Microcombustion of ng Amounts of Carbon in Non-Volatile Materials for isotope Ratio Evaluation

Abstract A novel microcombustion technique for carbon isotopic analysis of nanogram amounts of carbon in non-volatile materials based on isotope ratio monitoring (irm) mass spectrometry is described. Liquid or solid samples placed in a quartz sleeve are combusted at 1000°C in a continuous stream of helium and oxygen. CO(2) removed from the carrier gas stream by cryogenic trapping is transferred onto a GC column. Following GC separation, the CO(2) is transferred via an open split to the ion source of a gas isotope ratio mass spectrometer. Reproducibility for samples >25 nmol carbon is <1‰. Problems associated with blanks from various sources and with reproducible deposition of small sample amounts led to variable accuracy, which was dependent on the compound class being analysed. Minimum sample size was in the range from 5 to 10 nmol carbon. Measurements of dissolved organic carbon (DOC) of groundwater from Germany yielded consistent values of δ(13)C = -28.8‰.     

Microcombustion of ng Amounts of Carbon in Non-Volatile Materials for isotope Ratio Evaluation

Abstract

A novel microcombustion technique for carbon isotopic analysis of nanogram amounts of carbon in non-volatile materials based on isotope ratio monitoring (irm) mass spectrometry is described. Liquid or solid samples placed in a quartz sleeve are combusted at 1000°C in a continuous stream of helium and oxygen. CO₂ removed from the carrier gas stream by cryogenic trapping is transferred onto a GC column. Following GC separation, the CO₂ is transferred via an open split to the ion source of a gas isotope ratio mass spectrometer. Reproducibility for samples >25 nmol carbon is <1‰. Problems associated with blanks from various sources and with reproducible deposition of small sample amounts led to variable accuracy, which was dependent on the compound class being analysed. Minimum sample size was in the range from 5 to 10 nmol carbon. Measurements of dissolved organic carbon (DOC) of groundwater from Germany yielded consistent values of δ¹³C = -28.8‰.     

Identification of the nitrate contamination sources of the Brusselian sands groundwater body (Belgium) using a dual-isotope approach

Isotopic fingerprinting is an advanced technique allowing the classification of the nitrate source pollution of groundwater, but needs further development and validation. In this study, we performed measurements of natural stable isotopic composition of nitrate ((15)N and (18)O) in the groundwater body of the Brussels sands (Belgium) and studied the spatial and temporal dynamics of the isotope signature of this aquifer. Potential nitrogen sources sampled in the region had isotopic signatures that fell within the corresponding typical ranges found in the literature. For a few monitoring stations, the isotopic data strongly suggest that the sources of nitrate are from mineral fertiliser origin, as used in agriculture and golf courses. Other stations suggest that manure leaching from unprotected stockpiles in farms, domestic gardening practices, septic tanks and probably cemeteries contribute to the nitrate pollution of this groundwater body. For most monitoring stations, nitrate originates from a mixing of several nitrogen sources. The isotopic signature of the groundwater body was poorly structured in space, but exhibited a clear temporal structure. This temporal structure could be explained by groundwater recharge dynamics and cycling process of nitrogen in the soil-nitrogen pool.     

Identification of the nitrate contamination sources of the Brusselian sands groundwater body (Belgium) using a dual-isotope approach

Isotopic fingerprinting is an advanced technique allowing the classification of the nitrate source pollution of groundwater, but needs further development and validation. In this study, we performed measurements of natural stable isotopic composition of nitrate (¹⁵N and ¹⁸O) in the groundwater body of the Brussels sands (Belgium) and studied the spatial and temporal dynamics of the isotope signature of this aquifer. Potential nitrogen sources sampled in the region had isotopic signatures that fell within the corresponding typical ranges found in the literature. For a few monitoring stations, the isotopic data strongly suggest that the sources of nitrate are from mineral fertiliser origin, as used in agriculture and golf courses. Other stations suggest that manure leaching from unprotected stockpiles in farms, domestic gardening practices, septic tanks and probably cemeteries contribute to the nitrate pollution of this groundwater body. For most monitoring stations, nitrate originates from a mixing of several nitrogen sources. The isotopic signature of the groundwater body was poorly structured in space, but exhibited a clear temporal structure. This temporal structure could be explained by groundwater recharge dynamics and cycling process of nitrogen in the soil-nitrogen pool.     

Identifying local and regional groundwater in basins: chemical and stable isotopic attributes of multivariate classification of hydrochemical data,the Lower Virgin River Basin,Nevada, Arizona and Utah,U.S.A

In the Basin and Range Province of the Southwestern U.S.A., deep carbonate groundwater has been suggested as a significant source to many overlying basin-fill alluvial aquifer systems. Notwithstanding, testing this hypothesis is limited by obtaining data from such considerable depths and complex geology.

This study uses δ²H and δ¹⁸O data from springs, rivers, and wells tapping shallow basin-fill groundwater to test the hydrochemical interpretation of deep regional carbonate groundwater flow into the basin-fill aquifers. Stable isotopic and major ion attributes of hydrochemical facies suggest basin-fill alluvial groundwater of the Lower Virgin River Basin is a mixture of precipitation recharge within the Lower Virgin River Basin or the Clover and Escalante Desert Basin northwards, and the deep carbonate flow. The data support the conclusions that in the Lower Virgin River Basin, deep carbonate groundwater is an important source to the alluvial aquifer system and likely accounts for approximately 50% of the alluvial aquifer groundwater. Na⁺, K⁺, and SO₄^2– increase in the basin-fill alluvial groundwaters outside the Virgin River floodplain appears to be related with upwelling of deep regional groundwater, and indicating that the chemical character of the basin-fill alluvial groundwaters are related to the deeper flow systems.     

Nachruf Prof. Hübner

Abstract

Representative water samples were collected from different oases in the western desert of Egypt to examine characteristics of Egyptian groundwater. Chemical data recognized two basic groundwater types; alkali bicarbonate and alkali chloride, where sodium has the highest concentration. For each region the median SD, δ¹⁸ 0 and ionic strength of water calculated from their chemical analyses are recorded and plotted.

Generally, the results of isotopic content measurements of the groundwaters from the different oases in the western desert indicate the fossile origin of these waters. The values of δD and δ¹⁸ 0 of the western desert oases' waters are characteristic of old paleowaters from the Nubian sand aquifer. Modern sparse rainfall data suggest that any precipitation will exhibit moderate to large positive isotopic content enrichments and cannot be a source for these waters. They have been in no connection with the Nile water, moreover the change in isotopic composition is due to evaporation which is now ineffective at deeper levels.     

Degradation pathways of dissolved carbon in landfill leachate traced with compound-specific (13)C analysis of DOC

The isotopic compositions of carbon compounds in landfill leachate provide insights into the biodegradation pathways that dominate the different stages of waste decomposition. In this study, the carbon geochemistry of different carbon pools, environmental stable isotopes and compound-specific isotope analysis (CSIA) of leachate dissolved organic carbon (DOC) fractions and gases show distinctions in leachate biogeochemistry and methane production between the young area of active waste emplacement and the old area of historical emplacement at the Trail Road Landfill (TRL). The active area leachate has low DOC concentrations (<200 mg l(-1)) dominated by fulvic acid (FA=160 mg l(-1)), and produces CH(4) dominantly by CO(2) reduction (D- excess=20.6 per thousand). Leachate generated in the area of older waste has high DOC (>4770 mg l(-1)) dominated by FA (4482 mg l(-1)) and simple fatty acids (acetic=1008 mg l(-1) and propionic=608 mg l(-1)), and produces CH(4) by the acetate fermentation pathway (D- excess=9.8 per thousand). CSIA shows an advanced degradation and a progressive accumulation of (13)C of fatty acids in leachate from the older area. The enriched (13)C value of FA (-20 and-26 per thousand for the older and active parts, respectively,) and of low molecular weight DOC (-8 and-27 per thousand) as well as of the bulk DOC (-21 and-25 per thousand) shows more advanced degradation in the older part of the landfill, which is consistent with the shift in the humic/FA ratios (0.05 and 0.18). The (13)C enrichment of acetate (-12 per thousand) above the (13)C of DOC (-21 per thousand) and of propionic acid (-19 per thousand), in older leachate, suggests that this acetate has not evolved from the simple degradation of larger organic molecules, but by homoacetogenesis from the enriched dissolved inorganic carbon (DIC) pool (8 per thousand) and H(2,) which produce a more enriched (13)C of acetate. In contrast, the (13)C of the minor acetate in the active area (-17 per thousand) indicates that CO(2)-reducing bacteria must be the primary consumers of H(2), which has resulted in enriched (13)C(DIC) (10 per thousand) and depleted (13)C(CH4) (-58 per thousand).     

Nitrate degradation without 15N enrichment: a hydrochemical and isotopic study of a fractured rock aquifer including embedded lakes

Water samples from three quarry lakes and the surrounding fractured rock aquifer were investigated for delta18O and delta2H (H2O), delta15N and delta18O (NO3-), as well as anions and cations. Lake water and groundwater can be distinguished by their different chemical and isotopic composition. Because of evaporation processes, 18O and 2H are enriched in the lake water and can be used as natural tracers for the water dynamic of the lakes. The groundwater is characterised by high nitrate concentrations (up to 120 mg/l). Lake internal processes reduce the nitrate concentration in the quarry lakes. However, no enrichment of delta15N and delta18O in nitrate, typical for microbial nitrate degradation, is observed in the lake water. Because of the complex flow paths in the fractured rock aquifer and the intense chemical transformations at the interface between groundwater and lake water, isotopic and hydrochemical data of lake water and groundwater alone do not conclusively explain hydrological and hydrochemical processes of the investigated lake-groundwater system.     

Isotopes to assess sustainability of overexploited groundwater in the Souss–Massa system (Morocco)

Groundwater depletion and changes in isotopic and chemical contents constitute the main indicators of overexploitation, recharge, and flow paths in the Souss–Massa aquifer. These indicators highlight processes concerning sustainability of water resources in the aquifer (e.g. surface/groundwater interaction, recharge processes, and marine intrusion). The spatial variation of stable and radioactive isotopic contents indicates a mixing of modern and old water within the system. Recent recharge was observed mainly along the Souss River (the major surface-water drainage in the study area) and in the irrigated areas. Mapping of chemical and isotopic variation shows that the area is affected by abstraction, irrigation water return, and the evolution of modern recharge in time and space. The processes, distribution, and timing of groundwater flow are influenced by short- and long-term effects; long-term recharge is dependent on climatic conditions. This study can be used to make informed decisions about water-resource allocation and alternative management practices.     

Seawater intrusion into groundwater aquifer through a coastal lake - complex interaction characterised by water isotopes 2H and 18O

The present study investigates the complex interactions among surface waters, groundwaters and a coastal lake in northeastern Greece, using their stable isotopic composition (δ¹⁸O, δ²H) in combination with hydrogeological and hydrochemical data. Seasonal and spatial trends of water isotopes were studied and revealed that all water bodies in the study area interact. It was also shown that the aquifer's increased salinity is not due to fossil water from past geological periods, but is attributed to brackish lake water intrusion into the aquifer induced by the extensive groundwater pumping for irrigation purposes. Quantification of the contribution of the lake to the aquifer was achieved using the simple dilution formula. The isotopic signatures of the seawater and the groundwaters are considerably different, so there is a very little possibility of direct seawater intrusion into the aquifer.     

Spatial distribution of electrical conductivity and stable isotopes in groundwater in large catchments: a geostatistical approach in the Quequén Grande River catchment,Argentina

Stable isotopes and electrical conductivity in groundwater were used as natural tracers to adjust the hydrogeological conceptual model in one of the largest catchments within the inter-mountainous Pampa plain, Argentina. Geostatistical tools were used to define the model that best fitted the spatial distribution of each tracer, and information was obtained in areas where there was a lack of data. The conventional isotopic analysis allowed the identification of three groundwater groups with different isotopic fingerprints. One group containing 56?% of the total groundwater samples suggested a well-mixed system and soil infiltration precipitation as the main recharge source to the aquifer. The other two groups included samples with depleted (25.5?%) and enriched (18.5?%) isotopic compositions, respectively. The combination of δ¹⁸O, δ²H and electrical conductivities maps suggested ascending regional flows and water transfer from the Quequén Grande River catchment to the Moro creek. The spatial interpretation of these tracers modified the conceptual hydrogeological model of the Quequén Grande River.     

Insights from stable isotopes of water and hydrochemistry to the evolutionary processes of groundwater in the Subei lake basin,Ordos energy base,Northwestern China

ABSTRACT

Changes in groundwater evolutionary processes due to aquifer overexploitation show a world-wide increase and have been of growing concern in recent years. The study aimed to improve the knowledge of groundwater evolutionary processes by means of stable water isotopes and hydrochemistry in a representative lake basin, Ordos energy base. Groundwater, precipitation, and lake water collected during four campaigns were analysed by stable isotopes and chemical compositions. Results showed that temperature effect predominated the isotope fractionation in precipitation, while evaporation and inadequate groundwater recharge were the key factors explaining high salinity and isotopic enrichment in lake water. Additionally, the Kuisheng Lake was a preferential area of groundwater recharge, while the Subei Lake received less sources from underlying aquifer due to the combined effects of low permeable zone and upstream groundwater captured by the production wells. The homogeneous isotope signatures of groundwater may be ascribed to the closely vertical hydraulic connectivity between the unconfined and the confined aquifers. Isotopically enriched groundwater pumping from well field probably promoted isotopic depletion in groundwater depression cone. These findings not only provide the conceptual framework for the inland basin, but also have important implications for sustainable groundwater management in other groundwater discharge basins with arid climate.     

Hydrogeochemical and isotopic characterisation of groundwater in a sand-dune phreatic aquifer on the northeastern coast of the province of Buenos Aires,Argentina

This contribution presents the hydrochemical and isotopic characterisation of the phreatic aquifer located in the Partido de la Costa, province of Buenos Aires, Argentina. In the sand-dune barrier geomorphological environment, groundwater is mainly a low-salinity Ca-HCO₃ and Na-HCO₃-type, being in general suitable for drinking, whereas in the continental plain (silty clay sediments), groundwater is a Na-Cl type with high salinity and unsuitable for human consumption. The general isotopic composition of the area ranges from?6.8 to?4.3 ‰ for δ¹⁸O and from?39 to?21 ‰ for δ²H, showing that rainwater rapidly infiltrates into the sandy substrate and reaches the water table almost without significant modification in its isotopic composition. These analyses, combined with other chemical parameters, made it possible to corroborate that in the eastern area of the phreatic aquifer, there is no contamination from marine salt water.     

Anomalous fluoride concentration in groundwater - is it natural or pollution? A stable isotope approach

Fluoride anomalies (up to 11 mg/l) have been detected in groundwater of the central region of Rio Grande do Sul State, Southern Brazil, in an area where fluorosis is endemic. Two hypotheses are investigated concerning the fluoride origin: lithochemical affiliation from regional rock or contamination by fertilisers application. These hypotheses are discussed based on the stable isotope data of water, nitrate, and sulphate, which indicates that the local precipitation is the main groundwater recharge source. The isotopic composition of groundwater sulphate is similar to that of fertiliser sulphate. However, a conclusive assignment of groundwater sulphate to fertiliser origin is not indicated because further possible sulphate sources fall into the same isotopic range. In contrast, the isotopic composition of dissolved nitrate suggests that there is no direct relationship to the use of NPK fertilisers. Hence, an origin of the high fluoride content in groundwater related to long-term rock-water interactions seems likely.     

Incorporation of carbon originating from CO2 into different compounds of soil microbial biomass and soil organic matter

In general, soils without the vegetation growing on them are regarded as sources of CO2. However, there are indications that CO2 is also fixed by soil microorganisms. Although this process is not significant from a quantitative point of view, it may change the isotopic composition of soil organic matter. Therefore, we conducted an incubation study with soil and 13C-labeled CO2 to investigate this process. We found that the label was transferred from CO2 into organic compounds in soil. At the end of a 61-day incubation period, 1.3 micromol C g(-1) soil, corresponding to approximately 0.08% of the soil organic carbon, had been fixed. CO2 may, therefore, be an additional source of soil organic carbon. Compound-specific analysis of amino sugars, amino acids, and fatty acids indicated that the label is incorporated into microbial, mainly bacterial, biomass. All groups of microorganisms were involved in the assimilation of CO2, but the relatively high enrichment of mono-unsaturated and mid-chain branched fatty acids indicates that gram negative bacteria and actinomycetes may be slightly more important in this process than other groups of microorganisms.     

Degradation pathways of dissolved carbon in landfill leachate traced with compound-specific 13C analysis of DOC

The isotopic compositions of carbon compounds in landfill leachate provide insights into the biodegradation pathways that dominate the different stages of waste decomposition. In this study, the carbon geochemistry of different carbon pools, environmental stable isotopes and compound-specific isotope analysis (CSIA) of leachate dissolved organic carbon (DOC) fractions and gases show distinctions in leachate biogeochemistry and methane production between the young area of active waste emplacement and the old area of historical emplacement at the Trail Road Landfill (TRL).

The active area leachate has low DOC concentrations (<200 mg l^?1) dominated by fulvic acid (FA=160 mg l^?1), and produces CH₄ dominantly by CO₂ reduction (D? excess=20.6‰). Leachate generated in the area of older waste has high DOC (>4770 mg l^?1) dominated by FA (4482 mg l^?1) and simple fatty acids (acetic=1008 mg l^?1 and propionic=608 mg l^?1), and produces CH₄ by the acetate fermentation pathway (D? excess=9.8‰). CSIA shows an advanced degradation and a progressive accumulation of ¹³C of fatty acids in leachate from the older area. The enriched ¹³C value of FA (?20 and?26‰ for the older and active parts, respectively,) and of low molecular weight DOC (?8 and?27‰) as well as of the bulk DOC (?21 and?25‰) shows more advanced degradation in the older part of the landfill, which is consistent with the shift in the humic/FA ratios (0.05 and 0.18). The ¹³C enrichment of acetate (?12‰) above the ¹³C of DOC (?21‰) and of propionic acid (?19‰), in older leachate, suggests that this acetate has not evolved from the simple degradation of larger organic molecules, but by homoacetogenesis from the enriched dissolved inorganic carbon (DIC) pool (8‰) and H_2, which produce a more enriched ¹³C of acetate. In contrast, the ¹³C of the minor acetate in the active area (?17‰) indicates that CO₂-reducing bacteria must be the primary consumers of H₂, which has resulted in enriched ¹³C_DIC (10‰) and depleted ¹³C_CH4 (?58‰).     

Anomalous fluoride concentration in groundwater—is it natural or pollution? A stable isotope approach

Fluoride anomalies (up to 11 mg/l) have been detected in groundwater of the central region of Rio Grande do Sul State, Southern Brazil, in an area where fluorosis is endemic. Two hypotheses are investigated concerning the fluoride origin: lithochemical affiliation from regional rock or contamination by fertilisers application. These hypotheses are discussed based on the stable isotope data of water, nitrate, and sulphate, which indicates that the local precipitation is the main groundwater recharge source. The isotopic composition of groundwater sulphate is similar to that of fertiliser sulphate. However, a conclusive assignment of groundwater sulphate to fertiliser origin is not indicated because further possible sulphate sources fall into the same isotopic range. In contrast, the isotopic composition of dissolved nitrate suggests that there is no direct relationship to the use of NPK fertilisers. Hence, an origin of the high fluoride content in groundwater related to long-term rock–water interactions seems likely.     

Using environmental isotopes 2H and 18O for identification of infiltration processes in floodplain ecosystems of the River Elbe

We examined a floodplain area in the middle section of the river Elbe Valley with regard to hydrogeological and hydrological processes using isotopic methods. Over two years, river water and groundwater have been analysed for temporal and spatial chemical and isotopic (delta2H and delta18O) changes. By these methods we assessed the flow dynamics of the river-groundwater infiltration system. At low and mean river stages there is a general hydraulic gradient from the higher areas at the margin of the valley towards the floodplain. During floods river water infiltrates into the adjacent aquifer not primarily through the river banks but first through surface water inflow from north to south, via depressions and gullies from the back of the floodplain. The early stage of river water infiltration is characterized by a sharp decrease in conductivity and in concentrations of SO4(2-) and Cl- in the hydraulically connected shallow aquifer. delta2H and delta18O values show a similar tendency. We observed a significant minimum in stable isotope ratios during the flood in March 1999. Using a simple mixing equation it was calculated that the groundwater in the upper, shallow aquifer consists of around 70% river water in the transition zone (well 13) during flooding.     

Effects of high pressure and temperature on carbon isotope compositions of some acyclic alkanes and preservation of organic matter

The effects of high pressure and temperature on carbon isotopic compositions of acyclic alkanes and the stability of the acyclic alkanes were experimentally investigated. The pyrolysis of lignite with water in a closed system was conducted at 400–700°C and 1–3 GPa. The carbon isotope data, variations of peak carbon and evident odd–even predominance of acyclic alkanes indicated that: (1) the high pressure retarded the maturation of organic matter and destruction of hydrocarbons, (2) n-C₁₂₊ hydrocarbons from biogenic sources could be preserved in the cool slab subducted into the upper mantle, and (3) some organic compounds might preserve the carbon isotope signals inherited from biogenic sources. The results favor tracing the origins of organic matter in mantle rocks and extraterrestrial organic matter in meteorites and the process of deep carbon cycle.     

Stable isotope mass balances versus concentration differences of dissolved inorganic carbon – implications for tracing carbon turnover in reservoirs

The aim of this study was to identify sources of carbon turnover using stable isotope mass balances. For this purpose, two pre-reservoirs in the Harz Mountains (Germany) were investigated for their dissolved and particulate carbon contents (dissolved inorganic carbon (DIC), dissolved organic carbon, particulate organic carbon) together with their stable carbon isotope ratios. DIC concentration depth profiles from March 2012 had an average of 0.33 mmol L^–1. Increases in DIC concentrations later on in the year often corresponded with decreases in its carbon isotope composition (δ¹³C_DIC) with the most negative value of –18.4?‰ in September. This led to a carbon isotope mass balance with carbon isotope inputs of ?28.5?‰ from DOC and ?23.4, ?31.8 and ?30.7?‰ from algae, terrestrial and sedimentary matter, respectively. Best matches between calculated and measured DIC gains were achieved when using the isotope composition of algae. This shows that this type of organic material is most likely responsible for carbon additions to the DIC pool when its concentrations and δ¹³C_DIC values correlate negatively. The presented isotope mass balance is transferable to other surface water and groundwater systems for quantification of organic matter turnover.