Pore water pressure

About: Pore water pressure is a research topic. Over the lifetime, 11455 publications have been published within this topic receiving 247670 citations. The topic is also known as: pwp.

Spatial and seasonal variations of sulphate, dissolved organic carbon, and nutrients in deep pore waters of intertidal flat sediments

Abstract: Spatial and seasonal variations of sulphate, dissolved organic carbon (DOC), nutrients and metabolic products were determined down to 5 m sediment depth in pore waters of intertidal flats located in NW Germany. The impact of sediment permeability, pore water flow, and organic matter supply on deep pore water biogeochemistry was evaluated. Low sediment permeability leads to an enrichment of remineralisation products in pore waters of clay-rich sediments. In permeable sandy sediments pore water biogeochemistry differs depending on whether tidal flat margins or central parts of the tidal flat are studied. Pore water flow in tidal flat margins increases organic matter input. Substrate availability and enhanced temperatures in summer stimulate sulphate reducers down to 3.5 m sediment depth. Sulphate, DOC, and nutrient concentrations exhibit seasonal variations in deep permeable sediments of the tidal flat margin. In contrast, seasonal variations are small in deep pore waters of central parts of the sand flat. This study shows for the first time that seasonal variations in pore water chemistry are not limited to surface sediments, but may be observed down to some metres depth in permeable tidal flat margin sediments. In such systems more organic matter seems to be remineralised than deduced from surface sediment studies.

Laboratory investigation of variable-density flow and solute transport in unsaturated-saturated porous media

Abstract: In many groundwater systems, fluid density and viscosity may vary in space and time as a function of changes in concentration and temperature of the fluid. When dense groundwater plumes interact with less dense ambient groundwater, these density variations can significantly affect flow and transport processes. Under certain conditions, gravitational instabilities in the form of lobe-shaped fingers can occur. This process is significant because it can lead to more rapid and spatially extensive solute transport. This paper presents new experiments carried out in a sand filled glass flow container under both fully saturated and variably-saturated conditions and focuses upon the processes that occur at the capillary fringe and below the water table, as affected by a dense contaminant plumes migration through the unsaturated zone. Source fluids stained with Rhodamine-WT were introduced at the upper boundary of the tank at a range of low and high densities. In addition to the fluid density gradients and porous medium permeability that determine the onset conditions for instabilities in fully saturated experiments, volumetric water content appears critical in the variably-saturated laboratory runs. Plume behaviour at the water table appears dependent upon the density of the fluid that accumulates there. For neutral and low density fluids, plumes accumulate at the water table and then spread laterally above it and the water table forms a barrier to further vertical flow as pore water velocities reduce with increasing water content. For medium and high density fluids, vertical movement continues as instabilities form at the capillary fringe and fingers begin to grow at the water table boundary and move downwards into the saturated zone. In these cases, lateral spreading of the plume is small. Despite these more qualitative observations, the exact nature of the relevant stability criteria for the onset and growth of instabilities in variably-saturated porous media presently remain unclear. All experimental results suggest, however, that the unsaturated zone and position of the water table must be considered in contaminant studies in order to predict the migration pathways, rates and ultimate fate of dense contaminant plumes. It is possible that the results of experiments presented in this paper could form a useful basis for the testing of variable-density (and variably-saturated) groundwater flow and solute transport numerical codes because they offer controlled physical laboratory analogs for comparison. They also provide a strong basis for the development of more rigorous mathematical formulations that are likely to be either developed or tested using numerical flow and solute transport simulators.

Processes and Signals of Nonsteady-State Diagenesis in Deep-Sea Sediments and their Pore Waters

Abstract: Nonsteady-state conditions — induced by changes in the fluxes of electron donors and acceptors and environmental conditions — are shown to have been and to be still widespread in sediments of the equatorial and South Atlantic Ocean. Typical diagenetic phenomena initiated under such nonsteady-state conditions comprise the fixation and downward progression of redox boundaries and reaction fronts. Intervals most severely altered by diagenetic overprint often occur cyclically within the sedimentary record and are mostly associated with full glacial/interglacial transitions. The extent of post-depositional oxidation of organic carbon as well as the dissolution and reprecipitation of minerals across these glacial terminations was shown to depend on the overall sedimentation rate and the magnitude of change encountered in the various depositional and geochemical factors. A sedimentation rate of about 2 cm/kyr was confirmed to be the critical value below which no significant amounts of non-refractory organic carbon are preserved. The influence of climatically induced variations in environmental conditions is not restricted to the geochemical boundaries in the vicinity of the sediment surface (e.g. oxic/post-oxic and Fe redox boundary) but well extends into much deeper sediment sections — namely into the zone of anaerobic oxidation of methane (AOM). In this way, processes within the zone of AOM can produce a further profound diagenetic alteration of the sediment composition up to hundreds of thousands of years after initial deposition and thus a significantly delayed chemical log-in. The long-term utility of all primary and secondary signals — also those formed and initially preserved across the oxic/post-oxic and Fe redox boundaries — is ultimately controlled by the geochemical processes within and below the sulfate/methane transition (SMT). While dissolution of authigenic and productivity-related barite takes place in sulfate-depleted sediment sections, iron sulfides as well as sulfurized organic matter and associated trace elements have a high potential to survive burial below the SMT. Nonsteady-state diagenesis can be triggered not only by changes in conditions at the sediment/water interface like TOC input, sedimentation rate or O2 content of bottom water but also by processes in the underlying sediment — namely the formation and/or liberation of methane. Apart from the distinct alteration of the solid-phase composition, variations in the upward flux of methane also have a considerable impact on the shape of sulfate pore water profiles. Modelling the effects of such variations in methane flux on sulfate profiles has illustrated that considering possible nonsteady-state situations in the sediment/pore water system is of utmost importance for the interpretation of pore water data.

Behavior of a fine-grained soil during the Loma Prieta earthquake

Abstract: Results of an investigation into the behavior of a fine-grained clayey soil at Moss Landing during the 1989 Loma Prieta earthquake are presented. A deposit of this soil underlies portions of the Mo...

Modeling of Coupled Thermo-Hydro-Mechanical Processes with Links to Geochemistry Associated with Bentonite-Backfilled Repository Tunnels in Clay Formations

Abstract: This paper presents simulation results related to coupled thermal-hydraulic-mechanical (THM) processes in engineered barrier systems (EBS) and clay host rock, in one case considering a possible link to geochemistry. This study is part of the US DOE Office of Nuclear Energy's used fuel disposition campaign, to investigate current modeling capabilities and to identify issues and knowledge gaps associated with coupled THMC processes and EBS- rock interactions associated with repositories hosted in clay rock. In this study, we simulated a generic repository case assuming an EBS design with waste emplacement in hor- izontal tunnels that are back-filled with bentonite-based swelling clay as a protective buffer and heat load, derived for one type of US reactor spent fuel. We adopted the Barcelona basic model (BBM) for modeling of the geo- mechanical behavior of the bentonite, using properties corresponding to the FEBEX bentonite, and we used clay host rock properties derived from the Opalinus clay at Mont Terri, Switzerland. We present results related to EBS host-rock interactions and geomechanical performance in general, as well as studies related to peak temperature, buffer resaturation and thermally induced pressurization of host rock pore water, and swelling pressure change owing to variation of chemical composition in the EBS. Our ini- tial THM modeling results show strong THM-driven interactions between the bentonite buffer and the low- permeability host rock. The resaturation of the buffer is delayed as a result of the low rock permeability, and the fluid pressure in the host rock is strongly coupled with the temperature changes, which under certain circumstances could result in a significant increase in pore pressure. Moreover, using the BBM, the bentonite buffer was found to have a rather complex geomechanical behavior that eventually leads to a slightly nonuniform density distribu- tion. Nevertheless, the simulation shows that the swelling of the buffer is functioning to provide an adequate increase in confining stress on the tunnel wall, leading to a stabil- ization of any failure that may occur during the tunnel excavation. Finally, we describe the application of a pos- sible approach for linking THM processes with chemistry, focusing on the evolution of primary and secondary swelling, in which the secondary swelling is caused by changes in ionic concentration, which in turn is evaluated using a transport simulation model.