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ABSTRACT: We developed a method for the vacuum extraction (VacEx) of volatile organic compounds (VOCs) from water samples for ultratrace determinations of carbon isotopic signatures. Our method permits compound-specific stable carbon isotope analysis (CSIA) at VOC concentrations as low as 0.03-1.34 microg/L. VacEx was developed to extract and preconcentrate VOCs for subsequent carbon-CSIA by the standard technique purge-and-trap (P&T) coupled to an isotope-ratio mass spectrometer (IRMS). Even without complete extraction, the delta(13)C signatures of VOCs determined by VacEx-P&T-IRMS were in good agreement (deviation <1 per thousand) with signatures determined by P&T-IRMS. This indicates that VacEx does not cause isotopic discrimination. Limits of quantification (LOQs) for delta(13)C analysis were: 0.03-0.06 microg/L for benzene, toluene, o-xylene, m-p-xylene and ethylbenzene, 0.09 microg/L for methyl tert-butyl ether (MTBE), and 0.18-0.27 microg/L for trans-DCE, cis-DCE, TCE and PCE. These are the lowest LOQs reported to date for continuous-flow isotope-ratio determinations using a commercially available and automated system. To our knowledge, analytical protocols adopted from noble gas analysis in water were applied for the first time to determine the isotope composition of organic contaminants. We applied VacEx in a field study to illustrate how the determination of VOC isotopic signatures at very low concentrations opens new avenues in the in situ assessment of these priority groundwater pollutants.
Environmental Science and Technology 02/2010; 44(3):1023-9. · 5.23 Impact Factor
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ABSTRACT: Quantitative information on microbial processes in the field is important. Here we propose a new field method, the "gas push-pull test" (GPPT) for the in-situ quantification of microbial activities in the vadose zone. To evaluate the new method, we studied microbial methane oxidation above an anaerobic, petroleum-contaminated aquifer. A GPPT consists of the injection of a gas mixture of reactants (e.g., methane, oxygen) and nonreactive tracer gases (e.g., neon, argon) into the vadose zone and the subsequent extraction of the injection gas mixture together with soil air from the same location. Rate constants of gas conversion are calculated from breakthrough curves of extracted reactants and tracers. In agreement with expectations from previously measured gas profiles, we determined first-order rate constants of 0.68 h(-1) at 1.1 m below soil surface and 2.19 h(-1) at 2.7 m, close to the groundwater table. Co-injection of a specific inhibitor (acetylene) for methanotrophs showed that the observed methane consumption was microbially mediated. This was confirmed by increases of stable carbon isotope ratios in methane by up to 42.6 %. In the future, GPPTs should provide useful quantitative information on a range of microbial processes in the vadose zone.
Environmental Science and Technology 02/2005; 39(1):304-10. · 5.23 Impact Factor
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ABSTRACT: Lake sediment pore water has been proposed as a noble gas archive for paleoenvironmental reconstruction, but appropriate experimental techniques have not been available until recently. Here we present noble gas concentrations measured in the sediment pore water of Lake Issyk-Kul (Kyrgyzstan) which demonstrate for the first time the value of the sediment pore water archive for paleoclimate reconstruction. The noble gas profiles in the sediment indicate that the salinity of the lake water during the mid-Holocene was more than twice its present value of 6.0 g/kg, implying a 200-m lower lake level.
First publ. in: Geophysical Research Letters 31 (2004), 4, L04202. 01/2004;
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ABSTRACT: The characteristic feature of the physical structure of Lac Pavin is a distinct and permanent chemically induced density increase between about 60 and 70 m depth. This chemocline separates the seasonally mixed mixolimnion from the monimolimnion, which is characterized by elevated temperature and salinity as well as complete anoxia. Previously published box-models of the lake postulated substantial groundwater input at the lake bottom, and consequently a short water residence time in the monimolimnion and high fluxes of dissolved constituents across the chemocline. We present a new view of the physical structure and dynamics of Lac Pavin, which is based on the results of high-resolution CTD profiles, transient and geochemical tracers (tritium, CFCs, helium), and numerical modeling. The CTD profiles indicate the existence of a sublacustrine spring above rather than below the chemocline. A stability analysis of the water column suggests that vertical turbulent mixing in the chemocline is very weak. A numerical one-dimensional lake model is used to reconstruct the evolution of transient tracer distributions over the past 50 years. Model parameters such as vertical diffusivity and size of sublacustrine springs are constrained by comparison with observed tracer data. Whereas the presence of a significant water input to the monimolimnion can clearly be excluded, the input to the mixolimnion – both at the surface and from the indicated sublacustrine spring – cannot be accurately determined. The vertical turbulent diffusivity in the chemocline is well constrained to K 510-8 m2 s-1, about a factor of three below the molecular diffusivity for heat. Assuming thus purely molecular heat transport, the heat flow through the chemocline can be estimated to between 30 and 40 mW m-2. With respect to dissolved constituents, the very weak turbulent diffusive exchange is equivalent to a stagnant monimolimnion with a residence time of nearly 100 years. Based on these results and vertical concentration profiles of dissolved species, diffusive fluxes between monimolimnion and mixolimnion can be calculated. A large excess of helium with a 3He/4He ratio of (9.09 0.01)10-6 (6.57 R
a) is present in the monimolimnion, clearly indicating a flux of magmatic gases into the monimolimnion. We calculate a flux of 1.010-12 mol m-2 s-1 for mantle helium and infer a flux of 1.210-7 mol m-2 s-1 (72 t year-1) for magmatic CO2. The monimolimnion appears to be in steady state with respect to these fluxes.
Hydrobiologia 10/2002; 487(1):111-136. · 1.78 Impact Factor
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ABSTRACT: The hydrogeological system of an ecologically sensitive alpine floodplain in the Valle di Blenio, Switzerland, was investigated using hydrochemical and 3H-3He dating methods. Water samples from six wells and from different surface locations were analyzed. The analysis of the concentrations of major ions in conjunction with age determination by the 3H-3He-method allowed the main hydrological properties of the system to be consistently characterized. Two geochemically distinct water zones can be distinguished: Ca-SO4-dominated water from the main river and Ca-HCO3-dominated floodplain water. The floodplain water component characterizes the whole floodplain including the surficial hillslope drainage system. Within the ground water samples, two spatially and temporally different types of water can be determined. A younger (age < 1.5 years), less mineralized water is found in the upper part of the aquifer during the summer season. The underlying aquifer zone contains older and more highly mineralized water. However, the general hydrochemical characterization of both types of ground water is similar. In winter, the water ages increase with decreasing ground water levels. Because precipitation is stored temporarily in the snow cover, the contribution of the younger near-surface ground water decreases, resulting in higher apparent water ages and higher mineralization in the upper zone of the aquifer. Water exchange between the main river and the ground water system is limited to ground water exfiltration from the shallow aquifer zone, whereas the hydrochemical separation of the deeper aquifer zone indicates the isolation of the deeper ground water from the main river.
Ground Water 10/2001; 39(6):841 - 852. · 1.78 Impact Factor
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ABSTRACT: We have developed a new method for the simultaneous analysis of CFC-11, CFC-12, N2, and Ar in water that allows field samples, collected in copper tubes and locked gas-tight, to be transported and stored for several weeks. In the laboratory the gases are extracted in a vacuum system and frozen onto cold traps. CFCs are measured with a GC-ECD, and N2 and Ar are measured with a GC-TCD. Detection limits are 0.007 pmol kg-1 for CFC-11, 0.017 pmol kg-1 for CFC-12, and 0.89 μmol kg-1 for N2 and Ar. The reproducibility of replicate samples with modern CFC concentrations (1σ) lies within ±1.8% for CFC-11, ±1.5% for CFC-12, ±0.6% for N2, and ±0.8% for Ar. The oxygen demand and anoxic degradation rates of CFCs in the copper tube samplers were determined with lake water. At room temperature (22 °C), the oxygen demand is 14.3 ± 7.6 μmol kg-1 d-1, and the CFC-11 anoxic degradation rate is 0.0026 ± 0.0006 pmol kg-1 d-1. For CFC-12, no anoxic degradation was detected within 200 days.
01/1998;
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ABSTRACT: In complex environmental systems, tracers are indispensable tools for the investigation of various physical, chemical, and biological processes. From the large variety of tracers employed by EAWAG in the aquatic environment, we present some examples relevant to groundwater research. Some tracers (e.g., 222Rn, 3H/3He, chlorofluorocarbons) allow the time since groundwater infiltration to be determined; this information can be used to quantify process rates such as flow velocities, recharge and reaction rates. Other tracers (e.g., conductivity, isotopes of oxygen or boron) can be used to quantify mixing ratios between waters of different origin, for instance, to study the admixture of leachate from a landfill to an aquifer. In contrast to these inert tracers, chemically reactive tracers (e.g., nitroaromatic compounds) can be used to study subsurface biogeochemical transformation processes, e.g. the reduction of contaminants under anaerobic conditions. The successful use of tracers in the study of environmental systems requires highly developed analytical facilities for a broad palette of tracer measurements and a careful choice of appropriate tracers for each specific problem.
CHIMIA International Journal for Chemistry 11/1997; 51(12):941-946. · 1.21 Impact Factor
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ABSTRACT: Knowing the travel-time distributions from infiltrating rivers to pumping wells is important in the management of alluvial aquifers. Commonly, travel-time distributions are determined by releasing a tracer pulse into the river and measuring the breakthrough curve in the wells. As an alternative, one may measure signals of a time-varying natural tracer in the river and in adjacent wells and infer the travel-time distributions by deconvolution. Traditionally this is done by fitting a parametric function such as the solution of the one-dimensional advection-dispersion equation to the data. By choosing a certain parameterization, it is impossible to determine features of the travel-time distribution that do not follow the general shape of the parameterization, i.e., multiple peaks. We present a method to determine travel-time distributions by nonparametric deconvolution of electric-conductivity time series. Smoothness of the inferred transfer function is achieved by a geostatistical approach, in which the transfer function is assumed as a second-order intrinsic random time variable. Nonnegativity is enforced by the method of Lagrange multipliers. We present an approach to directly compute the best nonnegative estimate and to generate sets of plausible solutions. We show how the smoothness of the transfer function can be estimated from the data. The approach is applied to electric-conductivity measurements taken at River Thur, Switzerland, and five wells in the adjacent aquifer, but the method can also be applied to other time-varying natural tracers such as temperature. At our field site, electric-conductivity fluctuations appear to be an excellent natural tracer.
Ground Water 45(3):318-28. · 1.78 Impact Factor
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ABSTRACT: The hydrogeological system of an ecologically sensitive alpine floodplain in the Valle di Blenio, Switzerland, was investigated using hydrochemical and 3 H-3 He dating methods. Water samples from six wells and from different surface locations were ana-lyzed. The analysis of the concentrations of major ions in conjunction with age determination by the 3 H-3 He-method allowed the main hydrological properties of the system to be consistently characterized. Two geochemically distinct water zones can be dis-tinguished: Ca-SO 4 -dominated water from the main river and Ca-HCO 3 -dominated floodplain water. The floodplain water component characterizes the whole floodplain including the surficial hillslope drainage system. Within the ground water samples, two spatially and temporally different types of water can be determined. A younger (age < 1.5 years), less mineralized water is found in the upper part of the aquifer during the summer season. The underlying aquifer zone contains older and more highly miner-alized water. However, the general hydrochemical characterization of both types of ground water is similar. In winter, the water ages increase with decreasing ground water levels. Because precipitation is stored temporarily in the snow cover, the contribution of the younger near-surface ground water decreases, resulting in higher apparent water ages and higher mineralization in the upper zone of the aquifer. Water exchange between the main river and the ground water system is limited to ground water exfiltration from the shallow aquifer zone, whereas the hydrochemical separation of the deeper aquifer zone indicates the isolation of the deeper ground water from the main river.
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ABSTRACT: Concentrations of molecular oxygen in Lake Baikal (Eastern Siberia), the deepest lake on earth (1634 m), are above 80% saturation in the whole water column suggesting fast deep-water renewal. A model is developed to describe vertical water exchange based on measured concentrations of tritium, 3He, and the chlorofluorocarbons CFC-11 (CCl3F) and CFC-12 (CCl2F2). Lake Baikal consists of three main basins (south, central, north) that are separated by sills reaching up to about 300 m depth. Each basin is vertically divided into two boxes: a surface box 300 m thick and a deepwater box below 300 m. Tracers are transported from/into the surface layer by gas exchange with the atmosphere during ice-free periods, by precipitation, by evaporation, and by rivers. The tracer distribution in the lake depends primarily on the three intrabasin vertical water exchange rates. These rates are simultaneously fitted by modeling the four tracers from 1900 to the present. Mean residence times in the south, central, and north basins are 11.2 plus minus 0.6, 10.4plus minus 0.5, and 6.2 plus minus 0.5 yr, respectively. Application of the transport model to measured oxygen concentrations yields O2 consumption rates in all deep-water boxes of 0.1 mg L-1 yr-1.
First publ. in: Environmental Science & Technology 31 (1997), 10, pp. 2973-2982.
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ABSTRACT: The 3H-3He age of a water mass is a measure of the time that has passed since the water mass was last in contact with the atmosphere. Between 1992 and 1995 a detailed study of3H-3He ages was conducted in Lake Baikal, the deepest and largest lake by volume on Earth, to investigate deep water renewal in its three major basins. Maximum 3H-3He ages are 14-17 years in the southern basin, 16-18 years in the central basin, and 10-11 years in the northern basin. Rates of renewal of deep water with surface water, deduced from volume-weighted mean3 H-3He ages below 250 m depth, are about 10% yr-l in the southern and central basins and 15% yr-l in the northern basin. In the southern basin the mean 3H-3He age below 250 m depth increased steadily from 9.6 years in 1992 to 11 years in 1995, indicating a slight diminution in deep water renewal during this time. Bottom water renewal by largescale advection was estimated from the mass balance of 3He in the 200 m thick bottom layer of each basin. Bottom water renewal rates in the northern basin were found to be between 80 and 150 km3 yr-l and in the central basin between 10 and 20 km3 yrl, whereas in the southern basin they were practically zero. Correlating oxygen and dissolved helium-4 concentrations with 3H-3He age aallowed us to determine the mean hypolimnetic oxygen depletion rate in the water column (4.5 nanomol L -1 yr-l), as well as mean helium fluxes from the lake bottom (2.8 x 1011 atoms m-2 s-l in the northern basin, and 1.3 x 10 11 atoms m-2 s-l in the central and southern basins). The helium isotope ratio of the terrigenic helium component injected from the lake bottom, determined from measurements of water from hydrothermal s rings in the vicinity of the lake, was found to be ~2.2xl0-7.
First publ. in: Journal of Geophysical Research 103 (1998), C6, pp. 12,823-12,838.
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ABSTRACT: Vertical turbulent diffusion coefficients, upwelling velocities, and oxygen depletion rates are estimated by inverse modeling of the concentrations of CFC-ll (CCI3F), CFC-12 (CCI2F2), 3H, 3He, and dissolved oxygen for the southern, central, and northern basin of Lake Baikal. A model is developed that considers two regions in each basin of Lake Baikal: (1) a surface mixed layer (SML) 400 m thick and (2) a dcepwater column (DWC) below 400 m. The SMLs are assumed to be well mixed. In each of the DWCs, passive tracers are transported by vertical turbulent diffusion and upwelling. Upwelling is generated by a depth-dependent source of water because of density plumes propagating from the SML downward to larger depths. This water is considered to contain the same tracer concentrations as the SML. The tracer concentrations in the SMLs of the three basins are coupled to the atmosphere by gas exchange (including water vapor transport) and precipitation to the catchment by river inflow and outflow and to the neighboring basins via diffusive exchange and advection. SMLs and DWCs of the same basin are connected by vertical turbulent diffusion, density-driven water transport, and upwelling. Beginning at the turn of this century, the tracers CFC-II, CFC-12, 3H and 3He are modeled simultaneously to predict modern concentrations. On the basis of the tracer data the vertical diffusion coefficient K2 is determined to be 4.6x 10 high minus 4 (Marion H.)m2 S-I ± 10% for the southern, 6.3x10high minus4 m2 s high minus 1, ± 10% for the central, and 1.7x 10.high minus 4 m2 s·high minus 1 ± 25% for tbe northern basin. The vertical advective flux of water at 400 m water depth is calculated as 110 km high 3 yr high minus1 in the southern, 70 km3 yr high minus1 in the central, and 290 km3 yr high minus 1 in the northern basin. Concentration of dissolved molecular oxygen is modeled by using the estimated transport parameters and by fitting for the unknown consumption rate. Inverse modeling of oxygen suggests that 02 depletion in the DWC can be described by a volume sink of 44 ± 3 mg02 m high minus3 yr high minus1 combined with an areal sink at the sediment water interface of 17000 ± 3000 mg02 m-2 yr high minus1.
First publ. in: Journal of Geophysical Research / Oceans 105 (2000), C6, pp. 3451-3464.
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ABSTRACT: Simultaneous profiles of the transient tracers sulfur hexafluoride (SF6), 3H-3He, and the chlorofluorocarbons CFC-11 and CFC-12 were measured in Lake Issyk-Kul, a large, deep lake in Kyrgyzstan. Apparent water ages derived from these measurements suggest rapid mixing, with a deepwater renewal rate > 10% yr21 at 650 m depth. SF6 and 3H-3He ages agree reasonably well, whereas CFC ages are significantly greater. The discrepancy is explained by the nonlinear relationship between tracer age and tracer concentration and by the flattening of the atmospheric growth curves for CFCs. Novel to physical limnology is the application of SF6 dating, which proves to be an excellent tool for the study of mixing in lakes, complementing 3H-3He and CFC dating techniques.
First publ. in: Limnology and Oceanography 47 (2002), 4, pp. 1210-1216.
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ABSTRACT: In order to quantify deep-water exchange in the Caspian Sea, the world's largest inland water body, water samples were analyzed for the transient tracers 3H, 3He, 4He, CFC-11, CFC-12 and atmospheric noble gases. Measurements of temperature, salinity (calculated from conductivity for the ionic composition of Caspian Sea water), and dissolved oxygen were employed to investigate the processes responsible for deep-water renewal. The Caspian Sea consists of two deep basins, the southern and central basins, separated by a sill, and a shallow northern basin. The deep water (below 200 m) accounts for almost 60% of the total water mass. Below 200 m the concentrations of 3H and 3He are much lower in the southern basin than at the same depths in the central basin, but this is not the case for either of the CFCs. However, apparent water ages calculated from 3H}3He and from CFC-12 concentrations are the same for the deep water of the southern and central basins, and yield deep-water exchange rates of approximately 7% per year for each of the two basins. This implies volume fluxes across the 200-m level of about 2220 km3 yr-1 within the southern basin and 770 km3 yr-1 within the central basin. Based on the apparent water ages, the oxygen depletion in the deep water is estimated to be about 0.35 mg l-1 yr-1. The processes responsible for deep-water exchange have not yet been identified conclusively. However, vertical temperature and salinity gradients observed during two expeditions, in September 1995 and 1996, suggest that within the southern and central basins large-scale convection cannot be triggered by seasonal cooling alone, but requires the surface water to be cold/saline or to contain high suspended sediment loads. In the central basin the increase in salinity occurring during ice formation in early winter is possibly sufficient to cause convection. In late summer, the horizontal transport of water from the upper 170 m of the central basin into the southern basin results in mixing down to 400 m. In winter this process might result in convection down to the maximum depth. Alternatively, the data are also consistent with the hypothesis that rare events cause large-scale convection down to the maximum depth in the southern and in the central basin simultaneously, followed by slight mixing that mainly affects only the top 500 m. According to apparent water ages from below 500 m, the last such major mixing event could have occurred in 1976 shortly before the water level of the Caspian Sea began to rise.
First publ. in: Deep Sea Research / 1, 47 (2000), pp. 621-654.
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ABSTRACT: The formation of an excess of dissolved gas (excess air) in quasi-saturated media was studied by analyzing and interpreting dissolved noble gas concentrations in laboratory column experiments. Using quartz sand filled columns of 1 m length, two different experimental designs were realized. In the first, groundwater recharge was simulated by a unidirectional vertical water flow through the columns. In the second, groundwater level fluctuations in an aquifer zone without active infiltration were reproduced by cyclic water level fluctuations in the columns. The reproducible generation of excess air under these defined, near natural conditions was successful. Partial or complete dissolution of air bubbles entrapped in the quartz sand could be identified as the mechanism responsible for the generation of excess air. Depending on the experimental design, supersaturation of the dissolved atmospheric noble gases ranging between 1.4% ∆Ne and 16.2% ∆Ne was found. The measured noble gas patterns were interpreted using inverse modeling and conceptual gas exchange models and were compared to results of numerical simulations of gas bubble dissolution in water filled soil columns. The gas composition in most of the samples resembles either unfractionated pure atmospheric excess air or is fractionated in accordance with closed-system equilibration between entrapped air and surrounding water. In addition to the amount of entrapped air, the hydrostatic pressure exerted on the entrapped air bubbles is the dominating parameter responsible for the total amount of dissolved air. The composition of the excess air component is controlled by the water flow regime, the bubble size distribution, the initially dissolved gas concentrations and the initially entrapped gas composition.
First publ. in: Geochimica et Cosmochimica Acta 66 (2002), 23, pp. 4103-4117.
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ABSTRACT: During two field campaigns on Lake Issyk-Kul in March and August 2001, 179 high-resolution conductivity-temperature-depth profiles were measured, along with profiles of dissolved oxygen and light transmission. On the basis of this extensive data set, we investigated the large-scale vertical advective processes responsible for deepwater renewal in Lake Issyk-Kul. At some locations sampled in March, variable peak structures in the tracer profiles were observed that indicate horizontally localized intrusions. These intrusions have their origin in density plumes that propagate along the bottom of the channels in the eastern shelf region. The cold dense water at the bottom of the channels is most likely generated by differential cooling of the water in the shallow shelf regions during winter. During summer, vertical advective processes appear to be limited to the upper 200 m, where upwelling in the open water leads to a doming structure. The August data support the results of earlier investigations that suggested that dynamic forcing by a basinwide gyre is responsible for the upwelling in the open water.
First publ. in: Limnology and Oceanography 48 (2003), 4, pp. 1419-1431.
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ABSTRACT: Here we present a new method for the sampling and quantitative extraction of dissolved He, Ne, Ar, Kr, and Xe from lake sediment samples leading to deter- minations of porewater noble gas concentration profiles and the isotopic ratios 3He/4He, 20Ne/22Ne, and 40Ar/36Ar. Bulk sediment is transferred from a sediment core into standard Cu sample tubes without exposure to the atmosphere or other gas reservoirs. The noble gases are then extracted from the porewater by degassing the sediment in an evacuated extraction vessel and analyzed following standard mass spectrometric procedures. In tests of the new method using 0.8 to 1.4 m long sediment cores from two Swiss lakes, analytical uncertainties were only slightly greater than those of standard water samples. The majority of porewater noble gas concentrations and isotopic ratios were found to correspond closely to those measured in the overlying lake water. Because these values reflect water temperature and salinity during atmospheric equilibration at the lake surface, historical conditions are expected to be archived further downcore in the sediment porewater. This method therefore has great potential for paleolimnological reconstructions. The formation of methane bubbles in anoxic sediment layers is one process that may alter gas distributions. However because the lighter noble gases are most sensitive to degassing effects, noble gas data can be used to detect this process. In addition, noble gas data can yield information on the transport processes occurring in the sediment pore space and on the input of water or gas to the sediment from external sources.
First publ. in: Limnology and Oceanography / Methods, 1 (2003), pp. 51-62.
