Natural gas of radiolytic origin: An overlooked component of shale gas.

Natural gas is an important fossil energy source that has historically been produced from conventional hydrocarbon reservoirs. It has been interpreted to be of microbial, thermogenic, or, in specific contexts, abiotic origin. Since the beginning of the 21st century, natural gas has been increasingly produced from unconventional hydrocarbon reservoirs including organic-rich shales. Here, we show, based on a careful interpretation of natural gas samples from numerous unconventional hydrocarbon reservoirs and results from recent irradiation experiments, that there is a previously overlooked source of natural gas that is generated by radiolysis of organic matter in shales. We demonstrate that radiolytic gas containing methane, ethane, and propane constitutes a significant end-member that can account for >25% of natural gas mixtures in major shale gas plays worldwide that have high organic matter and uranium contents. The consideration of radiolytic gas in natural gas mixtures provides alternative explanations for so-called carbon isotope reversals and suggests revised interpretations of some natural gas origins. We submit that considering natural gas of radiolytic origin as an additional component in uranium-bearing shale gas formations will lead to a more accurate determination of the origins of natural gas.

Domino Three-Component N-Acylation/[4 + 2] Cycloaddition/Alder-ene Synthesis of Polysubstituted Benzo[f]isoindole-4-carboxylic Acids.

Diversely substituted, partially saturated benzo[f]isoindole-4-carboxylic acids were synthesized by a new three-component reaction (3CR) starting from cinnamic amines (3-arylallylamines), maleimides, and maleic anhydride. The process consists of N-acylation of the amines by maleic anhydride, intramolecular [4 + 2] cycloaddition in vinylarenes (the IMDAV reaction), and the concluding Alder-ene reaction between Diels-Alder intermediates and maleimides. All of the reaction steps proceed in a highly regio- and stereoselective manner, furnishing five adjacent chiral centers and leading to a single diastereoisomer of the title compound. The efficiency of the transformation is secured by thermal conditions or utilization of soft Lewis acids (Yb(OTf)3) as catalysts. The kinetics and mechanism of the 3CR were studied by using dynamic 19F NMR. Based on the NMR data and density functional theory (DFT) calculations, the IMDAV, not the Alder-ene, reaction is the rate-limiting step of the entire process.

Nitrate sources and transformation processes in groundwater of a coastal area experiencing various environmental stressors.

In coastal salinized groundwater systems, contamination from various nitrate (NO3) inputs combined with complex hydrogeochemical processes make it difficult to distinguish NO3 sources and identify potential NO3 transformtation processes. Effective field-based NO3 studies in coastal areas are needed to improve the understanding of NO3 contamination dynamics in groundwater of such complex coastal systems. This study focuses on a typical Mediterranean coastal agricultural area, located in Tunisia, experiencing substantial NO3 contamination from multiple anthropogenic sources. Here, multiple isotopic tracers (δ18OH2O, δ2HH2O, δ15NNO3, δ18ONO3, and δ11B) combined with a Bayesian isotope MixSIAR model are used (i) to identify the major NO3 sources and their contributions, and (ii) to describe the potential NO3 transformation processes. The measured NO3 concentrations in groundwater are above the natural baseline threshold, suggesting anthropogenic influence. The measured isotopic composition of NO3 indicates that manure, soil organic matter, and sewage are the potential sources of NO3, while δ11B values constrain the NO3 contamination to manure; a finding that is supported by the results of MixSIAR model revealing that manure-derived NO3 dominates over other likely sources. Nitrate derived from manure in the study area is attributed to organic fertilizers used to promote crop growth, and livestock that deposit manure directly on the ground surface. Evidence for ongoing denitrification in groundwaters of the study area is supported by an enrichment in both 15N and 18O in the remaining NO3, although isotopic mass balances between the measured and the theoretical δ18ONO3 values also suggest the occurrence of nitrification. The simultaneous occurrence of these biogeochemical processes with heterogeneous distribution across the study area reflect the complexity of interactions within the investigated coastal aquifer. The multiple isotopic tracer approach used here can identify the effect of multiple NO3 anthropogenic activities in coastal environments, which is fundamental for sustainable groundwater resources management.

Potential Impacts of Shale Gas Development on Inorganic Groundwater Chemistry: Implications for Environmental Baseline Assessment in Shallow Aquifers.

The potential contamination of shallow groundwater with inorganic constituents is a major environmental concern associated with shale gas extraction through hydraulic fracturing. However, the impact of shale gas development on groundwater quality is a highly controversial issue. The only way to reliably assess whether groundwater quality has been impacted by shale gas development is to collect pre-development baseline data against which subsequent changes in groundwater quality can be compared. The objective of this paper is to provide a conceptual and methodological framework for establishing a baseline of inorganic groundwater quality in shale gas areas, which is becoming standard practice as a prerequisite for evaluating shale gas development impacts on shallow aquifers. For this purpose, this paper first reviews the potential sources of inorganic contaminants in shallow groundwater from shale gas areas. Then, it reviews the previous baseline studies of groundwater geochemistry in shale gas areas, showing that a comprehensive baseline assessment includes documenting the natural sources of salinity, potential geogenic contamination, and potential anthropogenic influences from legacy contamination and surface land use activities that are not related to shale gas development. Based on this knowledge, best practices are identified in terms of baseline sampling, selection of inorganic baseline parameters, and definition of threshold levels.

Metaproteomics method to determine carbon sources and assimilation pathways of species in microbial communities.

Measurements of stable carbon isotope ratios (δ13C) are widely used in biology to address questions regarding food sources and metabolic pathways used by organisms. The analysis of these so-called stable isotope fingerprints (SIFs) for microbes involved in biogeochemical cycling and microbiota of plants and animals has led to major discoveries in environmental microbiology. Currently, obtaining SIFs for microbial communities is challenging as the available methods either only provide low taxonomic resolution, such as the use of lipid biomarkers, or are limited in throughput, such as nanoscale secondary ion MS imaging of single cells. Here we present "direct protein-SIF" and the Calis-p software package (https://sourceforge.net/projects/calis-p/), which enable high-throughput measurements of accurate δ13C values for individual species within a microbial community. We benchmark the method using 20 pure culture microorganisms and show that the method reproducibly provides SIF values consistent with gold-standard bulk measurements performed with an isotope ratio mass spectrometer. Using mock community samples, we demonstrate that SIF values can also be obtained for individual species within a microbial community. Finally, a case study of an obligate bacteria-animal symbiosis shows that direct protein-SIF confirms previous physiological hypotheses and can provide unexpected insights into the symbionts' metabolism. This confirms the usefulness of this approach to accurately determine δ13C values for different species in microbial community samples.

Methane oxidation and methylotroph population dynamics in groundwater mesocosms.

Extraction of natural gas from unconventional hydrocarbon reservoirs by hydraulic fracturing raises concerns about methane migration into groundwater. Microbial methane oxidation can be a significant methane sink. Here, we inoculated replicated, sand-packed, continuous mesocosms with groundwater from a field methane release experiment. The mesocosms experienced thirty-five weeks of dynamic methane, oxygen and nitrate concentrations. We determined concentrations and stable isotope signatures of methane, carbon dioxide and nitrate and monitored microbial community composition of suspended and attached biomass. Methane oxidation was strictly dependent on oxygen availability and led to enrichment of 13 C in residual methane. Nitrate did not enhance methane oxidation under oxygen limitation. Methylotrophs persisted for weeks in the absence of methane, making them a powerful marker for active as well as past methane leaks. Thirty-nine distinct populations of methylotrophic bacteria were observed. Methylotrophs mainly occurred attached to sediment particles. Abundances of methanotrophs and other methylotrophs were roughly similar across all samples, pointing at transfer of metabolites from the former to the latter. Two populations of Gracilibacteria (Candidate Phyla Radiation) displayed successive blooms, potentially triggered by a period of methane famine. This study will guide interpretation of future field studies and provides increased understanding of methylotroph ecophysiology.

Distinguishing Natural from Anthropogenic Sources of Acid Extractable Organics in Groundwater near Oil Sands Tailings Ponds.

Distinguishing between naphthenic acids (NAs) associated with oil sands process-affected water (OSPW) and those found naturally in groundwaters in contact with the bituminous McMurray Formation poses a considerable analytical challenge to environmental research in Canada's oil sands region. Previous work addressing this problem combined high-resolution Orbitrap mass spectrometry with carbon isotope values generated by online pyrolysis (δ13Cpyr) to characterize and quantify the acid extractable organics (AEOs) fraction containing NAs in the subsurface near an oil sands tailings pond. Here, we build upon this work through further development and application of these techniques at two different study sites near two different tailings ponds, in conjunction with the use of an additional isotopic tool-sulfur isotope analysis (δ34S) of AEOs. The combined use of both δ13Cpyr and δ34S allowed for discrimination of AEOs into the three end-members relevant to ascertaining the NA environmental footprint within the region: (1) OSPW; (2) McMurray Formation groundwater (i.e., naturally occurring bitumen), and; (3) naturally occurring non-bitumen. A Bayesian isotopic mixing model was used to determine the relative proportions of these three sources in groundwater at both study sites. Although background levels of OSPW-derived AEOs were generally low, one sample containing 49-99% (95% credibility interval) OSPW-derived AEOs was detected within an inferred preferential flow-path, highlighting the potential for this technique to track tailings pond seepage.

A Probabilistic Approach for Predicting Methane Occurrence in Groundwater.

Aqueous geochemistry datasets from regional groundwater monitoring programs can be a major asset for environmental baseline assessment (EBA) in regions with development of natural gases from unconventional hydrocarbon resources. However, they usually do not include crucial parameters for EBA in areas of shale gas development such as methane concentrations. A logistic regression (LR) model was developed to predict the probability of methane occurrence in aquifers in Alberta (Canada). The model was calibrated and tested using geochemistry data including methane concentrations from two groundwater monitoring programs. The LR model correctly predicts methane occurrence in 89.8% (n = 234 samples) and 88.1% (n = 532 samples) of groundwater samples from each monitoring program. Methane concentrations strongly depend on the occurrence of electron donors such as sulfate and to a lesser extent on well depth and the total dissolved solids of groundwater. The model was then applied to a province-wide public health groundwater monitoring program (n = 52,849 samples) providing aqueous geochemistry data but no methane concentrations. This approach allowed the prediction of methane occurrence in regions where no groundwater gas data are available, thereby increasing the resolution of EBA in areas of shale gas development by using basic hydrochemical parameters measured in high-density groundwater monitoring programs.

Domino Insertion-Coupling Synthesis of Solid-State Luminescent Propynylidene Indolones.

The compound class of 3-arylpropynylidene indolones, key intermediates in consecutive three-component syntheses of various indole-based chromophores with peculiar emission characteristics, are readily synthesized by a domino insertion-coupling synthesis with an electronically diverse substitution pattern in moderate to excellent yields. The title compounds are formed in E/Z-ratios from 100:0 to 0:100. Besides structure elucidation by NMR-spectroscopy and X-ray structure analysis, DFT calculations have been employed to rationalize the observed stereoselectivity. The photophysical properties of 3-arylpropynylidene indolones are characterized by intense, tunable, solid-state emission of N-substituted derivatives as quantified for drop-cast films. The electronic ground state structure was corroborated by DFT and TD-DFT calculations, showing that gradient-corrected exchange and correlation PBE (Perdew-Burke-Ernzerhof) functionals can be successfully employed to correctly reproduce the observed absorption characteristics of merocyanine derivatives. The huge Stokes shifts strongly depend on the electronic substitution pattern as supported by Hammett-Taft correlations.

Nitrogen, Sulfur, and Oxygen Isotope Ratios of Animal- and Plant-Based Organic Fertilizers Used in South Korea.

Organic fertilizers are increasingly used in agriculture in Asia and elsewhere. Tracer techniques are desirable to distinguish the fate of nutrients added to agroecosystems with organic fertilizers from those contained in synthetic fertilizers. Therefore, we determined the nitrogen, sulfur, and oxygen isotope ratios of nitrogen- and sulfur-bearing compounds in animal- and plant-based organic fertilizers (ABOF and PBOF, respectively) used in South Korea to evaluate whether they are isotopically distinct. The δN values of total and organic nitrogen for ABOF ranged from +7 to +19‰ and were higher than those of PBOF (generally <+6‰). This suggests that ABOFs have distinct δN values suitable for tracing these fertilizer compounds in the plant-soil-water system, whereas PBOFs have similar δN values to synthetic fertilizers. However, δO values for nitrate (δO) from organic fertilizer samples (<+17.0‰) were consistently lower than those of synthetic nitrate-containing fertilizers. The δS values of total sulfur, organic sulfur compounds (e.g., carbon-bonded sulfur and hydriodic acid-reducible sulfur), and sulfate for ABOFs yielded wide and overlapping ranges of +0.3 to +6.3, +0.9 to +7.2, and -2.6 to +14.2‰, whereas those for PBOFs varied from -3.4 to +7.7, +1.4 to +9.4, and -4.1 to +12.5‰, respectively, making it challenging to distinguish the fate of sulfur compounds from ABOF and PBOF in the environment using sulfur isotopes. We conclude that the δN values of ABOFs and the O values of organic fertilizers are distinct from those of synthetic fertilizers and are a promising tool for tracing the fate of nutrients added by organic fertilizers to agroecosystems.

Use of δ15N and δ18O Values for Nitrate Source Identification under Irrigated Crops: A Cautionary Vadose Zone Tale.

Source nitrogen (N) identification of leachate or groundwater nitrate is complicated by N source mixing and N and oxygen (O) isotope fractionation caused by microbial N transformations. This experiment examined the δN and δO values in leachate collected over 1 yr at 55 cm below raspberry ( L.) plots receiving either synthetic fertilizer (FT) or poultry manure (MT). The large ranges of δN (FT: -2.4 to +8.7‰, MT: +1.6 to +9.6‰) and δO (FT: -9.9 to -0.3‰, MT: -10.9 to +1.7‰) values in leachate collected under crop rows prohibited the reliable identification of the applied N sources on individual sampling dates. However, the mass-weighted average δN (FT: +3.2‰, MT: +7.3‰) values in leachate were significantly different and can be explained by accounting for the estimated contributions of nitrate and δN values of the various N sources, including applied fertilizer (-0.7‰) or manure (+7.9‰), nitrate-rich irrigation water (+9.0‰), and nitrate from soil N mineralization and nitrification (FT: +3.7‰, MT: +4.6‰; the seasonal timing of which is unknown). This study illustrates the importance of characterizing all major N sources and considering the seasonal variation of these sources and of N cycling processes, as they contribute to the δN values of leachate.

Long-term fate of nitrate fertilizer in agricultural soils.

Increasing diffuse nitrate loading of surface waters and groundwater has emerged as a major problem in many agricultural areas of the world, resulting in contamination of drinking water resources in aquifers as well as eutrophication of freshwaters and coastal marine ecosystems. Although empirical correlations between application rates of N fertilizers to agricultural soils and nitrate contamination of adjacent hydrological systems have been demonstrated, the transit times of fertilizer N in the pedosphere-hydrosphere system are poorly understood. We investigated the fate of isotopically labeled nitrogen fertilizers in a three-decade-long in situ tracer experiment that quantified not only fertilizer N uptake by plants and retention in soils, but also determined to which extent and over which time periods fertilizer N stored in soil organic matter is rereleased for either uptake in crops or export into the hydrosphere. We found that 61-65% of the applied fertilizers N were taken up by plants, whereas 12-15% of the labeled fertilizer N were still residing in the soil organic matter more than a quarter century after tracer application. Between 8-12% of the applied fertilizer had leaked toward the hydrosphere during the 30-y observation period. We predict that additional exports of (15)N-labeled nitrate from the tracer application in 1982 toward the hydrosphere will continue for at least another five decades. Therefore, attempts to reduce agricultural nitrate contamination of aquatic systems must consider the long-term legacy of past applications of synthetic fertilizers in agricultural systems and the nitrogen retention capacity of agricultural soils.

Thiophene-forming one-pot synthesis of three thienyl-bridged oligophenothiazines and their electronic properties.

The pseudo five-component Sonogashira-Glaser cyclization synthesis of symmetrically 2,5-diaryl-substituted thiophenes is excellently suited to access thienyl-bridged oligophenothiazines in a one-pot fashion. Three thienyl-bridged systems were intensively studied by UV-vis and fluorescence spectroscopy as well as by cyclic voltammetry. The oxidation proceeds with lower oxidation potentials and consistently reversible oxidations can be identified. The Stokes shifts are large and substantial fluorescence quantum yields can be measured. Computational chemistry indicates lowest energy conformers with sigmoidal and helical structure, similar to oligophenothiazines. TD-DFT and even semiempirical ZINDO calculations reproduce the trends of longest wavelengths absorption bands and allow the assignment of these transitions to possess largely charge-transfer character from the adjacent phenothiazinyl moieties to the central thienyl unit.

Lake Louise water (USGS47): a new isotopic reference water for stable hydrogen and oxygen isotope measurements.

RATIONALE: Because of the paucity of isotopic reference waters for daily use, a new secondary isotopic reference material has been prepared from Lake Louise water from Alberta, Canada, for international distribution. METHODS: This water was filtered, homogenized, loaded into glass ampoules, sealed with a torch, autoclaved to eliminate biological activity, and measured by dual-inlet isotope-ratio mass spectrometry. This isotopic reference water is available by the case of 144 glass ampoules containing 5 mL of water in each ampoule. RESULTS: The δ(2)H and δ(18)O values of this reference water are -150.2 ± 0.5‰ and -19.80 ± 0.02‰, respectively, relative to VSMOW, on scales normalized such that the δ(2)H and δ(18)O values of SLAP reference water are, respectively, -428 and -55.5‰. Each uncertainty is an estimated expanded uncertainty (U = 2uc) about the reference value that provides an interval that has about a 95% probability of encompassing the true value. CONCLUSIONS: This isotopic reference material, designated as USGS47, is intended as one of two isotopic reference waters for daily normalization of stable hydrogen and stable oxygen isotopic analysis of water with a mass spectrometer or a laser absorption spectrometer. Published in 2013. This article is a U.S. Government work and is in the public domain in the USA.

Isotopic evidence for determining the sources of dissolved organic sulfur in a forested catchment.

Understanding sulfur (S) biogeochemistry, especially in those watersheds subject to elevated levels of atmospheric S inputs, is needed for determining the factors that contribute to acidification, nutrient losses and the mobilization of toxic solutes (e.g., monomeric aluminum and methylmercury). S is found in a variety of both organic and inorganic forms undergoing a range of biotic and abiotic transformations. In watersheds with decreasing atmospheric S inputs, internal cycling is becoming dominant in affecting whether there is net loss or retention of S. Little attention has been given to the role of dissolved organic S (DOS) in affecting S biogeochemistry. DOS originates from assimilatory and bacterial dissimilatory S reduction (BDSR), the latter of which produces (34)S-depleted S. Within groundwater of the Archer Creek Catchment in the Adirondack Mountains (New York) there was reoxidation of reduced S, which was an important source of SO4(2-). DOS in surface waters had a higher variation of δ(34)S-DOS values (-6.0 to +8.4‰) than inorganic S with δ(34)S-SO4(2-) values ranging from +1.0 to +5.8‰. Inverse correlations between δ(34)S values of SO4(2-) and DOS suggested that BDSR played an important role in producing DOS.

Dispersion of surface floating plastic marine debris from Indonesian waters using hydrodynamic and trajectory models.

Plastic waste has become the most significant component of marine debris, while research on traces of marine plastic waste related to the condition of Indonesian waters is still limited. Therefore, this study examines the movement patterns of plastic marine debris in Indonesian waters. Simulations were carried out for one year (2013) using the HAMburg Shelf Ocean Model (hydrodynamic model) and Lagrange trajectory. The approach treated the simulated particles as conservative particles floating at sea. Even though the percentage is small (±16 %), Indonesia contributes to transboundary marine debris in the Pacific Ocean, Indian Ocean, and the South China Sea due to the influence of the south equatorial currents moving from north and south of Indonesia. Most of the plastic marine debris remains in Indonesian waters (±80-84 %) and moves back and forth due to the influence of the monsoon currents. Therefore, the Java Sea becomes a crossing point and accumulates plastic marine debris.

High diversity, abundance, and expression of hydrogenases in groundwater.

Hydrogen may be the most important electron donor available in the subsurface. Here we analyse the diversity, abundance and expression of hydrogenases in 5 proteomes, 25 metagenomes, and 265 amplicon datasets of groundwaters with diverse geochemistry. A total of 1545 new [NiFe]-hydrogenase gene sequences were recovered, which considerably increased the number of sequences (1999) in a widely used database. [NiFe]-hydrogenases were highly abundant, as abundant as the DNA-directed RNA polymerase. The abundance of hydrogenase genes increased with depth from 0 to 129 m. Hydrogenases were present in 481 out of 1245 metagenome-assembled genomes. The relative abundance of microbes with hydrogenases accounted for ~50% of the entire community. Hydrogenases were actively expressed, making up as much as 5.9% of methanogen proteomes. Most of the newly discovered diversity of hydrogenases was in "Group 3b", which has been associated with sulfur metabolism. "Group 3d", facilitating the interconversion of electrons between hydrogen and NAD, was the most abundant and mainly observed in methanotrophs and chemoautotrophs. "Group 3a", associated with methanogenesis, was the most abundant in proteomes. Two newly discovered groups of [NiFe]-hydrogenases, observed in Methanobacteriaceae and Anaerolineaceae, further expanded diversity. Our results highlight the vast diversity, abundance and expression of hydrogenases in groundwaters, suggesting a high potential for hydrogen oxidation in subsurface habitats.

The effects of heavy rain on the fate of urban and agricultural pollutants in the riverside area around weirs using multi-isotope, microbial data and numerical simulation.

The increase in extreme heavy rain due to climate change is a critical factor in the fate of urban and agricultural pollutants in aquatic system. Nutrients, including NO3- and PO43-, are transported with surface and seepage waters into rivers, lakes and aquifers and can eventually lead to algal blooms. δ15N-NO3-, δ18O-NO3-, and δ11B combined with hydrogeochemical and microbial data for groundwater and surface water samples were interpreted to evaluate the fate of nutrients in a riverside area around weirs in Daegu, South Korea. Most of the ions showed similar concentrations in the groundwater samples before and after heavy rain while concentrations of major ions in surface water samples were diluted after heavy rain. However, Si, PO43-, Zn, Ce, La, Pb, Cu and a number of waterborne pathogens increased in surface water after heavy rain. The interpretation of δ11B, δ15N-NO3-, and δ18O-NO3- values using a Bayesian mixing model revealed that sewage and synthetic fertilizers were the main sources of contaminants in the groundwater and surface water samples. δ18O and SiO2 interpreted using the Bayesian mixing model indicated that the groundwater component in the surface water increased from 4.4 % to 17.9 % during the wet season. This is consistent with numerical simulation results indicating that the direct surface runoff and the groundwater baseflow contributions to the river system had also increased 6.4 times during the wet season. The increase in proteobacteria and decrease of actinobacteria in the surface water samples after heavy rain were also consistent with an increase of surface runoff and an increased groundwater component in the surface water. This study suggests that source apportionment based on chemical and multi-isotope data combined with numerical modeling approaches can be useful for identifying main hydrological and geochemical processes in riverside areas around weirs and can inform suggestions of effective methods for water quality management.

Cyclohexene-Embedded Dicyanomethylene Merocyanines - Consecutive Three-Component Coupling-Addition Synthesis and Chromophore Characteristics.

A concise and efficient consecutive three-component alkynylation-addition synthesis of cyclohexene-embedded dicyanomethylene merocyanines furnishes a small library of dyes in moderate to excellent yield. The dyes possess strong absorption coefficients of the longest wavelength absorption bands. According to the crystal structure, the small bond length alternations account for a highly delocalized electronic ground state. The electronic structure of the absorption bands is qualitatively rationalized by TDDFT calculations, which explain that intense HOMO-LUMO transitions along the merocyanine axis lead to cyanine similar Stokes shifts.

Evaluating the sources and fate of nitrate in riparian aquifers under agricultural land using in situ-measured noble gases, stable isotopes, and metabolic genes.

Riparian zones with their buffering ability and abundant water supply are often subjected to intensive agricultural activities. We investigated a riparian aquifer located near a stream in South Korea that recently experienced sharply decreasing groundwater levels and elevated nitrate (NO3-) concentrations, which were attributed to local agricultural activities. Our goal was to identify the predominant nitrogen sources and NO3- removal processes. Multiple approaches including geochemical and isotopic tracers, land-use analysis, metabolic gene quantification, and inert gas tracers were used to elucidate groundwater and nutrient dynamics in stream-side granitic aquifers. The dual isotopic composition of NO3- identified manure and sewage as the major sources of NO3- contamination. Denitrification was the dominant NO3- removal process in the aquifer, as demonstrated by the negative relationship between δ15N and δ18O values in NO3-and NO3-/Cl-. Denitrification and anammox genes were also observed in microbial communities of the aquifer throughout the study site, suggesting that these processes support effective natural NO3- attenuation in groundwater. A mixing model constructed using a catchment-scale dataset including SiO2 concentrations and δ18O-H2O suggested that mixing with paddy soil water was the major driver of denitrification in the aquifer at the study site, where impervious layers provided anaerobic conditions for natural NO3- attenuation. Denitrification reduced the NO3- flux into the nearby stream by up to 114.4 NO3- kg/ha/y (26 kg N/ha/y). The N2 generated by denitrification did not accumulate in the groundwater, but mostly escaped from groundwater to the atmosphere, as demonstrated by the degassed signature of dissolved inert gases below the air saturated water level. This study identified the predominant NO3- sources and conceptualized N cycling in the heavily developed agricultural riparian aquifer using multiple tracers, demonstrating that NO3- is partially removed through denitrification and possibly anammox while N2 mostly escapes into the atmosphere.