Die quantitative Erfassung von Methan im Seewasser

A preparation method for the quantitative analysis of methane in lake water samples has been developed. The method is based on the equilibrium reactions of gases in a closed two-phase (headspace-water) system, in which the headspace gas is analyzed gaschromatographically. The method shows a standard deviation of ±5%, including sampling and sample preparation and can also be used for the quantitative determination of other dissolved gases.      

An In Situ Device to Measure Oxygen in the Vadose Zone and in Ground Water: Laboratory Testing and Field Evaluation

Oxygen probes developed to measure in situ oxygen concentrations in gaseous and aqueous environments were evaluated in laboratory tests and during long-term field evaluation trials at contaminated sites. The oxygen probes were shown to have a linear calibration and to be accurate compared to conventional dissolved oxygen electrodes and gas chromatography, both of which require labor-intensive sample collection and processing. The probes, once calibrated, required no maintenance or recalibration for up to a period of 7 years in low-oxygen environments, demonstrating long-term stability. Times to achieve 90% of the stabilized concentration ( t ₉₀) after a step change in aqueous oxygen concentration were 100–120 min in laboratory experiments and up to 180 min in field experiments, which is adequate for monitoring subsurface changes. Field application data demonstrated that the oxygen probes could monitor oxygen concentrations in hydrocarbon-contaminated ground water to a depth of 20 m below the water table or in pyritic vadose zones over extended periods. During bioremediation field trials, oxygen monitoring enabled estimation of oxygen utilization rates by microorganisms and hydrocarbon biodegradation rates. Also, probes were able to monitor the development of ground water desaturation during air sparging trials, enabling the automated assessment of the distribution of injected air.     

Evaluation of Noble Gas Recharge Temperatures in a Shallow Unconfined Aquifer

Water table temperatures inferred from dissolved noble gas concentrations (noble gas temperatures, NGT) are useful as a quantitative proxy for air temperature change since the last glacial maximum. Despite their importance in paleoclimate research, few studies have investigated the relationship between NGT and actual recharge temperatures in field settings. This study presents dissolved noble gas data from a shallow unconfined aquifer heavily impacted by agriculture. Considering samples unaffected by degassing, NGT calculated from common physically based interpretive gas dissolution models that correct measured noble gas concentrations for "excess air" agreed with measured water table temperatures (WTT). The ability to fit data to multiple interpretive models indicates that model goodness-of-fit does not necessarily mean that the model reflects actual gas dissolution processes. Although NGT are useful in that they reflect WTT, caution is recommended when using these interpretive models. There was no measurable difference in excess air characteristics (amount and degree of fractionation) between two recharge regimes studied (higher flux recharge primarily during spring and summer vs. continuous, low flux recharge). Approximately 20% of samples had dissolved gas concentrations below equilibrium concentration with respect to atmospheric pressure, indicating degassing. Geochemical and dissolved gas data indicate that saturated zone denitrification caused degassing by gas stripping. Modeling indicates that minor degassing (<10% ΔNe) may cause underestimation of ground water recharge temperature by up to 2°C. Such errors are problematic because degassing may not be apparent and degassed samples may be fit by a model with a high degree of certainty.     

A Portable Membrane Contactor Sampler for Analysis of Noble Gases in Groundwater

To enable a wider use of dissolved noble gas concentrations and isotope ratios in groundwater studies, we have developed an efficient and portable sampling device using a commercially available membrane contactor. The device separates dissolved gases from a stream of water and collects them in a small copper tube (6 mm in diameter and 100 mm in length with two pinch‐off clamps) for noble gas analysis by mass spectrometry. We have examined the performance of the sampler using a tank of homogeneous water prepared in the laboratory and by field testing. We find that our sampling device can extract heavier noble gases (Ar, Kr, and Xe) more efficiently than the lighter ones (He and Ne). An extraction time of about 60 min at a flow rate of 3 L/min is sufficient for all noble gases extracted in the sampler to attain equilibrium with the dissolved phase. The extracted gas sample did not indicate fractionation of helium (³He/⁴He) isotopes or other noble gas isotopes. Field performance of the sampling device was tested using a groundwater well in Vienna and results were in excellent agreement with those obtained from the conventional copper tube sampling method.     

Patterns of Entrapped Air Dissolution in a Two‐Dimensional Pilot‐Scale Synthetic Aquifer

Past studies of entrapped air dissolution have focused on one‐dimensional laboratory columns. Here the multidimensional nature of entrapped air dissolution was investigated using an indoor tank (180 × 240 × 600 cm³) simulating an unconfined sand aquifer with horizontal flow. Time domain reflectometry (TDR) probes directly measured entrapped air contents, while dissolved gas conditions were monitored with total dissolved gas pressure (P_TDG) probes. Dissolution occurred as a diffuse wedge‐shaped front from the inlet downgradient, with preferential dissolution at depth. This pattern was mainly attributed to increased gas solubility, as shown by P_TDG measurements. However, compression of entrapped air at greater depths, captured by TDR and leading to lower quasi‐saturated hydraulic conductivities and thus greater velocities, also played a small role. Linear propagation of the dissolution front downgradient was observed at each depth, with both TDR and P_TDG, with increasing rates with depth (e.g, 4.1 to 5.7× slower at 15 cm vs. 165 cm depth). P_TDG values revealed equilibrium with the entrapped gas initially, being higher at greater depth and fluctuating with the barometric pressure, before declining concurrently with entrapped air contents to the lower P_TDG of the source water. The observed dissolution pattern has long‐term implications for a wide variety of groundwater management issues, from recharge to contaminant transport and remediation strategies, due to the persistence of entrapped air near the water table (potential timescale of years). This study also demonstrated the utility of P_TDG probes for simple in situ measurements to detect entrapped air and monitor its dissolution.     

Geochemical precursors to seismic activity

Studies of earthquake precursory phenomena during the last several decades have found that significant geophysical and geochemical changes can occur prior to intermediate and large earthquakes. Among the more intensely investigated geochemical phenomena have been: (1) changes in the concentrations of dissolved ions and gases in groundwaters and (2) variations in the concentrations of crustal and mantle volatiles in ground gases. The concentration changes have typically showed no conanomalies trend (either increasing or decreasing), and the spatial and temporal distribution of the observed anomalies have been highly variable. As a result, there is little agreement on the physical or chemical processes responsible for the observed anomalies. Mechanisms proposed to account for precursory groundwater anomalies include ultrasonic vibration, pressure sensitive solubility, pore volume collapse, fracture induced increases in reactive surfaces, and aquifer breaching/fluid mixing. Precursory changes in soil gas composition have been suggested to result from pore volume collapse, micro-fracture induced exposure of fresh reactive silicate surfaces, and breaching of buried gas-rich horizons. An analysis of the available field and laboratory data suggests that the aquifer breaching/fluid mixing (AB/FM) model can best account for many of the reported changes in temperature, dissolved ion and dissolved gas concentrations in groundwater. Ultrasonic vibration and pressure sensitive solubility models cannot reasonably account for the geochemical variations observed and, although the pore collapse model could explain some of the observed chemical changes in groundwater and ground gas, uncertainties remain regarding its ability to generate anomalies of the magnitude observed. Other geochemical anomalies, in particular those associated with hydrogen and radon, seem best accounted for by increases in reactive surface areas (IRSA model) that may accompany precursory deformation around the epicenter of an impending earthquake. Analysis of the probable response of these models to the earthquake preparation process, as well as to other environmental factors, suggests that geochemical monitoring programs can provide information that may be valuable in forecasting the probability of an earthquake; however, because of the complexity of the earthquake preparation process, the absolute prediction of seismic events using geochemical methods alone, does not presently appear to be feasible.     

Low-Cost Apparatus for On-Site Monitoring of Methane in Ground Water

An upsurge in oil- and gas-well drilling in northwestern Pennsylvania and western New York has been accompanied by several incidents of contamination of ground water by methane. Determining which well is causing the contamination is extremely difficult if more than one gas or oil well is present in the area.
The fact that the solubility of methane decreases as the pressure on ground water decreases provides a quantitative basis for monitoring changes in the amount of methane in the ground water. Quantitative measurements of the volume of methane given off by ground water pumped from a well as the water enters atmospheric pressure permit detection of temporal changes in the gas content which are too subtle to be detected visually. These gas volume changes may, in some cases, be correlated with variations in the pressure of methane in the annulus of nearby individual gas/oil wells and thus may provide a means of pinpointing the gas/oil well that is causing the methane contamination.
The basic principle of the gas-volume monitoring apparatus (GVMA) described in this paper is that as a measured amount of ground water enters atmospheric pressure the gas which comes out of solution is trapped and measured. The GVMA can be constructed of materials costing less than $100 and requires no special skills to assemble or operate. In a recent study conducted in a western New York village, four homeowners were able to collect quantitative gas-volume data from their household water wells daily in about one-half hour. Unlike laboratory analyses for dissolved methane, there is no cost involved in monitoring with the GVMA beyond the initial instrument cost and operator time. Another advantage is that the data are available immediately.     

Free-Phase Gas Detection in Groundwater Wells via Water Pressure and Continuous Field Parameters

Detection of free-phase gas (FPG) in groundwater wells is critical for accurate assessment of dissolved gas concentrations and the occurrence of FPG in the subsurface, with consequent implications for understanding groundwater contamination and greenhouse gas emissions. However, identifying FPG is challenging during routine groundwater monitoring and there is poor agreement on the best approach to detect the occurrence of FPG in groundwater. In this study, laboratory experiments in a water column were designed to mimic nonflowing and flowing conditions in a groundwater well to evaluate how the presence of FPG affects water pressure and commonly used continuous field parameters. The laboratory results were extrapolated to interpret field data at an abandoned exploration well with episodic release of free-gas CO₂. The FPG effect on water pressure varied between flowing and nonflowing wells, and depending on whether the FPG was above or below the sensor. Electrical conductivity values were decreased and/or behaved erratically when FPG was present in the water column. Findings from this study have shown that the combined measurement of water pressure, electrical conductivity, and total dissolved gas pressure can provide information about the occurrence of FPG in groundwater wells. Measurement of these parameters at different depths can also provide information about relative depths and amounts of FPG within the well water column. This approach can be used for long-term monitoring of groundwater gases, managing gas-locking in production wells with gassy groundwater, and measuring fugitive greenhouse gas emissions from groundwater wells.     

In-Well Degassing Issues for Measurements of Dissolved Gases in Groundwater

Measurement of dissolved gases in groundwater is becoming increasingly common and important. Many of these measurements involve monitoring or sampling within wells or from water pumped from wells. We used total dissolved gas pressure (TDGP) sensors placed in the screened section of various wells (4 to 72 m deep) to assess the dissolved gas conditions for open wells compared to the conditions when sealed (i.e., isolated from the atmosphere) with a hydraulic packer (one well) or when pumped. When the packer was installed (non-pumping conditions), TDGP rose from <1.7 to >3.1 atm (<172 to >314 kPa), with declines noted when the packer was removed or deflated. While pumping, TDGP measured in many of the wells rose to substantially higher levels, up to 4.0 atm (408 kPa) in one case. Thus, when groundwater is gas charged, the background aquifer TDGP, and likewise the dissolved gas concentrations, may be substantially higher than initially measured in open wells, indicating significant in-well degassing. This raises concerns about past and current methods of measuring the dissolved gases in groundwater. Additional procedures that may be required to obtain representative measurements from wells include (1) installing in-well hydraulic packers to seal the well, or (2) pumping to bring in fresh groundwater. However, observed transient decreased TDGPs during pumping, believed to result from gas bubble formation induced by drawdown in the well below a critical pressure (relative to TDGP), may disrupt the measurements made during or after pumping. Thus, monitoring TDGP while pumping gas-charged wells is recommended.     

Changes in Entrapped Gas Content and Hydraulic Conductivity with Pressure

Water table fluctuations continuously introduce entrapped air bubbles into the otherwise saturated capillary fringe and groundwater zone, which reduces the effective (quasi‐saturated) hydraulic conductivity, K_quasi, thus impacting groundwater flow, aquifer recharge and solute and contaminant transport. These entrapped gases will be susceptible to compression or expansion with changes in water pressure, as would be expected with water table (and barometric pressure) fluctuations. Here we undertake laboratory experiments using sand‐packed columns to quantify the effect of water table changes of up to 250 cm on the entrapped gas content and the quasi‐saturated hydraulic conductivity, and discuss our ability to account for these mechanisms in ground water models. Initial entrapped air contents ranged between 0.080 and 0.158, with a corresponding K_quasi ranging between 2 and 6 times lower compared to the K_s value. The application of 250 cm of water pressure caused an 18% to 26% reduction in the entrapped air content, resulting in an increase in K_quasi by 1.16 to 1.57 times compared to its initial (0 cm water pressure) value. The change in entrapped air content measured at pressure step intervals of 50 cm, was essentially linear, and could be modeled according to the ideal gas law. Meanwhile, the changes in K_quasi with compression–expansion of the bubbles because of pressure changes could be adequately captured with several current hydraulic conductivity models.     

北京太平庄井流体物理化学多项动态的对比观测研究

在北京太平庄井详细观察观测与研究了井水位、水温、流量、水化学离子、溶解气与逸出气及井区土壤气与气象因素等３７项地下流体及其影响因素的短期动态。根据试验观察结果，本文初步解释出以地固体潮、大气大无畏民不动力条件的变动为共同基础的该井各测项短期－动态的内在联系，文中还对比分析了溶解气－逸出气－土壤气，水温－地温－气温等同类测项动态间的联系。     

和硕5.0级地震气体地球化学异常特征

１９９５年３月１９日，新疆和硕县那音克乡发生Ｍｓ５．０级地震。震前，乌鲁木齐地区地下水中溶解气体Ａｒ、ＣＨ４、ＣＯ２、Ｈ２Ｓ等和库尔勒地区的断层气Ｒｎ相继出现明显的异常。异常具有短期临震性质，主要表现为短期趋势升高，临震突跳。     

山西夏县中心地震台断层逸出气氡、溶解水氡测值干扰因素分析

山西夏县中心震台断层逸出气氡及溶解水氡分别观测中条山山前断裂土壤逸出气氡及热水井的水中溶解氡值。观测灵敏度高、信息量大,但易受其他因素的干扰。通过对观测值进行分析,发现,夏县台断层逸出气氡测值与气温呈正相关,与气压呈负相关;气温、气压对夏县台热水井溶解水氡测值几乎没有影响,但受水位的影响较明显。     

Total Dissolved Gas Pressure Measurements for Assessing Contaminant Degradation Processes in Groundwater

The total dissolved gas pressure (P_TDG ) probe has been used in groundwater studies for over a decade, but rarely in assessing contaminant degradation, despite the many degradation reactions that produce or consume dissolved gases. Here we present three studies to demonstrate the application of P_TDG measurements to groundwater experiencing contaminant degradation, with discussion of its benefits and limitations. The first study is a pilot‐scale laboratory experiment simulating dissolved ethanol contamination of an anaerobic sand aquifer. Continuous monitoring of P_TDG showed the rapid onset of microbial hydrocarbon degradation via denitrification and fermentation. The subsequent formation of a gas phase was revealed when P_TDG began mimicking the bubbling pressure (P_G *; sum of hydrostatic and atmospheric pressure), fluctuating with atmospheric pressure. Some deviations of P_TDG above P_G * occurred also, which may hold promise for signalling substantial changes in the rate or type of degradation process (here, the onset of methanogenesis). In the second study, synoptic field measurements at a petroleum plume site demonstrated how elevated P_TDG could identify wells with evidence of hydrocarbon degradation (denitrification and/or methanogenesis). And finally, combined field measurements of dissolved oxygen (DO) and P_TDG in monitoring wells of a nitrate‐contaminated aquifer (Abbottsford‐Sumas) revealed areas where denitrification was likely occurring. Limitations to P_TDG use identified in these studies included the masking of degradation processes by the presence of a gas phase, as when trapped following water table fluctuations or formed from rigorous degradation reactions, and confounded assessment of P_TDG patterns from other natural or anthropogenic processes that can also influence groundwater P_TDG .     

Effects of Process Control Changes on Aquifer Oxygenation Rates During In Situ Air Sparging in Homogeneous Aquifers

In situ air sparging is used to remediate petroleum fuels and chlorinated solvents present as submerged contaminant source /ones and dissolved contaminant plumes, or to provide barriers to dissolved contaminant plume migration. Contaminant removal occurs through a combination of volatilization and aerobic biodegradation: thus, the performance at any given site depends on the contaminant and oxygen mass transfer rates induced by the air injection. It has been hypothesized that these rates are sensitive to changes in process flow conditions and site lithology, but no data is available to identify trends or the magnitude of the changes. In this work, oxygenation rates were measured for a range of air injection rates, ground water flow rates, and pulsing frequencies using a laboratory-scale two-dimensional physical model constructed to simulate a homogeneous hydrogeologic setting. Experiments were conducted with water having low chemical and biochemical oxygen demand. Results suggest the following: that there is an optimum air injection rate: advective How of ground water can be a significant factor when ground water velocities are > 0.3 m/d: and pulsing the air injection had little effect on the oxygenation rate relative lo the continuous air injection case.     

Measurement of Carbon Dioxide in Soil Gases for Indication of Subsurface Hydrocarbon Contamination

A preliminary field evaluation of a new application of soil-gas measurement for delineation of subsurface organic contamination is described. The method measures carbon dioxide concentrations in soil gases and is based on the hypothesis that carbon dioxide concentrations from subsurface oxidation of organic compounds will be porportional to the extent of organic contamination. A correlation coefficient (r) of 0.81 (n=6) was observed between ground water dissolved organic carbon ground water concentrations and carbon dioxide concentrations in the overlying soil gases at one site. Soil-gas carbon dioxide concentrations measured ranged from 0.09 percent to 0.45 percent.     

Measuring Concentrations of Dissolved Methane and Ethane and the 13C of Methane in Shale and Till

Baseline characterization of concentrations and isotopic values of dissolved natural gases is needed to identify contamination caused by the leakage of fugitive gases from oil and gas activities. Methods to collect and analyze baseline concentration‐depth profiles of dissolved CH₄ and C₂H₆ and δ¹³C‐CH₄ in shales and Quaternary clayey tills were assessed at two sites in the Williston Basin, Canada. Core and cuttings samples were stored in Isojars^® in a low O₂ headspace prior to analysis. Measurements and multiphase diffusion modeling show that the gas concentrations in core samples yield well‐defined and reproducible depth profiles after 31‐d equilibration. No measurable oxidative loss or production during core sample storage was observed. Concentrations from cuttings and mud gas logging (including IsoTubes^®) were much lower than from cores, but correlated well. Simulations suggest the lower concentrations from cuttings can be attributed to drilling time, and therefore their use to define gas concentration profiles may have inherent limitations. Calculations based on mud gas logging show the method can provide estimates of core concentrations if operational parameters for the mud gas capture cylinder are quantified. The δ¹³C‐CH₄ measured from mud gas, IsoTubes^®, cuttings, and core samples are consistent, exhibiting slight variations that should not alter the implications of the results in identifying the sources of the gases. This study shows core and mud gas techniques and, to a lesser extent, cuttings, can generate high‐resolution depth profiles of dissolved hydrocarbon gas concentrations and their isotopes.     

Noble gas abundance patterns in deep-sea basalts — Primordial gases from the mantle

Rapidly cooled portions of eleven samples of mid-ocean ridge tholeiitic basalt pillows have noble gas abundance patterns which resemble the solar rare gas pattern rather than the noble gas pattern of the terrestrial atmosphere. We conclude that these samples contain primordial noble gases. In contrast, holocrystalline samples and a sample from the interior of a basalt pillow have noble gas abundance patterns which resemble the sea water pattern. Whereas the quenched glossy margins of basalt pillows record a non-atmospheric gas reservoir, these slowly cooled samples apparently have undergone exchange of their noble gases with those dissolved in sea water.     

New Tape For Ground Water Measurements

A composite tape has been developed which can be the link to various sensing devices for measuring ground water parameters. The tape consists of a standard steel engineering tape with two conductors at the edges of the tape, similar to a TV antenna wire. The tape coating is clear Tefzel®, which has outstanding chemical and physical resistance. At the bottom end of the tape is a probe which can sense either water level by conductivity, a layer of organic liquid by an optical device, temperature by advanced semiconductor temperature transducers, pH, dissolved oxygen, absolute conductivity, or any other measurement that can be made with a small in situ probe.
The steel tape itself provides one of three conductors and, because of its thin, flat shape, provides an excellent ground plane for electronic noise reduction. A multiplexing circuit commonly used in the electronics industry is placed inside the probe tip housing and routinely allows up to 16 signals to be transmitted to the hub at ground level. Most probes can be powered with a common 9V rechargeable battery. Signals are given either by low power-consuming light-emitting diodes (LEDs) ∼ 5 milliamperes (mA), piezo-audible alarms ∼ 3mA, or liquid crystal displays ∼ l-2mA
This configuration allows for extremely accurate depth profiles of the desired parameters.     

The Application of In Situ Air Sparging as an Innovative Soils and Ground Water Remediation Technology

Vapor extraction (soil venting) has been demonstrated to be a successful and cost-effective remediation technology for removing VOCs from the vadose (unsaturated) zone. However, in many cases, seasonal water table fluctuations, drawdown associated with pump-and-treat remediation techniques, and spills involving dense, non-aqueous phase liquids (DNAPLS) create contaminated soil below the water table. Vapor extraction alone is not considered to be an optimal remediation technology to address this type of contamination.
An innovative approach to saturated zone remediation is the use of sparging (injection) wells to inject a hydrocarbon-free gaseous medium (typically air) into the saturated zone below the areas of contamination. The contaminants dissolved in the ground water and sorbed onto soil particles partition into the advective air phase, effectively simulating an in situ air-stripping system. The stripped contaminants are transported in the gas phase to the vadose zone, within the radius of influence of a vapor extraction and vapor treatment system.
In situ air sparging is a complex multifluid phase process, which has been applied successfully in Europe since the mid-1980s. To date, site-specific pilot tests have been used to design air-sparging systems. Research is currently underway to develop better engineering design methodologies for the process. Major design parameters to be considered include contaminant type, gas injection pressures and flow rates, site geology, bubble size, injection interval (areal and vertical) and the equipment specifications. Correct design and operation of this technology has been demonstrated to achieve ground water cleanup of VOC contamination to low part-per-billion levels.