Groundwater Gas Research Articles

Diffusive helium emissions as a precursory sign of volcanic unrest

Significant increases in helium emissions from the soil and 3 He/ 4 He ratios in groundwater on El Hierro Island (Canary Islands, Spain) were observed prior to the 2011–2012 submarine eruption off the coast of the island. The changes of diffusive helium emissions rate were observed one month prior to the submarine eruption onset (12 October 2011) and the major increase preceded increases in seismic energy release during the volcanic unrest. Measured 3 He/ 4 He ratios in groundwaters from a well in El Hierro Island increased from 2–3 R A to 7.2 R A (R A = 3 He/ 4 He ratio in air) 1 month prior to the eruption onset, and reached a peak of 8.2 R A , indicating a dominant magmatic contribution to the dissolved gases in ground waters. 3 He/ 4 He values and diffusive helium emission studies have been extremely important for forecasting the onset of the volcanic unrest and subsequent volcanic eruption. An aseismic exsolution of magmatic gases from magma bodies beneath El Hierro Island through fractures and vertical permeability structures increased the diffusive helium emission rate prior to episodes of seismic energy release associated with the volcanic unrest.

Response of noble gas partial pressures in soil air to oxygen depletion

Soil air composition is known to fluctuate both temporally and spatially. These fluctuations are mainly caused by microbiological activity, leading to oxygen depletion. The exact composition of soil air is of importance to the reconstruction of paleotemperatures using dissolved noble gases in ground water, yet few data exist on the impact of soil air composition changes on the noble gas concentrations. This study documents the evolution of soil air composition, including noble gases, over a period of 17months at a test site with clay dominated soil in Germany. It confirms that O2 and CO2 concentrations within the soil air vary strongly between a minimum of the sum of O2 and CO2 of 16.5% and a maximum of 24.5%. Soil air noble gas composition deviated from atmospheric composition as expected, i.e. noble gas concentrations increased when O2+CO2 decreased, and vice versa. The highest observed increase in soil air argon mixing ratio was to 106.4% of the atmospheric air mixing ratio. This maximum increase of noble gas mixing ratios would cause an underestimation of paleotemperatures by 1.5°C when employing the current models neglecting the response of noble gases to oxygen depletion. Due to the observed annual variation, however, the mean effect is much smaller.

Origins of hydrocarbon gas seeping out from offshore mud volcanoes in the Nile delta

This paper discusses the origin of gas seepages (free gas or dissolved gas in ground water or brine) sampled with the Nautile submarine during the Nautinil cruise at the seafloor of the deep water area of the Nile turbiditic system on different mud volcanoes and brine pools. Generally, the gas is wet and includes C1, C2, C3, iC4, nC4, CO2. These gas samples show no evidence of biodegradation which is not the case of the gas present in the deep hydrocarbon accumulations at depth. It indicates that the gas expelled by the mud volcanoes is not issued from direct leakages from deep gas fields. The collected gas samples mainly have a thermogenic origin and show different maturities. Some samples show very high maturities indicating that these seepages are sourced from great depths, below the Messinian salt. Moreover, the different chemical compositions of the gas samples reflect not only differences in maturity but also the fact that the gas finds its origin in different deep source rocks. Carbon dioxide has an organic signature and cannot result from carbonate decomposition or mantle fluids. The crustal-derived radiogenic isotopes show that the analyzed gas samples have suffered a fractionation processes after the production of the radiogenic isotopes, due either to oil occurrence at depth interacting with the flux of gas, and/or fractionation during the fluid migration.

A new noble gas paleoclimate record in Texas — Basic assumptions revisited

A generally accepted basic principle in relation to the use of the noble gas thermometer in groundwater flow systems is that high-frequency noble gas climatic signals are lost due to the effect of dispersion. This loss of signal, combined with 14C dating issues, makes it only suited to identify major climatic events such as the Last Glacial Maximum (LGM). Consequently, the identification of significant noble gas temperature (NGT) cooling (≥ 5 °C) with respect to present time has systematically been associated with the occurrence of the LGM even when reasonable water age controls were unavailable. It has also become apparent at a number of studied sites that modern NGTs estimated through standard models [M. Stute, P. Schlosser, Principles and applications of the noble gas paleothermometer, in: P.K. Swart, K.C. Lohmann, J.A. McKenzie, S. Savin, (Eds), Climate change in continental isotopic records, Geophysical monograph 78, AGU (1993) 89-100.; W. Aeschbach-Hertig, F. Peeters, U. Beyerle, R. Kipfer, Paleotemperature reconstruction from noble gases in ground water taking into account equilibration with entrapped air, Nature 405(6790) (2000) 1040-1044.] are unable to reproduce ground temperatures at the interface with the unsaturated zone, a basic requirement for proper paleoclimate reconstruction through noble gases. Instead, a systematic bias to low NGTs in recharge areas is observed. The Carrizo aquifer, in which the LGM was previously identified [M. Stute, P. Schlosser, J.F. Clark, W.S. Broecker, Paleotemperatures in the Southwestern United States derived from noble gases in ground water, Science 256(5059) (1992) 1000-1001.] and which presents an NGT bias of over 4 °C, is an ideal setting to analyze and revise basic principles and assumptions in relation with the use of the noble gas thermometer. Here, we present a new noble gas data set (49 measurements) collected at 20 different locations in the Carrizo aquifer. This new data set together with previously published data (20 measurements) was used to calibrate a 3-D groundwater flow and 4He transport model in which simulations of groundwater age were subsequently carried out. These account for mixing processes due to advection, dispersion, diffusion, and cross-formational flow. We first show that samples previously attributed to the LGM belong in fact to the middle Holocene. Through a step-by-step approach we then proceed to carry out a comparative analysis of both the impact of dispersion on high frequency climatic signals and assumptions underlying competing NGT models. Our combined analysis indicates that groundwater flow systems, at least those with similar characteristics to that of the Carrizo, do have the ability to preserve short term (100–200 yrs) climatic fluctuations archived by noble gases. It also shows that abrupt climate shifts during the mid-late Holocene which are associated with significant NGT changes (≥ 5 °C) do not reflect equally important changes in the mean annual atmospheric temperature (MAAT). Instead, these reflect the combined effect of atmospheric temperature changes, seasonality of recharge and, above all, significant variations of the water table depth which result from shifts between humid and arid regimes. Together with NGTs, our excess air record plays a critical role in identifying such abrupt climate changes. Specifically, the Carrizo combined data set indicates an abrupt shift from a cool, humid regime to a warmer, arid one at ∼ 1 kyrs BP. A major Holocene (∼ 6 kyrs BP) NGT change of 7.7 °C with respect to present now identified is mostly the result of a dramatic water table drop which occurred during the ∼ 1 kyrs BP transition period. Current NGTs in the Carrizo recharge area do not appear to be recording atmospheric changes. Rather, these are recording ground conditions reflecting mostly the impact of heat flow in the area. We also show that observed systematic offsets in NGT recharge areas can be reconciled through NGT estimation models which account for a noble gas partial pressure increase in the unsaturated zone, potentially due to O 2 depletion.

Oxidation of fugitive methane in ground water linked to bacterial sulfate reduction

When fugitive methane migrates upward along boreholes of oil and gas wells, it may migrate into shallow ground water or pass through overlying soil to the atmosphere. Prior to this study, there was little information on the fate of fugitive methane that migrates into ground water. In a field study near Lloydminster, Alberta, Canada, we found hydrogeochemical evidence that fugitive methane from an oil well migrated into a shallow aquifer but has been attenuated by dissimilatory bacterial sulfate reduction at low temperature ( approximately 5 degrees C) under anaerobic conditions. Evidence includes spatial and temporal trends in concentrations of methane and sulfate in ground water and associated trends in concentrations of bicarbonate and sulfide. Within 10 m of the oil well, sulfate concentrations were low, and sulfate was enriched in both 34S and 18O. Sulfate concentrations had a strong positive correlation with delta13C values of bicarbonate, and sulfide was depleted in 34S compared to sulfate. These data indicate that bacterial sulfate reduction occurred near the production well. Near the oil well, elevated concentrations of bicarbonate were observed, and the bicarbonate was depleted in 13C. Modeling indicates that the main source of this excess 13C-depleted bicarbonate is oxidized methane. In concert with the sulfate concentration and isotope data, these results support an interpretation that in situ bacterial oxidation of methane has occurred, linked to bacterial sulfate reduction. Bacterial sulfate reduction may play a major role in bioattenuation of fugitive natural gas in ground water in western Canada.

DISSOLVED GAS ANALYSIS FOR ASSESSING THE FATE OF NITRATE IN WETLANDS1

ABSTRACT: Dissolved gas analysis permits direct detection of ground water denitrification, a technique we used in this study to assess the fate of nitrate in a riparian wetland. Dissolved argon (Ar) and dinitrogen (N2) were measured in transects of nested piezometers installed at different depths within upwelling regions of a riparian wetland. The method uses the Ar content in the water as a natural inert tracer for assessing background content of N2 from the previous air/water equilibrium. Within the wetland under study, anoxic to suboxic ground water became more oxic in piezometers close to the aquifer layer, indicating upwelling of oxic ground water. Assessment of loss of nitrate and Ar in ground water within an upwelling zone indicated that shallow piezometers had significant N2 loss through degassing. Most of the measured nitrate‐nitrogen (NO3−‐N) loss of 205 μM in a piezometer nest could be accounted for by total N2‐N produced (169 μM N), calculated from changes in dissolved N2 and estimated N2 degassed. Degassing due to methane (CH4) production was also detected in some shallow piezometers within nests. This technique for analysis of dissolved gases in ground water can be applied to detect small changes in N gas concentration and aids in assessing the fate of nitrate along a ground water flow path.

Palaeotemperature reconstruction from noble gases in ground water taking into account equilibration with entrapped air

Noble-gas concentrations in ground water have been used as a proxy for past air temperatures, but the accuracy of this approach has been limited by the existence of a temperature-independent component of the noble gases in ground water, termed 'excess air' whose origin and composition is poorly understood. In particular, the evidence from noble gases in a Brazilian aquifer for a cooling of more than 5 C in tropical America during the Last Glacial Maximum has been called into question. Here we propose a model for dissolved gases in ground water, which describes the formation of excess air by equilibration of ground water with entrapped air in quasi-saturated soils. Our model predicts previously unexplained noble-gas data sets, including the concentration of atmospheric helium, and yields consistent results for the non-atmospheric helium isotopes that are used for dating ground water. Using this model of excess air, we re-evaluate the use of noble gases from ground water for reconstructing past temperatures. Our results corroborate the inferred cooling in Brazil during the Last Glacial Maximum, and indicate that even larger cooling took place at mid-latitudes.

Site Characterization and Containment Assessment with Dissolved Gases

A passive in situ headspace sampler has been developed for the collection of dissolved gases in ground water during tracer tests using dissolved, inert gases. The sampler is constructed of a semipermeable membrane and copper tubing equipped with a Schrader valve. Because the headspace in the sampler equilibrates with the dissolved gases in the ground water, no water is collected, which can be a great advantage in use at contaminated sites. The injection of the dissolved gases into the ground water is accomplished by diffusion through a semipermeable membrane. Oak Ridge National Laboratory routinely uses this technology for site characterization. The passive-diffusive technology also has potential as a low-cost, low-maintenance, and long-term method for containment assessment.

PALEOTEMP: a Mathematica® program for evaluating paleotemperatures from the concentration of atmosphere-derived noble gases in ground water

During the recharge of an aquifer, atmosphere-derived noble gases are dissolved in ground water. The dissolved amount of gas depends on the relative noble gas solubilities, which in turn are inversely proportional to the ambient temperature. Therefore, paleotemperatures prevailing at the time of the aquifer recharge can be calculated from precise measurements of the amount of noble gases in ground water samples. However, the initial noble gas concentrations can be modified by several physical factors, such as the NaCl content of water, the elevation of the recharge area, the radiogenic in situ production of noble gas isotopes, air contamination and degassing, all of which make paleotemperatures difficult to estimate. The amount of air added or gas lost are unknown parameters which must be evaluated through iterative methods. Here, we present a program written with Mathematica®, which calculates paleotemperatures using atmosphere-derived noble gas corrected from the above mentioned factors. The program involves root finding methods and minimization of functions which has been specifically developed for this application.

Retardation of Dissolved Oxygen Due to a Trapped Gas Phase in Porous Media

AbstractInformation on the transport of dissolved gases in ground water is needed to design ways to increase dissolved gas concentrations in ground water for use in in situ bioremediation (e.g., O2 and CH4) and to determine if dissolved gases are conservative tracers of ground‐water flow (e.g., He). A theoretical model was developed to describe the effect of small quantities of trapped gas bubbles on the transport of dissolved gases in otherwise saturated porous media. Dissolved gas transport in porous media can be retarded by gas partitioning between the mobile aqueous phase and a stationary trapped gas phase. The model assumes equilibrium partitioning where the retardation factor is defined as R = 1 + H′(Vg/Vw) where H' is the dimensionless Henry's Law constant for the dissolved gas, and Vg and Vw are the volumes of the trapped gas and water phases, respectively. At 15°C and with Vg/ Vw= 0.05, the predicted retardation factors for He, O2, and CH4 are 5.8, 2.4, and 2.3, respectively. The validity of the model was tested for dissolved oxygen in small‐scale column experiments over a range of trapped gas volumes. Retardation factors of dissolved oxygen increased from 1 to 6.6 as Vg/Vw increased from 0 to 0.123 and are in general agreement with model predictions except for the larger values of Vg/Vw. The theoretical and experimental results suggest that gas partitioning between the aqueous phase and a trapped gas phase can greatly influence rates of dissolved gas transport in ground water.

Paleotemperatures in the Southwestern United States Derived from Noble Gases in Ground Water

A paleotemperature record based on measurements of atmospheric noble gases dissolved in ground water of the Carrizo aquifer (Texas) shows that the annual mean temperature in the southwestern United States during the last glacial maximum was about 5 degrees C lower than the present-day value. In combination with evidence for fluctuations in mountain snow lines, this cooling indicates that the glacial lapse rate was approximately the same as it is today. In contrast, measurements on deep-sea sediments indicate that surface temperatures in the ocean basins adjacent to our study area decreased by only about 2 degrees C. This difference between continental and oceanic records poses questions concerning our current understanding of paleoclimate and climate-controlling processes.

Dissolved radioelements and inert gases in geothermal investigations

The geochemical, radiochemical and hydrological factors which influence the solution of the natural radioelements and radiogenic inert gases in ground waters are discussed. Enhanced 234U/ 238U activity ratios for dissolved uranium are caused by either preferential chemical solution of 234U relative to 238U or by 234U solution following α-recoil ejection of 234Th at the rock — water interface. The dependence of activity ratio on residence time is discussed for recoil-dominated ground waters. 222Rn solution is dependent upon both the uranium content and physical properties of the aquifer rock, and ground water 222Rn contents may be used to assess the variability of these parameters in an aquifer. The solution of 226Ra is partly controlled by an α-recoil process and it generally increases with ground water residence time. Radiogenic 4He is generated by radioelement decay within the aquifer, and ground water 4He content is determined by its residence time and by the extent of 4He transport by fracture fluids or by diffusion. The 4He/ 3He ratio of dissolved helium changes from that characteristic of the atmosphere to a value characteristic of the aquifer as both radiogenic 4He and 4He are generated within it. The application of radiochemical methods to the investigation of some aquifers in the U.K. for their geothermal potential is discussed.