Denitrification in a Sand and Gravel Aquifer

Upscaling of Denitrification Rates from Point to Catchment Scales for Modeling of Nitrate Transport and Retention

The spatial and temporal variability of denitrification makes it challenging to integrate conceptual, process-based understandings of nitrate transport and retention into numerical modeling at the catchment scale, although it is critical for the realism and predictive power of the model. In this study, we propose a novel approach where the conceptual understandings of the spatial structure of denitrification zones and the corresponding representative denitrification rates are transformed into a form that can be integrated into a multi-point statistical simulation framework. This is done by constructing a denitrification training image (TI) coupled to a geophysically based TI of the hydrogeological structure. The field observations and laboratory analyses of denitrification rates and the chemistry of water and sediment revealed that the study catchment's subsurface can be characterized by three zones: (1) the oxic zone with no nitrate reduction; (2) the slow-denitrification zone (mean of ln-transformed rate = -1.19 ± 0.52 mg N L^-1 yr^-1); and (3) the high-denitrification zone (mean of ln-transformed rate = 3.86 ± 1.96 mg N L^-1 yr^-1). The underlying controls on the spatial distribution of these zones and the representativeness of denitrification rates were investigated. Then, a TI illustrating the subsurface structure of the denitrification zone was constructed by synthesizing the results of these geochemical interpretations and the hydrogeology TI.

Wetland buffer zones for nitrogen and phosphorus retention: Impacts of soil type, hydrology and vegetation

Wetland buffer zones (WBZs) are riparian areas that form a transition between terrestrial and aquatic environments and are well-known to remove agricultural water pollutants such as nitrogen (N) and phosphorus (P). This review attempts to merge and compare data on the nutrient load, nutrient loss and nutrient removal and/or retention from multiple studies of various WBZs termed as riparian mineral soil wetlands, groundwater-charged peatlands (i.e. fens) and floodplains. Two different soil types ('organic' and 'mineral'), four different main water sources ('groundwater', 'precipitation', 'surface runoff/drain discharge', and 'river inundation') and three different vegetation classes ('arboraceous', 'herbaceous' and 'aerenchymous') were considered separately for data analysis. The studied WBZs are situated within the temperate and continental climatic regions that are commonly found in northern-central Europe, northern USA and Canada. Surprisingly, only weak differences for the nutrient removal/retention capability were found if the three WBZ types were directly compared. The results of our study reveal that for example the nitrate retention efficiency of organic soils (53 ± 28%; mean ± sd) is only slightly higher than that of mineral soils (50 ± 32%). Variance in load had a stronger influence than soil type on the N retention in WBZs. However, organic soils in fens tend to be sources of dissolved organic N and soluble reactive P, particularly when the fens have become degraded due to drainage and past agricultural usage. The detailed consideration of water sources indicated that average nitrate removal efficiencies were highest for ground water (76 ± 25%) and lowest for river water (35 ± 24%). No significant pattern for P retention emerged; however, the highest absolute removal appeared if the P source was river water. The harvesting of vegetation will minimise potential P loss from rewetted WBZs and plant biomass yield may promote circular economy value chains and provide compensation to land owners for restored land now unsuitable for conventional farming.

Assessing the feasibility of sequential aerobic respiration and heterotrophic denitrification of a high-strength mixture of phenol and its derivatives in the field single-well-drift test

Biological degradation of high strength phenol and its derivatives in groundwater is problematic because these compounds are toxic to human and microbes. To evaluate the feasibility of in situ bioremediation using sequential aerobic respiration and heterotrophic denitrification, a field single-well-drift test (SWDT) was conducted in groundwater contaminated with coal tar distillates. To stimulate indigenous phenol degrading microorganisms, a 1400 L of oxygen-saturated test solution containing bromide (3.96 ± 0.179 mmol-Br/L) and nitrate (5.34 ± 0.187 mmol NO₃^--N/L) was injected into an aquifer. After injection of the test solution, significant consumption of dissolved oxygen (DO) was immediately observed; then, degradation of the methyl derivatives o-cresol and m,p-cresol was observed with average zero-order rate coefficients of 0.047 mmol/L/d and 0.23 mmol/L^/d, respectively. After 73% of the injected DO was consumed, significant NO₃^- consumption was observed along with degradation of phenol and the dimethyl derivatives 2,4-xylenol and 3,5-xylenol, which had average zero-order rate coefficients of 0.17 mmol/L/d, 0.060 mmol/L/d, and 0.018 mmol/L/d, respectively. The production of CO₂, NO₂^-, and N₂O along with significant consumption of DO and NO₃^- suggest that phenolic compounds were biologically degraded by sequential aerobic respiration and heterotrophic denitrification. The results of 16s RNA analysis revealed that, after injection of the test solution, a bacterium that shared a 99% 16s rRNA sequence similarity with an uncultured bacterium revealed to be Pseudomonas stutzeri, a facultative heterotrophic denitrifier, was found in the aquifer. Thus, these results suggest that simultaneous injection of DO and NO₃^- is an appropriate in situ bioremediation strategy for degrading mixtures of high-strength phenolic compounds in an aquifer.

Insights into Biodegradation Related Metabolism in an Abnormally Low Dissolved Inorganic Carbon (DIC) Petroleum-Contaminated Aquifer by Metagenomics Analysis

In petroleum-contaminated aquifers, biodegradation is always associated with various types of microbial metabolism. It can be classified as autotrophic (such as methanogenic and other carbon fixation) and heterotrophic (such as nitrate/sulfate reduction and hydrocarbon consumption) metabolism. For each metabolic type, there are several key genes encoding the reaction enzymes, which can be identified by metagenomics analysis. Based on this principle, in an abnormally low dissolved inorganic carbon (DIC) petroleum-contaminated aquifer in North China, nine groundwater samples were collected along the groundwater flow, and metagenomics analysis was used to discover biodegradation related metabolism by key genes. The major new finding is that autotrophic metabolism was revealed, and, more usefully, we attempt to explain the reasons for abnormally low DIC. The results show that the methanogenesis gene, Mcr, was undetected but more carbon fixation genes than nitrate reduction and sulfate genes were found. This suggests that there may be a considerable number of autotrophic microorganisms that cause the phenomenon of low concentration of dissolved inorganic carbon in contaminated areas. The metagenomics data also revealed that most heterotrophic, sulfate, and nitrate reduction genes in the aquifer were assimilatory sulfate and dissimilatory nitrate reduction genes. Although there was limited dissolved oxygen, aerobic degrading genes AlkB and Cdo were more abundant than anaerobic degrading genes AssA and BssA. The metagenomics information can enrich our microorganic knowledge about petroleum-contaminated aquifers and provide basic data for further bioremediation.

Identification of Sulfate Sources and Biogeochemical Processes in an Aquifer Affected by Peatland: Insights from Monitoring the Isotopic Composition of Groundwater Sulfate in Kampinos National Park, Poland

: Temporal and spatial variations of the concentration and the isotopic composition of groundwater sulfate in an unconfined sandy aquifer covered by peatland have been studied to better understand the sources and biogeochemical processes that affect sulfate distribution in shallow groundwater systems influenced by organic rich sediments. The groundwater monitoring was carried out for one year at hydrogeological station Pożary located within the protected zone of the Kampinos National Park. Sulfur (34SSO4) and oxygen (18OSO4) isotopic composition of dissolved sulfates were analyzed together with oxygen (18OH2O) and hydrogen (2HH2O) isotopic composition of water and major ions concentration at monthly intervals. The research revealed three main sources of sulfates dissolved in groundwater, namely, (a) atmospheric sulfates—supplied to the aquifer by atmospheric deposition (rain and snow melt), (b) sulfates formed by dissolution of evaporite sulfate minerals, mainly gypsum—considerably enriched in 34S and 18O, and (c) sulfate formed during oxidation of reduced inorganic sulfur compounds (RIS), mainly pyrite—depleted in 34S and 18O. The final isotopic composition and concentration of dissolved SO42− in groundwater are the result of overlapping processes of dissimilatory sulfate reduction, oxidation of sulfide minerals, and mixing of water in aquifer profile.

Identification of active denitrifiers by DNA-stable isotope probing and amplicon sequencing reveals Betaproteobacteria as responsible for attenuation of nitrate contamination in a low impacted aquifer

Groundwater reservoirs constitute important freshwater resources. However, these ecosystems are highly vulnerable to contamination and have to rely on the resident microbiota to attenuate the impact of this contamination. Nitrate is one of the main contaminants found in groundwater, and denitrification is the main process that removes the compound. In this study, the response to nutrient load on indigenous microbial communities in groundwater from a low impacted aquifer in Uruguay was evaluated. Denitrification rates were measured in groundwater samples from three different sites with nitrate, acetate and pyrite amendments. Results showed that denitrification is feasible under in situ nitrate and electron donor concentrations, although the lack of readily available organic energy source would limit the attenuation of higher nitrate concentrations. DNA-stable isotope probing, combined with amplicon sequencing of 16S rRNA, nirS and nirK genes, was used to identify the active denitrifiers. Members of the phylum Betaproteobacteria were the dominant denitrifiers in two of three sites, with different families being observed; members of the genus Vogesella (Neisseriaceae) were key denitrifiers at one site, while the genera Dechloromonas (Rhodocyclaceae) and Comamonas (Comamonadaceae) were the main denitrifiers detected at the other sites.

Hydrologic Controls on Nitrogen Cycling Processes and Functional Gene Abundance in Sediments of a Groundwater Flow-Through Lake

The fate and transport of inorganic nitrogen (N) is a critically important issue for human and aquatic ecosystem health because discharging N-contaminated groundwater can foul drinking water and cause algal blooms. Factors controlling N-processing were examined in sediments at three sites with contrasting hydrologic regimes at a lake on Cape Cod, MA. These factors included water chemistry, seepage rates and direction of groundwater flow, and the abundance and potential rates of activity of N-cycling microbial communities. Genes coding for denitrification, anaerobic ammonium oxidation (anammox), and nitrification were identified at all sites regardless of flow direction or groundwater dissolved oxygen concentrations. Flow direction was, however, a controlling factor in the potential for N-attenuation via denitrification in the sediments. Potential rates of denitrification varied from 6 to 4500 pmol N/g/h from the inflow to the outflow side of the lake, owing to fundamental differences in the supply of labile organic matter. The results of laboratory incubations suggested that when anoxia and limiting labile organic matter prevailed, the potential existed for concomitant anammox and denitrification. Where oxic lake water was downwelling, potential rates of nitrification at shallow depths were substantial (1640 pmol N/g/h). Rates of anammox, denitrification, and nitrification may be linked to rates of organic N-mineralization, serving to increase N-mobility and transport downgradient.

Remediation of nitrate-contaminated water by solid-phase denitrification process-a review

The paper presents a compilation of various autotrophic and heterotrophic ways of solid-phase denitrification. It covers a complete understanding of various pathways followed during denitrification process. The paper gives a brief review on various governing factors on which the process depends. It focuses mainly on the solid-phase denitrification process, its applicability, efficiency, and disadvantages associated. It presents a critical review on various methodologies associated with denitrification process reported in past years. A comparative study has also been carried out to have a better understanding of advantages and disadvantages of a particular method. We summarize the various organic and inorganic substances and various techniques that have been used for enhancing denitrification process and suggest possible gaps in the research areas whi'ch are worthy of future research.

Feeding strategies for groundwater enhanced biodenitrification in an alluvial aquifer: chemical, microbial and isotope assessment of a 1D flow-through experiment

Nitrate-removal through enhanced in situ biodenitrification (EISB) is an existing alternative for the recovery of groundwater quality, and is often suggested for use in exploitation wells pumping at small flow-rates. Innovative approaches focus on wider-scale applications, coupling EISB with water-management practices and new monitoring tools. However, before this approach can be used, some water-quality issues such as the accumulation of denitrification intermediates and/or of reduced compounds from other anaerobic processes must be addressed. With such a goal, a flow-through experiment using 100mg-nitrate/L groundwater was built to simulate an EISB for an alluvial aquifer. Heterotrophic denitrification was induced through the periodic addition of a C source (ethanol), with four different C addition strategies being evaluated to improve the quality of the denitrified water. Chemical, microbial and isotope analyses of the water were performed. Biodenitrification was successfully stimulated by the daily addition of ethanol, easily achieving drinking water standards for both nitrate and nitrite, and showing an expected linear trend for nitrogen and oxygen isotope fractionation, with a εN/εO value of 1.1. Nitrate reduction to ammonium was never detected. Water quality in terms of remaining C, microbial counts, and denitrification intermediates was found to vary with the experimental time, and some secondary microbial respiration processes, mainly manganese reduction, were suspected to occur. Carbon isotope composition from the remaining ethanol also changed, from an initial enrichment in (13)C-ethanol compared to the value of the injected ethanol (-30.6‰), to a later depletion, achieving δ(13)C values well below the initial isotope composition (to a minimum of -46.7‰). This depletion in the heavy C isotope follows the trend of an inverse fractionation. Overall, our results indicated that most undesired effects on water quality may be controlled through the optimization of the C/N ratio determined from the amounts of injected ethanol vs. the amount of nitrate in groundwater, with a smaller C/N ratio causing a lower level of undesired impurities. Furthermore, the authors suggest that the biofilm life-time has a direct effect on microbial population and hence affects biodenitrification performance, influencing the accumulation of nitrite over time.

Effects of enhanced denitrification on hydrodynamics and microbial community structure in a soil column system

Enhanced heterotrophic denitrification by adding glucose was investigated by means of a soil column experiment which simulated the groundwater flow. The carbon-to-nitrogen ratio was the main factor determining denitrification potential under experimental conditions. The influence of stimulated denitrification on the autochthonous microbial community was investigated by quantitative PCR (qPCR), and denaturing gradient gel electrophoresis (DGGE). The qPCR detection of the nosZ genes encoding nitrous oxide reductase, and the comparison of the abundances of 16S rRNA genes revealed that the addition of glucose enhanced denitrification leading to an increase in both the total eubacteria and, in particular, in the ratio of denitrifying bacteria, which represented the 21% of the total native eubacteria on the basis of nosZ/16S rRNA gene ratio. Microbial community profiling by DGGE indicated that ribotypes closely related to the genera Acidovorax and Hydrogenophaga (Comamonadaceae family) became enriched in the soil column. The effects of biomass occurrence in the column system on soil hydrodynamics, assessed by tracer studies, revealed a reduction of porosity and a significant increase of dispersivity that could be caused by the appearance of new functional microbial biomass in the aquifer material under enhanced denitrifying conditions. The importance of investigating the microbial growth in relation to the hydrodynamic effects, during enhanced denitrification, has been revealed in the column system experiments associated with the bioremediation. Combining microbial characterisation and hydrodynamic data in a soil column system permits us to gain an insight to the limiting factors of different stimulation strategies that can be applied in the field.

The impact of relict organic materials on the denitrification capacity in the unsaturated-saturated zone continuum of three volcanic profiles

The denitrification capacity of wetlands, riparian zones, and aquifers in glacial outwash areas is well documented, but little or no information exists for volcanic profiles, particularly those containing relict organic matter contained in or on top of paleosols (old soils buried by volcanic deposits) below the groundwater table. Relict carbon contained in these layers could provide the necessary electrons to fuel heterotrophic denitrification. To the best of our knowledge, this is the first study investigating the denitrification capacity in both the unsaturated and saturated zone of volcanic profiles. Samples from three profile types with differing organic matter distribution were amended with N-enriched nitrate (NO-) and incubated in the laboratory under anaerobic conditions. Dinitrogen (N) dominated the N gas fluxes; averaged across all samples, it accounted for 96% of the total N (nitrous oxide [NO] and N) gas fluxes. Dinitrogen fluxes were generally highest in the A horizon samples (4.1-6.2 nmol N g h), but substantial fluxes were also observed in some paleosol layers (up to 0.72 nmol N g h). A significant correlation ( < 0.001) was found between the concentration of extractable dissolved organic carbon and the total N gas flux produced in samples from below the A horizon, suggesting that heterotrophic denitrification was the dominant NO attenuation process in this study. Extrapolation of lab-derived denitrification capacities to field conditions suggests that the denitrification capacity of profiles containing relict soil organic matter in the saturated zone exceeds the estimated N leaching from the root zone.

Online measurement of denitrification rates in aquifer samples by an approach coupling an automated sampling and calibration unit to a membrane inlet mass spectrometry system

Aquifers within agricultural catchments are characterised by high spatial heterogeneity of their denitrification potential. Therefore, simple but sophisticated methods for measuring denitrification rates within the groundwater are crucial for predicting and managing N-fluxes within these anthropogenic ecosystems. Here, a newly developed automated online (15)N-tracer system is presented for measuring (N(2)+N(2)O) production due to denitrification in aquifer samples. The system consists of a self-developed sampler which automatically supplies sample aliquots to a membrane-inlet mass spectrometer. The developed system has been evaluated by a (15)N-nitrate tracer incubation experiment using samples (sulphidic and non-sulphidic) from the aquifer of the Fuhrberger Feld in northern Germany. It is shown that the membrane-inlet mass spectrometry (MIMS) system successfully enabled nearly unattended measurement of (N(2)+N(2)O) production within a range of 10 to 3300 µg N L(-1) over 7 days of incubation. The automated online approach provided results in good agreement with simultaneous measurements obtained with the well-established offline approach using isotope ratio mass spectrometry (IRMS). In addition, three different (15)N-aided mathematical approaches have been evaluated for their suitability to analyse the MIMS raw data under the given experimental conditions. Two approaches, which rely on the measurement of (28)N(2), (29)N(2) and (30)N(2), exhibit the best reliability in the case of a clear (15) N enrichment of evolved denitrification gases. The third approach, which uses only the ratio of (29)N(2)/(28)N(2), overestimates the concentration of labelled denitrification products under these conditions. By contrast, at low (15)N enrichments and low fractions of denitrified gas, the latter approach is on a par with the other two approaches. Finally, it can be concluded that the newly developed system represents a comprehensive and simply applicable tool for the determination of denitrification in aquifers.

Effects of the antimicrobial sulfamethoxazole on groundwater bacterial enrichment

The effects of "trace" (environmentally relevant) concentrations of the antimicrobial agent sulfamethoxazole (SMX) on the growth, nitrate reduction activity, and bacterial composition of an enrichment culture prepared with groundwater from a pristine zone of a sandy drinking-water aquifer on Cape Cod, MA, were assessed by laboratory incubations. When the enrichments were grown under heterotrophic denitrifying conditions and exposed to SMX, noticeable differences from the control (no SMX) were observed. Exposure to SMX in concentrations as low as 0.005 μM delayed the initiation of cell growth by up to 1 day and decreased nitrate reduction potential (total amount of nitrate reduced after 19 days) by 47% (p=0.02). Exposure to 1 μM SMX, a concentration below those prescribed for clinical applications but higher than concentrations typically detected in aqueous environments, resulted in additional inhibitions: reduced growth rates (p=5×10(-6)), lower nitrate reduction rate potentials (p=0.01), and decreased overall representation of 16S rRNA gene sequences belonging to the genus Pseudomonas. The reduced abundance of Pseudomonas sequences in the libraries was replaced by sequences representing the genus Variovorax. Results of these growth and nitrate reduction experiments collectively suggest that subtherapeutic concentrations of SMX altered the composition of the enriched nitrate-reducing microcosms and inhibited nitrate reduction capabilities.

Autotrophic, hydrogen-oxidizing, denitrifying bacteria in groundwater, potential agents for bioremediation of nitrate contamination

Addition of hydrogen or formate significantly enhanced the rate of consumption of nitrate in slurried core samples obtained from an active zone of denitrification in a nitrate-contaminated sand and gravel aquifer (Cape Cod, Mass.). Hydrogen uptake by the core material was immediate and rapid, with an apparent K(m) of 0.45 to 0.60 muM and a V(max) of 18.7 nmol cm h at 30 degrees C. Nine strains of hydrogen-oxidizing denitrifying bacteria were subsequently isolated from the aquifer. Eight of the strains grew autotrophically on hydrogen with either oxygen or nitrate as the electron acceptor. One strain grew mixotrophically. All of the isolates were capable of heterotrophic growth, but none were similar to Paracoccus denitrificans, a well-characterized hydrogen-oxidizing denitrifier. The kinetics for hydrogen uptake during denitrification were determined for each isolate with substrate depletion progress curves; the K(m)s ranged from 0.30 to 3.32 muM, with V(max)s of 1.85 to 13.29 fmol cell h. Because these organisms appear to be common constituents of the in situ population of the aquifer, produce innocuous end products, and could be manipulated to sequentially consume oxygen and then nitrate when both were present, these results suggest that these organisms may have significant potential for in situ bioremediation of nitrate contamination in groundwater.

Nitrate reduction in a groundwater microcosm determined by N gas chromatography-mass spectrometry

Aerobic and anaerobic groundwater continuous-flow microcosms were designed to study nitrate reduction by the indigenous bacteria in intact saturated soil cores from a sandy aquifer with a concentration of 3.8 mg of NO(3)-N liter. Traces of NO(3) were added to filter-sterilized groundwater by using a Darcy flux of 4 cm day. Both assimilatory and dissimilatory reduction rates were estimated from analyses of N(2), N(2)O, NH(4), and N-labeled protein amino acids by capillary gas chromatography-mass spectrometry. N(2) and N(2)O were separated on a megabore fused-silica column and quantified by electron impact-selected ion monitoring. NO(3) and NH(4) were analyzed as pentafluorobenzoyl amides by multiple-ion monitoring and protein amino acids as their N-heptafluorobutyryl isobutyl ester derivatives by negative ion-chemical ionization. The numbers of bacteria and their [methyl-H]thymidine incorporation rates were simultaneously measured. Nitrate was completely reduced in the microcosms at a rate of about 250 ng g day. Of this nitrate, 80 to 90% was converted by aerobic denitrification to N(2), whereas only 35% was denitrified in the anaerobic microcosm, where more than 50% of NO(3) was reduced to NH(4). Assimilatory reduction was recorded only in the aerobic microcosm, where N appeared in alanine in the cells. The nitrate reduction rates estimated for the aquifer material were low in comparison with rates in eutrophic lakes and coastal sediments but sufficiently high to remove nitrate from an uncontaminated aquifer of the kind examined in less than 1 month.

Estimation of indirect nitrous oxide emissions from a shallow aquifer in northern Germany

Ground water is considered to be an important source for indirect N2O emissions. We investigated indirect N2O emissions from a shallow aquifer in Germany over a 1-yr period. Because N2O accumulated in considerable amounts in the surface ground water (mean, 52.86 microg N2O-N L(-1)) and corresponding fluxes were high (up to 34 microg N2O-N m(-2) h(-1)), it was hypothesized that significant indirect N2O emissions would occur via the vertical and the lateral emission pathway. Vertical N2O emissions were investigated by measuring N2O concentrations and calculating fluxes from the surface ground water to the unsaturated zone and at the soil surface. Lateral N2O fluxes were investigated by measuring ground water N2O and NO3- concentrations at five multilevel wells and at a waterworks well. Negligible amounts of N2O were emitted vertically into the unsaturated zone; most of it was convectively transported into the deeper autotrophic denitrification zone. Only a ground water level fall and rise triggered the emission of N2O (up to 3 microg N2O-N m(-2) h(-1)) into the unsaturated zone. Ground water-derived N2O was probably reduced during the upward diffusion, and soil surface emissions were governed by topsoil processes. Along the lateral pathway, N2O and NO3- concentrations decreased with increasing depth in the aquifer. Discharging ground water was almost free of N2O and NO3-, and indirect N2O emissions were small.

Determining the probability of arsenic in groundwater using a parsimonious model

Spatial distributions of groundwater quality are commonly heterogeneous, varying with depths and locations, which is important in assessing the health and ecological risks. Owing to time and cost constraints, it is not practical or economical to measure arsenic everywhere. A predictive model is necessary to estimate the distribution of a specific pollutant in groundwater. This study developed a logistic regression (LR) model to predict the residential well water quality in the Lanyang plain. Six hydrochemical parameters, pH, NO3- -N, NO2- -N, NH+ -N, Fe, and Mn, and a regional variable (binary type) were used to evaluate the probability of arsenic concentrations exceeding 10 microg/L in groundwater. The developed parsimonious LR model indicates that four parameters in the Lanyang plain aquifer, (pH, NH4+, Fe(aq), and a component to account for regional heterogeneity) can accurately predict probability of arsenic concentration > or =1 microg/Lin groundwater. These parameters provide an explanation for release of arsenic by reductive dissolution of As-rich FeOOH in NH4+ containing groundwater. A comparison of LR and indicator kriging (IK) show similar results in modeling the distributions of arsenic. LR can be applied to assess the probability of groundwater arsenic at sampled sites without arsenic concentration data apriori. However, arsenic sampling is still needed and required in arsenic-assessment stages in other areas, and the need for long-term monitoring and maintenance is not precluded.

Microbial characterization of nitrification in a shallow, nitrogen-contaminated aquifer, Cape Cod, Massachusetts and detection of a novel cluster associated with nitrifying Betaproteobacteria

Groundwater nitrification is a poorly characterized process affecting the speciation and transport of nitrogen. Cores from two sites in a plume of contamination were examined using culture-based and molecular techniques targeting nitrification processes. The first site, located beneath a sewage effluent infiltration bed, received treated effluent containing O2 (>300 microM) and NH4+ (51-800 microM). The second site was 2.5 km down-gradient near the leading edge of the ammonium zone within the contaminant plume and featured vertical gradients of O2, NH4+, and NO3- (0-300, 0-500, and 100-200 microM with depth, respectively). Ammonia- and nitrite-oxidizers enumerated by the culture-based MPN method were low in abundance at both sites (1.8 to 350 g(-1) and 33 to 35,000 g(-1), respectively). Potential nitrifying activity measured in core material in the laboratory was also very low, requiring several weeks for products to accumulate. Molecular analysis of aquifer DNA (nested PCR followed by cloning and 16S rDNA sequencing) detected primarily sequences associated with the Nitrosospira genus throughout the cores at the down-gradient site and a smaller proportion from the Nitrosomonas genus in the deeper anoxic, NH4+ zone at the down-gradient site. Only a single Nitrosospira sequence was detected beneath the infiltration bed. Furthermore, the majority of Nitrosospira-associated sequences represent an unrecognized cluster. We conclude that an uncharacterized group associated with Nitrosospira dominate at the geochemically stable, down-gradient site, but found little evidence for Betaproteobacteria nitrifiers beneath the infiltration beds where geochemical conditions were more variable.

Occurrence and turnover of nitric oxide in a nitrogen-impacted sand and gravel aquifer

Little is known about nitric oxide (NO) production or consumption in the subsurface, an environment which may be conducive to NO accumulation. A study conducted in a nitrogen-contaminated aquifer on Cape Cod, Massachusetts assessed the occurrence and turnover of NO within a contaminant plume in which nitrification and denitrification were known to occur. NO (up to 8.6 nM) was detected in restricted vertical zones located within a nitrate (NO3-) gradient and characterized by low dissolved oxygen (< 10 microM). NO concentrations correlated best with nitrite (NO2-) (up to 35 microM), but nitrous oxide (N2O) (up to 1 microM) also was present. Single-well injection tests were used to determine NO production and consumption in situ within these zones. First-order rate constants for NO consumption were similar (0.05-0.08 h(-1)) at high and low (260 and 10 nM) NO concentrations, suggesting a turnover time at in situ concentrations of 10-20 h. Tracer tests with 15N[NO] demonstrated that oxidation to 15N[NO2-] occurred only during the initial stages, but after 4 h reduction to 15N[N2O] was the primary reaction product. Added NO2- (31 microM) or NO3- (53 microM) resulted in a linear NO accumulation at 2.4 and 1.0 nM h(-1) for the first 6 h of in situ tests. These results suggest that NO was primarily produced by denitrification within this aquifer.

Nitrate attenuation in groundwater: a review of biogeochemical controlling processes

Biogeochemical processes controlling nitrate attenuation in aquifers are critically reviewed. An understanding of the fate of nitrate in groundwater is vital for managing risks associated with nitrate pollution, and to safeguard groundwater supplies and groundwater-dependent surface waters. Denitrification is focused upon as the dominant nitrate attenuation process in groundwater. As denitrifying bacteria are essentially ubiquitous in the subsurface, the critical limiting factors are oxygen and electron donor concentration and availability. Variability in other environmental conditions such as nitrate concentration, nutrient availability, pH, temperature, presence of toxins and microbial acclimation appears to be less important, exerting only secondary influences on denitrification rates. Other nitrate depletion mechanisms such as dissimilatory nitrate reduction to ammonium and assimilation of nitrate into microbial biomass are unlikely to be important in most subsurface settings relative to denitrification. Further research is recommended to improve current understanding on the influence of organic carbon, sulphur and iron electron donors, physical restrictions on microbial activity in dual porosity aquifers, influences of environmental condition (e.g. pH in poorly buffered environments and salinity in coastal or salinized soil settings), co-contaminant influences (particularly the contrasting inhibitory and electron donor influences of pesticides) and improved quantification of denitrification rates in the laboratory and field.

Assessment of nitrification potential in ground water using short term, single-well injection experiments

Nitrification was measured within a sand and gravel aquifer on Cape Cod, MA, using a series of single-well injection tests. The aquifer contained a wastewater-derived contaminant plume, the core of which was anoxic and contained ammonium. The study was conducted near the downgradient end of the ammonium zone, which was characterized by inversely trending vertical gradients of oxygen (270 to 0 microM) and ammonium (19 to 625 microM) and appeared to be a potentially active zone for nitrification. The tests were conducted by injecting a tracer solution (ambient ground water + added constituents) into selected locations within the gradients using multilevel samplers. After injection, the tracers moved by natural ground water flow and were sampled with time from the injection port. Rates of nitrification were determined from changes in nitrate and nitrite concentration relative to bromide. Initial tests were conducted with (15)N-enriched ammonium; subsequent tests examined the effect of adding ammonium, nitrite, or oxygen above background concentrations and of adding difluoromethane, a nitrification inhibitor. In situ net nitrate production exceeded net nitrite production by 3- to 6- fold and production rates of both decreased in the presence of difluoromethane. Nitrification rates were 0.02-0.28 mumol (L aquifer)(-1) h(-1) with in situ oxygen concentrations and up to 0.81 mumol (L aquifer)(-1) h(-1) with non-limiting substrate concentrations. Geochemical considerations indicate that the rates derived from single-well injection tests yielded overestimates of in situ rates, possibly because the injections promoted small-scale mixing within a transport-limited reaction zone. Nonetheless, these tests were useful for characterizing ground water nitrification in situ and for comparing potential rates of activity when the tracer cloud included non-limiting ammonium and oxygen concentrations.

Model parameters for simulating fate and transport of on-site wastewater nutrients

This paper presents a critical review of model-input parameters for transport of on-site wastewater treatment system (OWS) pollutants. Approximately 25% of the U.S. population relies on soil-based OWS for effective treatment and protection of public health and environmental quality. Mathematical models are useful tools for understanding and predicting the transport and fate of wastewater pollutants and for addressing water-budget issues related to wastewater reclamation from site to watershed scales. However, input parameters for models that simulate fate and transport of OWS pollutants are not readily obtained. The purpose of this analysis is to illustrate an objective, statistically supported method for choosing model-input parameters related to nitrogen (N) and phosphorus (P). Data were gathered from existing studies reported in the literature. Cumulative frequency distributions (CFDs) are provided for OWS effluent concentrations of N and P, nitrification and denitrification rates, and linear sorption isotherm constants for P. When CFDs are not presented, ranges and median values are provided. Median values for model-input parameters are as follows: total N concentration (44 mg/L), nitrate-N (0.2 mg/L), ammonium (60 mg/L), phosphate-P (9 mg/L), organic N (14 mg/L), zero-order nitrification rate (264 mg/L/d), first-order nitrification (2.9/d), first-order dentrification (0.025/d), maximum soil capacity for P uptake (237 mg/kg), linear sorption isotherm constant for P (15.1 L/kg), and OWS effluent flow rates (260 L/person/d).

In situ ground water denitrification in stratified, permeable soils underlying riparian wetlands

The ground water denitrification capacity of riparian zones in deep soils, where substantial ground water can flow through low-gradient stratified sediments, may affect watershed nitrogen export. We hypothesized that the vertical pattern of ground water denitrification in riparian hydric soils varies with geomorphic setting and follows expected subsurface carbon distribution (i.e., abrupt decline with depth in glacial outwash vs. negligible decline with depth in alluvium). We measured in situ ground water denitrification rates at three depths (65, 150, and 300 cm) within hydric soils at four riparian sites (two per setting) using a 15N-enriched nitrate "push-pull" method. No significant difference was found in the pattern and magnitude of denitrification when grouping sites by setting. At three sites there was no significant difference in denitrification among depths. Correlations of site characteristics with denitrification varied with depth. At 65 cm, ground water denitrification correlated with variables associated with the surface ecosystem (temperature, dissolved organic carbon). At deeper depths, rates were significantly higher closer to the stream where the subsoil often contains organically enriched deposits that indicate fluvial geomorphic processes. Mean rates ranged from 30 to 120 microg N kg(-1) d(-1) within 10 m versus <1 to 40 microg N kg(-1) d(-1) at >30 m from the stream. High denitrification rates observed in hydric soils, down to 3 m within 10 m of the stream in both alluvial and glacial outwash settings, argue for the importance of both settings in evaluating the significance of riparian wetlands in catchment-scale N dynamics.

Nitrate-consuming processes in a petroleum-contaminated aquifer quantified using push-pull tests combined with 15N isotope and acetylene-inhibition methods

Nitrate consumption in aquifers may result from several biogenic and abiotic processes such as denitrification, assimilatory NO3- reduction, dissimilatory NO3- reduction to ammonium (DNRA), or abiotic NO3- (or NO2-) reduction. The objectives of this study were to investigate the fate of NO3- in a petroleum-contaminated aquifer, and to assess the feasibility of using single-well push-pull tests (PPTs) in combination with 15N isotope and C2H2 inhibition methods for the quantification of processes contributing to NO3- consumption. Three consecutive PPTs were performed in a monitoring well of a heating oil-contaminated aquifer in Erlen, Switzerland. For each test, we injected 500 l of test solution containing 0.5 mM Br- as conservative tracer and either 0.5 mM unlabeled NO3- or approximately 0.3 mM 15N-labeled NO3- as reactant. Test solutions were sparged during preparation and injection with either N2, Ar or 10% C2H2 in Ar. After an initial incubation period of 1.5-3.2 h, we extracted the test solution/groundwater mixtures from the same location and measured concentrations of relevant species including Br-, NO3-, NO2-, N2O, N2, and NH4+. In addition, we determined the 15N contents of N2, N2O, NH4+, and suspended biomass from 15N/14N isotope-ratio measurements. Average total test duration was 50.5 h. First-order rate coefficients (k) were computed from measured NO3- consumption, N2-15N production and N2O-15N production. From measured NO3- consumption we obtained nearly identical estimates of k for all PPTs with small 95% confidence intervals, indicating good reproducibility and accuracy for the tests. Estimates of k from N2-15N production and N2O-15N production indicated that denitrification accounted for only 46-49% of observed NO3- consumption. Production of N2-15N in the presence of C2H2 was observed during one of the tests, which may be an indicator for abiotic NO3- reduction. Moreover, 15N isotope analyses confirmed occurrence of assimilatory NO3- reduction (0.58 at.% 15N in suspended biomass) and to a smaller extent DNRA (up to 4 at.% 15N in NH4+). Our results indicated that the combination of PPTs, 15N-isotope and C2H2 inhibition methods provided improved information on denitrification as well as alternative fates of NO3- in this aquifer.

A novel in situ technology for the treatment of nitrate contaminated groundwater

A novel in situ membrane technology was developed to remove nitrate (NO3-) from groundwater. Membrane-fed hydrogen gas (H2) was used as an electron donor to stimulate denitrification. A flow-through reactor fit with six hollow-fiber membranes (surface area = 93 cm2) was designed to simulate groundwater flowing through an aquifer with a velocity of 0.3 m/day. This membrane technology supported excellent NO3- and nitrite (NO2-) removal once H2 and carbon limitations were corrected. The membrane module achieved a maximum H2 flux of 1.79 x 10(-2) mg H2/m2 s, which was sufficient to completely remove 16.4 mg/L NO3(-)-N from a synthetic groundwater with no NO2- accumulation. In addition, this model in situ treatment process produced a high quality water containing <0.5 mg/L total organic carbon.

Transport behavior of groundwater protozoa and protozoan-sized microspheres in sandy aquifer sediments

Transport behaviors of unidentified flagellated protozoa (flagellates) and flagellate-sized carboxylated microspheres in sandy, organically contaminated aquifer sediments were investigated in a small-scale (1 to 4-m travel distance) natural-gradient tracer test on Cape Cod and in flow-through columns packed with sieved (0.5-to 1.0-mm grain size) aquifer sediments. The minute (average in situ cell size, 2 to 3 (mu)m) flagellates, which are relatively abundant in the Cape Cod aquifer, were isolated from core samples, grown in a grass extract medium, labeled with hydroethidine (a vital eukaryotic stain), and coinjected into aquifer sediments along with bromide, a conservative tracer. The 2-(mu)m flagellates appeared to be near the optimal size for transport, judging from flowthrough column experiments involving a polydispersed (0.7 to 6.2 (mu)m in diameter) suspension of carboxylated microspheres. However, immobilization within the aquifer sediments accounted for a log unit reduction over the first meter of travel compared with a log unit reduction over the first 10 m of travel for indigenous, free-living groundwater bacteria in earlier tests. High rates of flagellate immobilization in the presence of aquifer sediments also was observed in the laboratory. However, immobilization rates for the laboratory-grown flagellates (initially 4 to 5 (mu)m) injected into the aquifer were not constant and decreased noticeably with increasing time and distance of travel. The decrease in propensity for grain surfaces was accompanied by a decrease in cell size, as the flagellates presumably readapted to aquifer conditions. Retardation and apparent dispersion were generally at least twofold greater than those observed earlier for indigenous groundwater bacteria but were much closer to those observed for highly surface active carboxylated latex microspheres. Field and laboratory results suggest that 2-(mu)m carboxylated microspheres may be useful as analogs in investigating several abiotic aspects of flagellate transport behavior in groundwater.

Effect of treated-sewage contamination upon bacterial energy charge, adenine nucleotides, and DNA content in a sandy aquifer on Cape Cod

Changes in adenylate energy charge (ECA) and in total adenine nucleotides (A(T) and DNA content (both normalized to the abundance of free-living, groundwater bacteria) in response to carbon loading were determined for a laboratory-grown culture and for a contaminated aquifer. The latter study involved a 3-km-long transect through a contaminant plume resulting from continued on-land discharge of secondary sewage to a shallow, sandy aquifer on Cape Cod, Mass. With the exception of the most contaminated groundwater immediately downgradient from the contaminant source, DNA and adenylate levels correlated strongly with bacterial abundance and decreased exponentially with increasing distance downgradient. ECAS (0.53 to 0.60) and the ratios of ATP to DNA (0.001 to 0.003) were consistently low, suggesting that the unattached bacteria in this groundwater study are metabolically stressed, despite any eutrophication that might have occurred. Elevated ECAS (up to 0.74) were observed in glucose-amended groundwater, confirming that the metabolic state of this microbial community could be altered. In general, per-bacterium DNA and ATP contents were approximately twofold higher in the plume than in surrounding groundwater, although ECA and per-bacterium levels of A(T) differed little in the plume and the surrounding uncontaminated groundwater. However, per-bacterium levels of DNA and A(T) varied six- and threefold, respectively, during a 6-h period of decreasing growth rate for an unidentified pseudomonad isolated from contaminated groundwater and grown in batch culture.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of existing denitrification enzyme activity by chloramphenicol

Chloramphenicol completely inhibited the activity of existing denitrification enzymes in acetylene-block incubations with (i) sediments from a nitrate-contaminated aquifer and (ii) a continuous culture of denitrifying groundwater bacteria. Control flasks with no antibiotic produced significant amounts of nitrous oxide in the same time period. Amendment with chloramphenicol after nitrous oxide production had begun resulted in a significant decrease in the rate of nitrous oxide production. Chloramphenicol also decreased (greater than 50%) the activity of existing denitrification enzymes in pure cultures of Pseudomonas denitrificans that were harvested during log-phase growth and maintained for 2 weeks in a starvation medium lacking electron donor. Short-term time courses of nitrate consumption and nitrous oxide production in the presence of acetylene with P. denitrificans undergoing carbon starvation were performed under optimal conditions designed to mimic denitrification enzyme activity assays used with soils. Time courses were linear for both chloramphenicol and control flasks, and rate estimates for the two treatments were significantly different at the 95% confidence level. Complete or partial inhibition of existing enzyme activity is not consistent with the current understanding of the mode of action of chloramphenicol or current practice, in which the compound is frequently employed to inhibit de novo protein synthesis during the course of microbial activity assays. The results of this study demonstrate that chloramphenicol amendment can inhibit the activity of existing denitrification enzymes and suggest that caution is needed in the design and interpretation of denitrification activity assays in which chloramphenicol is used to prevent new protein synthesis.

Isolation and characterization of a nitrite reductase gene and its use as a probe for denitrifying bacteria

The dissimilatory nitrite reductase gene (nir) from denitrifying bacterium Pseudomonas stutzeri JM300 was isolated and sequenced. In agreement with recent sequence information from another strain of P. stutzeri (strain ZoBell), strain JM300 nir is the first gene in an operon and is followed immediately by a gene which codes for a tetraheme protein; 2.5 kb downstream from the nitrite reductase carboxyl terminus is the cytochrome c551 gene. P. stutzeri JM300 nir is 67% homologous to P. aeruginosa nir and 88% homologous to P. stutzeri ZoBell nir. Within the nitrite reductase promoter region is an fnr-like operator very similar to an operator upstream of a separate anaerobic pathway, that for arginine catabolism in P. aeruginosa. The denitrification genes in P. stutzeri thus may be under the same regulatory control as that found for other anaerobic pathways of pseudomonads. We have generated gene probes from restriction fragments within the nitrite reductase operon to evaluate their usefulness in ecology studies of denitrification. Probes generated from the carboxyl terminus region hybridized to denitrifying bacteria from five separate genera and did not cross-hybridize to any nondenitrifying bacteria among six genera tested. The denitrifier probes were successful in detecting denitrifying bacteria from samples such as a bioreactor consortium, aquifer microcosms, and denitrifying toluene-degrading enrichments. The probes also were used to reveal restriction fragment length polymorphism patterns indicating the diversity of denitrifiers present in these mixed communities.

In situ measurement of methane oxidation in groundwater by using natural-gradient tracer tests

Methane oxidation was measured in an unconfined sand and gravel aquifer (Cape Cod, Mass.) by using in situ natural-gradient tracer tests at both a pristine, oxygenated site and an anoxic, sewage-contaminated site. The tracer sites were equipped with multilevel sampling devices to create target grids of sampling points; the injectate was prepared with groundwater from the tracer site to maintain the same geochemical conditions. Methane oxidation was calculated from breakthrough curves of methane relative to halide and inert gas (hexafluroethane) tracers and was confirmed by the appearance of 13C-enriched carbon dioxide in experiments in which 13C-enriched methane was used as the tracer. A Vmax for methane oxidation could be calculated when the methane concentration was sufficiently high to result in zero-order kinetics throughout the entire transport interval. Methane breakthrough curves could be simulated by modifying a one-dimensional adevection-dispersion transport model to include a Michaelis-Menten-based consumption term for methane oxidation. The Km values for methane oxidation that gave the best match for the breakthrough curve peaks were 6.0 and 9.0 microM for the uncontaminated and contaminated sites, respectively. Natural-gradient tracer tests are a promising approach for assessing microbial processes and for testing in situ bioremediation potential in groundwater systems.