B.M. van Breukelen

Relationships between Microbial Community Structure and Hydrochemistry in a Landfill Leachate-Polluted Aquifer

ABSTRACT Knowledge about the relationship between microbial community structure and hydrogeochemistry (e.g., pollution, redox and degradation processes) in landfill leachate-polluted aquifers is required to develop tools for predicting and monitoring natural attenuation. In this study analyses of pollutant and redox chemistry were conducted in parallel with culture-independent profiling of microbial communities present in a well-defined aquifer (Banisveld, The Netherlands). Degradation of organic contaminants occurred under iron-reducing conditions in the plume of pollution, while upstream of the landfill and above the plume denitrification was the dominant redox process. Beneath the plume iron reduction occurred. Numerical comparison of 16S ribosomal DNA (rDNA)-based denaturing gradient gel electrophoresis (DGGE) profiles of Bacteria andArchaea in 29 groundwater samples revealed a clear difference between the microbial community structures inside and outside the contaminant plume. A similar relationship was not evident in sediment samples. DGGE data were supported by sequencing cloned 16S rDNA. Upstream of the landfill members of the β subclass of the class Proteobacteria(β-proteobacteria) dominated. This group was not encountered beneath the landfill, where gram-positive bacteria dominated. Further downstream the contribution of gram-positive bacteria to the clone library decreased, while the contribution of δ-proteobacteria strongly increased and β-proteobacteria reappeared. The β-proteobacteria (Acidovorax,Rhodoferax) differed considerably from those found upstream (Gallionella, Azoarcus). Direct comparisons of cloned 16S rDNA with bands in DGGE profiles revealed that the data from each analysis were comparable. A relationship was observed between the dominant redox processes and the bacteria identified. In the iron-reducing plume members of the familyGeobacteraceae made a strong contribution to the microbial communities. Because the only known aromatic hydrocarbon-degrading, iron-reducing bacteria areGeobacter spp., their occurrence in landfill leachate-contaminated aquifers deserves more detailed consideration.

Analysis of Microbial Communities in a Landfill Leachate Polluted Aquifer using a New Method for Anaerobic Physiological Profiling and 16S rDNA Based Fingerprinting.

A bstractDatabases containing information regarding presence and activity of microbial communities will be very useful for determination of the potential for intrinsic bioremediation in landfill leachate polluted aquifers. Simple analyses such as community-level physiological profiling (CLPP) and denaturing gradient gel electrophoresis (DGGE) of 16S rDNA fragments yield large sets of data for inclusion into such databases. In this study we describe the development of a method for anaerobic CLPP, using commercially available Biolog plates. Incubation at the in situ temperature of the aquifer (10°C) for 28 days was optimal for obtaining a specific, reproducible physiological profile. Anaerobic incubation was essential for profiling anaerobic communities. The anaerobic cultivation-dependent CLPP method and cultivation-independent DGGE were applied to groundwater and sediment samples from the aquifer near the Coupépolder landfill in The Netherlands. A combination of computer-assisted CLPP and DGGE analysis of both groundwater and sediment samples yielded the best separating power for characterizing microbial communities in the aquifer. Communities in groundwater were significantly different from those in the corresponding sediment. Microbial communities present in subsamples from sediment cores usually were similar for the various sampling locations. Variation was observed for the heterogeneous sediment beneath the landfill. Both anaerobic CLPP and DGGE analysis clearly separated microbial communities from the polluted aquifer underneath the landfill from those in the less or not polluted aquifer downstream and upstream of the landfill.

Transverse hydrodynamic dispersion effects on isotope signals in groundwater chlorinated solvents’ plumes.

The effects of transverse hydrodynamic dispersion on altering transformation-induced compound-specific isotope analysis (CSIA) signals within groundwater pollution plumes have been assessed with reactive transport modeling accommodating diffusion-induced isotope fractionation (DIF) and implementing different parameterizations of local transverse dispersion. The model reproduced previously published field data showing a negative carbon isotope pattern (-2 ‰) at the fringes of a nondegrading PCE plume. We extended the study to reactive transport scenarios considering vinyl chloride as a model compound and assessing, through a detailed sensitivity analysis, the coupled effects of transverse hydrodynamic dispersion (with and without DIF) and aerobic fringe degradation on the evolution of carbon and chloride isotope ratios. Transformation-induced positive isotope signals were increasingly attenuated with distance from the source and higher degradation rate. The effect of DIF on the overall isotope signal attenuation was greatest near the source and for low values of groundwater flow velocity, transverse dispersion coefficient, molecular weight, rate constant, and isotope fractionation factor, α, of the degradation reaction. Models disregarding DIF underestimate the actual α. The approximately twice larger DIF effect for chlorine than for carbon together with the low α for oxidation resulted in strong chlorine CSIA depletions for VC at the plume fringe.

Hydrogeochemical transport modeling of 24 years of Rhine water infiltration in the dunes of the Amsterdam Water Supply.

Abstract Water quality changes were modelled along a flowpath in a plume of artificially recharged, pretreated Rhine water in the dunes of the Amsterdam Water Supply, after 24 years of infiltration. The hydrogeochemical transport model PHREEQC was extended with dispersion/diffusion and kinetics for selected chemical reactions. In the model the following reactions were included: cation-exchange, calcite dissolution and precipitation, and kinetic oxygen consumption and denitrification by oxidation of organic matter. Monthly-averaged values were used for the infiltration water quality. Traveltimes from infiltration area to sampling points were determined with chloride and tritium, and used to place the 3D field-observations in the 1D column-model. Values for CEC were variable for seven layers in the model. Infiltration of pretreated Rhine water in the dune aquifer can be considered an intrusion of more saline water. It caused desorption of Ca2+, in exchange for Na+, K+ and Mg2+ from Rhine water. Because of variations in total solute concentrations in infiltration water, local small scale freshening fronts (Ca2+ sorption, Na+ desorption) were created by seasonally decreasing salt concentrations. The undersaturation with respect to calcite in the infiltration water, and the CO2 produced during consumption of oxygen, resulted in dissolution of calcite. Precipitation of calcite occurred in response to desorption of calcium from the exchanger in the downstream parts. Overall, a net dissolution of calcite was simulated. Good results were generally achieved for all components: sulfate, nitrate, chloride, alkalinity, calcium, magnesium, potassium, sodium, 3H and O2. The contributions of the different geochemical reactions to the water quality are illustrated with computer simulations for the individual processes.

The impact of low-temperature seasonal aquifer thermal energy storage (SATES) systems on chlorinated solvent contaminated groundwater: Modeling of spreading and degradation

Groundwater systems are increasingly used for seasonal aquifer thermal energy storage (SATES) for periodic heating and cooling of buildings. Its use is hampered in contaminated aquifers because of the potential environmental risks associated with the spreading of contaminated groundwater, but positive side effects, such as enhanced contaminant remediation, might also occur. A first reactive transport study is presented to assess the effect of SATES on the fate of chlorinated solvents by means of scenario modeling, with emphasis on the effects of transient SATES pumping and applicable kinetic degradation regime. Temperature effects on physical, chemical, and biological reactions were excluded as calculations and initial simulations showed that the small temperature range commonly involved (ΔT<15 °C) only caused minor effects. The results show that a significant decrease of the contaminant mass and (eventually) plume volume occurs when degradation is described as sediment-limited with a constant rate in space and time, provided that dense non-aqueous phase liquid (DNAPL) is absent. However, in the presence of DNAPL dissolution, particularly when the dissolved contaminant reaches SATES wells, a considerably larger contaminant plume is created, depending on the balance between DNAPL dissolution and mass removal by degradation. Under conditions where degradation is contaminant-limited and degradation rates depend on contaminant concentrations in the aquifer, a SATES system does not result in enhanced remediation of a contaminant plume. Although field data are lacking and existing regulatory constraints do not yet permit the application of SATES in contaminated aquifers, reactive transport modeling provides a means of assessing the risks of SATES application in contaminated aquifers. The results from this study are considered to be a first step in identifying the subsurface conditions under which SATES can be applied in a safe or even beneficial manner.

Impacts of shallow geothermal energy production on redox processes and microbial communities

Shallow geothermal systems are increasingly being used to store or harvest thermal energy for heating or cooling purposes. This technology causes temperature perturbations exceeding the natural variations in aquifers, which may impact groundwater quality. Here, we report the results of laboratory experiments on the effect of temperature variations (5-80 °C) on redox processes and associated microbial communities in anoxic unconsolidated subsurface sediments. Both hydrochemical and microbiological data showed that a temperature increase from 11 °C (in situ) to 25 °C caused a shift from iron-reducing to sulfate-reducing and methanogenic conditions. Bioenergetic calculations could explain this shift. A further temperature increase (>45 °C) resulted in the emergence of a thermophilic microbial community specialized in fermentation and sulfate reduction. Two distinct maxima in sulfate reduction rates, of similar orders of magnitude (5 × 10(-10) M s(-1)), were observed at 40 and 70 °C. Thermophilic sulfate reduction, however, had a higher activation energy (100-160 kJ mol(-1)) than mesophilic sulfate reduction (30-60 kJ mol(-1)), which might be due to a trade-off between enzyme stability and activity with thermostable enzymes being less efficient catalysts that require higher activation energies. These results reveal that while sulfate-reducing functionality can withstand a substantial temperature rise, other key biochemical processes appear more temperature sensitive.

Iron-Reducing Microorganisms in a Landfill Leachate-Polluted Aquifer: Complementing Culture-Independent Information with Enrichments and Isolations

Using culture-independent 16S rRNA gene-based methods, we previously observed that Geobacteraceae were a major component of the microbial communities in the iron-reducing aquifer polluted by the Banisveld landfill, The Netherlands. However, phylogenetic information does not tell about the functional potential of the detected Geobacteraceae, nor can phylogenetic information easily be used to establish the presence of other iron-reducers. Therefore, we enriched for iron-reducing consortia using a range of culturing media, with various electron donors and acceptors and varying incubation conditions (pH, temperature), and by applying dilution-to-extinction culturing. Enrichments and strains isolated from these enrichments were characterized by 16S rRNA gene-based methods. The number of culturable iron-reducers was less than 110 iron-reducing bacteria per gram of sediment. The Geobacter phylotype that was previously found to constitute a major part of the microbial communities in a part of the aquifer where organic matter was attenuated at a relatively high rate, was not isolated. The isolation of another Geobacter strain and Serratia, Clostridium, Rhodoferax and Desulfitobacteriumstrains suggest the presence of a diverse iron-reducing community. Physiological capabilities of the isolates are described and discussed in relation to the hydrogeochemistry and the high abundance of Geobacteraceae in the aquifer polluted by the Banisveld landfill.

Reactive transport modeling of subsurface arsenic removal systems in rural Bangladesh.

Subsurface Arsenic Removal (SAR) is a technique for in-situ removal of arsenic from groundwater. Extracted groundwater is aerated and re-injected into an anoxic aquifer, where the oxygen in the injected water reacts with ferrous iron in the aquifer to form hydrous ferric oxide (HFO). Subsequent extraction of groundwater contains temporarily lower As concentrations, because As sorbs onto the HFO. Injection, storage, and extraction together is called a cycle. A reactive transport model (RTM) was developed in PHREEQC to determine the hydrogeochemical processes responsible for As (im)mobilization during experimental SAR operation performed in Bangladesh. Oxidation of Fe(II) and As(III) were modeled using kinetic-rate expressions. Cation exchange, precipitation of HFO, and surface complexation, were modeled as equilibrium processes. A best set of surface complexation reactions and corresponding equilibrium constants was adopted from previous studies to simulate all 20 cycles of a SAR experiment. The model gives a reasonable match with observed concentrations of different elements in the extracted water (e.g., the r(2) value of As was 0.59 or higher). As concentrations in the extracted water are governed by four major processes. First, As concentration decreases in response to the elevated pH of injection water and likewise increases when native neutral pH groundwater flows in. Second, the sorption capacity for As increases due to the gradual buildup of HFO. Third, As sorption is enhanced by preferential removal of As(V). Fourth, competitive sorption of Si limits the capacity of freshly precipitated HFO for As sorption. Transferability of the developed reactive transport model was demonstrated through successful application of the model, without further calibration, to two additional SAR sites in Bangladesh. This gives confidence that the model could be useful to assess potential SAR performance at locations in Bangladesh based on local hydrogeochemical conditions.

Combined Source Apportionment and Degradation Quantification of Organic Pollutants with CSIA: 1. Model Derivation

Compound-specific stable isotope analysis (CSIA) serves as a tool for source apportionment (SA) and for the quantification of the extent of degradation (QED) of organic pollutants. However, simultaneous occurrence of mixing of sources and degradation is generally believed to hamper both SA and QED. On the basis of the linear stable isotope mixing model and the Rayleigh equation, we developed the stable isotope sources and sinks model, which allows for simultaneous SA and QED of a pollutant that is emitted by two sources and degrades via one transformation process. It was shown that the model necessitates at least dual-element CSIA for unequivocal SA in the presence of degradation-induced isotope fractionation, as illustrated for perchlorate in groundwater. The model also enables QED, provided degradation follows instantaneous mixing of two sources. If mixing occurs after two sources have degraded separately, the model can still provide a conservative estimate of the overall extent of degradation. The model can be extended to a larger number of sources and sinks as outlined. It may aid in forensics and natural attenuation assessment of soil, groundwater, surface water, or atmospheric pollution.

Combined source apportionment and degradation quantification of organic pollutants with CSIA: 2. Model validation and application.

Compound-specific stable isotope analysis (CSIA) has proven a useful tool for the quantification of the extent of degradation (QED), and for source identification and source apportionment (SA) in contaminated environmental systems. However, the simultaneous occurrence of degradation processes and mixing of emission sources complicates the use of CSIA in combined SA and QED. In a companion study, we developed a mathematical model that allows for combined SA and QED of organic pollutants (and inorganic compounds such as nitrate) in a scenario of two emission sources and degradation via one reaction pathway. This work presents a validation of the model against virtual data from a two-dimensional reactive transport model. The model calculations for SA and QED were in good agreement with the simulation results, which suggests the correctness of the model assumptions. However, the application of the model to field data of benzene contamination was challenged by large uncertainties in CSIA data and the unknown interplay between competing degradation pathways. Nonetheless, the use of the model allowed for the identification of a prevailing contribution of one emission source and revealed a low overall extent of degradation at the field site. This indicates that the model can, for example, facilitate the characterization of air pollution or aquifer contamination with organic pollutants.