Biogeochemistry and isotope geochemistry of a landfill leachate plume

Using a combination of δ13CDIC-DOC-difference in dissolved inorganic and organic carbon, δ2H, and δ18O to localize leachate leaks at landfill sites in China

The investigation of leachate leakage at numerous landfill sites is urgently needed. This study presents an exploration of environmental tracing methods using δ2H and δ13C-difference in dissolved carbon (δ13CDIC-DOC) to localize leachate leak points at landfill sites. δ2H, δ13CDIC, δ13CDOC, δ18O, and an array of physicochemical indices (e.g., total dissolved solids, temperature, and oxidation reduction potential) were monitored in both leachate and groundwater from different zones of a landfill site in China during the year of 2021-2023. Moreover, data for these parameters (i.e., the isotopic composition and physicochemical indices) from twelve published landfill cases were also collected, and these groundwater/leachate data points were located within 1 km away from the landfill boundary. Then statistical analyses, such as Pearson correlation analysis and redundancy analysis (RDA), were performed using both the detected and collected parameters at landfill sites. Consequently, the intensity of interaction between leachate and background groundwater was found to significantly control the isotopic fractionation features of hydrogen and carbon, and both the content of major contamination indicators (total dissolved solids, chemical oxygen demand, and ammoniacal nitrogen) and the oxidation reduction potential were the key impact factors. Accordingly, the water type used to indicate leachate leakage points was determined to be leachate that significantly interacted with the background groundwater or precipitation (LBGP). δ2H showed a perfect linear correlation (0.81 ≤ r2 < 1.0) with δ13CDIC-DOC in leachate under highly anaerobic landfill conditions, and the δ2H & δ13CDIC-DOC combinations in the LBGP were significantly different from those in the other water types. For groundwater with total dissolved solids lower than 1400 mg/L at landfill sites, a strong positive linear correlation (r = 0.83) was revealed between δ13CDIC and δ13CDOC. Based on these insights, δ2H versus δ13CDIC-DOC plots and RDA using δ2H and δ13CDIC-DOC as response variables were proposed to localize leak points at both lined landfills and leachate facilities. These findings further understanding of the isotopic fractionation features of hydrogen, carbon, and oxygen and provide novel environmental tracer methods for investigating leachate leak points at MSW landfill sites.

Tracing sulfate sources in a tropical agricultural catchment with a stable isotope Bayesian mixing model

Sulfate (SO42-) is an essential anion in drinking water and a vital macronutrient for plant growth. However, elevated sulfate levels can impact ecosystem or human health and could be an important indicator of acid rock drainage or pollution. Therefore, monitoring SO42- sources and transport is important for water quality assessments. This study focused on exploring the sources and transformations of SO42- as well as estimating the proportional contribution of the potential SO42- pollutant sources to groundwater and surface water in a tropical river basin, the Densu River Basin. The study used major ions combined with stable sulfur and oxygen isotope compositions and a Bayesian isotope mixing model, MixSIAR. The major ion characteristics indicate that SO42- concentrations remain stable throughout the rainy and dry seasons but originate from diverse sources. The multi-isotope model (δ34SSO4, δ18OSO4) identified four potential SO42- sources: detergent, precipitation, sewage, and sulfate fertilizer. However, the δ34SSO4 and δ18OSO4 values of the fertilizer source signatures overlapped with those of precipitation and sewage. Nevertheless, the contributions from each source were disentangled using the MixSIAR model, which revealed sewage as the most dominant SO42- pollutant in the Densu Basin, accounting for ~47 % of sulfate in groundwater and ~ 56 % of sulfate in surface water. Sulfate fertilizer (~33 %) was the second most important source after sewage for groundwater, while detergent (~23 %) was the second most important source for surface water. The redox processes of bacterial sulfate reduction and sulfide oxidation were determined to have a minimal impact on the sulfur isotope fractionation within the basin. This study highlights the benefits of combining major ions, sulfur isotopes and the MixSIAR model for identifying sources of sulfate. This approach accounts for uncertainties in source contributions which allows for more robust and reliable apportionment of sulfate sources. The study emphasizes the need for effective waste management and pollution control measures to protect water quality and provides vital guidelines on how to partition sulfate sources on a large catchment scale and evidence for making pollution management decisions on water resources.

A comprehensive monitoring approach for a naturally anoxic aquifer beneath a controlled landfill

The processes leading to high levels of arsenic (As), iron (Fe), and manganese (Mn) in groundwater, in a naturally reducing aquifer at a controlled municipal landfill site, are investigated. The challenge is to distinguish the natural water-rock interaction processes, that allow these substances to dissolve in groundwater, from direct pollution or enhanced dissolution of hydroxides as undesired consequences of the anthropic activities above. Ordinary groundwater monitoring of physical-chemical parameters and inorganic compounds (major and trace elements) was complemented by environmental isotopes of groundwater (tritium, deuterium, oxygen-18 and carbon-13) and dissolved gases (carbon-13 of methane and carbon dioxide and carbon-14 of methane). Pearson/Spearman correlation indices, as well as Principal Component Analysis (PCA), were used to determine the main correlations among variables. The concurrent presence of As, Fe and CH4, as reported in similar anoxic environments, suggests that anaerobic oxidation of methane could drive the reductive dissolution of As-rich Fe(III)(hydro)oxides. Manganese is more sensitive to carbon dioxide, possibly due to a decrease in pH which accelerates the dissolution of Mn-oxides. Finally, we found that tritium and deuterium, which have been used for decades as leachate tracer in groundwater, may be subject to false positives due to the reuse of water recovered from leachate treatment (which has the same isotopic signature of leachate) within the plants, to comply with the requirements of the circular economy. The integration of the environmental isotope analysis into the traditional monitoring approach can effectively support the comprehension of processes. However, this strategy needs to be complemented by a good conceptual hydrogeological model and expert evaluation to avoid misinterpretations.

Anaerobic oxidation of methane in landfill and adjacent groundwater environments: Occurrence, mechanisms, and potential applications

Landfills remain the predominant means of solid waste management worldwide. Widespread distribution and significant stockpiles of waste in landfills make them a significant source of methane emissions, exacerbating climate change. Anaerobic oxidation of methane (AOM) has been shown to play a critical role in mitigating methane emissions on a global scale. The rich methane and electron acceptor environment in landfills provide the necessary reaction conditions for AOM, making it a potentially low-cost and effective strategy for reducing methane emissions in landfills. However, compared to other anaerobic habitats, research on AOM in landfill environments is scarce, and there is a lack of analysis on the potential application of AOM in different zones of landfills. Therefore, this review summarizes the existing knowledge on AOM and its occurrence in landfills, analyzes the possibility of AOM occurrence in different zones of landfills, discusses its potential applications, and explores the challenges and future research directions for AOM in landfill management. The identification of research gaps and future directions outlined in this review encourages further investigation and advancement in the field of AOM, paving the way for more effective waste stabilization, greenhouse gas reduction, and pollutant mitigation strategies in landfills.

Potential for Natural Attenuation of Domestic and Agricultural Pollution in Karst Groundwater Environments

In karst areas, anthropogenic contaminants reach the subsurface with detrimental effects on the groundwater ecosystem and downstream springs, which often serve as drinking water sources for the local human communities. We analyzed the water chemistry and microbial community composition in upstream and downstream locations of five hydrokarst systems (HKS) during four seasons. Conductivity and nitrates were higher in the downstream springs than in the pre-karst waters, whereas the concentration of organic matter, considered here as a pollution indicator, was lower. The microbial community composition varied largely between upstream and downstream locations, with multiple species of potentially pathogenic bacteria decreasing in the HKS. Bacteria indicative of pollution decreased as well when passing through the HKS, but potential biodegraders increased. This suggests that the HKS can filter out part of the polluting organic matter and, with it, part of the associated microorganisms. Nevertheless, the water quality, including the presence of pathogens in downstream springs, must be further monitored to control whether the water is appropriate for consumption. In parallel, the human populations located upstream must be advised of the risks resulting from their daily activities, improper stocking of their various wastes and dumping of their refuse in surface streams.

New method for simultaneous determination of dissolved organic carbon and its stable carbon isotope ratio in liquid samples: environmental applications

This study reports the development of an all-in-one elemental analyser isotope ratio mass spectrometry (EA-IRMS) system modified for simultaneous analysis of dissolved organic carbon (DOC) concentration and its stable carbon isotope footprint (δ¹³C_DOC) in aqueous samples. The method involves a quantitative oxidation of DOC in a 200 µL liquid sample to CO₂, after sample acidification and stripping by nitrogen. The detection limit of the method for DOC quantification was 0.2 mg C/L with an analytical precision of 12 %. Uncertainty of stable isotope determinations was 2 % at 0.2 mg DOC/L, while decreasing to 0.3 % at 20 mg DOC/L. Quantitative oxidation of DOC in aqueous samples was validated by using ring test water samples and Deep Sea reference seawater. The method performances of isotope analysis were evaluated by analysing different isotopic standard solutions. The applicability of the method was tested through the analysis of different environmental types of water, showing that δ¹³C_DOC ranged from - 23.30 to -31.85 ‰, allowing to characterize samples of different environmental origin. The developed method offers several advantages including rapidity, use of small sample volumes and minimal sample pre-treatment, making it a valuable tool for routine DOC concentration measurements paired with isotopic characterization.

Application of stable isotopes and dissolved ions for monitoring landfill leachate contamination

We evaluated groundwater contamination by landfill leachate at a municipal landfill and characterized isotopic and hydrogeochemical evidence of the degradation and natural attenuation of buried organic matter at the study site. Dissolved ion content was generally much higher in the leachate than in the surrounding groundwater. The leachate was characterized by highly elevated bicarbonate and ammonium levels and a lack of nitrate and sulfate, indicating generation under anoxic conditions. Leachate δD and δ¹³C_DIC values were much higher than those of the surrounding groundwater; some groundwater samples near the landfill showed a significant contamination by the leachate plume. Hydrochemical characteristics of the groundwater suggest that aquifer geology in the study area plays a key role in controlling the natural attenuation of leachate plumes in this oxygen-limited environment.

Identification of groundwater pollution sources in a landfill site using artificial sweeteners, multivariate analysis and transport modeling

In this study, sources of groundwater pollution in a landfill site were identified, using artificial sweeteners as chemical tracers, multivariate statistical analysis and a quantitative analysis of the groundwater flow system through particle tracking and transport modeling. The study area, located in northern Italy, hosts an older unlined landfill and a newer lined municipal solid waste landfill placed downstream of the former. Groundwater, surface water, treated wastewater, and leachate samples were collected in March 2017 for analysis of the artificial sweeteners saccharin, cyclamate, acesulfame and sucralose together with major cations and anions, inorganic nitrogen compounds, total phosphorus, COD and some further parameters. The interpretation of the results suggests that two main leachate leaks/spills are affecting the study area. The first one concerns leachate probably spilling out of the leachate collection system serving the younger lined landfill, the other one involves leachate from the older unlined landfill that also seems to affect an area downstream of the lined landfill. Direct leachate leaks from the lined landfill seem unlikely, although they cannot be definitively excluded. This work underlines the importance of a multi-methods approach, which integrates here chemical tracers, multivariate analysis and transport modeling, for assessing groundwater pollution sources generated from complex landfill sites, where multiple and different sources may exist. In particular, this work highlights how artificial sweeteners can be used for tracing leachate plumes from landfills. The methodology applied in this study can have a broad applicability also in other polluted landfill sites worldwide.

Rural solid waste-characteristics and leachate pollution assessment for different precipitation levels, China

Open dumping adversely affects the environment and remains the most widely used method for waste disposal in many developing rural areas in China. Information regarding the impact of rural solid waste (RSW) on the environment remains limited. The objectives of this study are to investigate the characteristics of RSW and the impact of different precipitation rates, and to evaluate the contamination potential of RSW using a leachate pollution index (LPI). The study showed that leachate concentration was significantly influenced by precipitation rates at the initial precipitation stages. Precipitation rates of 42.00 mm/day appeared to have the largest dilution effects. In contrast, the concentrations of leachate at rainfall rates of 24.00 mm/day and soaking were steady, and no similar trends were observed. The highest amounts of pollutants in leachate were the result of soaking. In the first week of our experiment, the LPI value for each rural area waste sample rapidly increased with rising precipitation rates from soaking to 42.00 mm/day. However, no significant change in LPI was observed thereafter (after 5 weeks) even with increasing precipitation rates. The values of chemical oxygen demand, biochemical oxygen demand, total nitrogen, and NH₃-N in the leachate after 10 weeks were 4.00, 7.34, 1.87, and 2.21 times higher, respectively, than those of the prescribed leachate quality standards in China. The results of our study suggest the following course of action for the three dump sites investigated: in Banqiao, given the size of the population and the size of the waste amount, landfill might be a suitable way for disposing of RSW. In Machen, building a standardized waste collection site would be an economical solution for reducing potential pollution risks. In Jiuduhe, increasing the transportation rate of solid waste might be an effective solution. The results of this study can help to improve the understanding of leachate pollution in Chinese rural areas.

Environmental monitoring of a landfill area through the application of carbon stable isotopes, chemical parameters and multivariate analysis

Leachate produced during an organic matter decomposition process has a complex composition and can cause contamination of surface and groundwaters adjacent to a landfill area. The monitoring of these areas is extremely important for the characterization of the leachate produced and to avoid or mitigate environmental damages. Thus, the present study has the objective of monitoring the area of a Brazilian landfill using conventional parameters (dissolved metals and anions in water) and alternative, stable carbon isotopes parameters (δ¹³C of dissolved organic and inorganic carbons in water) in addition to multivariate analysis techniques. The use of conventional and alternative parameters together with multivariate analysis showed that cells of the residues are at different phases of stabilization of the organic matter and probably already at C3 of the methanogenic phase of decomposition. In addition, the data showed that organic matter stabilization ponds present in the landfill are efficient and improve the quality of the leachate. Enrichment of the heavy ¹³C isotope in both surface and groundwater suggested contamination in two sampling sites.

Response and recovery of a pristine groundwater ecosystem impacted by toluene contamination - A meso-scale indoor aquifer experiment

Microbial communities are the driving force behind the degradation of contaminants like aromatic hydrocarbons in groundwater ecosystems. However, little is known about the response of native microbial communities to contamination in pristine environments as well as their potential to recover from a contamination event. Here, we used an indoor aquifer mesocosm filled with sandy quaternary calciferous sediment that was continuously fed with pristine groundwater to study the response, resistance and resilience of microbial communities to toluene contamination over a period of almost two years, comprising 132days of toluene exposure followed by nearly 600days of recovery. We observed an unexpectedly high intrinsic potential for toluene degradation, starting within the first two weeks after the first exposure. The contamination led to a shift from oxic to anoxic, primarily nitrate-reducing conditions as well as marked cell growth inside the contaminant plume. Depth-resolved community fingerprinting revealed a low resistance of the native microbial community to the perturbation induced by the exposure to toluene. Distinct populations that were dominated by a small number of operational taxonomic units (OTUs) rapidly emerged inside the plume and at the plume fringes, partially replacing the original community. During the recovery period physico-chemical conditions were restored to the pristine state within about 35days, whereas the recovery of the biological parameters was much slower and the community composition inside the former plume area had not recovered to the original state by the end of the experiment. These results demonstrate the low resilience of sediment-associated groundwater microbial communities to organic pollution and underline that recovery of groundwater ecosystems cannot be assessed solely by physico-chemical parameters.

A preliminary report of indigenous fungal isolates from contaminated municipal solid waste site in India

Municipal solid waste (MSW) containing harmful substances is a major concern in waste management and can cause adverse effects on diversity of fungi in soil. The main objective was to evaluate the fungal diversity inhabiting in the soil nearby MSW disposal site. The fungal strains were isolated in potato dextrose agar (PDA), media at temperatures 28 ± 1 °C by using standard serial dilution pour plate method, and appeared fungal colonies identified based on morphological characteristics. The overall most fungal diversity was found in soil sample collected from S5, followed by S4, S3, S1, and least in S2 site. A total of 24 fungal isolates recovered from the different MSW sites and Aspergillus sp., Fusarium sp., and Curvularia sp. genus has isolated from all the samples. In addition, the metal tolerance index performed because it needs to classify the fungus for their best use as potential agent for environmental protection. The metal tolerance outcomes revealed that both metals (cadmium and chromium) has appeared as the highest growth inhibitor for most strains and even fungal colonies did not propagate very well on the surface of media. Therefore, these findings suggest that the pre-adapted indigenous fungal isolates have proven remarkable tolerance ability to both metals. Furthermore, these highly metal-tolerant fungal strains are recommended for detail research or can use in pilot-scale bioremediation application to treat contaminated site.

Identification of groundwater contamination zone around a reclaimed landfill using carbon isotopes

Chemical and isotopic analyses of groundwater from piezometers localized around a reclaimed landfill were performed in order to identify the boundaries of groundwater contamination zone. Spatial distribution of dissolved inorganic carbon (DIC) concentration and stable carbon isotopes in the groundwater was used to distinguish the piezometers localized within the contaminated aquifer. Background groundwater was characterized by low DIC concentration (from 1.8 to 5.0 mmol/L) and negative values of δ¹³C_DIC (from -20.6‰ to -12.4‰). Higher DIC concentrations (from 6.0 to 12.5 mmol/L) and higher values of δ¹³C_DIC (from -10.9 to +3.6‰) were determined in groundwater contaminated by landfill leachate. The study confirmed that δ¹³C_DIC value in the groundwater was a useful tracer in determining the extent of the contamination zone around the landfill. In general, upgradient from the landfill, carbon isotopic composition of groundwater depended on natural sources of carbon and δ¹³C_DIC values were negative. Downgradient from the landfill, where groundwater was contaminated by the landfill leachate, δ¹³C_DIC values were higher, sometimes even positive.

Determination of the origin of dissolved inorganic carbon in groundwater around a reclaimed landfill in Otwock using stable carbon isotopes

Chemical and isotopic analyses of groundwater from piezometers located around a reclaimed landfill in Otwock (Poland) were performed in order to trace the origin of dissolved inorganic carbon (DIC) in the groundwater. Due to differences in the isotopic composition of carbon from different sources, an analysis of stable carbon isotopes in the groundwater, together with the Keeling plot approach and a two-component mixing model allow us to evaluate the relative contributions of carbon from these sources in the groundwater. In the natural (background) groundwater, DIC concentrations and the isotopic composition of DIC (δ(13)CDIC) comes from two sources: decomposition of organic matter and carbonate dissolution within the aquifer sediments, whereas in the leachate-contaminated groundwater, DIC concentrations and δ(13)CDIC values depend on the degradation of organic matter within the aquifer sediments and biodegradation of organic matter stored in the landfill. From the mixing model, about 4-54% of the DIC pool is derived from organic matter degradation and 96-46% from carbonate dissolution in natural conditions. In the leachate-contaminated groundwater, about 20-53% of the DIC is derived from organic matter degradation of natural origin and 80-47% from biodegradation of organic matter stored in the landfill. Partial pressure of CO2 (P CO2) was generally above the atmospheric, hence atmospheric CO2 as a source of carbon in DIC pool was negligible in the aquifer. P CO2 values in the aquifer in Otwock were always one to two orders of magnitude above the atmospheric P CO2, and thus CO2 escaped directly into the vadose zone.

Stable isotope signatures for characterising the biological stability of landfilled municipal solid waste

Stable isotopic signatures of landfill leachates are influenced by processes within municipal solid waste (MSW) landfills mainly depending on the aerobic/anaerobic phase of the landfill. We investigated the isotopic signatures of δ(13)C, δ(2)H and δ(18)O of different leachates from lab-scale experiments, lysimeter experiments and a landfill under in situ aeration. In the laboratory, columns filled with MSW of different age and reactivity were percolated under aerobic and anaerobic conditions. In landfill simulation reactors, waste of a 25year old landfill was kept under aerobic and anaerobic conditions. The lysimeter facility was filled with mechanically shredded fresh waste. After starting of the methane production the waste in the lysimeter containments was aerated in situ. Leachate and gas composition were monitored continuously. In addition the seepage water of an old landfill was collected and analysed periodically before and during an in situ aeration. We found significant differences in the δ(13)C-value of the dissolved inorganic carbon (δ(13)C-DIC) of the leachate between aerobic and anaerobic waste material. During aerobic degradation, the signature of δ(13)C-DIC was mainly dependent on the isotopic composition of the organic matter in the waste, resulting in a δ(13)C-DIC of -20‰ to -25‰. The production of methane under anaerobic conditions caused an increase in δ(13)C-DIC up to values of +10‰ and higher depending on the actual reactivity of the MSW. During aeration of a landfill the aerobic degradation of the remaining organic matter caused a decrease to a δ(13)C-DIC of about -20‰. Therefore carbon isotope analysis in leachates and groundwater can be used for tracing the oxidation-reduction status of MSW landfills. Our results indicate that monitoring of stable isotopic signatures of landfill leachates over a longer time period (e.g. during in situ aeration) is a powerful and cost-effective tool for characterising the biodegradability and stability of the organic matter in landfilled municipal solid waste and can be used for monitoring the progress of in situ aeration.

Stable carbon isotope analysis of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in natural waters--results from a worldwide proficiency test

RATIONALE

Stable carbon isotope ratios of dissolved inorganic (DIC) and organic carbon (DOC) are of particular interest in aquatic geochemistry. The precision for this type of analysis is typically reported in the range of 0.1‰ to 0.5‰. However, there is no published attempt that compares δ(13)C measurements of DIC and DOC among different laboratories for natural water samples.

METHODS

Five natural water samples (lake water, seawater, two geothermal waters, and petroleum well water) were analyzed for δ(13)CDIC and δ(13)CDOC values by five laboratories with isotope ratio mass spectrometry (IRMS) in an international proficiency test.

RESULTS

The reported δ(13)CDIC values for lake water and seawater showed fairly good agreement within a range of about 1‰, whereas geothermal and petroleum waters were characterized by much larger differences (up to 6.6‰ between laboratories). δ(13)CDOC values were only comparable for seawater and showed differences of 10 to 21‰ for other samples.

CONCLUSIONS

This study indicates that scatter in δ(13)CDIC isotope data can be in the range of several per mil for samples from extreme environments (geothermal waters) and may not yield reliable information with respect to dissolved carbon (petroleum wells). The analyses of lake water and seawater also revealed a larger than expected difference and researchers from various disciplines should be aware of this. Evaluation of analytical procedures of the participating laboratories indicated that the differences cannot be explained by analytical errors or different data normalization procedures and must be related to specific sample characteristics or secondary effects during sample storage and handling. Our results reveal the need for further research on sources of error and on method standardization.

Effects of solution chemistry on the removal reaction between calcium carbonate-based materials and Fe(II)

Elevated iron concentrations have been observed in the groundwater underlying and surrounding several Florida landfill sites. An in situ groundwater remediation method for iron (present as soluble ferrous iron) using a permeable reactive barrier composed of calcium carbonate-based materials (CCBMs), such as limestone, was examined as a potentially effective and low-cost treatment technique. The effects of various environmental factors (i.e., pH, co-existing cations, and natural organic matter (NOM)) on the removal reaction were investigated using laboratory batch studies. Solution pH had a minor effect on iron removal, with superior iron removal observed in the highest pH solution (pH of 9). Sodium and calcium tended to impede the iron removal process by increasing the ionic strength of the solution. Manganese competes with iron ions at the adsorption sites on CCBMs; therefore, the presence of manganese prohibits iron removal and reduces removal effectiveness. NOM was found to decrease Fe(II) uptake by CCBMs and reduce the removal effectiveness by complexing Fe(II), most likely through the carboxyl group, thereby maintaining Fe(II) mobility in the aqueous phase.

Evidence for iron-mediated anaerobic methane oxidation in a crude oil-contaminated aquifer

In a methanogenic crude oil contaminated aquifer near Bemidji, Minnesota, the decrease in dissolved CH(4) concentrations along the groundwater flow path, along with the positive shift in δ(13) C(CH) (4) and negative shift in δ(13) C(DIC) , is indicative of microbially mediated CH(4) oxidation. Calculations of electron acceptor transport across the water table, through diffusion, recharge, and the entrapment and release of gas bubbles, suggest that these processes can account for at most 15% of the observed total reduced carbon oxidation, including CH(4) . In the anaerobic plume, the characteristic Fe(III)-reducing genus Geobacter was the most abundant of the microbial groups tested, and depletion of labile sediment iron is observed over time, confirming that reduced carbon oxidation coupled to iron reduction is an important process. Electron mass balance calculations suggest that organic carbon sources in the aquifer, BTEX and non-volatile dissolved organic carbon, are insufficient to account for the loss in sediment Fe(III), implying that CH(4) oxidation may also be related to Fe(III) reduction. The results support a hypothesis of Fe(III)-mediated CH(4) oxidation in the contaminated aquifer.

Using S and Pb isotope ratios to trace leaching of toxic substances from an acid-impacted industrial-waste landfill (Pozdatky, Czech Republic)

Slightly elevated concentrations of toxic species in waters sampled in the surroundings of a leaky landfill may be both a sign of an approaching contaminant plume, or a result of water-rock interaction. Isotopes can be instrumental in distinguishing between anthropogenic and geogenic species in groundwater. We studied sulfur and lead isotope ratios at an abandoned industrial-waste landfill, located in a densely populated part of Central Europe. Stable isotope variability in space and time was used to follow the movement of a groundwater plume, contaminated with toxic metals (Cd, Cr, Be), in fractured granitoids. Toxic metals had been mobilized from industrial waste by a strong pulse of sulfuric acid, also deposited in the landfill. Both tracers exhibited a wide range of values (δ(34)S between +2.6 and +18.9‰; (206)Pb/(207)Pb between 1.16 and 1.39), which facilitated identification of mixing end-members, and made it possible to assess the sources of the studied species. In situ fractionations did not hinder source apportionment. Influx of contaminated groundwater was observed neither in irrigation wells in a nearby village, nor at distances greater than 300 m from the landfill. Combination of stable isotope tracers can be used as part of an early-warning system in landscapes affected by landfills.

Environmental isotopes and major ions for tracing leachate contamination from a municipal landfill in Metro Manila, Philippines

The surface water and groundwater sources in the vicinity of a major municipal landfill in Metro Manila, Philippines were investigated to determine contamination by landfill leachate. Tritium, stable isotopes of hydrogen and oxygen, and major ions in the leachate and freshwater within the landfill environment were determined. The leachate contained elevated tritium activities and high concentrations of sodium, chloride, potassium, and calcium. The concentrations of tritium and the leachate related ions in the affected surface water were significantly higher than the non-impacted water and correlated strongly with distance from the leachate source, following a negative exponential relationship, providing evidence of leachate transport along the affected surface water. Enrichment in deuterium was exhibited by leachate in the holding pond but not by the effluent leachate. The stable isotope signature of leachate is masked in the surface water due to dilution by stream water. Dilution similarly masked the effect of leachate in the shallow groundwater which was strongly influenced by precipitation. Evidence of leachate contamination in the deep groundwater was sporadic. In isolated cases, elevated tritium concentrations coincided with enrichment in deuterium. In the same case, leachate related ions, Na, Ca, Mg, and Cl, varied with rainfall but generally increased from 2003 to 2009. The effect on the groundwater of methane produced within the landfill was seen in the depletion in deuterium in groundwater in the drier months.

Tracing solid waste leachate in groundwater using δ13 C from dissolved inorganic carbon

Tracers can be used to monitor emissions of leachate from landfills in order to detect hydrological pathways and to evaluate environmental pollution. We investigated the stable carbon isotope ratio (δ(13)C-Σ CO (2)) in dissolved inorganic carbon and tritium ((3)H) in water, in addition to the tracers of pollution commonly found in relatively high concentrations in leachate, such as chloride (Cl), organic matter (COD), nitrogen (total and NH(4)-N), iron (Fe), electrical conductivity (EC) and pH. The sampling was performed at seven landfills in the south-eastern part of Norway during a period of 5 years. The objective was to evaluate the potential for tracing leachate in the environment with emphasis on groundwater pollution. By measuring the δ(13)C-Σ CO (2) in leachates, groundwaters and surface waters, the influence of leachate can be identified. The value of δ(13)C-Σ CO (2) varied from-5.5 to 25.9 ‰ in leachate, from-25.4 to 14.7 ‰ in groundwater and from-19.7 to-13.1 ‰ in creeks. A comparison of the carbon isotope ratio with COD, EC and the concentrations of total and NH (4)-N, Cl and Fe showed that δ(13)C-Σ CO (2) is a good tracer for leachate due to higher sensitivity compared to other parameters. The mean concentrations of all the studied parameters were higher in the leachate samples; however, only the carbon isotope ratio showed significant differences between all the groups with strong and middle pollution and samples with low pollution, showing that it can be used as a convenient tracer for leachate in groundwater and surface water. The carbon isotope ratio showed strong correlation between nitrogen, EC and bicarbonate, but not with pH. Tritium was only sporadically found in measureable concentrations and is not considered as a suitable tracer at the sampled locations.

Bioattenuation in groundwater impacted by landfill leachate traced with δ13C

The impact on groundwater imparted by the infiltration of high dissolved organic carbon (DOC) leachate from capped, unlined landfills can be attenuated by biogeochemical reactions beyond the waste source, although such reactive loss in the aquifer is difficult to distinguish from conservative advective dispersion. Compound-specific measurement of δ(13)C in carbon species, including CH(4), dissolved inorganic carbon (DIC), and the major DOC compounds (acetate, humic acid, and fulvic acid) provides a constraint in this assessment that can assist in exercises of modeling and prediction of leachate transport. The Trail Road municipal landfill near Ottawa, Ontario, Canada, hosts an unlined sector which produces a highly enriched leachate (DOC >4500 mg/L) that provides a good site to examine reactive attenuation within the receptor aquifer. Acetate, a sentinel component of leachate DOC (~1000 mg C/L), is absent in impacted groundwater. Mass balance calculations together with reaction modeling suggest continued acetate fermentation with calcite control on DIC and δ(13)C(DIC) evolution. In groundwater within 50 m of the landfill, methane concentrations are elevated (~10 mg/L), consistent with acetate fermentation, whereas δ(13)C(CH4) measurements in deeper groundwater range down to -51‰ compared with -60‰ in the landfill demonstrating oxidative loss. DOC in the deep aquifer is remarkably depleted to values less than -40‰ suggesting methanotrophic bacteria selectively consume isotopically light CH(4) to fix carbon. Continued reaction of leachate DOC in groundwater is demonstrated by evolution away from conservative mixing lines on diagrams of δ(13)C vs. concentrations of DIC and DOC.

Natural attenuation processes in landfill leachate plumes at three Danish sites

This article provides an overview of comprehensive core and fringe field studies at three Danish landfill sites. The goal of the research activities is to provide a holistic description of core and fringe attenuation processes for xenobiotic organic compounds in landfill leachate plumes. The approach used is cross-disciplinary, encompassing integration of field-scale observations at different scales, field injection experiments, laboratory experiments, and reactive solute transport modeling. This is illustrated in examples from the most recently investigated site-the Sjoelund Landfill. The research performed serves as good case studies to conceptualize natural attenuation processes in landfill leachate plumes and also supports the notion that monitored natural attenuation (MNA) may be a possible remediation strategy at landfills. However, landfill leachate plumes challenge traditional approaches and tools used in the application of MNA. In particular, the use of in situ indicators to document mass removal in landfill leachate plumes is emphasized. In this article, we advocate the application of conceptual and numerical models as tools for the integration of data and testing of hypotheses.

Biogeochemical evolution of a landfill leachate plume, Norman, Oklahoma

Leachate from municipal landfills can create groundwater contaminant plumes that may last for decades to centuries. The fate of reactive contaminants in leachate-affected aquifers depends on the sustainability of biogeochemical processes affecting contaminant transport. Temporal variations in the configuration of redox zones downgradient from the Norman Landfill were studied for more than a decade. The leachate plume contained elevated concentrations of nonvolatile dissolved organic carbon (NVDOC) (up to 300 mg/L), methane (16 mg/L), ammonium (650 mg/L as N), iron (23 mg/L), chloride (1030 mg/L), and bicarbonate (4270 mg/L). Chemical and isotopic investigations along a 2D plume transect revealed consumption of solid and aqueous electron acceptors in the aquifer, depleting the natural attenuation capacity. Despite the relative recalcitrance of NVDOC to biodegradation, the center of the plume was depleted in sulfate, which reduces the long-term oxidation capacity of the leachate-affected aquifer. Ammonium and methane were attenuated in the aquifer relative to chloride by different processes: ammonium transport was retarded mainly by physical interaction with aquifer solids, whereas the methane plume was truncated largely by oxidation. Studies near plume boundaries revealed temporal variability in constituent concentrations related in part to hydrologic changes at various time scales. The upper boundary of the plume was a particularly active location where redox reactions responded to recharge events and seasonal water-table fluctuations. Accurately describing the biogeochemical processes that affect the transport of contaminants in this landfill-leachate-affected aquifer required understanding the aquifer's geologic and hydrodynamic framework.

Energetic constraints on H2-dependent terminal electron accepting processes in anoxic environments: a review of observations and model approaches

Microbially mediated terminal electron accepting processes (TEAPs) to a large extent control the fate of redox reactive elements and associated reactions in anoxic soils, sediments, and aquifers. This review focuses on thermodynamic controls and regulation of H2-dependent TEAPs, case studies illustrating this concept, and the quantitative description of thermodynamic controls in modeling. Other electron transfer processes are considered where appropriate. The work reviewed shows that thermodynamics and microbial kinetics are connected near thermodynamic equilibrium. Free energy thresholds for terminal respiration are physiologically based and often near -20 kJ mol(-1), depending on the mechanism of ATP generation; more positive free energy values have been reported under "starvation conditions" for methanogenesis and lower values for TEAPs that provide more energy. H2-dependent methanogenesis and sulfate reduction are under direct thermodynamic control in soils and sediments and generally approach theoretical minimum energy thresholds. If H2 concentrations are lowered by thermodynamically more potent TEAPs, these processes are inhibited. This principle is also valid for TEAPS providing more free energy, such as denitrification and arsenate reduction, but electron donor concentration cannot be lowered so that the processes reach theoretical energy thresholds. Thermodynamics and kinetics have been integrated by combining traditional descriptions of microbial kinetics with the equilibrium constant K and reaction quotient Q of a process, taking into account process-specific threshold energies. This approach is dynamically evolving toward a general concept of microbially driven electron transfer in anoxic environments and has been used successfully in applications ranging from bioreactor regulation to groundwater and sediment biogeochemistry.

A strictly anaerobic betaproteobacterium Georgfuchsia toluolica gen. nov., sp. nov. degrades aromatic compounds with Fe(III), Mn(IV) or nitrate as an electron acceptor

A bacterium (strain G5G6) that grows anaerobically with toluene was isolated from a polluted aquifer (Banisveld, the Netherlands). The bacterium uses Fe(III), Mn(IV) and nitrate as terminal electron acceptors for growth on aromatic compounds. The bacterium does not grow on sugars, lactate or acetate. Phylogenetic analysis of the 16S rRNA gene sequence indicated that strain G5G6 belonged to the Betaproteobacteria. Its closest, but only distantly related, cultured relative is Sterolibacterium denitrificans Chol-1S(T) (94.6% similarity of the 16S rRNA genes), a cholesterol-oxidizing, denitrifying bacterium. Strain G5G6 possesses the benzylsuccinate synthase A (bssA) gene encoding the alpha-subunit of Bss, which catalyzes the first step in anaerobic toluene degradation. The deduced BssA amino acid sequence is closely related to those of Azoarcus and Thauera species, which also belong to the Betaproteobacteria. Strain G5G6 is the first toluene-degrading, iron-reducing bacterium that does not belong to the Geobacteraceae within the Deltaproteobacteria. Based on phylogenetic and physiological comparison, strain G5G6 could not be assigned to a described species. Therefore, strain G5G6 (DSMZ 19032(T)=JCM 14632(T)) is a novel taxon of the Betaproteobacteria. We propose the name Georgfuchsia toluolica gen. nov., sp. nov.

How Geobacteraceae may dominate subsurface biodegradation: physiology of Geobacter metallireducens in slow-growth habitat-simulating retentostats

Geobacteraceae dominate many iron-reducing subsurface environments and are associated with biodegradation of organic pollutants. In order to enhance the understanding of the environmental role played by Geobacteraceae, the physiology of Geobacter metallireducens was investigated at the low growth rates found in its subsurface habitat. Cultivation in retentostats (a continuous culturing device with biomass retention) under electron acceptor and electron donor limitation enabled growth rates as low as 0.0008 h(-1). The maximum growth yield was between 0.05 and 0.09 C-mol biomass per C-mol acetate and comparable to that observed in batch experiments. Maintenance energy demand is among the lowest reported for heterotrophic bacteria, under both acetate and AQDS limitation. The cells were able to use alternative electron acceptors directly, without requiring de novo protein synthesis. We discuss how the extremely low maintenance energy demand and the ability to readily use alternative electron acceptors may help Geobacter species to become ubiquitous and dominant microorganisms in many iron-reducing subsurface settings.

Seasonal dynamics in leachate hydrochemistry and natural attenuation in surface run-off water from a tropical landfill

Open waste dump systems are still widely used in Indonesia. The Jatibarang landfill receives 650-700 tons of municipal waste per day from the city of Semarang, Central Java. Some of the leachate from the landfill flows via several natural and collection ponds to a nearby river. The objectives of the study were to identify seasonal landfill leachate characteristics in this surface water and to determine the occurrence of natural attenuation, in particular the potential for biodegradation, along the flow path. Monthly measurements of general landfill leachate parameters, organic matter-related factors and redox-related components revealed that leachate composition was influenced by seasonal precipitation. In the dry season, electrical conductivity and concentrations of BOD, COD, N-organic matter, ammonia, sulphate and calcium were significantly higher (1.1-2.3 fold) than during the wet season. Dilution was the major natural attenuation process acting on leachate. Heavy metals had the highest impact on river water quality. Between the landfill and the river, a fivefold dilution occurred during the dry season due to active springwater infiltration, while rainwater led to a twofold dilution in the wet season. Residence time of leachate in the surface leachate collection system was less than 70 days. Field measurements and laboratory experiments showed that during this period hardly any biodegradation of organic matter and ammonia occurred (less than 25%). However, the potential for biodegradation of organic matter and ammonia was clearly revealed during 700 days of incubation of leachate in the laboratory (over 65%). If the residence time of leachate discharge can be increased to allow for biodegradation processes and precipitation reactions, the polluting effects of leachate on the river can be diminished.

Degassing of 3H/3He, CFCs and SF6 by denitrification: measurements and two-phase transport simulations

The production of N2 gas by denitrification may lead to the appearance of a gas phase below the water table prohibiting the conservative transport of tracer gases required for groundwater dating. We used a two-phase flow and transport model (STOMP) to study the reliability of 3H/3He, CFCs and SF6 as groundwater age tracers under agricultural land where denitrification causes degassing. We were able to reproduce the amount of degassing (R2=69%), as well as the 3H (R2=79%) and 3He (R2=76%) concentrations observed in a 3H/3He data set using simple 2D models. We found that the TDG correction of the 3H/3He age overestimated the control 3He/3He age by 2.1 years, due to the accumulation of 3He in the gas phase. The total uncertainty of degassed 3H/3He ages of 6 years (+/-2 sigma) is due to the correction of degassed 3He using the TDG method, but also due to the travel time in the unsaturated zone and the diffusion of bomb peak 3He. CFCs appear to be subject to significant degradation in anoxic groundwater and SF6 is highly susceptible to degassing. We conclude that 3H/3He is the most reliable method to date degassed groundwater and that two-phase flow models such as STOMP are useful tools to assist in the interpretation of degassed groundwater age tracer data.

Degradation pathways of dissolved carbon in landfill leachate traced with compound-specific (13)C analysis of DOC

The isotopic compositions of carbon compounds in landfill leachate provide insights into the biodegradation pathways that dominate the different stages of waste decomposition. In this study, the carbon geochemistry of different carbon pools, environmental stable isotopes and compound-specific isotope analysis (CSIA) of leachate dissolved organic carbon (DOC) fractions and gases show distinctions in leachate biogeochemistry and methane production between the young area of active waste emplacement and the old area of historical emplacement at the Trail Road Landfill (TRL). The active area leachate has low DOC concentrations (<200 mg l(-1)) dominated by fulvic acid (FA=160 mg l(-1)), and produces CH(4) dominantly by CO(2) reduction (D- excess=20.6 per thousand). Leachate generated in the area of older waste has high DOC (>4770 mg l(-1)) dominated by FA (4482 mg l(-1)) and simple fatty acids (acetic=1008 mg l(-1) and propionic=608 mg l(-1)), and produces CH(4) by the acetate fermentation pathway (D- excess=9.8 per thousand). CSIA shows an advanced degradation and a progressive accumulation of (13)C of fatty acids in leachate from the older area. The enriched (13)C value of FA (-20 and-26 per thousand for the older and active parts, respectively,) and of low molecular weight DOC (-8 and-27 per thousand) as well as of the bulk DOC (-21 and-25 per thousand) shows more advanced degradation in the older part of the landfill, which is consistent with the shift in the humic/FA ratios (0.05 and 0.18). The (13)C enrichment of acetate (-12 per thousand) above the (13)C of DOC (-21 per thousand) and of propionic acid (-19 per thousand), in older leachate, suggests that this acetate has not evolved from the simple degradation of larger organic molecules, but by homoacetogenesis from the enriched dissolved inorganic carbon (DIC) pool (8 per thousand) and H(2,) which produce a more enriched (13)C of acetate. In contrast, the (13)C of the minor acetate in the active area (-17 per thousand) indicates that CO(2)-reducing bacteria must be the primary consumers of H(2), which has resulted in enriched (13)C(DIC) (10 per thousand) and depleted (13)C(CH4) (-58 per thousand).

Spatial heterogeneity in sediment-associated bacterial and eukaryotic communities in a landfill leachate-contaminated aquifer

Heterogeneity in eukaryotic and bacteria community structure in surface and subsurface sediment samples downgradient of the Banisveld landfill (The Netherlands) was studied using a culturing-independent molecular approach. Along a transect covering the part of the aquifer most polluted by landfill leachate, sediment was sampled at 1-m depth intervals, until a depth of 5.5 m, at four distances from the landfill. Two drillings were placed in a nearby clean area as a reference. Denaturing gradient gel electrophoresis banding patterns revealed high bacterial and eukaryotic diversity and complex community structures. Bacteria and eukaryotic community profiles in polluted samples grouped different from those in clean samples. Bacteria community profiles in surface samples clustered together and separately from subsurface community profiles. Subsurface bacteria profiles clustered in a location-specific manner. Eukaryotic community structure did not significantly relate to distance from the landfill or depth. No significant spatial autocorrelation of bacteria or eukaryotic communities was observed over 1-m depth intervals per sampling location. Spatial heterogeneity in sediment-associated bacterial communities appears to be much larger than in groundwater. We discuss how on the one hand, spatial heterogeneity may complicate the assessment of microbial community structure and functioning, while on the other it may provide better opportunities for natural attenuation.

Eukaryotic diversity in an anaerobic aquifer polluted with landfill leachate

Eukaryotes may influence pollutant degradation processes in groundwater ecosystems by activities such as predation on bacteria and recycling of nutrients. Culture-independent community profiling and phylogenetic analysis of 18S rRNA gene fragments, as well as culturing, were employed to obtain insight into the sediment-associated eukaryotic community composition in an anaerobic sandy aquifer polluted with landfill leachate (Banisveld, The Netherlands). The microeukaryotic community at a depth of 1 to 5 m below the surface along a transect downgradient (21 to 68 m) from the landfill and at a clean reference location was diverse. Fungal sequences dominated most clone libraries. The fungal diversity was high, and most sequences were sequences of yeasts of the Basidiomycota. Sequences of green algae (Chlorophyta) were detected in parts of the aquifer close (<30 m) to the landfill. The bacterium-predating nanoflagellate Heteromita globosa (Cercozoa) was retrieved in enrichments, and its sequences dominated the clone library derived from the polluted aquifer at a depth of 5 m at a location 21 m downgradient from the landfill. The number of culturable eukaryotes ranged from 10(2) to 10(3) cells/g sediment. Culture-independent quantification revealed slightly higher numbers. Groundwater mesofauna was not detected. We concluded that the food chain in this polluted aquifer is short and consists of prokaryotes and fungi as decomposers of organic matter and protists as primary consumers of the prokaryotes.

The opposing effects of bacterial activity and gas production on anaerobic TCE degradation in soil columns

This laboratory study explores the effect of growth substrate concentration on the anaerobic degradation of trichloroethylene (TCE) in sand packed columns. In all columns the growth substrate rapidly degraded to gas, that formed a separate phase. Biomass accumulated in the 0-4.8 cm section of the columns in proportion to the influent growth substrate concentration and biomass concentrations in the remaining sections of all columns were similar to the column receiving the lowest substrate concentration. Increases in growth substrate concentration up to 3030 mg-CODl(-1) promoted TCE degradation, but a further increase to 14300 mg-CODl(-1) reduced the amount of TCE completely dechlorinated but did not affect the production of chlorinated TCE intermediates. The mathematical model developed here satisfactorily described the enhancement in TCE dehalogenation for substrate concentration up to 3030 mg-CODl(-1); reproducing TCE dehalogenation for 14300 mg-CODl(-1) required that the moisture content used in simulation be lowered to 0.1. The study shows that volatilization of TCE can be significant and volatilization losses should be taken into account when anaerobic activity in in-situ bioremediation applications is stimulated via addition of growth substrates. An implication of the modeling simulations is that maintaining a lower, but uniform, substrate concentration over the contaminated region may lead to faster contaminant degradation.

Dominance of Geobacteraceae in BTX-degrading enrichments from an iron-reducing aquifer

Microbial community structure was linked to degradation potential in benzene-, toluene- or xylene- (BTX) degrading, iron-reducing enrichments derived from an iron-reducing aquifer polluted with landfill leachate. Enrichments were characterized using 16S rRNA gene-based analysis, targeting of the benzylsuccinate synthase-encoding bssA gene and phospholipid fatty acid (PLFA) profiling in combination with tracking of labelled substrate. 16S rRNA gene analysis indicated the dominance of Geobacteraceae, and one phylotype in particular, in all enrichments inoculated with polluted aquifer material. Upon cultivation, progressively higher degradation rates with a concomitant decrease in species richness occurred in all primary incubations and successive enrichments. Yet, the same Geobacteraceae phylotype remained common and dominant, indicating its involvement in BTX degradation. However, the bssA gene sequences in BTX degrading enrichments differed considerably from those of Geobacter isolates, suggesting that the first steps of toluene, but also benzene and xylene oxidation, are carried out by another member of the enrichments. Therefore, BTX would be synthrophically degraded by a bacterial consortium in which Geobacteraceae utilized intermediate metabolites. PLFA analysis in combination with (13)C-toluene indicated that the enriched Geobacteraceae were assimilating carbon originally present in toluene. Combined with previous studies, this research suggests that Geobacteraceae play a key role in the natural attenuation of each BTX compound in situ.

Modelling of iron cycling and its impact on the electron balance at a petroleum hydrocarbon contaminated site in Hnevice, Czech Republic

Over a period of several decades multiple leaks of large volumes from storage facilities located near Hnevice (Czech Republic) have caused the underlying Quaternary aquifer to be severely contaminated with nonaqueous phase liquid (NAPL) petroleum hydrocarbons. Beginning in the late 1980's the NAPL plume started to shrink as a consequence of NAPL dissolution exceeding replenishment and due to active remediation. The subsurface was classified geochemically into four different zones, (i) a contaminant-free zone never occupied by NAPL or dissolved contaminants, (ii) a re-oxidation zone formerly occupied by NAPL, (iii) a zone currently occupied by NAPL, and (iv) a lower fringe zone between the overlying NAPL and the deeper underlying contaminant-free zone. The study investigated the spatial and temporal variability of the redox zonation at the Hnevice site and quantified the influence of iron-cycling on the overall electron balance. As a first step inverse geochemical modelling was carried out to identify possible reaction models and mass transfer processes. In a subsequent step, two-dimensional (forward) multi-component reactive transport modelling was performed to evaluate and quantify the major processes that control the geochemical evolution at the site. The study explains the observed enrichment of the lower fringe zone with ferrihydrite as a result of the re-oxidation of ferrous iron. It suggests that once the NAPL zone started to shrink the dissolution of previously formed siderite and FeS by oxygen and nitrate consumed a significant part of the oxidation capacity for a considerable time period and therefore limited the penetration of electron acceptors into the NAPL contaminated zone.

Degradation of BTX by dissimilatory iron-reducing cultures

The ability of indigenous bacteria to anaerobically degrade monoaromatic hydrocarbons has received attention as a potential strategy to remediate polluted aquifers. Despite the fact that iron-reducing conditions are often dominating in contaminated sediment, most of the studies have focussed on degradation of this class of pollutants with other terminal acceptors. In this work, we enriched bacteria from an iron-reducing aquifer in which a plume of pollution has developed over several decades and we show that benzene, toluene, meta- and para-xylene (BTX) could be degraded by the enriched cultures containing intrinsic iron-reducing microorganisms. To our knowledge, this is the first time that para-xylene degradation by dissimilatory iron-reducing bacteria has been reported in sediment free enrichment cultures. BTX degradation rates in enrichment cultures progressively increased in time and were found in good agreement with theoretical values calculated assuming complete BTX oxidation with Fe(II) as final electron acceptor. In addition, using labelled ((13)C(1)) benzene and toluene we could unambiguously identify intermediates of their respective degradation pathways. We provide evidence for benzene degradation via phenol formation under iron-reducing conditions, whereas toluene and meta-xylene were transformed into the corresponding benzylsuccinates.

A steady state redox zone approach for modeling the transport and degradation of xenobiotic organic compounds from a landfill site

A redox zonation approach is used as a framework for obtaining biodegradation rate constants of xenobiotic compounds in a landfill plume (Grindsted, Denmark). The aquifer is physically heterogeneous in terms of a complex zonation of different geological units close to the landfill and biogeochemically heterogeneous in terms of a specified redox zonation. First-order degradation rates of six organic compounds (benzene, toluene, ethylbenzene, o-xylene, m/p-xylene, and naphthalene) were calculated in the methanogenic/sulfate- and Fe-reducing zones. The numerical simulations show that all compounds are anaerobically biodegraded, but at very different rates. High rates of biodegradation of most of the compounds (except benzene) were found in the Fe-reducing zone. These rates generally agree with previously published rates. Only o-xylene and toluene were significantly biodegraded in the methanogenic/sulfate-reducing environment. All rates in this redox zone are generally much lower than previously published rates.

Investigating the role of gas bubble formation and entrapment in contaminated aquifers: Reactive transport modelling

In many natural and contaminated aquifers, geochemical processes result in the production or consumption of dissolved gases. In cases where methanogenesis or denitrification occurs, the production of gases may result in the formation and growth of gas bubbles below the water table. Near the water table, entrapment of atmospheric gases during water table rise may provide a significant source of O(2) to waters otherwise depleted in O(2). Furthermore, the presence of bubbles will affect the hydraulic conductivity of an aquifer, resulting in changes to the groundwater flow regime. The interactions between physical transport, biogeochemical processes, and gas bubble formation, entrapment and release is complex and requires suitable analysis tools. The objective of the present work is the development of a numerical model capable of quantitatively assessing these processes. The multicomponent reactive transport code MIN3P has been enhanced to simulate bubble growth and contraction due to in-situ gas production or consumption, bubble entrapment due to water table rise and subsequent re-equilibration of the bubble with ambient groundwater, and permeability changes due to trapped gas phase saturation. The resulting formulation allows for the investigation of complex geochemical systems where microbially mediated redox reactions both produce and consume gases as well as affect solution chemistry, alkalinity, and pH. The enhanced model has been used to simulate processes in a petroleum hydrocarbon contaminated aquifer where methanogenesis is an important redox process. The simulations are constrained by data from a crude oil spill site near Bemidji, MN. Our results suggest that permeability reduction in the methanogenic zone due to in-situ formation of gas bubbles, and dissolution of entrapped atmospheric bubbles near the water table, both work to attenuate the dissolved gas plume emanating from the source zone. Furthermore, the simulations demonstrate that under the given conditions more than 50% of all produced CH(4) partitions to the gas phase or is aerobically oxidised near the water table, suggesting that these processes should be accounted for when assessing the rate and extent of methanogenic degradation of hydrocarbons.

Investigating ebullition in a sand column using dissolved gas analysis and reactive transport modeling

Ebullition of gas bubbles through saturated sediments can enhance the migration of gases through the subsurface, affect the rate of biogeochemical processes, and potentially enhance the emission of important greenhouse gases to the atmosphere. To better understand the parameters controlling ebullition, methanogenic conditions were produced in a column experiment and ebullition through the column was monitored and quantified through dissolved gas analysis and reactive transport modeling. Dissolved gas analysis showed rapid transport of CH4 vertically through the column at rates several times faster than the bromide tracer and the more soluble gas CO2, indicating that ebullition was the main transport mechanism for CH4. An empirically derived formulation describing ebullition was integrated into the reactive transport code MIN3P allowing this process to be investigated on the REV scale in a complex geochemical framework. The simulations provided insights into the parameters controlling ebullition and show that, over the duration of the experiment, 36% of the CH4 and 19% of the CO2 produced were transported to the top of the column through ebullition.

Identification of a reactive degradation zone at a landfill leachate plume fringe using high resolution sampling and incubation techniques

Vertical small-scale variation in phenoxy acid herbicide degradation across a landfill leachate plume fringe was studied using laboratory degradation experiments. Sediment cores (subdivided into 5 cm segments) were collected in the aquifer and the sediment and porewater were used for microcosm experiments (50 experiments) and for determination of solid organic carbon, solid-water partitioning coefficients, specific phenoxy acid degraders and porewater chemistry. Results from a multi-level sampler installed next to the cores provided information on the plume position and oxygen concentration in the groundwater. Oxygen concentration was controlled individually in each microcosm to mimic the conditions at their corresponding depths. A highly increased degradation potential existed at the narrow plume fringe (37.7 to 38.6 masl), governed by the presence of phenoxy acids and oxygen. This resulted in the proliferation of a microbial population of specific phenoxy acid degraders, which further enhanced the degradation potential for phenoxy acids at the fringe. The results illustrate the importance of fringe degradation processes in contaminant plumes. Furthermore, they highlight the relevance of using high-resolution sampling techniques as well as controlled microcosm experiments in the assessment of the natural attenuation capacity of contaminant plumes in groundwater.

Changes in organic matter biodegradability influencing sulfate reduction in an aquifer contaminated by landfill leachate

In situ experiments were conducted to measure sulfate reduction rates and identify rate-limiting factors in a shallow, alluvial aquifer contaminated with municipal landfill leachate. Single-well, push-pull tests conducted in a well adjacent to the landfill with > 8 mM dissolved organic carbon (DOC) exhibited a sulfate reduction rate of 3.2 mumol SO4(-2) (L sediment)(-1) day(-1), a value in close agreement with laboratory-derived estimates. Identical tests conducted in wells located 90 m downgradient where DOC levels remained high (> 3 mM) showed no detectable sulfate consumption, and laboratory assays confirmed this observation. However, the rates of sulfate reduction in sediment samples obtained from this site were three times larger when they were amended with filter-sterilized groundwater from the upgradient location. The effect of various amendments on sulfate reduction rates was further examined in laboratory incubations using sediment collected from the downgradient site amended with 35S sulfate. Unamended sediments showed only weak conversion of the tracer to 35S sulfide (5 to 7 cpm/cm2), whereas the addition of Desulfovibrio cells increased 35S sulfide production to 44 cpm/cm2. However, the application of heat-killed Desulfovibrio had a similar stimulatory effect, as did a lactate amendment. Collectively, these findings indicate that the lack of measurable sulfate reduction at the downgradient site was not due to the absence of the necessary metabolic potential, the presence of lower sulfate concentration, or the quantity of electron donor, but by its biodegradability. The findings also indicate that field bioaugmentation attempts should be interpreted with caution.

Geobacteraceae community composition is related to hydrochemistry and biodegradation in an iron-reducing aquifer polluted by a neighboring landfill

Relationships between community composition of the iron-reducing Geobacteraceae, pollution levels, and the occurrence of biodegradation were established for an iron-reducing aquifer polluted with landfill leachate by using cultivation-independent Geobacteraceae 16S rRNA gene-targeting techniques. Numerical analysis of denaturing gradient gel electrophoresis (DGGE) profiles and sequencing revealed a high Geobacteraceae diversity and showed that community composition within the leachate plume differed considerably from that of the unpolluted aquifer. This suggests that pollution has selected for specific species out of a large pool of Geobacteraceae. DGGE profiles of polluted groundwater taken near the landfill (6- to 39-m distance) clustered together. DGGE profiles from less-polluted groundwater taken further downstream did not fall in the same cluster. Several individual DGGE bands were indicative of either the redox process or the level of pollution. This included a pollution-indicative band that dominated the DGGE profiles from groundwater samples taken close to the landfill (6 to 39 m distance). The clustering of these profiles and the dominance by a single DGGE band corresponded to the part of the aquifer where organic micropollutants and reactive dissolved organic matter were attenuated at relatively high rates.

A multivariate statistical approach to spatial representation of groundwater contamination using hydrochemistry and microbial community profiles

Managers of landfill sites are faced with enormous challenges when attempting to detect and delineate leachate plumes with a limited number of monitoring wells, assess spatial and temporal trends for hundreds of contaminants, and design long-term monitoring (LTM) strategies. Subsurface microbial ecology is a unique source of data that has been historically underutilized in LTM groundwater designs. This paper provides a methodology for utilizing qualitative and quantitative information (specifically, multiple water quality measurements and genome-based data) from a landfill leachate contaminated aquifer in Banisveld, The Netherlands, to improve the estimation of parameters of concern. We used a principal component analysis (PCA) to reduce nonindependent hydrochemistry data, Bacteria and Archaea community profiles from 16S rDNA denaturing gradient gel electrophoresis (DGGE), into six statistically independent variables, representing the majority of the original dataset variances. The PCA scores grouped samples based on the degree or class of contamination and were similar over considerable horizontal distances. Incorporation of the principal component scores with traditional subsurface information using cokriging improved the understanding of the contaminated area by reducing error variances and increasing detection efficiency. Combining these multiple types of data (e.g., genome-based information, hydrochemistry, borings) may be extremely useful at landfill or other LTM sites for designing cost-effective strategies to detect and monitor contaminants.

Biogeochemical processes at the fringe of a landfill leachate pollution plume: potential for dissolved organic carbon, Fe(II), Mn(II), NH4, and CH4 oxidation

Various redox reactions may occur at the fringe of a landfill leachate plume, involving oxidation of dissolved organic carbon (DOC), CH4, Fe(II), Mn(II), and NH4 from leachate and reduction of O2, NO3 and SO4 from pristine groundwater. Knowledge on the relevance of these processes is essential for the simulation and evaluation of natural attenuation (NA) of pollution plumes. The occurrence of such biogeochemical processes was investigated at the top fringe of a landfill leachate plume (Banisveld, the Netherlands). Hydrochemical depth profiles of the top fringe were captured via installation of a series of multi-level samplers at 18, 39 and 58 m downstream from the landfill. Ten-centimeter vertical resolution was necessary to study NA within a fringe as thin as 0.5 m. Bromide appeared an equally well-conservative tracer as chloride to calculate dilution of landfill leachate, and its ratio to chloride was high compared to other possible sources of salt in groundwater. The plume fringe rose steadily from a depth of around 5 m towards the surface with a few meters in the period 1998-2003. The plume uplift may be caused by enhanced exfiltration to a brook downstream from the landfill, due to increased precipitation over this period and an artificial lowering of the water level of the brook. This rise invoked cation exchange including proton buffering, and triggered degassing of methane. The hydrochemical depth profile was simulated in a 1D vertical reactive transport model using PHREEQC-2. Optimization using the nonlinear optimization program PEST brought forward that solid organic carbon and not clay minerals controlled retardation of cations. Cation exchange resulted in spatial separation of Fe(II), Mn(II) and NH4 fronts from the fringe, and thereby prevented possible oxidation of these secondary redox species. Degradation of DOC may happen in the fringe zone. Re-dissolution of methane escaped from the plume and subsequent oxidation is an explanation for absence of previously present nitrate and anaerobic conditions in pristine groundwater above the plume. Stable carbon isotope (delta13C) values of methane confirm anaerobic methane oxidation immediately below the fringe zone, presumably coupled to reduction of sulfate, desorbed from iron oxide. Methane must be the principle reductant consuming soluble electron-acceptors in pristine groundwater, thereby limiting NA for other solutes including organic micro-pollutants at the fringe of this landfill leachate plume.

Reactive transport modelling of biogeochemical processes and carbon isotope geochemistry inside a landfill leachate plume

The biogeochemical processes governing leachate attenuation inside a landfill leachate plume (Banisveld, the Netherlands) were revealed and quantified using the 1D reactive transport model PHREEQC-2. Biodegradation of dissolved organic carbon (DOC) was simulated assuming first-order oxidation of two DOC fractions with different reactivity, and was coupled to reductive dissolution of iron oxide. The following secondary geochemical processes were required in the model to match observations: kinetic precipitation of calcite and siderite, cation exchange, proton buffering and degassing. Rate constants for DOC oxidation and carbonate mineral precipitation were determined, and other model parameters were optimized using the nonlinear optimization program PEST by means of matching hydrochemical observations closely (pH, DIC, DOC, Na, K, Ca, Mg, NH4, Fe(II), SO4, Cl, CH4, saturation index of calcite and siderite). The modelling demonstrated the relevance and impact of various secondary geochemical processes on leachate plume evolution. Concomitant precipitation of siderite masked the act of iron reduction. Cation exchange resulted in release of Fe(II) from the pristine anaerobic aquifer to the leachate. Degassing, triggered by elevated CO2 pressures caused by carbonate precipitation and proton buffering at the front of the plume, explained the observed downstream decrease in methane concentration. Simulation of the carbon isotope geochemistry independently supported the proposed reaction network.