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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 NO 3 - and PO 4 3- , are transported with surface and seepage waters into rivers, lakes and aquifers and can eventually lead to algal blooms. δ 15 N-NO 3 - B 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, PO 18 O-NO 3 - , and δ 11 B 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, PO 4 3- 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. δ 11 B, δ 15 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.3 - , and δ 18 O-NO 3 - 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. δ 18 O and SiO 2 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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

In coastal salinized groundwater systems, contamination from various nitrate (NO 3 ) inputs combined with complex hydrogeochemical processes make it difficult to distinguish NO 3 sources and identify potential NO 3 transformtation processes. Effective field-based NO 3 studies in coastal areas are needed to improve the understanding of NO 3 contamination dynamics in groundwater of such complex coastal systems. This study focuses on a typical Mediterranean coastal agricultural area, located in Tunisia, experiencing substantial NO 3 contamination from multiple anthropogenic sources. Here, multiple isotopic tracers (δ 18 O H2O , δ 2 H H2O , δ 15 N NO3 , δ 18 O NO3 , and δ 11 B) combined with a Bayesian isotope MixSIAR model are used (i) to identify the major NO 3 sources and their contributions, and (ii) to describe the potential NO 3 transformation processes. The measured NO 3 concentrations in groundwater are above the natural baseline threshold, suggesting anthropogenic influence. The measured isotopic composition of NO 3 indicates that manure, soil organic matter, and sewage are the potential sources of NO 3 , while δ 11 B values constrain the NO 3 contamination to manure; a finding that is supported by the results of MixSIAR model revealing that manure-derived NO 3 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 15 N and 18 O in the remaining NO 3 , although isotopic mass balances between the measured and the theoretical δ 18 O NO3 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 NO 3 anthropogenic activities in coastal environments, which is fundamental for sustainable groundwater resources management., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Around 50% of humankind relies on groundwater as a source of drinking water. Here we investigate the age, geochemistry, and microbiology of 138 groundwater samples from 95 monitoring wells (<250 m depth) located in 14 aquifers in Canada. The geochemistry and microbiology show consistent trends suggesting large-scale aerobic and anaerobic hydrogen, methane, nitrogen, and sulfur cycling carried out by diverse microbial communities. Older groundwaters, especially in aquifers with organic carbon-rich strata, contain on average more cells (up to 1.4 × 10 7  mL -1 ) than younger groundwaters, challenging current estimates of subsurface cell abundances. We observe substantial concentrations of dissolved oxygen (0.52 ± 0.12 mg L -1 [mean ± SE]; n = 57) in older groundwaters that seem to support aerobic metabolisms in subsurface ecosystems at an unprecedented scale. Metagenomics, oxygen isotope analyses and mixing models indicate that dark oxygen is produced in situ via microbial dismutation. We show that ancient groundwaters sustain productive communities and highlight an overlooked oxygen source in present and past subsurface ecosystems of Earth., (© 2023. The Author(s).)

The excessive input of nutrients into groundwater can accelerate eutrophication in associated surface water systems. This study combined hydrogeochemistry, multi isotope tracers, and microbiological data to estimate nutrient sources and the effects of groundwater-surface water interactions on the spatiotemporal variation of nutrients in groundwater connected to a large weir-regulated river in South Korea. δ 11 B and δ 15 N-NO 3 values, in combination with a Bayesian mixing model, revealed that manure and sewage contributed 40 % and 25 % respectively to groundwater nitrate, and 42 % and 27 % to nitrate in surface water during the wet season. In the dry season, the source apportionment was similar for groundwater while the sewage contribution increased to 52 % of nitrate in river water. River water displayed a high correlation between NO - concentration and cyanobacteria (Microcystis and Prochlorococcus) in the wet season. The mixing model using multiple isotopes indicated that manure-derived nutrients delivered with increased contributions of groundwater to the river during the wet season governed the occurrence of cyanobacterial blooms in the river. We postulate that the integrated approach using multi-isotopic and microbiological data is highly effective for evaluating nutrient sources and for delineating hydrological interactions between groundwater and surface water, as well as for investigating surface water quality including eutrophication in riverine and other surface water systems.3 - concentration and cyanobacteria (Microcystis and Prochlorococcus) in the wet season. The mixing model using multiple isotopes indicated that manure-derived nutrients delivered with increased contributions of groundwater to the river during the wet season governed the occurrence of cyanobacterial blooms in the river. We postulate that the integrated approach using multi-isotopic and microbiological data is highly effective for evaluating nutrient sources and for delineating hydrological interactions between groundwater and surface water, as well as for investigating surface water quality including eutrophication in riverine and other surface water systems., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

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 (NO 3 - 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 NO 3 - 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 NO 3 - identified manure and sewage as the major sources of NO 3 - contamination. Denitrification was the dominant NO 3 - removal process in the aquifer, as demonstrated by the negative relationship between δ 15 attenuation in groundwater. A mixing model constructed using a catchment-scale dataset including SiO 18 O values in NO 3 - and NO 3 - /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 NO 3 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 NO - attenuation in groundwater. A mixing model constructed using a catchment-scale dataset including SiO 2 concentrations and δ 18 O-H 2 O 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 NO 3 - attenuation. Denitrification reduced the NO 3 - flux into the nearby stream by up to 114.4 NO 3 - kg/ha/y (26 kg N/ha/y). The N 2 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 NO 3 - sources and conceptualized N cycling in the heavily developed agricultural riparian aquifer using multiple tracers, demonstrating that NO 3 - is partially removed through denitrification and possibly anammox while N 2 mostly escapes into the atmosphere., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

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.

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., (© 2020 The Authors. Environmental Microbiology published by Society for Applied Microbiology and John Wiley & Sons Ltd.)

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 (δ 13 C pyr ) 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 (δ 34 S) of AEOs. The combined use of both δ 13 C pyr and δ 34 S 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.

Oil and gas development can result in natural gas migration into shallow groundwater. Methane (CH 4 ), the primary component of natural gas, can subsequently react with solutes and minerals in the aquifer to create byproducts that affect groundwater chemistry. Hydro-biogeochemical processes induced by fugitive gas from leaky oil and gas wells are currently not well understood. We monitored the hydro-biogeochemical responses of a controlled natural gas release into a well-studied Pleistocene beach sand aquifer (Canadian Forces Base Borden, Ontario, Canada). Groundwater samples were collected before, during, and up to 700 days after gas injection and analyzed for pH, major and minor ions, alkalinity, dissolved gases, stable carbon isotope ratios of CO 2 and CH 4 , and microbial community composition. Gas injection resulted in a dispersed plume of free and dissolved phase natural gas, affecting groundwater chemistry in two distinct temporal phases. Initially (i.e. during and immediately after gas injection), pH declined and major ions and trace elements fluctuated; at times increasing above baseline concentrations. Changes in the short-term were due to invasion of deep groundwater with elevated total dissolved solids entrained with the upward migration of free phase gas and, reactions that were instigated through the introduction of constituents other than CH 4 present in the injected gas (e.g. CO 2 ). At later times, more pronounced aerobic and anaerobic CH 4 oxidation led to subtle increases in major ions (e.g. Ca 2+ , H 4 SiO 4 ) and trace elements (e.g. As, Cr). Microbial community profiling indicated a persistent perturbation to community composition with a conspicuous ingrowth of taxa implicated in aerobic CH 4 oxidation as well anaerobic S, N and Fe species metabolism., (Copyright © 2019 Elsevier B.V. All rights reserved.)

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.

Due to increasing concerns over the potential impact of shale gas and coalbed methane (CBM) development on groundwater resources, it has become necessary to develop reliable tools to detect any potential pollution associated with hydrocarbon exploitation from unconventional reservoirs. One of the key concepts for such monitoring approaches is the establishment of a geochemical baseline of the considered groundwater systems. However, the detection of methane is not enough to assess potential impact from CBM and shale gas exploitation since methane in low concentrations has been found to be naturally ubiquitous in many groundwater systems. The objective of this study was to determine the methane sources, the extent of potential methane oxidation, and gas-water-rock-interactions in shallow aquifers by integrating chemical and isotopic monitoring data of dissolved gases and aqueous species into a geochemical PHREEQC model. Using data from a regional groundwater observation network in Alberta (Canada), the model was designed to describe the evolution of the concentrations of methane, sulfate and dissolved inorganic carbon (DIC) as well as their isotopic compositions (δ 34 S SO4 , δ 13 C CH4 and δ 13 C DIC ) in groundwater subjected to different scenarios of migration, oxidation and in situ generation of methane. Model results show that methane migration and subsequent methane oxidation in anaerobic environments can strongly affect its concentration and isotopic fingerprint and potentially compromise the accurate identification of the methane source. For example elevated δ 13 C CH4 values can be the result of oxidation of microbial methane and may be misinterpreted as methane of thermogenic origin. Hence, quantification of the extent of methane oxidation is essential for determining the origin of methane in groundwater. The application of this model to aquifers in Alberta shows that some cases of elevated δ 13 C CH4 values were due to methane oxidation resulting in pseudo-thermogenic isotopic fingerprints of methane. The model indicated no contamination of shallow aquifers by deep thermogenic methane from conventional and unconventional hydrocarbon reservoirs under baseline conditions. The developed geochemical and multi-isotopic model describing the sources and fate of methane in groundwater is a promising tool for groundwater assessment purposes in areas with shale gas and coalbed methane development., (Copyright © 2019. Published by Elsevier B.V.)

Fugitive gas comprised primarily of methane (CH 4 ) with traces of ethane and propane (collectively termed C 1-3 ) may negatively impact shallow groundwater when unintentionally released from oil and natural gas wells. Currently, knowledge of fugitive gas migration, subsurface source identification and oxidation potential in groundwater is limited. To advance understanding, a controlled release experiment was performed at the Borden Research Aquifer, Canada, whereby 51m 3 of natural gas was injected into an unconfined sand aquifer over 72days with dissolved gases monitored over 323days. During active gas injection, a dispersed plume of dissolved C 1-3 evolved in a depth discrete and spatially complex manner. Evolution of the dissolved gas plume was driven by free-phase gas migration controlled by small-scale sediment layering and anisotropy. Upon cessation of gas injection, C 1-3 concentrations increased to the greatest levels observed, particularly at 2 and 6m depths, reaching up to 31.5, 1.5 and 0.1mg/L respectively before stabilizing and persisting. At no time did groundwater become fully saturated with natural gas at the scale of sampling undertaken. Throughout the experiment the isotopic composition of injected methane (δ 13 C of -42.2‰) and the wetness parameter (i.e. the ratio of C 1 to C 2+ ) constituted excellent tracers for the presence of fugitive gas at concentrations >2mg/L. At discrete times C 1-3 concentrations varied by up to 4 orders of magnitude over 8m of aquifer thickness (e.g. from <0.01 to 30mg/L for CH 4 ), while some groundwater samples lacked evidence of fugitive gas, despite being within 10m of the injection zone. Meanwhile, carbon isotope ratios of dissolved CH 4 showed no evidence of oxidation. Our results show that while impacts to aquifers from a fugitive gas event are readily detectable at discrete depths, they are spatially and temporally variable and dissolved methane has propensity to persist., (Copyright © 2017 Elsevier B.V. All rights reserved.)

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., (Copyright © by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc.)

An integrated study based on hydrogeochemical, microbiological and dual isotopic approaches for nitrate and sulfate was conducted to elucidate sources and biogeochemical reactions governing groundwater contaminants in different seasons and under different land use in a basin of Korea. The land use in the study area is comprised of forests (58.0%), vegetable fields (27.6%), rice paddy fields (11.4%) and others (3.0%). The concentrations of NO3-N and SO4(2-) in groundwater in vegetable fields were highest with 4.2-15.2 mg L(-1) and 1.6-19.7 mg L(-1) respectively, whereas under paddy fields NO3-N concentrations ranged from 0 to 10.7 mg L(-1) and sulfate concentrations were ~15 mg L(-1). Groundwater with high NO3-N concentrations of >10mgL(-1) had δ(15)N-NO3(-) values ranging from 5.2 to 5.9‰ and δ(18)O values of nitrate between 2.7 and 4.6‰ suggesting that the nitrate was mineralized from soil organic matter that was amended by fertilizer additions. Elevated concentrations of SO4(2-) with δ(34)S-SO4(2-) values between 1 and 6‰ in aquifers in vegetable fields indicated that a mixture of sulfate from atmospheric deposition, mineralization of soil organic matter and from synthetic fertilizers is the source of groundwater sulfate. Elevated δ(18)O-NO3(-) and δ(18)O-SO4(2-) values in samples collected from the paddy fields indicated that denitrification and bacterial sulfate reduction are actively occurring removing sulfate and nitrate from the groundwater. This was supported by high occurrences of denitrifying and sulfate reducing bacteria in groundwater of the paddy fields as evidenced by 16S rRNA pyrosequencing analysis. This study shows that dual isotope techniques combined with microbial data can be a powerful tool for identification of sources and microbial processes affecting NO3(-) and SO4(2-) in groundwater in areas with intensive agricultural land use., (Copyright © 2015 Elsevier B.V. All rights reserved.)

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.

The dual-isotope method of measuring both the δ(34)S and δ(18)O values of dissolved sulphate to assess the origin and fate of groundwater sulphate at the Ključ groundwater source, Serbia is applied. A sampling campaign was conducted in September 2007 during low-flow conditions, obtaining river water and groundwater from observation wells completed in a shallow aquifer formed in alluvial sandy-gravelly sediments. In the shallow groundwater, sulphate concentrations ranged from 56.2 to 165.0 mg l(-1). The δ(34)S values of sulphate varied from-5.5 to+3.0 ‰ and [Formula: see text] values from+1.4 to+4.2 ‰. Oxidation of pedospheric (organic soil S) and lithogenic sulphur sources (e.g. pyrite) were identified as the main causes for the increasing sulphate concentrations in the analysed groundwater. This study shows that combining hydrological, chemical, and isotopic techniques is a powerful approach to identifying sources and processes that control sulphate in water resources.

The groundwater of Jeju Island (Republic of Korea) is vulnerable to contamination because its aquifers are mainly composed of highly permeable geological units and its agricultural fields are often exposed to excessive use of predominantly synthetic fertilizers. In the Gosan area of Jeju Island, we investigated nitrate contamination in both a perched aquifer above an impermeable clay bed and the regional groundwater beneath this aquitard. The δO and δD values indicate that the perched groundwater is recharged by local precipitation, whereas the regional groundwater is recharged mainly by regional flow from an adjacent mountainous region. The perched groundwater contained very high NO-N concentrations of up to 87 mg/L. The isotopic composition of nitrate in the perched groundwater showed that synthetic fertilizers applied in high excesses of crop N needs were the main cause of aquifer pollution. Elevated nitrate concentrations were also observed in the regional groundwater especially after precipitation events. Concentration and isotopic data revealed that the inflow of shallow perched groundwater along the poorly cemented or uncemented annulus of regional groundwater wells was one of the main reasons for the nitrate contamination observed in the regional groundwater. In both aquifers, δN and δO values showed that the sources of nitrate were derived from synthetic fertilizers that had been recycled in the soil zone by nitrification and in some portions of the perched aquifer (dissolved oxygen concentrations <2 mg/L) indicated that denitrification occurred locally., (Copyright © by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc.)

This paper describes the use of the dual isotope method involving δ(15)N and δ(18)O measurements of dissolved nitrates to assess the origin and fate of groundwater nitrate at the Ključ groundwater source, Serbia. A sampling campaign was conducted in September 2007 during flow conditions obtaining groundwater from observation wells and river water fed by a shallow aquifer hosted in alluvial (sandy-gravel) sediments. Nitrate isotope ratios ranged from +5.3 to +16.9‰ and δ(18)O(NO(3)) values varied from -2.3 to +5.0‰. Two major contamination sources were identified with isotopic compositions characteristic for nitrate derived from nitrification of soil organic nitrogen (+5.3 to +7.8‰ for δ(15)N) resulting in nitrate concentrations of 33.6 and 78.8 mg/L and nitrate derived from animal wastes or human sewage, e.g. via septic systems, yielding δ(15)N values of +9.9 to +11.9‰ and elevated nitrate concentrations of 31.2-245.8 mg/L. The occurrence of nitrification and denitrification was also revealed based on concentration and isotope data for dissolved nitrate.

Petroleum resource development is creating a global legacy of active and inactive onshore energy wells. Unfortunately, a portion of these wells will exhibit gas migration (GM), releasing fugitive gas (FG) into adjacent geologic formations and overlying soils. Once mobilized, FG may traverse the critical zone, impact groundwater, and emit to the atmosphere, contributing to greenhouse‐gas emissions. Understanding of GM and FG has increased in recent years but significant gaps persist in knowledge of (1) the incidence and causes of GM, (2) subsurface baseline conditions in regions of development required to delineate GM and FG, and (3) the migration, impacts, and fate of FG. Here we provide an overview of these knowledge gaps as well as the occurrence of GM and FG as currently understood in British Columbia (BC), Canada, a petroleum‐producing region hosting significant reserves. To address the identified knowledge gaps within BC, the Energy and Environment Research Initiative (EERI) at the University of British Columbia is implementing several field‐focused research projects including: (1) statistical analyses of regulatory data to elucidate the incidence and causes of GM, (2) characterization of regional hydrogeology and shallow subsurface conditions in the Peace Region of the Montney resource play, and (3) investigation of the migration, impacts, and fate of FG in the shallow subsurface through controlled natural‐gas release. Together, the EERI investigations will advance understanding of GM and FG, provide scientific data that can inform regulations, and aid development of effective monitoring and detection methodologies for BC and beyond. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The extent of denitrification in a small agricultural area near a river in Yangpyeong, South Korea, was determined using multiple isotopes, groundwater age, and physicochemical data for groundwater. The shallow groundwater at one monitoring site had high concentrations of NO3-N (74-83 mg L−1). The δ15N-NO3 values for groundwater in the study area ranged between +9.1 and +24.6‰ in June 2014 and +12.2 to +21.6‰ in October 2014. High δ15N-NO3 values (+10.7 to +12.5‰) in both sampling periods indicated that the high concentrations of nitrate in the groundwater originated from application of organic fertilizers and manure. In the northern part of the study area, some groundwater samples showed elevated δ15N-NO3 and δ18O-NO3 values, which suggest that nitrate was removed from the groundwater via denitrification, with N isotope enrichment factors ranging between −4.8 and −7.9‰ and O isotope enrichment factors varying between −3.8 and −4.9‰. Similar δD and δ18O values of the surface water and groundwater in the south appear to indicate that groundwater in that area was affected by surface-water infiltration. The mean residence times (MRTs) of groundwater showed younger ages in the south (10-20 years) than in the north (20-30 years). Hence, it was concluded that denitrification processes under anaerobic conditions with longer groundwater MRT in the northern part of the study area removed considerable amounts of nitrate. This study demonstrates that multi-isotope data combined with physicochemical data and age-dating information can be effectively applied to characterize nitrate contaminant sources and attenuation processes. [ABSTRACT FROM AUTHOR]

Transformations of precipitation into groundwater and streamflow are fundamental hydrological processes, critical to irrigated agriculture, hydroelectric power generation, and ecosystem health. Our understanding of the timing of groundwater recharge and streamflow generation remains incomplete, limiting our ability to predict fresh water, nutrient, and contaminant fluxes, especially in large basins. Here, we analyze thousands of rain, snow, groundwater, and streamflow δ18O and δ2H values in the Nelson River basin, which covers 1.2 million km2 of central Canada. We show that the fraction of precipitation that recharges aquifers is ~1.3-5 times higher for precipitation falling during cold months with subzero mean monthly temperatures than for precipitation falling during warmer months. The near-ubiquity of cold-season-biased groundwater recharge implies that changes to winter water balances may have disproportionate impacts on annual groundwater recharge rates. We also show that young streamflow-defined as precipitation that enters a river in less than ~2.3 months-comprises ~27% of annual streamflow but varies widely among tributaries in the Nelson River basin (1-59%). Young streamflow fractions are lower in steep catchments and higher in flatter catchments such as the transboundary Red River basin. Our findings imply that flat, lower permeability, heavily tiled landscapes favor more rapid transmission of precipitation into rivers, possibly mobilizing excess soluble fertilizers and exacerbating eutrophication events in Lake Winnipeg. [ABSTRACT FROM AUTHOR]

Chemical compositions and stable isotope ratios of water and sulphate were used to characterise sources and processes responsible for elevated concentrations of sulphate and other constituents in groundwater from aquifers at Colima State along Mexico's Pacific Coast. The δO and δH values of the groundwater were similar to those of precipitation indicating a meteoric origin, and recharge processes are relatively uniform in large parts of the study area with only slight local evaporation effects. δS and δO analyses indicated that high sulphate concentrations of up to 1,480 mg/l are mainly due to dissolution of evaporites and volcanic exhalations. Chloride is largely related to sources other than seawater. The Marabasco sub basin is affected by anthropogenic contamination through manganese and iron ore mining activities. The obtained knowledge regarding sources and areas of contamination will be essential for the development and design of a water quality monitoring program in the study area. [ABSTRACT FROM AUTHOR]

Groundwater samples collected in a shallow oxic and reduced deep groundwater system revealed the influence of dissolved sulfide on the chemical and isotopic composition of fulvic acid associated sulfur. Stable isotope compositions of `groundwater sulfate and fulvic acid sulfur and sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy data were used to determine the sources and processes affecting fulvic acid sulfur in the aquifer. A δ34S value of 2.2 ‰ for the shallow groundwater sulfate and a δ34S value of fulvic acids of 4.9 ‰ accompanied by a contribution of up to 49% of the most oxidized sulfur species (S+6) documented that fulvic acid sulfur is mainly derived from soil S compounds such as ester sulfates, with δ34S values similar to those of atmospheric sulfate deposition. In contrast, in the deep groundwater system with elevated δ34S values in groundwater sulfate of up to 20 ‰ due to bacterial sulfate reduction, δ34S values in fulvic acid sulfur were negative and were up to 22‰ lower compared to those of groundwater sulfate. Furthermore, reduced sulfur compounds constituted a significantly higher proportion of total fulvic acid sulfur in the deep groundwater compared to fulvic acids in shallow groundwater, supporting the hypothesis that fulvic acids act as a sink for dissolved hydrogen sulfide in the deep aquifer. Our results suggest that the combination of sulfur K edge XANES spectroscopy and stable isotope analysis on fulvic acids represents a powerful tool to elucidate the role of fulvic acids in the sulfur cycle in groundwater. [ABSTRACT FROM AUTHOR]

The article discusses the isotope hydrology of the Oldman River basin in southern Alberta, Canada. The author states that hydormetric instruments indicate that flow in the Oldman River has decreased by 34 percent between 1913 and 2003. It is predicted that water withdrawals will increase by 2027. The aim of the study was to determine whether isotope ratio measurements can give more insight into the water dynamics of the Oldman River and its tributaries. The study used water samples from the surface which were collected every month between December 2000 and March 2003 as well as groundwater samples taken from 58 wells during one-time sampling trips.

The downward flux of sulfate in a weathered clayey aquitard to a hydrocarbon plume in ground water was investigated at a gas well site in Alberta, Canada. The dissolved hydrocarbon plume includes monoaromatic BTEX compounds and extends less than 20 m downgradient from a natural gas condensate source in silt- and clay-rich deposits. The vadose zone at the site has high concentrations of water soluble sulfate, which is apparently derived from pyrite oxidation. Seasonal infiltration of snowmelt water, most notably during the “wet” spring of 2003 and also in 2004, was focused in an excavation and recharged the water table. This recharge replenished the contaminated zone with sulfate that was flushed downward from the vadose zone. Recharge in 2003 resulted in episodic increases in sulfate concentrations in the ground water condensate plume of up to 1000 mg/L. After episodic increases in sulfate concentrations within the core of the contaminant plume associated with recharge, concentrations then decreased to less than 1 mg/L, accompanied by isotopic enrichment of the residual sulfate in 34S. The monitoring data provide strong field evidence that bacterial sulfate reduction is a dominant terminal electron-accepting process in the natural attenuation of the hydrocarbon plume in the ground water. This result suggests that engineered infiltration and replenishment of sulfate to petroleum hydrocarbon plumes in ground water might enhance biodegradation of these plumes. [ABSTRACT FROM AUTHOR]

The ionic and isotopic compositions (δD, δ18O, and 3H) of urban groundwaters have been monitored in Seoul to examine the water quality in relation to land-use. High tritium contents (6.1–12.0 TU) and the absence of spatial/seasonal change of O–H isotope data indicate that groundwaters are well mixed within aquifers with recently recharged waters of high contamination susceptibility. Statistical analyses show a spatial variation of major ions in relation to land-use type. The major ion concentrations tend to increase with anthropogenic contamination, due to the local pollutants recharge. The TDS concentration appears to be a useful contamination indicator, as it generally increases by the order of forested green zone (average 151 mg/l), agricultural area, residential area, traffic area, and industrialized area (average 585 mg/l). With the increased anthropogenic contamination, the groundwater chemistry changes from a Ca–HCO3 type toward a Ca–Cl(+NO3) type. The source and behavior of major ions are discussed and the hydrochemical backgrounds are proposed as the basis of a groundwater management plan. [ABSTRACT FROM AUTHOR]

Chemical and isotope analyses on groundwater sulfate and ³H measurements on ground water were used to determine the sulfate sources and sulfur transformation processes in a heterogeneous karst aquifer of the Franconian Alb, southern Germany. Sulfate was found to be derived from atmospheric deposition. Young groundwater was characterized by high sulfate concentrations and δ34S values similar to those of recent atmospheric sulfate deposition. However, the δ18O values of groundwater SO42- were depleted by several per mu with respect to those of atmospheric deposition. This isotopic shift is indicative of mineralization of carbon-bonded S in the vadose zone of the karst system. In groundwater with mean residence times of more than 60 years, a trend of increasing δ34S values and δ18O values with decreasing sulfate concentrations was observed. This trend could not be solely explained by preindustrial atmospheric sulfate deposition with higher δ34S values, and hence, we conclude that bacterial (dissimilatory) sulfate reduction in the porous matrix of the karst aquifer must have occurred. This process has the potential to contribute to long-term biodegradation of contaminants in the porous rock matrix representing the dominant water reservoir of the fissured porous karst aquifer. [ABSTRACT FROM AUTHOR]

When fugitive methane migrates upward along boreholes of oil and gas wells, it may migrate into shallow ground water or pass through overlying soil to the atmosphere. Prior to this study, there was little information on the fate of fugitive methane that migrates into ground water. In a field study near Lloydminster, Alberta, Canada, we found hydrogeochemical evidence that fugitive methane from an oil well migrated into a shallow aquifer but has been attenuated by dissimilatory bacterial sulfate reduction at low temperature (∼5°C) under anaerobic conditions. Evidence includes spatial and temporal trends in concentrations of methane and sulfate in ground water and associated trends in concentrations of bicarbonate and sulfide. Within 10 m of the oil well, sulfate concentrations were low, and sulfate was enriched in both 34S and 18O. Sulfate concentrations had a strong positive correlation with δ13C values of bicarbonate, and sulfide was depleted in 34S compared to sulfate. These data indicate that bacterial sulfate reduction occurred near the production well. Near the oil well, elevated concentrations of bicarbonate were observed, and the bicarbonate was depleted in 13C. Modeling indicates that the main source of this excess 13C-depleted bicarbonate is oxidized methane. In concert with the sulfate concentration and isotope data, these results support an interpretation that in situ bacterial oxidation of methane has occurred, linked to bacterial sulfate reduction. Bacterial sulfate reduction may play a major role in bioattenuation of fugitive natural gas in ground water in western Canada. [ABSTRACT FROM AUTHOR]

Nitrate (NO 3−) is one of the main pollutants in agriculturally impacted groundwater systems. The availability and reactivity of electron donors control the prevalent redox conditions in aquifers and past nitrate contamination of groundwater can be ameliorated if denitrification occurs. Using aqueous geochemistry data and the stable isotope composition of dissolved nitrate (δ15N and δ18O), we found that nitrate concentrations above the World Health Organization drinking water guideline were caused predominantly by manure and to a lesser extent by synthetic fertilizer applications and that denitrification was not a significant nitrate removal process in an aquifer in southern Germany underlying agricultural land with intensive hog farming. We also applied environmental isotopes (δ2H and δ18O, 3H/3He, and 14C) linked with a lumped parameter approach to determine apparent mean transit times (MTT) of groundwater that ranged from <5 years to >100 years. Furthermore, we determined low reduction rates of dissolved oxygen (O2) of 0.015 1/year for first‐order kinetics. By extrapolating the O2 reduction rates beyond the apparent MTT ranges of sampled groundwater, denitrification lag times (time prior to commencement of denitrification) of approximately 114 years were determined. This suggests that it will take many decades to significantly reduce nitrate concentrations in the porous aquifer via denitrification, even if future nitrate inputs were significantly reduced. Key Points: Low O2 reduction rates result in low denitrification potentialDenitrification lag times are in the order of approximately one centuryEstimation of denitrification lag times improves understanding of the self‐purification potential of groundwater ecosystems [ABSTRACT FROM AUTHOR]

The Kettle River Basin in South central British Columbia (Canada) is under increasing anthropogenic pressures affecting both water quantity and quality of surface waters and aquifers. We investigated water quality and sources and processes influencing NO 3 – and SO 4 2– in the Kettle River Basin using a combination of chemical and isotopic techniques. The dominant water type in the Kettle River Basin is Ca–HCO 3 with surface waters having total dissolved solids (TDS) concentrations of < 115 mg/L and groundwaters having TDS values of up to 572 mg/L. Based on δ 15 N NO 3 and δ 18 O NO 3 values and concentration data, NO 3 – in surface waters originates primarily from natural soil nitrification processes, with additional influences from anthropogenic activities, such as waste water effluents at sampling locations downstream from population centres. The source of NO 3 – in groundwater was predominantly nitrification of soil organic matter, although nitrate in a few groundwater samples originated from anthropogenic sources, including manure or septic systems. The dominant source of SO 4 2– in surface water and groundwater samples was the natural oxidation of sulfide minerals. With increasing distance downstream, surface water δ 18 O SO 4 values increase beyond the range of oxidation of sulfide minerals and into the range of soil and atmospheric-derived SO 4 2– that is in part derived from anthropogenic emissions. Hence, we conclude that recent anthropogenic impacts have affected water quality only marginally at only few sites in the Kettle River Basin. The presented data will serve as an excellent baseline against which future impacts can be assessed. [ABSTRACT FROM AUTHOR]

To determine the contributions of various sources to nitrate loads in groundwater ( n = 103) from three farmed rural sites in South Korea, we assessed the dual isotopic composition of nitrate, coupled with hydrochemical parameters. The measured isotopic values of nitrate varied widely, with 10th and 90th percentiles of 2.8‰ and 14.2‰ for δ 15 N, and 0.7‰ and 8.9‰ for δ 18 O, respectively and suggested that nitrification of anthropogenic N compounds is a predominant process of nitrate pollution. To overcome the known uncertainties of quantitative source apportionment for groundwater nitrate, arising from isotope fractionation during N transformations and the significant overlap in isotopic values of different sources, the combined hydrochemical and isotopic datasets were interpreted with orthogonal regression of a principal component analysis (PCA) and a Bayesian mixing model. The PCA projected the observed δ 15 N NO 3 values onto a mixing subspace in the multivariate variability of the dataset, and the regression fits of the sample data were presumed to be conservative mixtures. This procedure also allowed for an assessment of the sample uncertainty, as influenced by natural nitrate contributions and denitrification. The Bayesian mixing model was used to estimate the probability distributions of the proportional contributions of three anthropogenic sources: chemical fertilizers, composted manure, and manure slurries. Nitrate is largely derived from chemical fertilizers with fractional contributions of 0.35–0.71, and organic fertilizers including composted manure with mixing fractions of 0.39–0.49. The relative contribution of nitrate from composted manure compared to chemical fertilizers increased with increasing nitrate concentrations, suggesting that composted manure significantly increases nitrate pollution and therefore its use should be carefully controlled to manage rural groundwater quality. This study also suggests that PCA and Bayesian isotope mixing models are effective for quantitative assessment of the sources of pollutants, such as nitrate. [ABSTRACT FROM AUTHOR]

Knowledge of the natural background concentrations of groundwater constituents is important for the management of groundwater resources, particularly for the assessment of groundwater contamination and the establishment of clean-up goals and regulatory target levels. In recent years, an increasing number of studies have assessed the natural background concentrations of dissolved constituents in groundwater using a variety of different methods, each with its own assumptions, advantages and limitations. The objective of this paper is to provide a methodological basis for improving the estimation of natural background concentrations of groundwater constituents. To this end, this paper critically reviews the different approaches used to determine natural background concentrations of dissolved constituents in groundwater. In addition, two regional case studies of fluoride in Canadian groundwater are presented to illustrate the estimation of background concentrations for natural groundwater constituents. The review of existing methods shows that the use of pristine groundwater samples is not possible in many cases, due to the widespread influence of human activities. The widely used pre-selection method can provide misleading results because of inadequate selection criteria and poor statistical significance associated with the reduction of the dataset. A variety of model-based methods have been developed, but these methods are all based on assumptions that cannot be verified. Relying on the user's experience and previous knowledge of the groundwater system, exploratory data analysis has many advantages and can be applied for both anthropogenic and natural constituents. The case studies show that the exploratory data analysis approach provides critical information to determine the sources of groundwater constituents and to properly delineate groundwater bodies for which background values will be established. Natural background concentrations should always be considered as theoretical values due to their spatio-temporal variability and scale dependence, and thresholds as concentration values above which further investigation is required. • Methods to determine natural background concentrations in groundwater are reviewed. • Two case studies are presented for geogenic fluoride in Canadian groundwater. • Exploratory data analysis is a powerful method to estimate background concentrations. • The delineation of groundwater bodies is key to determine relevant background values. • The sources of constituents need to be considered to determine suitable thresholds. [ABSTRACT FROM AUTHOR]

Abstract: Based on hydrochemical and environmental isotope data (δ15N and δ18O of , and δ34S of ) of depth-specific groundwater samples from multi-level samplers, the source(s) and biogeochemical behavior of and in a shallow (<25 m below ground level) sandy alluvial aquifer underneath a riverside agricultural area in South Korea were evaluated. The groundwater in the study area was characterized by a large variability in the concentrations of (0.02 to ∼35mg/L ml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si7.gif" overflow="scroll"> SO 4 2 - (0.14 to ∼130mg/L). A distinct vertical redox zoning was observed sub-dividing an oxic groundwater at shallow depths (<8–10m below ground surface) from sub-oxic groundwater at greater depths. The δ15N and δ18O values indicated that elevated concentrations in the oxic groundwater are due to manure-derived and nitrification of urea- and ammonia-containing fertilizers used on agricultural fields. Chemical and isotopic data also revealed that groundwater concentrations significantly decrease due to denitrification in the lower oxic and sub-oxic groundwater. The δ34Ssulfate values of the oxic groundwater ranged from −14.4‰ to +2.4‰. The relationship between δ34Ssulfate values and concentrations with depth showed that increasing concentrations were caused by S-bearing fertilizers, not pyrite oxidation. Bacterial (dissimilatory) reduction occurred locally in the sub-oxic groundwater, as indicated by increasing δ34Ssulfate values (up to 64.1‰) with concomitant decreases of concentrations. This study shows that isotope data are very effective for discriminating different sources for the waters with high and low concentrations in the lower oxic zone. It is also suggested that the use of N- and S-containing fertilizers should be better controlled to limit nitrate and contamination of shallow groundwater. [Copyright &y& Elsevier]

Abstract: The production of coalbed methane (CBM) represents a vital new source of natural gas supply in Western Canada. There are, however, concerns over potential negative environmental impacts on shallow groundwater resources in the hypothetical case that leakage of fluids and gases from CBM operations occurs. This paper compares major ion and isotope geochemistry data for produced fluids or gases from two major coal deposits in Western Canada (Mannville Formation and the Horseshoe Canyon/Belly River Group) with similar data collected for shallow groundwater in south-central Alberta. The objective was to generate comprehensive baseline geochemical data to determine the key geochemical characteristics and differences of produced fluids and gases from two coal deposits and shallow groundwater in Alberta and to find parameters that are suitable for identifying potential leakage of fluids or gases into shallow groundwater. Shallow groundwater had average total dissolved solids (TDS) of 1037mg/L. Most samples belonged to the Na–HCO3–SO4 water type and average concentrations were 185mg/L. The Horseshoe Canyon/Belly River Group swabbing fluids had average TDS of 5427mg/L, a Na–HCO3 water type, and average concentrations of 47.7mg/L. The produced fluids from the Mannville Formation had average TDS contents of 74,500mg/L, negligible and a Na–Cl water type. Shallow groundwater and produced fluids from the Horseshoe Canyon Formation and the Mannville group had distinct δ 2H and δ 18O values and plotting δ 18O values versus total dissolved solids was found to be an effective approach to distinguish the waters. Sulfur isotope data revealed the occurrence of bacterial (dissimilatory) SO4 reduction in some shallow groundwater samples and in the produced fluids from the Horseshoe Canyon/Belly River Group. Methane was found in several shallow groundwater samples and its average δ 13C (−72.1±6.8‰) and δ 2H values (−297±17‰) indicated a biogenic origin predominantly from CO2 reduction. Dissolved gases from the Horseshoe Canyon Formation fluids with average δ 13C values of methane of −54.0±4.1‰ and ethane of −36.5±2.4‰ and traces of higher alkanes suggest a mixture of predominantly biogenic and some thermogenic gas. Dissolved hydrocarbon gas from the Mannville Formation had the highest average δ 13C values of −49.4±3.6‰ for methane, −28.8±2.1‰ for ethane and −26.9±1.1‰ for propane. The presence of higher alkanes suggests that the Mannville produced fluids contain an appreciable thermogenic gas component. The biogenic gas component in the Horseshoe Canyon and the Mannville Formation was mainly formed via acetate fermentation according to α CO2–CH4 values between 1.02 and 1.07. It is concluded that δ 18O values of the fluids in concert with total dissolved solids, and the isotopic compositions of methane and ethane are sufficiently distinct in shallow groundwater and produced fluids from the Horseshoe Canyon and the Mannville Formations that they may serve as tracers for evaluating potential contamination of shallow groundwater with produced fluids or gases. [Copyright &y& Elsevier]

High fluoride concentrations (median=4.4 mg/L) in deep bedrock groundwater of South Korea prevent the usage of it as a drinking water source. The hydrogeochemistry of deep thermal groundwaters (N =377) in diverse bedrocks has been studied in order to evaluate the geologic and geochemical controls on fluoride concentrations in groundwater. The groundwater samples were clustered geologically, and the average and median concentrations of fluoride were compared by the Mann–Whitney U test. The order of median fluoride concentration with respect to geology is as follows: metamorphic rocks≥granitoids≥complex rock≫volcanic rocks≥sedimentary rocks. This result indicates that the geological source of fluoride in groundwater is related to the mineral composition of metamorphic rocks and granitoids. With respect to groundwater chemistry, the fluoride concentration was highest in Na-HCO3 type groundwater and lowest in Ca-HCO3 type groundwater. Ionic relationships also imply that the geochemical behavior of fluoride in groundwater is related to the geochemical process releasing Na and removing Ca ions. The thermodynamic relationship between the activities of Ca and F indicates that fluoride concentration is controlled by the equilibrium of fluorite (CaF2). In other words, the upper limits of fluoride concentration are determined by the Ca ion; i.e., Ca concentrations play a crucial role in fluoride behavior in deep thermal groundwater. The result of this study suggests that the high fluoride in groundwater originates from geological sources and fluoride can be removed by fluorite precipitation when high Ca concentration is maintained. This provides a basis for a proper management plan to develop the deep thermal groundwater and for treatment of high fluoride groundwater frequently found in South Korea. [Copyright &y& Elsevier]

The sulfur cycle plays a key role in the hydrochemical development of lignite and coal mining districts, which are often characterized by acid mine drainage. Identification of sulfur sources and transformations is essential if we are to understand the current status of acid lakes and propose successful remediation strategies. We utilized stable isotope (δ34Ssulfate, δ18Osulfate, δ34Sdissolved sulfide, δ18Owater) and hydrochemical data of lake and groundwater samples as well as the concentrations and isotope ratios of various sedimentary sulfur compounds (acid-soluble sulfate [ASS], acid-volatile sulfur [AVS], chromium-reducible sulfur [CRS]) in aquifer and lake sediments for assessing the biogeochemical sulfur cycle in a Lusatian acidic mining lake (ML111) and the surrounding area.Pyrite oxidation is the dominant source of sulfate and iron in the two aquifers west and east of mining lake ML111. Concentration and isotope data for dissolved sulfate suggest that bacterial (dissimilatory) sulfate reduction became an increasingly important process in the flow direction of the western aquifer, thereby improving water quality. In contrast, bacterial sulfate reduction was only of local importance in the dump aquifer on the eastern side of the lake. Oxygen isotope ratios of the lake water sulfate, and mass and isotope balances demonstrate that ∼80% of the sulfate in mining lake ML111 is derived via groundwater influx from the dump aquifer, with the remainder contributed by the western aquifer. In-lake pyrite oxidation or bacterial sulfate reduction in the water column are insignificant. The latter process is restricted to the monimolimnion, which represents less than 1% of the lake volume. Hence, the potential for natural generation of alkalinity by bacterial (dissimilatory) sulfate reduction is presently low. For successful remediation of ML111, it is essential to reduce significantly the continuous input of sulfate and acidity from inflowing dump groundwater. [Copyright &y& Elsevier]

Analyses of the sulfur (S) and oxygen (O) stable isotope ratios (δ34S SO4 and δ18O SO4) of dissolved sulfate in streams and groundwater in Yukon Territory (YT, Canada) indicates that the dominant source of this major ion is oxidation of sulfide minerals. In these streams, sulfate has a large range in δ34S SO4 values (−19 to +10‰), which is consistent with the lower portion of the documented range for δ34S of sulfides in bedrock in YT (∼ −35 to +55‰). Furthermore, the large majority of the δ18O SO4 data plot within the expected field for sulfide oxidation in cross-plots of δ18O SO4 versus the O isotope ratios of ambient water (δ18O H2O). δ18O SO4 values plotting above that field were likely affected by microbial sulfate reduction, a process that enriches residual sulfate in both 34S and 18O. The stable isotope data indicate that dissolution of marine-evaporite gypsum, which has higher δ34S SO4 and δ18O SO4 values compared to all stream and groundwater samples, is a minor to negligible source of sulfate in the YT streams. Association of sulfate with other solutes indicates release of various metals from sulfide minerals, and suggests dominance of silicate weathering in response to oxidation of the sulfides. Variations in sulfate concentrations in YT streams are largely related to geology, while terrain and climate, including extent of permafrost, are also important factors. Long-term monitoring of sulfate concentrations in YT streams indicates increasing concentrations over time, in both streams impacted by mining, and in streams that have not been affected much by historical in-catchment industrial activities. Increases were largest in streams in northern and central-south YT in the presence of thawing continuous to discontinuous permafrost. [Display omitted] • Sulfate concentrations in Yukon Territory streams have increased over time. • Sulfide oxidation is the dominant source of sulfate in streams and groundwater. • Marine-evaporite gypsum is a minor to negligible source of stream sulfate. • Other solutes suggest silicate weathering as dominant response to sulfide oxidation. • Variations in sulfate concentrations in streams are largely related to geology. [ABSTRACT FROM AUTHOR]

We report the results of a synoptic ("snapshot") sampling of inorganic and isotopic geochemistry of surface water and groundwater during base flow in Red Canyon Creek watershed (Wyoming, USA) to evaluate how mixing of waters and geochemical processes modify stream-water chemistry. Our synoptic approach of studying the geochemistry of a stream mimics and has the same strengths of that widely used to characterize geochemical processes in groundwater systems. Gypsum dissolution, carbonate precipitation, and the influx of tributary and groundwater all affect Red Canyon Creek stream-water chemistry. Identical ranges of hydrogen and oxygen isotopes show good hydraulic connection between surface water and shallow groundwater. In contrast, δ34SSO4 values of groundwater (15.2‰ ± 1.6‰) and surface water (11.1‰ ± 1.6‰) reflect sulfate mostly contributed by gypsum dissolution. Calcium contributed by gypsum dissolution causes calcite to rapidly precipitate in crusts observed in the streambed. This process occurs even though residence time of water in the stream is orders of magnitude less than that found in groundwater systems showing the same kind of common ion effect. Although microbes oxidize organic matter by sulfate reduction behind beaver dams and in the hyporheic zone, too little of this water reaches the stream to measurably affect the sulfur isotopic signature of its water. The results of this study suggest that calcareous accumulations in the fluvial sedimentary record, in association with gypsum, may possibly be paleohydrologic proxies for environments where mixtures of surface water with different chemical composition occurred. [ABSTRACT FROM AUTHOR]

Abstract: The production of coalbed methane (CBM) represents a vital new source of natural gas supply in Western Canada. There are, however, concerns over potential negative environmental impacts on groundwater resources caused by potential contamination with fluids and gases from coal-bearing strata. This paper provides a brief description of the geochemistry and a detailed discussion of trace element and rare earth element (REE) concentrations of the produced fluids from two major coal deposits in Western Canada, the Mannville Formation and the Horseshoe Canyon/Belly River Group (HSCN/BRG), and shallow groundwater (SGW) in this region. It evaluates how the depositional environment, redox conditions, and water–rock interactions influence the trace element and rare earth element distributions in the produced fluids and SGW of the study area. Produced waters from the Mannville Formation are a saline Na–Cl type and a high boron content is indicative of a brackish/marine depositional environment. In contrast, the HSCN/BRG produced fluids are a Na–HCO3 water type and have lower total dissolved solids (TDS) and lower boron concentrations. Shallow groundwater has a Na–HCO3 to Na–HCO3–SO4 water type with often high concentrations of sulphate and low TDS. Some groundwater samples, many HSCN/BRG and all Mannville fluids had low sulphate concentrations often with elevated δ34S values indicating that bacterial sulphate reduction had occurred making the redox environment suitable for methanogenesis. Dissolved gas from the Mannville Formation had a notable thermogenic component, whereas dissolved gas from the HSCN/BRG and free gas in shallow groundwater contained methane predominantly or exclusively of biogenic origin. Mannville produced waters are up to 70 times more concentrated in trace elements compared to the HSCN/BRG waters and up to 300 times more concentrated than shallow groundwater. In the Mannville fluids, trace elements, such as As, Se and Pb occurred in concentrations often exceeding drinking water guidelines (5 μg/L, 10 μg/L and 10 μg/L, respectively), whereas most samples in the HSCN/BRG contained only Se above drinking water guidelines, and a few samples contained As and Pb slightly above the maximum allowable concentrations for drinking water. Therefore, the potential risk of trace metal contamination of shallow groundwater with HSCN/BRG swabbing fluids is low because of their comparatively low trace metal content. Rare earth elements in the Mannville produced fluids indicate interaction with silicate dominated host rocks. In contrast, the REEs of the HSCN/BRG produced fluids show interaction with carbonate-rich material. The shallow groundwater shows a similar pattern to the HSCN/BRG, suggesting that REEs are not a suitable parameter to monitor potential contamination of shallow groundwater with CBM fluids from the HSCN/BRG. However, several other geochemical and isotopic parameters are sufficiently distinct between shallow groundwater, HSCN/BRG, and Mannville fluids so that cross-contamination with produced fluids or gases should be identifiable with a suitable geochemical monitoring program. [Copyright &y& Elsevier]

Abstract: In the Jungwon area, South Korea, two contrasting types of deep thermal groundwater (around 20–33 °C) occur together in granite. Compared to shallow groundwater and surface water, thermal groundwaters have significantly lower δ 18O and δD values (>1‰ lower in δ 18O) and negligible tritium content (mostly <2 TU), suggesting a relatively high age of these waters (at least pre-thermonuclear period) and relatively long subsurface circulation. However, the hydrochemical evolution yielded two distinct water types. CO2-rich water (P CO2 =0.1 to 2 atm) is characterized by lower pH (5.7–6.4) and higher TDS content (up to 3300 mg/L), whereas alkaline water (P CO2 =10−4.1–10−4.6 atm) has higher pH (9.1–9.5) and lower TDS (<254 mg/L). Carbon isotope data indicate that the CO2-rich water is influenced by a local supply of deep CO2 (potentially, magmatic), which enhanced dissolution of silicate minerals in surrounding rocks and resulted in elevated concentrations of Ca2+, Na+, Mg2+, K+, HCO3 − and silica under lower pH conditions. In contrast, the evolution of the alkaline water was characterized by a lesser degree of water–rock (granite) interaction under the negligible inflow of CO2. The application of chemical thermometers indicates that the alkaline water represents partially equilibrated waters coming from a geothermal reservoir with a temperature of about 40 °C, while the immature characteristics of the CO2-rich water resulted from the input of CO2 in Na–HCO3 waters and subsequent rock leaching. [Copyright &y& Elsevier]

Abstract: The geochemistry and isotopic composition (H, O, S, Osulfate, C, Sr) of groundwater from the Nubian Sandstone (Kurnub Group) aquifer in the Negev, Israel, were investigated in an attempt to reconstruct the origin of the water and solutes, evaluate modes of water–rock interactions, and determine mean residence times of the water. The results indicate multiple recharge events into the Nubian sandstone aquifer characterized by distinctive isotope signatures and deuterium excess values. In the northeastern Negev, groundwater was identified with deuterium excess values of ∼16‰, which suggests local recharge via unconfined areas of the aquifer in the Negev anticline systems. The δ 18OH2O and δ 2H values (−6.5‰ and −35.4‰) of this groundwater are higher than those of groundwater in the Sinai Peninsula and southern Arava valley (−7.5‰ and −48.3‰) that likewise have lower deuterium excess values of ∼10‰. Based on the geochemical differences between groundwater in the unconfined and confined zones of the aquifer, a conceptual geochemical model for the evolution of the groundwater in the Nubian sandstone aquifer has been reconstructed. The isotopic composition of shallow groundwater from the unconfined zone indicates that during recharge oxidation of pyrite to SO4 (δ 34SSO4 ∼−13‰; δ 18OSO4 ∼+7.7‰) and dissolution of CaCO3 (87Sr/86Sr ∼0.70787; δ 13CDIC =−3.7‰) occur. In the confined zone of the aquifer, bacterial SO4 reduction removes a significant part of dissolved , thereby modifying its isotopic composition (δ 34SSO4 ∼−2‰; δ 18OSO4 ∼+8.5‰) and liberating dissolved inorganic C that contains little or no radiocarbon (14C-free) with low δ 13CDIC values (<−12‰). In addition to local recharge, the Sr and S isotopic data revealed contribution of external groundwater sources to the Nubian Sandstone aquifer, resulting in further modifications of the groundwater chemical and isotopic signatures. In the northeastern Negev, it is shown that SO4-rich groundwater from the underlying Jurassic aquifer contributes significantly to the salt budget of the Nubian Sandstone aquifer. The unique chemical and isotopic composition of the Jurassic groundwater (δ 34SSO4 ∼+14‰; δ 18OSO4 ∼14‰; 87Sr/86Sr ∼0.70764) is interpreted as reflecting dissolution of Late Triassic marine gypsum deposits. In the southern Arava Valley the authors postulate that SO4-rich groundwater with distinctively high Br/Cl (3×10−3) low 87Sr/86Sr (0.70734), and high δ 34SSO4 values (+15‰) is derived from mixing with underlying brines from the Paleozoic units. The radiocarbon measurements reveal low 14C activities (0.2–5.8 pmc) in both the northeastern Negev and southern Arava Valley. Taking into account dissolution of carbonate rocks and bacterial SO4 reduction in the unconfined area, estimated mean residence times of groundwater in the confined zone in the northeastern Negev are on the order of 21–38 ka, which suggests recharge predominantly during the last glacial period. The 14C signal in groundwater from the southern Arava Valley is equally low but due to evidence for mixing with external water sources the residence time estimates are questionable. [Copyright &y& Elsevier]

Abstract: The chemical composition of groundwater in the Jordan Valley, along the section between the Sea of Galilee and the Dead Sea, is investigated in order to evaluate the origin of the groundwater resources and, in particular, to elucidate the role of deep brines on the chemical composition of the regional groundwater resources in the Jordan Valley. Samples were collected from shallow groundwater in research boreholes on two sites in the northern and southern parts of the Jordan Valley, adjacent to the Jordan River. Data is also compiled from previous published studies. Geochemical data (e.g., Br/Cl, Na/Cl and SO4/Cl ratios) and B, O, Sr and S isotopic compositions are used to define groundwater groups, to map their distribution in the Jordan valley, and to evaluate their origin. The combined geochemical tools enabled the delineation of three major sources of solutes that differentially affect the quality of groundwater in the Jordan Valley: (1) flow and mixing with hypersaline brines with high Br/Cl (>2×10−3) and low Na/Cl (<0.8) ratios; (2) dissolution of highly soluble salts (e.g., halite, gypsum) in the host sediments resulting in typically lower Br/Cl signal (<2×10−3); and (3) recharge of anthropogenic effluents, primarily derived from evaporated agricultural return flow that has interacted (e.g., base-exchange reactions) with the overlying soil. It is shown that shallow saline groundwaters influenced by brine mixing exhibit a north–south variation in their Br/Cl and Na/Cl ratios. This chemical trend was observed also in hypersaline brines in the Jordan valley, which suggests a local mixing process between the water bodies. [Copyright &y& Elsevier]

Abstract: The Pocheon spa-land area, South Korea occurs in a topographically steep, fault-bounded basin and is characterized by a hydraulic upwelling flow zone of thermal water (up to 44°C) in its central part. Hydrogeochemical and environmental isotope data for groundwater in the study area suggested the occurrence of two distinct water types, a Ca-HCO3 type and a Na-HCO3 type. The former water type is characterized by relatively high concentrations of Ca, SO4 and NO3, which show significant temporal variation indicating a strong influence by surface processes. In contrast, the Na-HCO3 type waters have high and temporally constant temperature, pH, TDS, Na, Cl, HCO3 and F, indicating the attainment of a chemical steady state with respect to the host rocks (granite and gneiss). Oxygen, hydrogen and tritium isotope data also indicate the differences in hydrologic conditions between the two groups: the relatively lower δ18O, δD and tritium values for Na-HCO3 type waters suggest that they recharged at higher elevations and have comparatively long mean residence times. Considering the geologic and hydrogeologic conditions of the study area, Na-HCO3 type waters possibly have evolved from Ca-HCO3 type waters. Mass balance modeling revealed that the chemistry of Na-HCO3 type water was regulated by dissolution of silicates and carbonates and concurrent ion exchange. Particularly, low Ca concentrations in Na-HCO3 water was mainly caused by cation exchange. Multivariate mixing and mass balance modeling (M3 modeling) was performed to evaluate the hydrologic mixing and mass transfer between discrete water masses occurring in the shallow peripheral part of the central spa-land area, where hydraulic upwelling occurs. Based on Q-mode factor analysis and mixing modeling using PHREEQC, an ideal mixing among three major water masses (surface water, shallow groundwater of Ca-HCO3 type, deep groundwater of Na-HCO3 type) was proposed. M3 modeling suggests that all the groundwaters in the spa area can be described as mixtures of these end-members. After mixing, the net mole transfer by geochemical reaction was less than that without mixing. Therefore, it is likely that in the hydraulic mixing zone geochemical reactions are of minor importance and, therefore, that mixing regulates the groundwater geochemistry. [Copyright &y& Elsevier]

Abstract: Recent geochemical and hydrological findings show that the water quality of the base flow of the Lower Jordan River, between the Sea of Galilee and the Dead Sea, is dependent upon the ratio between surface water flow and groundwater discharge. Using water quality data, mass-balance calculations, and actual flow-rate measurements, possible management scenarios for the Lower Jordan River and their potential affects on its salinity are investigated. The predicted scenarios reveal that implementation of some elements of the Israel–Jordan peace treaty will have negative effects on the Jordan River water salinity. It is predicted that removal of sewage effluents dumped into the river (∼13 MCM/a) will significantly reduce the river water’s flow and increase the relative proportion of the saline groundwater flux into the river. Under this scenario, the Cl content of the river at its southern point (Abdalla Bridge) will rise to almost 7000mg/L during the summer. In contrast, removal of all the saline water (16.5 MCM/a) that is artificially discharged into the Lower Jordan River will significantly reduce its Cl concentration, to levels of 650–2600 and 3000–3500mg/L in the northern and southern areas of the Lower Jordan River, respectively. However, because the removal of either the sewage effluents or the saline water will decrease the river’s discharge to a level that could potentially cause river desiccation during the summer months, other water sources must be allocated to preserve in-stream flow needs and hence the river’s ecosystem. [Copyright &y& Elsevier]