Differentiating and Mitigating Methane Emissions from Fugitive Leaks from Natural Gas Distribution, Historic Landfills, and Manholes in Montréal, Canada.

Rapidly reducing urban methane (CH4) emissions is a critical component of strategies aimed at limiting climate change. Individual source measurements provide the details necessary to develop actionable mitigation strategies and are highly complementary to mobile surveys and other top-down methods. Here, we perform 615 individual source measurements in Montréal, Canada, to quantify CH4 emissions from historic landfills, manholes, and fugitive emissions from natural gas (NG) distribution systems. We find that in 2020, historic landfills produced 901 (452 to 1541, 95% c.i.) tons of CH4, manholes emitted 786 (32 to 2602, 95% c.i.) tons of CH4, and NG distribution systems emitted 451 (176-843, 95% c.i.) tons of CH4, placing them all within the top four CH4 sources in Montréal. Methane emissions from both historic landfills and manholes are not accounted for in any greenhouse gas inventory. We find that geochemistry alone cannot positively identify source subcategories (e.g., type of manhole or NG infrastructure) in almost all cases, although C2/C1 ratios can distinguish NG distribution sources from biogenic sources (historic landfills and manholes). Using our individual source measurement data, we show that historic landfills have the greatest potential for CH4 reductions but the highest mitigation costs, unless we target the highest emitting landfills. In contrast, CH4 emissions from manholes can be reduced at low costs, but reduction methods are commercially unavailable. For NG distribution, methods such as increasing repair rates for high-emitting industrial meters can greatly reduce mitigation costs and emissions. Overall, our results highlight the role of individual source measurements in developing actionable CH4 mitigation strategies to meet municipal, regional, and national climate action plans.

Provenances, preponderances, and distribution of humic acids and organic pollutants in hydro-geosphere: The co-existence, interaction and isotopic biomarkers in the riverine ecosystem.

This paper aims to critically review the importance of geochemical fingerprinting and tracing using biomarkers and stable isotopes in the riverine ecosystem and depicts that isotopic ratios of δ13C, δ15N, and δ34S can be used for tracing pollution sources. Stable isotopes like carbon, hydrogen, nitrogen, oxygen, and sulfur are being used for this purpose, and their isotopic signatures are primarily used to distinguish close sources of organic matter through dual isotopes. The present review is articulated to bridge the critical research gaps of the previous and contemporary documented literature on the genesis and transport of OM between freshwater and marine systems. This review comprehensively provides methods and techniques in geochemical tracing and discusses the future directions to address the challenges of the current methods to enhance the knowledge about the source identification of organic matter in the riverine environment. Tracer geochemistry emphasizes the implications of elemental abundances and isotope ratio variations in geologic substances to track natural earth processes, anthropogenic contaminants, and geochemical signatures in the hydrologic system. The principal constituent of organic matter comprises humic substances like humic acid, fulvic acid, and humin, and these comprise 50-75% of the sediments and DOC in natural waters. Their structural and functional characterization is required to elucidate the transport and fate of organic matter, which are often influenced by several paleoenvironmental factors.

Insights on Geochemical, Isotopic, and Volumetric Compositions of Produced Water from Hydraulically Fractured Williston Basin Oil Wells.

Tracing produced water origins from wells hydraulically fractured with freshwater-based fluids is sometimes predicated on assumptions that (1) each geological formation contains compositionally unique brine and (2) produced water from recently hydraulically fractured wells resembles fresher meteoric water more so than produced water from older wells. These assumptions are not valid in Williston Basin oil wells sampled in this study. Although distinct average 228Ra/226Ra ratios were found in water produced from the Bakken and Three Forks Formations, average δ2H, δ18O, specific gravity, and conductivity were similar but exhibited significant variability across five oil fields within each formation. Furthermore, initial produced water ("flowback") was operationally defined based on the presence of glycol ether compounds and water from wells that had produced <56% of the amount of fluids injected and sampled within 160 days of fracturing. Flowback unexpectedly exhibited higher temperature, specific gravity, conductivity, δ2H, and δ18O, but lower oxidation-reduction potential and δ11B, relative to the wells thought to be producing formation brines (from wells with a produced-to-injected water ratio [PIWR] > 0.84 and sampled more than 316 days after fracturing). As such, establishing an overall geochemical and isotopic signature of produced water compositions based solely on chemical similarity to meteoric water and formation without the consideration of well treatments, well completion depth, or lateral location across the basin could be misleading if these signatures are assumed to be applicable across the entire basin. These findings have implications for using produced water compositions to understand the interbasin fluid flow and trace sources of hydraulic fracturing fluids.

Investigating the importance of recreational roads as a sediment source in a mountainous catchment using a fingerprinting procedure with different multivariate statistical techniques and a Bayesian un-mixing model.

Road construction associated with land development generally increases erosion and sediment yields. Construction of unpaved roads has the potential to alter hydro-sedimentological behavior and catchment sediment source dynamics and, to date, this has largely been investigated in forested environments. The objective of this study, therefore, was to assess the relative importance of unpaved recreational roads as a sediment source alongside hillslope surface soils and stream channel banks in a non-forested mountainous catchment in northern Tehran, Iran, using a fingerprinting procedure. Eleven geochemical tracers were measured on 27 samples collected to characterise the sediment sources and five suspended sediment samples collected at the study catchment outlet. The statistical analysis employed to select three different composite fingerprints for discriminating the sediment sources comprised: (1) the Kruskal-Wallis H test (KW-H), (2) a combination of KW-H and discriminant function analysis (DFA), and (3) a combination of KW-H and principal components & classification analysis (PCCA). A Bayesian un-mixing model was used to ascribe sediment source contributions using the three composite fingerprints. Using the KW-H composite signature, the respective relative contributions (with uncertainty ranges) from recreational roads, hillslope surface soils and channel banks were estimated as 64.5% (57.7-73.1), 1.1% (0.1-4.9), and 33.9% (24.9-41.0), compared to 55.3% (45.5-68.5), 1.9% (0.1-7.9) and 42.1% (27.8-52.4) using a composite signature selected using a combination of KW-H and DFA, or 82.0% (69.7-93.8), 8.2% (0.7-22.7) and 7.3% (0.7-21.0) using a fingerprint selected using KW-H and PCCA. The root mean square difference between the apportionment results using the fingerprints identified on the basis of the three different statistical approaches ranged from 5.5% to 25.7%, highlighting the sensitivity of source estimates to the tracers used. Regardless, the different composite signatures all suggested that unpaved recreational roads were the dominant source of the suspended sediment samples, underscoring the need for mitigation measures targeting these anthropogenic features of the catchment system, including closure to permit re-vegetation, surface ripping and/or mulching to improve infiltration or gravel re-surfacing to reduce exposure of bare surfaces to sediment mobilisation.

Anatomy of a glacial meltwater discharge event in an Antarctic cove.

Glacial meltwater discharge from Antarctica is a key influence on the marine environment, impacting ocean circulation, sea level and productivity of the pelagic and benthic ecosystems. The responses elicited depend strongly on the characteristics of the meltwater releases, including timing, spatial structure and geochemical composition. Here we use isotopic tracers to reveal the time-varying pattern of meltwater during a discharge event from the Fourcade Glacier into Potter Cove, northern Antarctic Peninsula. The discharge is strongly dependent on local air temperature, and accumulates into an extremely thin, buoyant layer at the surface. This layer showed evidence of elevated turbidity, and responded rapidly to changes in atmospherically driven circulation to generate a strongly pulsed outflow from the cove to the broader ocean. These characteristics contrast with those further south along the Peninsula, where strong glacial frontal ablation is driven oceanographically by intrusions of warm deep waters from offshore. The Fourcade Glacier switched very recently to being land-terminating; if retreat rates elsewhere along the Peninsula remain high and glacier termini progress strongly landward, the structure and impact of the freshwater discharges are likely to increasingly resemble the patterns elucidated here.This article is part of the theme issue 'The marine system of the West Antarctic Peninsula: status and strategy for progress in a region of rapid change'.

Volatile organic compounds (VOCs) from diffuse degassing areas: Interstitial soil gases as message bearers from deep hydrothermal reservoirs.

The chemical composition of volatile organic compounds (VOCs) in interstitial soil gases from hydrothermal areas is commonly shaped by both deep hydrothermal conditions (e.g., temperature, redox, sulfur fugacity) and shallow secondary processes occurring near the soil-atmosphere interface. Caldara di Manziana and Solfatara di Nepi, i.e., two hydrothermal systems characterized by diverse physicochemical conditions located in the Sabatini Volcanic District and Vicano-Cimino Volcanic District, respectively (Central Italy), were investigated to evaluate the capability of VOCs in soil gases to preserve information from the respective feeding deep fluid reservoirs. Hierarchical cluster analyses and robust principal component analyses allowed recognition of distinct groups of chemical parameters of soil gases collected from the two study areas. The compositional dissimilarities from the free-gas discharges were indeed reflected by the chemical features of soil gases collected from each site, despite the occurrence of shallow processes, e.g., air mixing and microbial degradation processes, affecting VOCs. Four distinct groups of VOCs were recognized suggesting similar sources and/or geochemical behaviors, as follows: (i) S-bearing compounds, whose abundance (in particular that of thiophenes) was strictly dependent on the sulfur fugacity in the feeding system; (ii) C4,5,7+ alkanes, n-hexane, cyclics and alkylated aromatics, related to relatively low-temperature conditions at the gas source; (iii) C2,3 alkanes, benzene, benzaldehyde and phenol, i.e., stable compounds and thermal degradation products; and (iv) aliphatic O-bearing compounds, largely influenced by shallow processes within the soil. However, they maintain a chemical speciation that preserves a signature derived from the supplying deep-fluids, with aldehydes and ketones becoming more enriched after intense interaction of the hypogenic fluids with shallow aquifers. Accordingly, the empirical results of this study suggest that the chemical composition of VOCs in soil gases from hydrothermal areas provides insights into both deep source conditions and fluid circulation dynamics, identifying VOCs as promising geochemical tracers for geothermal exploration.

Hydrogeochemical characteristics, stable isotopes, positive matrix factorization, source apportionment, and health risk of high fluoride groundwater in semiarid region.

Chronic exposure to high fluoride (F-) levels in groundwater causes community fluorosis and non-carcinogenic health concerns in local people. This study described occurrence, dental fluorosis, and origin of high F-groundwater using δ2H and δ18O isotopes at semiarid Gilgit, Pakistan. Therefore, groundwater (n = 85) was collected and analyzed for F- concentrations using ion-chromatography. The lowest F- concentration was 0.4 mg/L and the highest 6.8 mg/L. F- enrichment is linked with higher pH, NaHCO3, NaCl, δ18O, Na+, HCO3-, and depleted Ca+2 aquifers. The depleted δ2H and δ18O values indicated precipitation and higher values represented the evaporation effect. Thermodynamic considerations of fluorite minerals showed undersaturation, revealing that other F-bearing minerals viz. biotite and muscovite were essential in F- enrichment in groundwater. Positive matrix factorization (PMF) and principal component analysis multilinear regression (PCAMLR) models were used to determine four-factor solutions for groundwater contamination. The PMF model results were accurate and reliable compared with those of the PCAMLR model, which compiled the overlapping results. Therefore, 28.3% exceeded the WHO permissible limit of 1.5 mg/L F-. Photomicrographs of granite rocks showed enriched F-bearing minerals that trigger F- in groundwater. The community fluorosis index values were recorded at > 0.6, revealing community fluorosis and unsuitability of groundwater for drinking.

A review of the distribution, sources, genesis, and environmental concerns of salinity in groundwater.

Awareness concerning the degradation of groundwater quality and their exacerbating adverse effects due to salinization processes is gaining traction, raising for adequate understanding of the distribution, sources, genesis, and environmental concerns of salinity in groundwater. Saline groundwater is widely distributed all over the world, with an area of 24 million km2 (16% of the total land area on earth) and 1.1 billion people living in the affected areas, especially the arid/semi-arid areas in developing countries. These large-scale groundwater salinization problems are sourced from two major ways: natural and anthropogenic. The natural sources are diversified from connate saline groundwater, seawater intrusion, evaporation, dissolution of soluble salts, membrane filtration process to geothermal origin. The anthropogenic sources include irrigation return flow, road deicing salts, industrial and agricultural wastewater, and gas and oil production activities. The integrated approach of geochemical tracers and multiple isotopes (δ18OH2O, δ2HH2O, δ11B, δ36Cl, δ34Ssulfate, 87Sr/86Sr, and δ7Li) is proved to be useful in the constraints of the origin and transport of solutes in groundwater. Groundwater salinization is often associated with high levels of some toxic elements like arsenic, fluoride, selenium, and boron. Four "triggers" lead to this association: salt effect, competing adsorption, microbial processes, and cation exchange.

Hydrochemistry of flowback water from Changning shale gas field and associated shallow groundwater in Southern Sichuan Basin, China: Implications for the possible impact of shale gas development on groundwater quality.

The worldwide expansion of shale gas production and increased use of hydraulic fracturing have raised public concerns about safety and risks of groundwater resources in shale gas extraction areas. China has the largest shale gas resources in the world, most of which are located in the Sichuan Basin. Shale gas extraction in the Sichuan Basin has been increasing rapidly in recent years. However, the potential impact on shallow groundwater quality has not yet been systematically investigated. In order to evaluate the possible impact of shale gas extraction on groundwater quality, we present, for the first time, the hydrochemistry and Sr isotopic data of shallow groundwater, as well as flowback and produced water (FP water) in the Changning shale gas field in Sichuan Basin, one of the major shale gas fields in China. The Changning FP water is characterized by high salinity (TDS of 13,100-53,500 mg/L), Br/Cl (2.76 × 10-3) and 87Sr/86Sr (0.71849), which are distinguished from the produced waters from nearby conventional gas fields with higher Br/Cl (4.5 × 10-3) and lower 87Sr/86Sr (0.70830-0.71235). The shallow groundwater samples were collected from a Triassic karst aquifer in both active and nonactive shale gas extraction areas. They are dominated by low salinity (TDS of 145-1100 mg/L), Ca-HCO3 and Ca-Mg-HCO3 types water, which are common in carbonate karst aquifers. No statistical difference of the groundwater quality was observed between samples collected in active versus nonactive shale gas extraction areas. Out of 66 analyzed groundwater, three groundwater samples showed relatively higher salinity above the background level, with low 87Sr/86Sr (0.70824-0.7110) and Br/Cl (0.5-1.8 × 10-3) ratios relatively to FP water, excluding the possibility of contamination from FP water. None of the groundwater samples had detected volatile organic compounds (VOCs). The integration of geochemical and statistical analysis shows no direct evidence of groundwater contamination caused by shale gas development.

Dual in-aquifer and near surface processes drive arsenic mobilization in Cambodian groundwaters.

Millions of people globally, and particularly in South and Southeast Asia, face chronic exposure to arsenic from reducing groundwaters in which. Arsenic release to is widely attributed largely to reductive dissolution of arsenic-bearing iron minerals, driven by metal reducing bacteria using bioavailable organic matter as an electron donor. However, the nature of the organic matter implicated in arsenic mobilization, and the location within the subsurface where these processes occur, remains debated. In a high resolution study of a largely pristine, shallow aquifer in Kandal Province, Cambodia, we have used a complementary suite of geochemical tracers (including 14C, 3H, 3He, 4He, Ne, δ18O, δD, CFCs and SF6) to study the evolution in arsenic-prone shallow reducing groundwaters along dominant flow paths. The observation of widespread apparent 3H-3He ages of <55years fundamentally challenges some previous models which concluded that groundwater residence times were on the order of hundreds of years. Surface-derived organic matter is transported to depths of >30m, and the relationships between age-related tracers and arsenic suggest that this surface-derived organic matter is likely to contribute to in-aquifer arsenic mobilization. A strong relationship between 3H-3He age and depth suggests the dominance of a vertical hydrological control with an overall vertical flow velocity of ~0.4±0.1m·yr-1 across the field area. A calculated overall groundwater arsenic accumulation rate of ~0.08±0.03µM·yr-1 is broadly comparable to previous estimates from other researchers for similar reducing aquifers in Bangladesh. Although apparent arsenic groundwater accumulation rates varied significantly with site (e.g. between sand versus clay dominated sequences), rates are generally highest near the surface, perhaps reflecting the proximity to the redox cline and/or depth-dependent characteristics of the OM pool, and confounded by localized processes such as continued in-aquifer mobilization, sorption/desorption, and methanogenesis.

Evidence for unmonitored coal ash spills in Sutton Lake, North Carolina: Implications for contamination of lake ecosystems.

Coal combustion residuals (CCRs, also known as "coal ash") contain high concentrations of toxic and carcinogenic elements that can pose ecological and human health risks upon their release into the environment. About half of the CCRs that are generated annually in the U.S. are stored in coal ash impoundments and landfills, in most cases adjacent to coal plants and waterways. Leaking of coal ash ponds and CCR spills are major environmental concerns. One factor which may impact the safety of CCRs stored in impoundments and landfills is the storage area's predisposition to flooding. The southeastern U.S., in particular, has a large number of coal ash impoundments located in areas that are vulnerable to flooding. In order to test for the possible presence of CCR solids in lake sediments following Hurricane Florence, we analyzed the magnetic susceptibility, microscopic screening, trace element composition, and strontium isotope ratios of bottom sediments collected in 2015 and in 2018 from Sutton Lake in eastern North Carolina and compared them to a reference lake. The results suggest multiple, apparently previously unmonitored, CCR spills into Sutton Lake from adjacent CCR storage sites. The enrichment of metals in Sutton Lake sediments, particularly those with known ecological impact such as As, Se, Cu, Sb, Ni, Cd, V, and Tl, was similar to or even higher than those in stream sediments impacted by the Tennessee Valley Authority (TVA) in Kingston, Tennessee, and the Dan River, North Carolina coal ash spills, and exceeded ecological screening standards for sediments. High levels of contaminants were also found in leachates extracted from Sutton Lake sediments and co-occurring pore water, reflecting their mobilization to the ambient environment. These findings highlight the risks of large-scale unmonitored spills of coal ash solids from storage facilities following major storm events and contamination of nearby water resources throughout the southeastern U.S.

Assessment of the metal contamination evolution in the Loire estuary using Cu and Zn stable isotopes and geochemical data in sediments.

In this work, a multi-elemental approach combining Cu and Zn stable isotopes is used to assess the metal contamination evolution in the Loire estuary bulk sediments. Elemental geochemical data indicate an increase of metal concentrations from the beginning of the industrial period peaking in the 1990s, followed by an attenuation of metal contamination inputs to the estuary. Zinc isotope compositions suggest a binary mixing process between Zn derived from terrigenous material and multi-urban anthropogenic sources. Copper isotope systematics indicate a single natural dominant source represented by weathered silicate particles from soils and rocks. This work demonstrates the applicability of Zn isotopes to identify anthropogenic Zn sources in coastal systems, even under a low to moderate degree of contamination. Further studies are required to constrain Cu sources and to elucidate possible effects of grain-size and mineralogy in the Cu isotope composition of sediment in the Loire estuary.

Fingerprinting sub-basin spatial suspended sediment sources by combining geochemical tracers and weathering indices.

Transport and deposition of fine-grained sediment, a pervasive nonpoint source pollutant, cause deleterious off-site impacts for water quality and aquatic ecosystems. Sediment fingerprinting provides one means of identifying the spatial sources of mobilised sediment delivered to fluvial systems in order to help target sediment control strategies and uptake of such source tracing procedures has been steadily increasing. Nonetheless, there remains a need to continue testing and comparing different composite signatures for source discrimination including the incorporation of physically grounded information relevant to erosion patterns. Accordingly, the objective of this study was to compare the discrimination and apportionment of sub-basin spatial suspended sediment sources in a mountainous basin in northern Tehran, Iran, using composite signatures comprising conventional geochemical tracers combined with lithological weathering indices or only the former. The list of conventional geochemical properties comprised Al, Ca, Cu, Fe, K, Mg, Mn, Na, Ni, Sr, Ti, and Zn whilst three weathering indices were included: the chemical index of alteration (CIA), the weathering index of Parker (WIP), and the indicator of recycling (IR) which were all calculated based on elemental oxides. Using a composite signature combining conventional geochemical tracers and one weathering index (IR), the relative contributions from the sub-basin spatial sources were estimated at 1 (Imamzadeh Davood; 1.4%), 2 (Taloon; 13.4%), 3 (Soleghan; 35.9%), and 4 (Keshar; 48.4%) compared with corresponding respective estimates of 0.7%, 45.5%, 40.2%, and 13.3% using conventional geochemical tracers alone. Wald-Wolfowitz Runs test pairwise comparisons of the posterior distributions of predicted source proportions generated using the two different composite signatures confirmed statistically significant differences. These differing proportions demonstrated the sensitivity of predicted source apportionment to the inclusion or exclusion of a weathering index providing information reflecting the relative coverage of more erodible lithological formations in each of the sub-basins (32.7% sub-basin 1, 53.6% sub-basin 2, 58.5% sub-basin 3, and 63.2% sub-basin 4). The outputs of this study will be used to target sediment mitigation strategies.

Tracing sediment sources in a mountainous forest catchment under road construction in northern Iran: comparison of Bayesian and frequentist approaches.

Development and land use change lead to accelerated soil erosion as a serious environmental problem in river catchments in Iran. Reliable information about the sources of sediment in catchments is therefore necessary to design effective control strategies. This study used a composite sediment source tracing procedure to determine the importance of forest road cuttings as a sediment source in a mountainous catchment located in northern Iran. A fallout radionuclide (137Cs) and 12 geochemical tracers (Ca, Cu, Fe, K, Mg, Mn, Na, Ni, OC, Pb, Sr and TN) were used to determine the relative contributions of three sediment source types (hillslopes, road cuttings and channel banks) to both suspended and bed sediment samples. Two mixing models based on different mathematical concepts were used to apportion the sediment sources: the mixture sampling importance resampling Bayesian model which incorporates the mass-balance matrix and a distribution model using normal and summed probability of normal distributions. The results of both mixing models indicated that sub-soil erosion from road cuttings and channel banks dominated the sources of river bed and suspended sediment samples, respectively. These results therefore highlight that conservation that works in the study area to remedy the sediment problem should initially focus on stabilisation and rehabilitation of road cuttings and channel banks. This successful application of a composite (radionuclide and geochemical) tracing technique for discriminating source end members characterised by different erosion processes underscores the importance of sub-soil erosion in this case study.

Bivalve shell formation in a naturally CO2-enriched habitat: Unraveling the resilience mechanisms from elemental signatures.

Marine bivalves inhabiting naturally pCO2-enriched habitats can likely tolerate high levels of acidification. Consequently, elucidating the mechanisms behind such resilience can help to predict the fate of this economically and ecologically important group under near-future scenarios of CO2-driven ocean acidification. Here, we assess the effects of four environmentally realistic pCO2 levels (900, 1500, 2900 and 6600 µatm) on the shell production rate of Mya arenaria juveniles originating from a periodically pCO2-enriched habitat (Kiel Fjord, Western Baltic Sea). We find a significant decline in the rate of shell growth as pCO2 increases, but also observe unchanged shell formation rates at moderate pCO2 levels of 1500 and 2900 µatm, the latter illustrating the capacity of the juveniles to partially mitigate the impact of high pCO2. Using recently developed geochemical tracers we show that M. arenaria exposed to a natural pCO2 gradient from 900 to 2900 µatm can likely concentrate HCO3- in the calcifying fluid through the exchange of HCO3-/Cl- and simultaneously maintain the pH homeostasis through active removal of protons, thereby being able to sustain the rate of shell formation to a certain extent. However, with increasing pCO2 beyond natural maximum the bivalves may have limited capacity to compensate for changes in the calcifying fluid chemistry, showing significant shell growth reduction. Findings of the present study may pave the way for elucidating the underlying mechanisms by which marine bivalves acclimate and adapt to high seawater pCO2.