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  • 11
    Language: English
    In: Isotopes in Environmental and Health Studies, 01 September 2010, Vol.46(3), pp.257-258
    Keywords: Chemistry ; Physics
    ISSN: 1025-6016
    E-ISSN: 1477-2639
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  • 12
    Language: English
    In: Environmental Earth Sciences, 2013, Vol.69(2), pp.349-358
    Description: Performing tracer tests using artificial tracer compounds is a common practice to characterize natural streams regarding their (reactive) transport properties. Recently, the fluorescent compound resazurin was introduced as a reactive stream tracer to quantify hyporheic exchange and metabolic activity of streams. This tracer, together with its reaction product resorufin and a conservative tracer (in our case fluorescein), provides additional information about transport properties of the stream and its hyporheic zone and can therefore overcome restrictions that are commonly affiliated with the use of conservative tracers alone. However, all previously published studies using this tracer system were based on manual sampling of the water. This usually limits the number of measurements and thus the achievable temporal resolution, and potentially endangers data quality due to inadequate handling of samples. In this paper, a modified version of the GGUN-FL30 on-line fluorometer is presented in which the optics have been modified to allow measuring the concentrations of all three tracers simultaneously at intervals of 10 s. Experiments under controlled and natural conditions showed that the performance of the on-line fluorometer regarding tracer separation efficiency and practical detection limits is comparable to a high-performance laboratory spectrofluorometer. Furthermore, suggestions are given on how to correct tracer signal fluctuations caused by temporal changes in temperature and pH that might occur during a field tracer test.
    Keywords: Fluorometer ; Tracer test ; Groundwater/surface water interaction ; Resazurin
    ISSN: 1866-6280
    E-ISSN: 1866-6299
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  • 13
    In: Water Resources Research, May 2013, Vol.49(5), pp.3024-3037
    Description: concurrent tests of conservative and reactive tracers in streams joint analysis of the tracers improved the estimation of hyporheic parameters Markov chain Monte Carlo methods used to infer parameter distributions Knowledge about the strength and travel times of hyporheic exchange is vital to predict reactive transport and biogeochemical cycling in streams. In this study, we outline how to perform and analyze stream tracer tests using pulse injections of fluorescein as conservative and resazurin as reactive tracer, which is selectively transformed to resorufin when exposed to metabolically active zones, presumably located in the hyporheic zone. We present steps of preliminary data analysis and apply a conceptually simple mathematical model of the tracer tests to separate effects of in‐stream transport from hyporheic exchange processes. To overcome the dependence of common parameter estimation schemes on the initial guess, we derive posterior parameter probability density functions using an adaptive Markov chain Monte Carlo scheme. By this, we can identify maximum‐likelihood parameter values of in‐stream transport, strength of hyporheic exchange, distribution of hyporheic travel times as well as sorption and reactivity coefficients of the hyporheic zone. We demonstrate the approach by a tracer experiment at River Goldersbach in southern Germany (60 L/s discharge). In‐stream breakthrough curves were recorded with online fluorometers and jointly fitted to simulations of a one‐dimensional reactive transport model assuming an exponential hyporheic travel‐time distribution. The findings show that the additional analysis of resazurin not only improved the physical basis of the modeling, but was crucial to differentiate between surface transport and hyporheic transient storage of stream solutes. Parameter uncertainties were usually small and could not explain parameter variability between adjacent monitoring stations. The latter as well as a systematic underestimation of the tailing are due to structural errors of the model, particularly the exponential hyporheic travel‐time distribution. Mean hyporheic travel times were in the range of 12 min, suggesting that small streambed structures dominate hyporheic exchange at the study site.
    Keywords: Hyporheic Exchange ; Stream Tracer Experiments ; Travel‐Time Distributions ; Resazurin ; Resorufin
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 14
    Language: English
    In: Quaternary International, 04 August 2014, Vol.338, pp.59-70
    Description: Miocene–Pliocene and Upper Cretaceous formations in Tunisia contain one of the most productive artesian aquifers in the country. They consist of Mio-Pliocene sands and Senonian/Turonian carbonates. Intensive pumping in Kébili and Nefzaoua regions over the past decades resulted in gradual lowering of water table at the rate of approximately 0.85 m y . Forty-two groundwater samples have been collected from different sectors of the studied aquifers for chemical and isotopic analyses. The observed large spatial variability of chemical composition of groundwater in the study area is most probably linked to two processes: (i) dissolution of dolomite and gypsum, combined with calcite precipitation (dedolomitisation), and (ii) partial evaporation of water. The first process plays an important role in the study area due to abundance of evaporites. Partial evaporation occurs in the upper part of the unsaturated zone during infiltration, especially for groundwater sampled in the Kebili and Djerid regions. Apart from these processes, there are others which influence the salinity of the aquifers. In the Mio-Pliocene aquifer, which behaves as an open system to gases and which receives inputs of CO gas derived from intensive tectonic activity in the area, the interaction of carbon dioxide with carbonate matrix of the aquifer produces an increase in the alkalinity of water. In the Senonian and Turonian aquifers, the process of dedolomitisation evolves in a closed system with respect to CO gas. Ca /Na cation exchange and halite dissolution processes are also important. Stable isotope composition of water (δ O, δ H) indicates that the recharge occurs from the Dahar upland. The C activity varies between 89.5 (±1.5) and 3.7 (±2.1) pmc. The C content in the total dissolved inorganic carbon (TDIC) range between −13.9 and −3.6‰. The calculated concentrations of C in the CO gas in equilibrium with the TDIC vary between −22 and −11‰, indicating two sources of carbon in the solution: carbonate matrix (δ C = −2‰) and soil CO (δ C from −25 to −21‰ for the cultivated areas). Mean residence times of water have been determined after correction of the initial C activities for C-dilution processes including carbonate dissolution, calcite precipitation and cation-exchange. The dilution processes were quantified on the basis of geochemical and C mass balance equations. The calculated mean residence times of water confirm modern recharge from Dahar upland and the mountains surrounding depressions, and indicate the presence of paleowaters in the east and south-west region, and in the discharge zone.
    Keywords: Tunisia ; Dissolution ; Evaporation ; Dilution ; Residence Time ; Geology
    ISSN: 1040-6182
    E-ISSN: 1873-4553
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  • 15
    Language: English
    In: Isotopes in Environmental and Health Studies, 01 September 2010, Vol.46(3), pp.312-324
    Description: In the framework of the investigation of enrichment processes of nitrate in groundwater of the Kalahari of Botswana near Serowe, recharge processes were investigated. The thick unsaturated zone extending to up to 100 m of mostly unconsolidated sediments and very low recharge rates pose a serious...
    Keywords: Aquifer ; Botswana ; CFC ; Helium-3 ; Hydrogen-3 ; Infiltration ; Isotope Ecology ; Isotope Hydrology ; Kalahari ; Recharge ; Soil Water ; Sulphur Hexafluoride ; Chemistry ; Physics
    ISSN: 1025-6016
    E-ISSN: 1477-2639
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  • 16
    Language: English
    In: Geochimica et Cosmochimica Acta, 1998, Vol.62(18), pp.3041-3045
    Description: The {sup 4}He, {sup 40}Ar, and {sup 136}Xe content dissolved in the pore water of sedimentary rock samples was measured on samples from borehole cores near the repository for nuclear waste in Morsleben, Germany. Due to the very low permeabilities of the rock formations, conventional groundwater sampling was almost impossible. Hence, the authors developed a new sampling method for noble gases in the pore water of freshly drilled rock cores. This method provides vertical noble gas profiles in high depth resolution, even in impermeable rocks. By application of the new technique quantitative age information of groundwater and pore water have been derived. The authors find palaeowaters from the last glaciation depleted in {delta}D and {delta}{sup 18}O with a {sup 4}He age of about 55 kyr. The high saline pore solutions below are at least 6 Mio years old. This has been concluded from the profiles of radiogenic {sup 4}He and {sup 40}Ar close to diffusion in steady-state and from xenon isotopes produced by spontaneous fission of {sup 238}U in the rocks. A {sup 4}He flux of 2 {center_dot} 10{sup {minus}7} cc STP/cm{sup 2} yr is derived from the profile, which is due to local {sup 4}He production within the investigated sediments.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 17
    Language: English
    In: Environmental Earth Sciences, 2013, Vol.69(2), pp.359-372
    Description: First results of a multi-disciplinary hyporheic monitoring study are presented from the newly established Steinlach Test Site in Southern Germany. The site is located in a bend of the River Steinlach (mean discharge of 1.8 m³/s) underlain by an alluvial sandy gravel aquifer connected to the stream. The overall objective is a better understanding of hyporheic exchange processes at the site and their interrelations with microbial community dynamics and biochemical reactions at the stream–groundwater interface. The present paper focuses on the distribution of lateral hyporheic exchange fluxes and their associated travel times at the Steinlach Test Site. Water level dynamics in various piezometers correspond to the different domains of hydraulic conductivity in the shallow aquifer and confirms hyporheic exchange of infiltrated stream water across the test site. Hydrochemical compositions as well as increased damping of continuous time series of electrical conductivity (EC) and temperature at the respective piezometers confirmed the inferred distribution of hyporheic flowpaths. Mean travel times ranging from 0.5 days close to the stream to more than 8 days in the upstream part of the test site could be estimated from deconvolution of EC and δ 18 O–H 2 O data. The travel times agree well with the presumed flowpaths. Mg/Ca ratios as well as model fits to the EC and δ 18 O data indicate the presence of an additional water component in the western part of the test site which most likely consists of hillslope water or groundwater. Based on the mean travel times, the total lateral hyporheic exchange flux at the site was estimated to be of the order of 1–2 L/s.
    Keywords: Hyporheic zone ; Stream–groundwater interaction ; Travel time distribution ; Deconvolution ; Monitoring
    ISSN: 1866-6280
    E-ISSN: 1866-6299
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  • 18
    Language: English
    In: Isotopes in Environmental and Health Studies, 01 September 2010, Vol.46(3), pp.259-278
    Description: Many problems related to groundwater supply and quality, as well as groundwater-dependent ecosystems require some understanding of the timescales of flow and transport. For example, increased concern about the vulnerabilities of 'young' groundwaters (less than ∼ 1000 years) to overexploitation,...
    Keywords: Environmental Tracers ; Groundwater Dating ; Groundwater Age ; Isotope Tracer Techniques ; Isotope Measurements ; Methods and Equipment ; Residence Time ; Chemistry ; Physics
    ISSN: 1025-6016
    E-ISSN: 1477-2639
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  • 19
    Language: English
    In: Geochimica et Cosmochimica Acta, 2009, Vol.73(4), pp.911-922
    Description: Groundwater is an important and often exclusive water resource in arid and semi-arid regions. The aim of the present paper was to gain insight into the processes and conditions that control the deterioration of groundwater quality in the semi-arid Kalahari of Botswana. Measurements of He, He, Ne, Ne, and of C of dissolved inorganic carbon (DIC) were combined with existing isotopic and hydrochemical data to investigate groundwater from the Ntane Sandstone Aquifer, which is affected by high nitrate concentrations of non-anthropogenic origin. All groundwater samples revealed neon concentrations in excess to air-saturated water, which we attributed to the addition of excess air during recharge. Neon concentrations ranged from values close to air saturation for C DIC rich samples (up to 80.5%MC) up to values of 90% in excess to air-saturated water for lower C DIC contents (2.6–61.3%MC). A strong linear correlation of excess Ne with nitrate concentrations suggests an intimate connection between groundwater quality and the processes and conditions during groundwater recharge. Low groundwater recharge rates under present-day semi-arid conditions are associated with low amounts of excess Ne and elevated nitrate concentrations. In contrast to this, higher excess Ne values in groundwater of lower C DIC and nitrate contents indicate that the high quality groundwater end-member presumably is related to higher groundwater table fluctuations during wetter climatic conditions in the past. We attribute the decline in groundwater quality with respect to nitrate to a decreasing rate and temporal variability of groundwater recharge, and to concurrent changes in biogeochemical activities following a transition to a drier climate during the Holocene. Under such conditions, a much stronger decrease in groundwater recharge compared to the release of nitrate from soil organic matter may result in elevated nitrate concentrations in the vadose zone and groundwater. This implies a strong impact of climate change on the transport of solutes like nitrate through the vadose zone which needs to be considered in predictions of future groundwater quality.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 20
    Language: English
    In: Science of the Total Environment, 10 March 2019, Vol.655, pp.1062-1070
    Description: The estimation of gas-exchange rates between streams and the atmosphere is of great importance for the fate of volatile compounds in rivers. For dissolved oxygen, this exchange process is called reaeration, and its accurate and precise estimation is essential for the quantification of metabolic rates. A common method for the determination of gas-exchange rates is through artificial gas-tracer tests with a proxy gas. We present the implementation of a portable gas-equilibrium membrane inlet mass spectrometer (GE-MIMS) to record concentrations of krypton and propane injected as tracer compound in the context of a gas-tracer test. The field-compatible GE-MIMS uses signals of atmospheric measurements for concentration standardization, and allows recording the dissolved-gas concentrations at a high temporal resolution, leading to overall low measurement uncertainty. Furthermore, the approach avoids loss of gas during the steps of sampling, transport, storage, and analysis required for gas measurements. We compare obtained gas-exchange rate coefficients, reaeration and derived metabolic rates from the measurements to results obtained from head-space sampling of propane followed by laboratory analysis, and find much lower uncertainties with the method.
    Keywords: Krypton ; Gas Tracer ; Gas-Equilibrium Membrane-Inlet Mass Spectrometry ; Reaeration ; Stream Metabolism ; In-Situ Recording of Dissolved Gases ; Environmental Sciences ; Biology ; Public Health
    ISSN: 0048-9697
    E-ISSN: 1879-1026
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