Denitrification In Aquifers Research Articles

Nitrate attenuation potential in karst conduits and aquifer matrix

A critical determinant of the effective denitrification potential in karst aquifers is the relative contribution of matrix and non-matrix (e.g., conduits) to the total groundwater flow. This work tests the hypothesis that observed karst aquifer-scale denitrification rates represent the superposition of the effects from matrix zones, which have low denitrification rates and long residence times, and non-matrix zones, which have higher denitrification rates and short residence times. To better evaluate karst whole-aquifer denitrification capacity, this study examined groundwater chemical data from 69 individual wells across the springshed of Silver Springs, FL, and conducted five push–pull tracer tests (PPTTs) in matrix and non-matrix aquifer sections. Mean residence times and mean zero-order denitrification rates (K0) were determined using reduced-complexity analytical models for matrix and non-matrix zones. Significant nitrate degradation at the local scale was observed in wells that sampled matrix portions of the aquifer (K0 ≈ 5 to 7 × 103 μmol N/L/d), indicating denitrification hotspots comparable to previous results employing the same PPTT method. However, PPTTs in matrix portions of the aquifer found no evidence of aquifer denitrification, likely due to the short residence times experienced during PPTTs. The analytical models coupled with the measured nitrate concentrations and excess N2 data in groundwater discharged at the spring vents showed whole-aquifer estimates of residence times tavg = 4.2 years and aquifer denitrification rate K0,avg = 0.01 ± 0.01 μmol N/L/d that were consistent with previous studies. However, the average denitrification rate for non-matrix zones K0,nm = 1.29 ± 0.92 μmol N/L/d was two orders of magnitude higher than that for matrix zones K0,m = 0.002 ± 0.001 μmol N/L/d. Whole-aquifer denitrification measures thus reflect the weighted contributions of low denitrification in non-matrix zones, which constitute the majority of the groundwater flow, and high-denitrification but low-flow matrix zones. The modeling approach with non-matrix networks from this study can provide insight into the catchment-scale N budget and transport in karst aquifers.

Genotypic and phenotypic characterization of hydrogenotrophic denitrifiers.

Stimulating litho-autotrophic denitrification in aquifers with hydrogen is a promising strategy to remove excess NO3 - , but it often entails accumulation of the cytotoxic intermediate NO2 - and the greenhouse gas N2 O. To explore if these high NO2 - and N2 O concentrations are caused by differences in the genomic composition, the regulation of gene transcription or the kinetics of the reductases involved, we isolated hydrogenotrophic denitrifiers from a polluted aquifer, performed whole-genome sequencing and investigated their phenotypes. We therefore assessed the kinetics of NO2 - , NO, N2 O, N2 and O2 as they depleted O2 and transitioned to denitrification with NO3 - as the only electron acceptor and hydrogen as the electron donor. Isolates with a complete denitrification pathway, although differing intermediate accumulation, were closely related to Dechloromonas denitrificans, Ferribacterium limneticum or Hydrogenophaga taeniospiralis. High NO2 - accumulation was associated with the reductases' kinetics. While available, electrons only flowed towards NO3 - in the narG-containing H. taeniospiralis but flowed concurrently to all denitrification intermediates in the napA-containing D. denitrificans and F. limneticum. The denitrification regulator RegAB, present in the napA strains, may further secure low intermediate accumulation. High N2 O accumulation only occurred during the transition to denitrification and is thus likely caused by delayed N2 O reductase expression.

Dynamic Mechanisms of Groundwater Quality by Residual Contaminants of the Tanghe Wastewater Reservoir in Xiong'an New Area

Tanghe wastewater reservoir(TWR) is located on the west side of Baiyangdian Lake in Xiong'an New Area, where sewage infiltration and irrigation has been taking place for 40 years, and a large number of contaminants have accumulated in the unsaturated zone. Identifying the mechanisms by which this combined system contributes to groundwater hydrochemical dynamics is important for the protection of the water environment in the area. Hydrogeochemical methods such as tracing and improved chlor-alkali index are used to analyze the spatial and temporal characteristics and evolution mechanisms of shallow groundwater. The study shows that the groundwater chemistry in the sewage reservoir area is SO4·HCO3-Na type, with an average sewage fraction of 48.4%, and the contribution of Na+ from ion exchange and halite dissolution is 29.9% and 8.6%, respectively. The chemical type of groundwater in the sewage irrigation area is SO4·HCO3-Na·Mg, the average sewage fraction is 58.3%, and Na+ consumption of ion exchange is 8.1%. The mix dilution of precipitation and irrigation leads to a reduction in the effluent fraction and saturation index in the groundwater, and promotes the adsorption of Na+ from groundwater into the soil. Denitrification in aquifers can effectively reduce groundwater nitrate pollution. In addition, the sewage fraction before and after the restoration of the reservoir was 61.5% and 49.3%, respectively. Pollutants retained in the sewage infiltration and irrigation combined system will continue to affect the quality of shallow groundwater with varying degrees of mixing and water-rock interaction driven by rainfall and irrigation.

A hillslope-scale aquifer-model to determine past agricultural legacy and future nitrate concentrations in rivers

The long-term fate of agricultural nitrate depends on rapid subsurface transfer, denitrification and storage in aquifers. Quantifying these processes remains an issue due to time varying subsurface contribution, unknown aquifer storage and heterogeneous denitrification potential. Here, we develop a parsimonious modelling approach that uses long-term discharge and river nitrate concentration time-series combined with groundwater age data determined from chlorofluorocarbons in springs and boreholes. To leverage their informational content, we use a Boussinesq-type equivalent hillslope model to capture the dynamics of aquifer flows and evolving surface and subsurface contribution to rivers. Nitrate transport was modelled with a depth-resolved high-order finite-difference method and denitrification by a first-order law. We applied the method to three heavily nitrate loaded catchments of a crystalline temperate region of France (Brittany). We found that mean water transit time ranged 10–32 years and Damköhler ratio (transit time/denitrification time) ranged 0.12–0.55, leading to limited denitrification in the aquifer (10–20%). The long-term trajectory of nitrate concentration in rivers appears determined by flows stratification in the aquifer. The results suggest that autotrophic denitrification is controlled by the accessibility of reduced minerals which occurs at the base of the aquifer where flows decrease. One interpretation is that denitrification might be an interfacial process in zones that are weathered enough to transmit flows and not too weathered to have remaining accessible reduced minerals. Consequently, denitrification would not be controlled by the total aquifer volume and related mean transit time but by the proximity of the active weathered interface with the water table. This should be confirmed by complementary studies to which the developed methodology might be further deployed.

Production, characterization, and evaluation of two types of slow-releasing carbon source tablets for in-situ heterotrophic nitrate denitrification in aquifers

Slow-releasing carbon source tablets were manufactured for an in-situ biological denitrification system. The average zero-order nitrate degradation rates seen, from highest to lowest, were in microcosms to which lactate, fumarate, propionate, and formate had been added. Fumarate was approximately 80% cheaper than lactate, and consequently was determined to be the most optimal slow-releasing carbon source in tablet form. The slow-releasing precipitating tablet (SRPT) and slow-releasing floating tablet (SRFT) were manufactured with hydroxypropyl methylcellulose (HPMC) as the agent of release control, microcrystalline cellulose pH 101 (MCC 101) as the binder, #8 sand as the precipitation agent, and calcium carbonate and citric acid as floating agents. Fourier transform infrared spectroscopy and powder X-ray diffraction indicated that the crystal arrangement in the SRPTs and SRFTs was maintained and ordered in a manner similar to raw excipients. SRFTs floated in water within 30 min and remained so for 5 d due to the buoyancy of carbon dioxide. The carbon source release rate was proportional to the quantity of HPMC added. The longevities of SRPT with 300 mg of HPMC and SRFT with 400 mg of HPMC were 25.4 d and 37.3 d, respectively. This study observed that SRPT and SRFT were manufactured effectively and are suitable for in-situ slow-releasing biological systems.

Spatially distributed denitrification in a karst springshed

AbstractKarst spring measurements assess biogeochemical processes occurring within groundwater contributing areas to springs (springsheds) but can only provide aggregated information. To better understand spatially distributed processes that comprise these aggregated measures, we investigated aquifer denitrification evidence in groundwater wells (n = 16) distributed throughout a springshed in the Upper Floridan aquifer in northern Florida. Aquifer geochemistry, nitrate isotopes, and dissolved gases were compared against similar measurements at the spring outlet to evaluate spatial heterogeneity of denitrification evidence in relation to land surface–aquifer connectivity. Sample locations spanned spatial variation in recharge processes (i.e., diffuse vs. focused recharge) and proximity to sources of denitrification reactants (e.g., wetlands). Although no distinct spatial pattern in denitrification was uncovered, excess dissolved N2 gas measurements were only above detection in the unconfined springshed, with some evidence of a wetland proximity effect. Measured oxidation–reduction potential and dissolved oxygen poorly predicted denitrification, indicating that measured denitrification may be occurring upgradient from sampled wells. Despite dramatic spatial chemical heterogeneity across wells, mean values for recharge nitrate concentrations (0.02 to 5.56 mg N L−1) and excess N2 from aquifer denitrification (below detection to 1.37 mg N L−1) corresponded reasonably with mean spring outlet measurements for initial nitrate (0.78 to 1.36 mg N L−1) and excess N2 (0.15 to 1.04 mg N L−1). Congruence between groundwater and spring measurements indicates that combining sampling at the spring outlet and across the springshed is useful for understanding spatial aquifer denitrification. However, this approach would be improved with a high‐density sampling network with transects of wells along distinct groundwater flow paths.

Using SWAT-LUD Model to Estimate the Influence of Water Exchange and Shallow Aquifer Denitrification on Water and Nitrate Flux

Numerous studies have pointed out the importance of groundwater and surface water interaction (SW–GW) in a river system. However; those functions have rarely been considered in large scale hydrological models. The SWAT-LUD model has been developed based on the Soil and Water Assessment Tool (SWAT) model; and it integrates a new type of subbasin; which is called subbasin-LU (SL); to represent the floodplain area. New modules representing SW–GW exchanges and shallow aquifer denitrification are developed in the SWAT-LUD model. In this study; the SWAT-LUD model was applied to the middle floodplain area of the Garonne catchment in France. The results showed that the SWAT-LUD model could represent the SW–GW exchange and shallow aquifer denitrification appropriately. An annual 44.1 × 107 m3 of water flowed into the river from the study area; but the annual exchanged water volume was 6.4 × 107 m3; which represented just 1% of the river discharge. A total of 384 tons of N-NO3− (0.023 t·ha−1) was consumed by denitrification in the floodplain shallow aquifer annually. The nitrate concentration (N-NO3−) decrease in the channel was 0.12 mg·L−1; but in the shallow aquifer it reached 11.40 mg·L−1; 8.05 mg·L−1; and 5.41 mg·L−1 in LU1; LU2; and LU3; respectively. Our study reveals that; in the Garonne floodplain; denitrification plays a significant role in the attenuation of nitrate associated with groundwater; but the impacts of denitrification on nitrate associated with river water is much less significant.

Anoxic nitrate reduction coupled with iron oxidation and attenuation of dissolved arsenic and phosphate in a sand and gravel aquifer

Nitrate has become an increasingly abundant potential electron acceptor for Fe(II) oxidation in groundwater, but this redox couple has not been well characterized within aquifer settings. To investigate this reaction and some of its implications for redox-sensitive groundwater contaminants, we conducted an in situ field study in a wastewater-contaminated aquifer on Cape Cod. Long-term (15year) geochemical monitoring within the contaminant plume indicated interacting zones with variable nitrate-, Fe(II)-, phosphate-, As(V)-, and As(III)-containing groundwater. Nitrate and phosphate were derived predominantly from wastewater disposal, whereas Fe(II), As(III), and As(V) were mobilized from the aquifer sediments. Multiple natural gradient, anoxic tracer tests were conducted in which nitrate and bromide were injected into nitrate-free, Fe(II)-containing groundwater. Prior to injection, aqueous Fe(II) concentrations were approximately 175μM, but sorbed Fe(II) accounted for greater than 90% of the total reactive Fe(II) in the aquifer. Nitrate reduction was stimulated within 1m of transport for 100μM and 1000μM nitrate additions, initially producing stoichiometric quantities of nitrous oxide (>300μMN). In subsequent injections at the same site, nitrate was reduced even more rapidly and produced less nitrous oxide, especially over longer transport distances. Fe(II) and nitrate concentrations decreased together and were accompanied by Fe(III) oxyhydroxide precipitation and decreases in dissolved phosphate, As(III), and As(V) concentrations. Nitrate N and O isotope fractionation effects during nitrate reduction were approximately equal (ε15N/ε18O=1.11) and were similar to those reported for laboratory studies of biological nitrate reduction, including denitrification, but unlike some reported effects on nitrate by denitrification in aquifers. All constituents affected by the in situ tracer experiments returned to pre-injection levels after several weeks. Additionally, Fe(II)-oxidizing, nitrate-reducing microbial enrichment cultures were obtained from aquifer sediments. Growth experiments with the cultures sequentially produced nitrite and nitrous oxide from nitrate while simultaneously oxidizing Fe(II). Field and culture results suggest that nitrogen oxide reduction and Fe(II) oxidation in the aquifer are a complex interaction of coupled biotic and abiotic reactions. Overall, the results of this study demonstrate that anoxic nitrate-dependent iron oxidation can occur in groundwater; that it could control iron speciation; and that the process can impact the mobility of other chemical species (e.g., phosphate and arsenic) not directly involved in the oxidation–reduction reaction.

Denitrification and inference of nitrogen sources in the karstic Floridan Aquifer

Abstract. Aquifer denitrification is among the most poorly constrained fluxes in global and regional nitrogen budgets. The few direct measurements of denitrification in groundwaters provide limited information about its spatial and temporal variability, particularly at the scale of whole aquifers. Uncertainty in estimates of denitrification may also lead to underestimates of its effect on isotopic signatures of inorganic N, and thereby confound the inference of N source from these data. In this study, our objectives are to quantify the magnitude and variability of denitrification in the Upper Floridan Aquifer (UFA) and evaluate its effect on N isotopic signatures at the regional scale. Using dual noble gas tracers (Ne, Ar) to generate physical predictions of N2 gas concentrations for 112 observations from 61 UFA springs, we show that excess (i.e. denitrification-derived) N2 is highly variable in space and inversely correlated with dissolved oxygen (O2). Negative relationships between O2 and δ15NNO3 across a larger dataset of 113 springs, well-constrained isotopic fractionation coefficients, and strong 15N:18O covariation further support inferences of denitrification in this uniquely organic-matter-poor system. Despite relatively low average rates, denitrification accounted for 32 % of estimated aquifer N inputs across all sampled UFA springs. Back-calculations of source δ15NNO3 based on denitrification progression suggest that isotopically-enriched nitrate (NO3–) in many springs of the UFA reflects groundwater denitrification rather than urban- or animal-derived inputs.

Characterization of Attachment and Growth of Thiobacillus denitrificans on Pyrite Surfaces

Anaerobic growth and attachment of the autotrophic denitrifying bacterium Thiobacillus denitrificans on pyrite surfaces were studied. Polished pyrite slabs were exposed to T. denitrificans for 1 to 9 weeks. The reacted pyrite surfaces were imaged with scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). Cells were observed as isolated attached cells, cells in division and cells forming microcolonies embedded in organic films. Bacteria began to colonize pyrite surfaces after 1 week, forming microcolonies after 3 weeks. The rate of colonization of the pyrite surface was around 35 cells mm−2 h−1 for the 3-week period. After 9 weeks, larger areas of the pyrite surface were covered by organic films. Bacterial enumeration on the pyrite surface and in solution showed that most of the cells were not attached to the mineral surface. Nevertheless, both attached and free-living bacteria probably contributed to pyrite-driven denitrification. The results may be applied to the natural environment to better understand pyrite-driven denitrification in aquifers and to improve the long-term performance of bioremediation processes using pyrite.

Aquifer denitrification and in situ mesocosms: Modeling electron donor contributions and measuring rates

Summary In situ denitrification rates were measured in a shallow unconfined glaciofluvial aquifer that had undergone large-scale nitrate contamination. Denitrification rates and isotopic enrichment factors, e , were measured using three tracer tests in two aquifer in situ mesocosms (ISMs). Denitrification rates were also measured using a mass balance method using water samples from multiport samplers. First-order kinetic rates ( k ) best described the denitrification rates measured. ISM kinetic rates ranged from 0.00049/d to 0.0031/d and e values ranged from −4.86‰ to −9.34‰; a linear relationship between k and e values showed greater fractionation (more negative e values) associated with higher rates. For the mass balance method, k values ranged from 0.0028/d to 0.0041/d. Combined mineralogical analysis, water quality data from the ISMs, and geochemical models using PHREEQC indicated that contributions of major electron donors to denitrification were 43–92% by organic carbon, 4–18% by pyrite, and 2–43% by non-pyritic ferrous iron, depending on the sample date and the type of amphibole used as the electron donor for ferrous iron. ISMs show promise as a tool for hydrogeochemical investigations. They are large enough to allow long-term sampling of aquifer denitrification tracer tests (>2 years), they may be used, with the modeling methodology shown herein, to estimate relative e − donor contributions, and they limit the influence of advection and mechanical dispersion on the amended water within the chamber.

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.

Denitrification in presence of acetate and glucose for bioremediation of nitrate‐contaminated groundwater

With the current increasing interest in aquifer denitrification, recent attention has been given to cost‐effective in‐situ treatments such as Enhanced In‐Situ Biological Denitrification (EISBD), which intends to stimulate the indigenous bacterial activity by injecting an external organic substrate and/or nutrients to the aquifer matrix. Within this context, laboratory batch assays have been conducted to develop a strategy for in‐situ denitrification of a nitrate‐contaminated aquifer in Argentona, Catalonia (Spain). The assays were run under aerobic and anaerobic conditions at a temperature of 17°C to better simulate the conditions of the aquifer. Acetate and glucose were added to assess their potential to promote heterotrophic denitrifying bacteria activity. Overall, the results revealed that indigenous micro‐organisms had the potential of reducing nitrate under appropriate conditions. Nitrate removal was complete and faster under anaerobic conditions, though high nitrate removals were also attained under initial aerobic conditions when a readily organic compound was amended at a sufficient dosage. The results also revealed that a significant amount of the available organic carbon was consumed by processes other than denitrification, namely aerobic oxidation and other microbial oxidation processes. To sum up, the results of this study demonstrated that addition of organic compounds into the groundwater is a promising method for in‐situ bioremediation of nitrate in the Argentona aquifer. This approach could potentially be applied to a number of situations in which nitrate concentration is elevated and where indigenous micro‐organisms with potential to reduce nitrate are present within the aquifer material.

Numerical and field investigation of enhanced in situ denitrification in a shallow-zone well-to-well recirculation system

This study investigated efficiency of in situ enhanced biological denitrification of nitrate-contaminated groundwater which employs a well-to-well circulation in a shallow zone where oxygen might give an adverse affect on the denitrification processes. The numerical model developed for the efficiency test included sequential aerobic and nitrate-based respiration, multi-Monod kinetics of reactive components, growth and decay of biomass, and denitrification inhibition associated with the presence of oxygen. Moreover, reaction kinetics for production of toxic intermediates such as nitrite and nitrous oxide were also included in the model. The developed model was applied to the analysis of enhanced in situ denitrification using an injection/extraction well pair. To evaluate the relative remediation effectiveness, comparisons were made between a continuous fumarate injection test (CFIT) system and a pulsed fumarate injection test (PFIT) system, where both systems had the same total fumarate mass injected into the aquifer. The PFIT system was preferable to the CFIT system because of the high possibility of occurrence of clogging in the latter case at the injection well, with no other significant advantages found in either the CFIT or the PFIT system. Accordingly, this developed numerical model is useful to predict and evaluate an in situ bioremediation by denitrification in aquifers.

Development of a procedure for sustainable in situ aquifer denitrification

AbstractDenitrification experiments have provided data showing the pitfalls and successes in developing a sustainable injection/extraction system in a sand and gravel aquifer. Experiments increase in complexity from continuous injection at one well to automated‐pulsed eight well injections. In both continuous and pulsed injection of organic carbon, 40 mg NO3‐N l−1 was reduced below the detection limit of < 0.1 mg NO3‐N l−1 in the denitrification zones. Under continuous injection, accumulation of bacterial exudates in the vicinity of the injection well resulted in injection well clogging within ten days. Periodic cleaning of the injection well and the adjacent gravel matrix was accomplished by using a tool developed to circulate a cleaning solution composed of 5 percent H2O2 and 0.02 percent NaOCl; but, biofouling could not be eliminated. In the later experiments, acetate became the carbon amendment because ethanol promoted more biomass development. A specialized pulse injection procedure was developed to separate nitrate from acetate‐C and was successful in alleviating the proliferation of bacterial exudates without affecting the performance of the denitrification system. Using pulsed injection, a maximum of 72 percent nitrate reduction was accomplished in the extraction well water, and denitrification was sustained for three months without clogging. © 2003 Wiley Periodicals, Inc.

Aquifer Denitrification as Interpreted from In Situ Microcosm Experiments

AbstractDenitrification of 40 mg L−1 NO3‐N groundwater in the vicinity of Central City, NE was stimulated with ethanol in in situ microcosms that were vibrated into the aquifer sediment and, thus, filled with a relatively undisturbed, saturated sand and gravel matrix. In microcosms receiving the 1.25 C/N ratio, nitrate disappeared within 40 h; however, NO2‐N accumulated and persisted after NO3‐N was depleted. Nitrite has been documented in groundwater redoxclines and is expected to be reported more frequently with increased use of ion chromatography and dense sampling networks. Dissolved oxygen was consumed to <2.0 mg L−1 within 15 h, and stoichiometric production of HCO−3 occurred. Average denitrification rate decreased from 28.5 to 19.3 and to 16 mg N L−1 d−1 for C/N ratios of 1.25, 2.5, and 5.0 mg L−1. Kinetic N‐isotope effects resulted in the δ15N enrichment of residual nitrate as denitrification progressed, thus, causing the initial δ15N value of ∼+6‰ to increase to > +20‰. The data support measuring additional parameters to prevent misinterpretations when using δ15N values in NO−3 source identification investigations. Apparent isotope‐enrichment factors, ε value, were derived from δ15N increases vs. NO−3 and NO3 plus NO−2 reduction. They ranged from −11 to −16‰ for δ15N vs. residual NO−3 plus NO−2 and from −2.5 to −9‰ for δ15N vs. NO−3 alone. Since NO−3 could not be resolved from NO−2 in the δ15N analysis, a δ15N depleted NO2 phase was not identified. Nevertheless, isotopically light enrichment factors are consistent with the presence of NO−2.

Research Watch: Microbial effects on aquifer denitrification

Research Watch: Microbial effects on aquifer denitrification

Natural denitrification in the saturated zone: A review

Denitrification is increasingly recognized for its ability to eliminate or reduce nitrate concentrations in groundwater. With this awareness comes a desire to predict the rate and extent of denitrification in aquifers. The limiting factor in making predictive models, however, is our limited knowledge of the physical characteristics of this process. This review synthesizes the published literature on natural aquifer denitrification. A background section discusses denitrification requirements and dissimilatory nitrate reduction to ammonium, which occurs in environments similar to those where denitrification occurs, and gives a historical perspective on denitrification. Other sections discuss denitrification with organic carbon serving as the electron donor (heterotrophic denitrification) and with reduced inorganic compounds serving as the electron donor (autotrophic denitrification). The section on heterotrophic denitrification is structured around two tables that summarize natural aquifer denitrification rates reported by laboratory studies and natural aquifer denitrification rates reported by field studies. The section on autotrophic denitrification discusses denitrification with reduced iron and reduced sulfur. Thus far, most studies only consider a single electron donor or donor type, whether heterotrophic or autotrophic. This review demonstrates, however, that multiple electron donors may be present in a given aquifer. Future research efforts are recommended to determine the factors affecting the availability of electron donors and their denitrification rates. Additional research is also suggested on how dissolved oxygen affects denitrification rates and on the factors influencing the partitioning of nitrate reduction products to nitrous oxide, a potential contributor to the destruction of the ozone layer, and to ammonium.

Denitrification in laboratory sand columns: Carbon regime, gas accumulation and hydraulic properties

Microbiological denitrification in a sandy matrix was studied by means of laboratory sand columns operated at continuous and pulse feed regimes. Gas production resulting from the biological activity played a major role in modifying the hydraulic properties of the column, leading to decreases in hydraulic conductivity and porosity, higher water velocities through the column, higher dispersion and anomalies in the head difference to flow rates ratios. All of these effects were more pronounced when formate, the carbon source used, was supplied continuously: microbial activity and gas production were concentrated at the top of the column, leading to almost complete clogging. When the formate was supplied in pulses, activity and gas production were dispersed, leading to relative uniformity in the physical parameters measured and a homogeneous appearance of the column. The results suggest that in a future in situ aquifer denitrification plant, pulse application of the carbon source is prefereable to a continuous supply regime.