, cell lysis by ultrasonication and purification of proteins by phenol extraction. The 2D-gels revealed a resolution of about 240 proteins spots and the spot patterns showed strong matrix dependence, but still differences were detectable between the metaproteomes obtained after growth on benzene and benzoate. Using direct data base search as well as de novo sequencing approaches we were able to identify several proteins. An enoyl-CoA hydratase with cross species homology to Azoarcus evansii, is known to be involved in the anoxic degradation of xenobiotics. Thereby the identification confirmed that this procedure has the capacity to analyse the metaproteome of an anoxic living microbial community.

In situ bioremediation of monoaromatic pollutants in groundwater: a review

Monoaromatic pollutants such as benzene, toluene, ethylbenzene and mixture of xylenes are now considered as widespread contaminants of groundwater. In situ bioremediation under natural attenuation or enhanced remediation has been successfully used for removal of organic pollutants, including monoaromatic compounds, from groundwater. Results published indicate that in some sites, intrinsic bioremediation can reduce the monoaromatic compounds content of contaminated water to reach standard levels of potable water. However, engineering bioremediation is faster and more efficient. Also, studies have shown that enhanced anaerobic bioremediation can be applied for many BTEX contaminated groundwaters, as it is simple, applicable and economical. This paper reviews microbiology and metabolism of monoaromatic biodegradation and in situ bioremediation for BTEX removal from groundwater under aerobic and anaerobic conditions. It also discusses the factors affecting and limiting bioremediation processes and interactions between monoaromatic pollutants and other compounds during the remediation processes.

Molecular characterization of bacterial communities mineralizing benzene under sulfate-reducing conditions

The microbial communities of in situ reactor columns degrading benzene with sulfate as an electron acceptor were analyzed based on clone libraries and terminal restriction fragment length polymorphism fingerprinting of PCR-amplified 16S rRNA genes. The columns were filled with either lava granules or sand particles and percolated with groundwater from a benzene-contaminated aquifer. The predominant organisms colonizing the lava granules were related to Magnetobacterium sp., followed by a phylotype affiliated to the genera Cryptanaerobacter/Pelotomaculum and several Deltaproteobacteria. From the sand-filled columns, a stable benzene-degrading consortium was established in sand-filled laboratory microcosms under sulfate-reducing conditions. It was composed of Delta- and Epsilonproteobacteria, Clostridia, Chloroflexi, Actinobacteria and Bacteroidetes. The most prominent phylotype of the consortium was related to the genus Sulfurovum, followed by Desulfovibrio sp. and the Cryptanaerobacter/Pelotomaculum phylotype. The proportion of the latter was similar in both communities and significantly increased after repeated benzene-spiking. During cultivation on aromatic substrates other than benzene, the Cryptanaerobacter/Pelotomaculum phylotype was outcompeted by other community members. Hence, this organism appears to be specific for benzene as a growth substrate and might play a key role in benzene degradation in both communities. Based on the possible functions of the community members and thermodynamic calculations, a functional model for syntrophic benzene degradation under sulfate-reducing conditions is proposed.

Enrichment of anaerobic benzene-degrading microorganisms by in situ microcosms

Microcosms filled with different solids (sand, lava, Amberlite XAD-7) were exposed for 67 days in the sulfidic part of a groundwater monitoring well downstream of the source zone of a benzene-contaminated aquifer and subsequently incubated in the laboratory. Benzene was repeatedly degraded in several microcosms accompanied by production of sulfide, leading to stable benzene-degrading enrichment cultures. In control microcosms without filling material, benzene was initially degraded, but the benzene-degrading capacity could not be sustained. The results indicate that long-term physiologically active benzene-degrading microorganisms were attached to surfaces of the solids. The biodiversity and attachment behavior of microorganisms in the in situ microcosms was assessed by confocal laser scanning microscopy and single-strand conformation polymorphism (SSCP) analysis, followed by sequencing of dominant SSCP bands. The microbial community was composed of several different Bacteria, representing members of Clostridia, Bacteroidales, all subgroups of the Proteobacteria, Verrucomicrobia, Nitrospira, Chloroflexi and Chlorobi. Only a few archaeal sequences could be retrieved from the communities. The majority of phylotypes were affiliated to bacterial groups with a possible functional relationship to the bacterial sulfur cycle, thus indicating that the microbial community in the investigated aquifer zone depends mainly on inorganic sulfur compounds as electron donors or acceptors, a finding that corresponds to the geochemical data.

Benzene oxidation under sulfate-reducing conditions in columns simulating in situ conditions

The oxidation of benzene under sulfate-reducing conditions was examined in column and batch experiments under close to in situ conditions. Mass balances and degradation rates for benzene oxidation were determined in four sand and four lava granules filled columns percolated with groundwater from an anoxic benzene-contaminated aquifer. The stoichiometry of oxidized benzene, produced hydrogen carbonate and reduced sulfate correlated well with the theoretical equation for mineralization of benzene with sulfate as electron acceptor. Mean retention times of water in four columns were determined using radon ((222)Rn) as tracer. The retention times were used to calculate average benzene oxidation rates of 8-36 microM benzene day(-1). Benzene-degrading, sulfide-producing microcosms were successfully established from sand material of all sand filled columns, strongly indicating that the columns were colonized by anoxic benzene-degrading microorganisms. In general, these data indicate a high potential for Natural Attenuation of benzene under sulfate-reducing conditions at the field site Zeitz. In spite of this existing potential to degrade benzene with sulfate as electron acceptor, the benzene plume at the field site is much longer than expected if benzene would be degraded at the rates observed in the column experiment, indicating that benzene oxidation under sulfate-reducing conditions is limited in situ.