Pore Water Gas Research Articles

Migration of calcium hydroxide compounds in construction waste soil

Migration of leachate generated through embankment of construction waste soil (CWS) in low-lying areas was studied through physical and chemical analysis. A leachate solution containing soluble cations from CWS was found to have a pH above 9.0. To determine the distribution coefficients in the alkali solution, column and migration tests were conducted in the laboratory. The physical and chemical properties of CWS satisfied environmental soil criteria; however, the pH was high. The effective diffusion coefficients for CWS ions fell within the range of <TEX>$0.725-3.3{\times}10^{-6}cm^2/s$</TEX>. Properties of pore water and the amount of undissolved gas in pore water influenced advection-diffusion behavior. Contaminants migrating from CWS exhibited time-dependent concentration profiles and an advective component of transport. Thus, the transport equations for CWS contaminant concentrations satisfied the differential equations in accordance with Fick's 2nd law. Therefore, the migration of the contaminant plume when the landfilling CWS reaches water table can be predicted based on pH using the effective diffusion coefficient determined in a laboratory test.

Molecular and isotopic composition of hydrate-bound and dissolved gases in the southern basin of Lake Baikal, based on an improved headspace gas method

Assessments of the molecular and isotopic composition of hydrate-bound and dissolved gases in pore water were conducted during the multi-phase gas hydrate project (MHP-09) cruise VER09-03 to the southern basin of Lake Baikal in September 2009. To avoid changes in gas composition during core sampling and transport, various headspace methods were investigated aimed at preserving the dissolved gases in pore water. When distilled water was added to the sediment samples, the concentrations of carbon dioxide and oxygen decreased because of dissolution into the water and/or microbial consumption. When the headspace was not flushed with inert gases, trace levels of hydrogen and ethylene were detected. The findings suggest that best preparation is achieved by flushing the headspace with helium, and adding a saturated aqueous solution of sodium chloride. This improved headspace method served to examine the molecular and isotopic compositions of gas samples retrieved at several new sites in the southern basin. Methane was the major component, and the proportion of ethane ranged widely from 0.0009 to 1.67 mol% of the total hydrocarbon gases. The proportions of propane and higher hydrocarbons were small or less than their detection limits. The carbon isotope signatures suggest that microbial-sourced methane and ethane were dominant in the Peschanka study area, whereas ethane was of thermogenic origin at all other study sites in the southern basin of Lake Baikal.

日本海東縁ガスハイドレート調査 (MD179) で得られた海底表層堆積物中の間隙水溶存ガス分析

We investigated molecular and stable isotope compositions of dissolved gas in pore water in subsurface sediment cores that were retrieved from the eastern margin of Japan Sea during the MD179 cruise onboard R/V Marion Dufresne in June 2010. Hydrate-bearing sediment cores retrieved from Umitaka Spur in the Joetsu Basin showed high 13C concentrations of methane, indicating its thermogenic origin, whereas those at Joetsu Knoll partly contained microbial methane because 13C and deuterium are both depleted. Other sediment cores without gas hydrates showed mainly microbial methane formed via CO2 reduction. The profiles of methane and CO2 in the sediments showed a minimum concentration of 13C at SMI (sulfate-methane interface) depth. The concentration of methane in the sediments increased dramatically beneath the shallow SMI depth in the Joetsu Basin, indicating active microbial methane generation.

Isotopic composition of gas hydrates in subsurface sediments from offshore Sakhalin Island, Sea of Okhotsk

Hydrate-bearing sediment cores were retrieved from recently discovered seepage sites located offshore Sakhalin Island in the Sea of Okhotsk. We obtained samples of natural gas hydrates and dissolved gas in pore water using a headspace gas method for determining their molecular and isotopic compositions. Molecular composition ratios C1/C2+ from all the seepage sites were in the range of 1,500–50,000, while δ13C and δD values of methane ranged from −66.0 to −63.2‰ VPDB and −204.6 to −196.7‰ VSMOW, respectively. These results indicate that the methane was produced by microbial reduction of CO2. δ13C values of ethane and propane (i.e., −40.8 to −27.4‰ VPDB and −41.3 to −30.6‰ VPDB, respectively) showed that small amounts of thermogenic gas were mixed with microbial methane. We also analyzed the isotopic difference between hydrate-bound and dissolved gases, and discovered that the magnitude by which the δD hydrate gas was smaller than that of dissolved gas was in the range 4.3–16.6‰, while there were no differences in δ13C values. Based on isotopic fractionation of guest gas during the formation of gas hydrate, we conclude that the current gas in the pore water is the source of the gas hydrate at the VNIIOkeangeologia and Giselle Flare sites, but not the source of the gas hydrate at the Hieroglyph and KOPRI sites.

Model of formation of double structure gas hydrates in Lake Baikal based on isotopic data

We measured the isotopic compositions of methane (C1) and ethane (C2) of hydrate‐bound gas and of dissolved gas in pore water retrieved from bottom sediments in Lake Baikal. Both structure I (sI:3%C2) and II (sII:14%C2) gas hydrates are observed in the same sediment cores in Kukuy K‐2 mud volcano. We found that C2δD of sI gas hydrate is larger than that of sII, whereas C1δ13C, C1δD and C2δ13C values are practically the same in both hydrate structures. δ13C of C1 and C2 of hydrate‐bound gas are several permil smaller than those in pore water, showing that the current pore water is not the source of gas hydrates. These findings lead to a new model where the sII gas hydrates were formed prior to the sI hydrates.

Identification of Oxygen-Depleting Components in MX-80 Bentonite

AbstractAfter closure, the near-field of a nuclear waste repository contains large amounts of oxygen in tunnels and deposition holes. The bentonite buffer/backfill will contain oxygen as a gas phase in unsaturated pores as well as dissolved gas in porewater. The redox conditions in the bentonite filling after post-closure will change towards reducing conditions. In the initial stage, the development of the redox state is mainly governed by the depletion of oxygen. The main mechanisms of oxygen depletion in the bentonite are: 1) diffusion into the surrounding rock and 2) reactions with accessory minerals and by microbial aerobic consumption of organic matter [1,2]. The reactions leading to oxygen depletion are not, however, well understood. The objective of this work was to gather new information concerning oxygen depletion in MX-80. This was done by measuring oxygen depletion and changes in the redox state in suspensions of 1) MX-80, 2) a heavy fraction of MX-80, or 3) a light fraction of MX-80.

Gas bubble growth in heavy oil-filled sand packs under undrained unloading

In situ oil sands are dense uncemented fine-grained sands that contain substantial amount of methane and carbon dioxide gases in pore water and heavy oil. Gas evolves when the pore pressure drops to the bubble point pressure (liquid–gas saturation pressure) due to a decrease in confining pressure or fluid production. The volume and pore pressure changes in live oil-filled sand specimens due to decrease in confining pressure under undrained condition were examined in laboratory. A mechanistic model based on kinetics of gas bubble growth due to solute diffusion in supersaturated oil liquid was formulated and presented to interpret the observed time-dependent non-thermodynamic equilibrium behaviour of pore pressure and volume changes. It was found that the bubble sizes could be estimated indirectly by matching the pore pressure response of the live-oil filled system.

Release of gas bubbles from lake sediment traced by noble gas isotopes in the sediment pore water

The release of gas bubbles from lacustrine or oceanic sediments into the overlying water (ebullition) is a major mechanism for the discharge of biogenic or geogenic gases into the water body. Ebullition of methane or carbon dioxide, for instance, contributes considerably to the release of these potent greenhouse gases through the sediment/water interface. Depending on the rate of ebullition, the pore water will show a depletion in dissolved atmospheric noble gases, because the poorly soluble noble gases escape from the pore water into the gas bubbles. In this study, the depletion of dissolved noble gases in sediment pore water was analyzed for the first time to study bubble formation and ebullition in sediments. The noble gases in the pore water of the sediments of Soppensee (Switzerland) show a distinct depletion due to ebullition of biologically produced methane. This depletion is lowest in the deep sediment and increases towards the sediment surface. The noble gas isotope ratios in the pore water indicate that vertical diffusion barely affects the observed noble gas profiles. The isotope ratios further show that the methane bubbles remain long enough in the sediment to attain noble gas solubility equilibrium before escaping into the overlying water. The volume of gas released from the sediment by ebullition can therefore be reconstructed from the extent of the noble gas depletion in the pore water using a simple gas-equilibration model. The noble gas profiles in the sediment indicate that ebullition increased in Soppensee during the Holocene, and that ebullition contributed strongly to the release of methane from the sediment. Our case study thus illustrates that noble gases are sensitive proxies for the release of gas from lacustrine and marine sediments or similar aquatic environments.

Dissolved noble gases in the porewater of lacustrine sediments as palaeolimnological proxies

This paper presents results from the numerical modelling of the transport of atmospheric noble gases (He, Ne, Ar, Kr, Xe), tritiated water and 3He produced by radioactive decay of 3H, in unconsolidated lacustrine sediment. Two case studies are discussed: (1) the evolution of 3H and 3He concentrations in the sediment porewater of Lake Zug (Switzerland) from 1953 up to the present; and (2) the response of dissolved atmospheric noble gas concentrations in the sediment porewater of a subtropical lake to an abrupt climatic change that occurred some 10 kyr before the present. (1) Modelled 3H and 3He porewater concentrations are compared with recent data from Lake Zug. An estimate of the effective diffusion coefficients in the sediment porewater is derived using an original approach which is also applicable also to lakes for which the historical 3H and 3He concentrations in the water column are unknown. (2) The air/water partitioning of atmospheric noble gases is sensitive to water temperature and salinity, and thus provides a mechanism by which these environmental variables are recorded in the concentrations of atmospheric noble gases in lakes. We investigate the feasibility of using noble gas concentrations in the porewater of lacustrine sediments as a proxy for palaeoenvironmental conditions in lakes. Numerical modelling shows that heavy noble gases in sediment porewater, because of their comparatively small diffusion coefficients and the strong temperature sensitivity of their equilibrium concentrations, can preserve concentrations corresponding to past lake temperatures over times on the order of 10 kyr. Noble gas analysis of sediment porewaters therefore promises to yield valuable quantitative information on the past environmental states of lakes.