•Time-series of water stable isotopes in major catchments in Germany were analyzed.•Spatial and temporal patterns in river isotopes were found similar to precipitation.•In complex river catchments, ...deviations from precipitation signals were observed.•Isotopic long-term averages are correlated with altitude and latitude.•Averages of d-excess were inversely related to flow length and catchment size.
Stable isotopes oxygen-18 (18O) and deuterium (2H) are commonly used to investigate hydrological processes in catchments. However, only a few isotope studies have been conducted on a large scale and rarely over long time periods. The objective of this study was to identify the spatial and seasonal variability of isotopic composition in river water and how it is affected by geographical and hydrological factors. The stable isotopic composition of river water has been measured in nine large river catchments in Germany for a time period of 12years or 26years. We conducted time series and correlation analyses to identify spatial and temporal patterns of the isotopic composition in the rivers. Further, we compared it to isotopic composition in local precipitation and catchments characteristics. In the majority of the rivers, the spatial and temporal patterns of precipitation were directly reflected in river water. The isotopic signals of the river water were time shifted and show attenuated amplitudes. Further deviations from isotopic compositions in local precipitation were observed in catchments with complex flow systems. These deviations were attributed to catchment processes and influences like evaporation, damming and storage. The seasonality of the isotopic composition was mainly determined by the discharge regimes of the rivers. We found correlations between isotopic long-term averages and catchment altitude as well as latitude and longitude, resulting in a northwest-southeast gradient. Furthermore, it was shown that long-term averages of d-excess were inversely related to flow length and catchment size, which indicates that evaporation enrichment has an impact on the isotopic composition even in catchments of humid climates. This study showed that isotopic composition in rivers can serve as a proxy for the local precipitation and can be utilized as an indicator for hydrological processes even in large river basins. In future, such long time series will help to also understand the impact of changes in the hydrological cycle on the larger scales. They can also be used for calibration and validation of flow and transport models at catchment and sub-catchment scale.
Core Ideas
To determine the water retention curve from inverse modeling, θ and ψ need to be monitored.
δ18O ratios contained information to inversely estimate soil hydraulic parameters.
Different ...observation types should be combined in a single OF to estimate parameters.
Averages of local measurement could be described using effective parameters.
Accurate measurement of fluxes at lysimeter boundaries improved model parameterization.
Accurate estimates of soil hydraulic parameters and dispersivities are crucial to simulate water flow and solute transport in terrestrial systems, particularly in the vadose zone. However, parameters obtained from inverse modeling can be ambiguous when identifying multiple parameters simultaneously and when boundary conditions are not well known. Here, we performed an inverse modeling study in which we estimated soil hydraulic parameters and dispersivities of layered soils from soil water content, matric potential, and stable water isotope (δ18O) measurements in weighable lysimeter systems. We used different optimization strategies to investigate which observation types are necessary for simultaneously estimating soil hydraulic and solute transport parameters. Combining water content, matric potential, and tracer (e.g., δ18O) data in one objective function (OF) was found to be the best strategy for estimating parameters that can simulate all observed water flow and solute transport variables. A sequential optimization, in which first an OF with only water flow variables and subsequently an OF with transport variables was optimized, performed slightly worse indicating that transport variables contained additional information for estimating soil hydraulic parameters. Hydraulic parameters that were obtained from optimizing OFs that used either water contents or matric potential could not predict non‐measured water flow variables. When a bromide (Br−) tracer experiment was simulated using the optimized parameters, the arrival time of the bromide pulse was underestimated. This suggested that Br− sorbed onto clay minerals and amorphous oxides under the prevailing geochemical conditions with low pH values. When accounting for anion adsorption in the simulation, Br− concentrations were well predicted, which validated the dispersivity parameterization.
Biodegradation is one of the most favored and sustainable means of removing organic pollutants from contaminated aquifers but the major steering factors are still surprisingly poorly understood. ...Growing evidence questions some of the established concepts for control of biodegradation. Here, we critically discuss classical concepts such as the thermodynamic redox zonation, or the use of steady state transport scenarios for assessing biodegradation rates. Furthermore, we discuss if the absence of specific degrader populations can explain poor biodegradation. We propose updated perspectives on the controls of biodegradation in contaminant plumes. These include the plume fringe concept, transport limitations, and transient conditions as currently underestimated processes affecting biodegradation.
•This study represents the first nitrate isotopic characterization of Mashhad urban aquifer.•Nitrate isotopic composition was combined with chemical data to track the pollution.•δ18O and δ15N values ...revealed sewage and human waste as the main origin of NO3−.•Negligible denitrification is limited natural remediation of NO3− in the groundwater.•Direct injection of NO3− to the ground caused by insufficiency of sewer system was found as primary means of pollution.
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Urban aquifer of Mashhad city, northeastern Iran.
Measuring the ratios of the stable nitrate isotopes and hydrogeochemistry of major components in order to specify the origin of nitrate contamination and potential transformation processes in the aquifer.
This study represents the first isotopic characterization of groundwater nitrate to identify the sources of excessive nitrate in the urban aquifer and provides a reference for the potential implementation of executive programs on groundwater quality and protection. Nitrate concentration exceeded the limits for nitrate in drinking water in most of the wells (110 out of 261). Isotopic composition of the oxygen and nitrogen of NO3− in groundwater indicated sewage as the primary source of nitrate contamination. Denitrification was identified as a non-significant process in the aquifer. Concentrations of anions (Cl−, SO42− and PO43−) and NO3− showed strong correlations confirming potential wastewater influence. Most of the groundwater was weakly acidic, and the chemistry of the groundwater was rather affected by urbanization and land-use than by aquifer rock interactions. The absence of a reliable sewer collecting system, particularly in the central and southern parts of the study area, directly resulted in the poor water quality. Considering the lack of natural attenuation processes in the groundwater, management options need to be considered for reducing nitrogen input into groundwater.
Vast regions of the northern hemisphere are exposed to snowfall and seasonal frost. This has large effects on spatiotemporal distribution of infiltration and groundwater recharge processes as well as ...on the fate of pollutants. Therefore, snow and frost need to be central inherent elements of risk assessment and management schemes. However, snow and frost are often neglected or treated summarily or in a simplistic way by groundwater modellers. Snow deposition is uneven, and the snow is likely to sublimate, be redistributed and partly melt during the winter influencing the mass and spatial distribution of snow storage available for infiltration, the presence of ice layers within and under the snowpack and, therefore, also the spatial distribution of depths and permeability of the soil frost. In steep terrain, snowmelt may travel downhill tens of metres in hours along snow layers. The permeability of frozen soil is mainly influenced by soil type, its water and organic matter content, and the timing of the first snow in relation to the timing of sub‐zero temperatures. The aim with this paper is to review the literature on snow and frost processes, modelling approaches with the purpose to visualize and emphasize the need to include these processes when modelling, managing and predicting groundwater recharge for areas exposed to seasonal snow and frost.
Water droplets in oil are microhabitats for microbial life Meckenstock, Rainer U.; von Netzer, Frederick; Stumpp, Christine ...
Science (American Association for the Advancement of Science),
08/2014, Letnik:
345, Številka:
6197
Journal Article
Recenzirano
Anaerobic microbial degradation of hydrocarbons, typically occurring at the oil-water transition zone, influences the quality of oil reservoirs. In Pitch Lake, Trinidad and Tobago—the world's largest ...asphalt lake—we found that microorganisms are metabolically active in minuscule water droplets (1 to 3 microliters) entrapped in oil. Pyrotag sequencing of individual droplet microbiomes revealed complex methanogenic microbial communities actively degrading the oil into a diverse range of metabolites, as shown by nuclear magnetic resonance and Fourier transform ion cyclotron resonance mass spectrometry. High salinity and water-stable isotopes of the droplets indicate a deep subsurface origin. The 13.5% water content and the large surface area of the droplets represent an underestimated potential for biodegradation of oil away from the oil-water transition zone.
The abstraction of groundwater is a global phenomenon that directly threatens groundwater ecosystems. Despite the global significance of this issue, the impact of groundwater abstraction and the ...lowering of groundwater tables on biota is poorly known. The aim of this study is to determine the impacts of groundwater drawdown in unconfined aquifers on the distribution of fauna close to the water table, and the tolerance of groundwater fauna to sediment drying once water levels have declined. A series of column experiments were conducted to investigate the depth distribution of different stygofauna (Syncarida and Copepoda) under saturated conditions and after fast and slow water table declines. Further, the survival of stygofauna under conditions of reduced sediment water content was tested. The distribution and response of stygofauna to water drawdown was taxon specific, but with the common response of some fauna being stranded by water level decline. So too, the survival of stygofauna under different levels of sediment saturation was variable. Syncarida were better able to tolerate drying conditions than the Copepoda, but mortality of all groups increased with decreasing sediment water content. The results of this work provide new understanding of the response of fauna to water table drawdown. Such improved understanding is necessary for sustainable use of groundwater, and allows for targeted strategies to better manage groundwater abstraction and maintain groundwater biodiversity.
Core Ideas
This special section focuses on stable isotopes approaches in the vadose zone.
Recent advances and challenges in
stable isotope approaches are summarized.
Isotopes are used to study flow, ...transport, geochemical, and ecohydrological processes.
The articles present key findings in analytical and modeling developments.
Results can improve understanding of water flow and contaminant transport in the critical zone.
Recent developments in the measurement, modeling, and application of stable isotope tools to provide insights on water flow, transport, biogeochemical, and ecological processes in the vadose zone have motivated a special section in Vadose Zone Journal. We give an overview of recent advances and challenges in stable isotope approaches in the vadose zone and present the key findings from the five manuscripts in this special section. These five manuscripts advance our understanding of (i) analytical developments in water stable isotope analysis, (ii) modeling of water and transport within the soil matrix using water stable isotopes, and (iii) using compound‐specific isotope analysis for identifying the fate of nonaqueous‐phase liquids in the unsaturated zone.
Core Ideas
Observed biases were gas matrix effects exclusively caused by biogenic headspace CO2.
Observed biases exceeded 30× (δ2H) and 65× (δ18O) accepted measurement uncertainties.
The proposed ...correction scheme uses data from repeated analyses of soil samples only.
Post‐correction quality of isotope data matches accepted measurement uncertainties.
The presented method helps to avoid grave misinterpretations of soil water isotope data.
The isotopic composition (δ2H, δ18O) of pore water is an invaluable tracer for the minimally invasive study of subsurface water flow and transport processes. Here, we evaluated a method for pore water isotope analysis that combines laser‐based isotope analyzers and water‐vapor isotope equilibration using evaporation‐proof metalized sample bags. We tested inflation atmospheres (dry air vs. pure N2) and the impact of biogenic gas (CO2, CH4) accumulation for storage times of up to 4 wk. Samples were analyzed with a water isotope analyzer (Picarro L2120‐i) and a gas chromatograph. Air‐inflated water vapor samples showed a greater range of gas matrix effects (δ18O: 9.63‰; δ2H: 21.7‰) than N2–inflated samples (δ18O: 7.49‰; δ2H: 10.6‰) induced by nonuniform buildup of biogenic CO2, starting immediately after sample preparation. However, only air‐inflated samples could be reliably corrected using instrument‐specific sensitivity factors that were empirically determined by interpretation of periodically repeated isotope measurements. Corrected water isotope data were confirmed by similarity with local precipitation and suction cup isotope data. Residual uncertainties were well below the natural variations of soil water isotope values and independent of storage time, thus allowing for consistently reliable interpretations of soil water isotope profiles. We conclude that, especially for pore water sampling that requires small sample volumes and/or long storage times, metalized sample bags should be used to prevent evaporation notwithstanding the enhanced buildup of biogenic gases. Further, if gas matrix effects cannot be excluded, air inflation is preferred over pure N2, as only in that case can reliable postcorrections be performed by using internal data only.
Understanding microbial community dynamics in the alpine cryosphere is an important step toward assessing climate change impacts on these fragile ecosystems and meltwater-fed environments downstream. ...In this study, we analyzed microbial community composition, variation in community alpha and beta diversity, and the number of prokaryotic cells and virus-like particles (VLP) in seasonal snowpack from two consecutive years at three high altitude mountain summits along a longitudinal transect across the European Alps. Numbers of prokaryotic cells and VLP both ranged around 10
and 10
per mL of snow meltwater on average, with variation generally within one order of magnitude between sites and years. VLP-to-prokaryotic cell ratios spanned two orders of magnitude, with median values close to 1, and little variation between sites and years in the majority of cases. Estimates of microbial community alpha diversity inferred from Hill numbers revealed low contributions of common and abundant microbial taxa to the total taxon richness, and thus low community evenness. Similar to prokaryotic cell and VLP numbers, differences in alpha diversity between years and sites were generally relatively modest. In contrast, community composition displayed strong variation between sites and especially between years. Analyses of taxonomic and phylogenetic community composition showed that differences between sites within years were mainly characterized by changes in abundances of microbial taxa from similar phylogenetic clades, whereas shifts between years were due to significant phylogenetic turnover. Our findings on the spatiotemporal dynamics and magnitude of variation of microbial abundances, community diversity, and composition in surface snow may help define baseline levels to assess future impacts of climate change on the alpine cryosphere.
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