Pore Water Evolution Research Articles

Numerical and experimental study on the effect of the installation meth-od on the lateral response of offshore monopiles

The successful deployment of offshore wind turbines requires the utilization of foundation installation methods that are fast, low-cost, and reliable. Offshore wind turbines foundations are commonly installed by impact driving. The noise generated during the driving process is a major constraint as it impacts marine life [1] and induces high stresses which reduce the fatigue lifetime of the foundation. Vibratory pile driving offers a promising alternative to traditional impact driving for the installation of monopiles [2], the limited data on its effects on the mechanical properties of the soil during installation in offshore conditions and consequently the mechanical response of the structure throughout its lifetime has prevented its adoption. During vibratory driving, the repeated and high intensity vibration can cause an increase in pore pressures and a decrease in both the inter-particle soil friction and the effective stress, which can eventually lead to liquefaction [3, 4].
 This research aims at investigating the influence of the installation method on the soil properties around monopiles and how it affects the lateral response of such structures. In order to comprehensively understand the different phenomena involved, this problem is investigated through both numerical modeling and laboratory testing.
 A numerical axisymmetric model based on the finite elements method that reflects on an offshore monopile was developed to describe the installation process using fixed boundaries or Perfectly Matched Layers (PML) [5] to realistically simulate the dynamic soil-pile interaction and ensure radiation of waves towards infinity. This model takes into account the reduction in soil strength around the pile shaft during the installation by impact or vibratory driving, along with the dynamic force applied by the vibro-hammer and the pore water evolution. Cyclic triaxial tests have been conducted on a sand extracted from the North Sea. They are used to calibrate the constitutive equations of the model. The results obtained provide insight into the onset and progression of liquefaction, the distribution of pore water pressures, and the resulting deformation of the soil-pile system and how the boundary conditons and the frequency of the imposed load affect those mechanisms.
 These numerical results are also compared with 1g reduced scale experiments composed of a large cylindrical box filled with dry and saturated sand, in which a fully instrumented model pile (tube 60mm in diameter) is vertically driven. The impact or vibrations are imposed at the pile head by a scaled vibratory hammer or scaled impact hammer. In addition, the relative density and the homogeneity of the sand is carefully controlled using the pluviation method [6] allowing to test the sand at different controlled densities. After the installation phase, the lateral behavior of the pile is investigated by imposing a horizontal force with a piston. The test results will also be compared to similar tests on large diameter piles (2 meters) than are planned for fall 2023.

Impact of clayey sediment compaction on pore water evolution and the release of iron

The groundwater contaminants caused by the clayey sediment compaction have been reported in numerous groundwater overextraction regions. But little is known about the impact of compaction on the evolution of pore water compacted from the clayey sediments. In this study, we collected clayey sediments and conducted a low-pressure indoor experiment to identify the pore water evolution and iron releasing mechanism. Pressure demonstrates a significant influence on the release and mobilization of iron in pore water and clayey sediments. Initially, Fe2+ concentration depletion in pore water is regulated by the reduction of Fe(III) (oxy) hydroxides and precipitation with S(-II) producing Fe–S phases. Subsequently, the increase of Fe2+ with the increased pressure is attributed to the transformation of FeS to FeS2. In the final stage of the pressure experiment, more formation of FeS2 and dissolution of siderite result in the fluctuation of Fe. In addition, a release of the trace metals (Fe, Mn, Cr, Cu, Ni and Zn) detected in pore water can be what creates a potential contaminant threat in the adjacent aquifers. The compaction of the clayey sediments results in a decrease of the water content and releases the pore water into the adjacent aquifers. This study reveals the pore water evolution, highlights the complex biogeochemical reactions of iron during clayey sediments compaction and reinforces the importance of clayey sediments compaction for the aquifers water quality.

Evolution of porewater in a Neogene sedimentary formation in the Horonobe area, Hokkaido, Japan: Modeling of burial diagenesis

Evolution of porewater in a Neogene sedimentary formation in the Horonobe area, Hokkaido, Japan: Modeling of burial diagenesis

Geodynamic seawater-sediment porewater evolution of the east central Atlantic Paleogene ocean margin revealed by U-Pb dating of sedimentary phosphates

Emerging evidence suggests that U-Pb and Lu-Hf ages of sedimentary apatite group minerals are often younger than their biostratigraphic ages. However, U-Pb dating of exquisitely preserved carbonate fluorapatite (CFA) is rare. The Upper Cretaceous/Paleogene marine sedimentary rocks of the Moroccan High Atlas host phosphate-rich sediments bracketed by calcareous nannofossil Zones (NP4-NP9) of late Danian to Thanetian age. Here, we use a laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to decipher whether CFA minerals are suitable for U-Pb chronostratigraphy and whether they can reveal the sedimentary and seawater history from which they formed. U-Pb dating of the CFA grains yields ages of 42.9 ± 1.3 Ma (MSWD = 2.3) and 35.7 ± 2.8 Ma (MSWD = 1.3) from three distinct phosphate-rich beds, being >15 million years younger than the expected biostratigraphic age. Combined scanning electron microscopy, X-ray diffraction, and infrared spectroscopy analyses, associate the Mg-rich clay minerals sepiolite and palygorskite, with micro-CFA crystals, while LA-ICP-MS trace element, rare earth element, and yttrium content for primary CFA grains, collectively point to long-term early diagenetic adsorption from oxygenated seawater-dominated porewater fluids. Authigenic clay minerals display a seawater-like pattern, with negligible U concentrations suggesting limited clay mineral influence on U-Pb dating of the CFA crystals. Considering the absence of extensive post-depositional alteration, we propose that because of their large surface area, the µm-sized CFA crystallites facilitated real-time surface adsorption and desorption of elements and diffusion processes. These conditions generated long-term open system connection of sediments with overlying seawater, enabling continuous U-Pb exchange for 15–25 Myr after phosphate precipitation. The data suggest that system closure was potentially associated with sediment lithification and the Atlas orogeny, pointing to stable oxygenation of shallow marine waters along the eastern passive margin of the central Atlantic Ocean in the Paleogene.

Progressive formation of authigenic carbonate with depth in siliciclastic marine sediments including substantial formation in sediments experiencing methanogenesis

Authigenic carbonate (AC) forms in siliciclastic marine sediments in part as a result of the degradation of organic matter and methane. Degradation pathways vary considerably and each impact the chemical evolution of sediments and porewater differently. The relative importance of these reactions in contributing to carbonate authigenesis are poorly understood, especially from a global perspective. Modern porewater geochemical data and authigenic carbonate carbon isotope compositions (δ13Cac) of globally distributed marine sediment sites allow direct assessment of the relative importance of diagenetic pathways. Common correlations between bulk δ13Cac and depth reveal that AC tends to form progressively by multiple reaction pathways, rather than within a particular sediment horizon. A general lack of shallow AC within sediments 1) containing porewater sulfate and 2) exhibiting decreasing dissolved inorganic carbon (DIC) δ13C with depth suggest that organotrophic sulfate reduction does not promote significant authigenesis. Instead, the anaerobic oxidation of methane (AOM) may (in part) account for most AC expressing 13C-depleted isotope compositions, including those with δ13C values between −25 (the approximate marine organic matter value) and 0‰ VPDB. Widespread increases in δ13Cac with depth in conjunction with values that exceed seawater DIC compositions indicate precipitation in sediments exhibiting methanogenesis, likely aided by contemporaneous marine silicate weathering. Deeper AC formation may occur in sediments experiencing thermal decarboxylation. When data from all sites are considered collectively, sediments experiencing AOM and methanogenesis emerge as the most significant in yielding AC. The relative importance of authigenesis pathways controls potential impacts of AC deposition on marine carbon budgets and provides insight into the geochemical signatures exhibited by ancient carbonate concretions.

Drainage chemistry of mine tailings from a carbonatite-hosted Nb-REE deposit, Oka, Québec, Canada

Potential environmental issues associated with the mining of carbonatites are receiving increased attention due to the importance of critical metals for green technologies. This study investigates the chemistry of tailings seepage at the former Saint Lawrence Columbium mine near Oka, Québec, Canada, which produced pyrochlore concentrate and ferroniobium from a carbonatite-hosted Nb-REE deposit. Detailed field sampling and laboratory methods were used to characterize the hydraulic properties of the tailings, their bulk chemistry, mineralogy, pore water and effluent chemistries. The tailings are composed of REE-enriched calcite (64–89 wt %) and fluorapatite (2–22 wt %), as well as biotite (6–17 wt %) and chlorite (0–7 wt %). Minor minerals include ankerite, pyrite, sphalerite, molybdenite, magnetite and unrecovered pyrochlore. Secondary minerals include gypsum, barite, strontianite and rhodochrosite. Geochemical mass balance modeling, constrained by speciation modeling, was used to identify dissolution, precipitation and exchange reactions controlling the chemical evolution of pore water along its flow path through the tailings impoundment. In the unsaturated zone, these reactions include sulfide oxidation and calcite dissolution with acid neutralization. Below the water table, gypsum dissolution is followed by sulfate reduction and FeS precipitation driven by the oxidation of organic carbon in the tailings. Incongruent dissolution of biotite and chlorite releases K, Mg, Fe, Mn, Ba and F and forms kaolinite and Ca-smectite. Cation exchange reactions further remove Ca from solution, increasing concentrations of Na and K. Fluoride concentrations reach 23 mg/L and 8 mg/L in tailings pore water and effluent, respectively. These values exceed Canadian guidelines for the protection of aquatic life. In the mildly alkaline (pH 8.3) pore waters, Mo is highly mobile and reaches an average concentration of 83 μg/L in tailings effluent, which slightly exceeds environmental guidelines. Concentrations (unfiltered) of Zn reach 1702 μg/L in tailings pore water although values in effluent are usually less than 20 μg/L. At the ambient pH, Zn is strongly adsorbed by Fe–Mn oxyhydroxides. Although U forms mobile complexes in tailings pore water, concentrations do not exceed 16 μg/L due to the low solubility of its pyrochlore host. Adsorption and the low solubility of pyrochlore limit concentrations of Nb to less than 49 μg/L. Cerium, from calcite dissolution, is strongly adsorbed although it reaches concentrations (unfiltered) in excess of 1 mg/L and 100 μg/L in pore water and effluent, respectively. Results of this study show that mine tailings from carbonatite deposits are enriched in a wide variety of incompatible elements with multiple mineral hosts of varying solubility. Some of these elements, such as F and Mo, may represent contaminants of concern because of their mobility in alkaline tailings waters.

Two-phase SPH modelling of a real debris avalanche and analysis of its impact on bottom drainage screens

Rapid flow-like landslides, particularly debris flows and debris avalanches, cause significant economic damage and many victims worldwide every year. They are usually extremely fast with the capability of travelling long distances in short times, sweeping away everything in their path. The principal objective of this paper is to test the ability of the ‘GeoFlow-SPH’ two-phase model developed by the authors, to reproduce the complex behaviour of natural debris avalanches where pore-water pressure evolution plays a key role. To reach this goal, the model is applied to reproduce the complex dynamic behaviour observed in Johnsons Landing debris avalanche including the observed bifurcation caused by the flowing out of part of the moving mass from the mid-channel. Initial thickness deposit trim-line, distribution of deposit volume, and the average velocities were provided for this real case, making it an appropriate case to validate the developed model. The paper also contributes to evaluate the SPH-FD model’s potentialities to simulate the structural countermeasure, like bottom drainage screens, used to reduce the impact of debris flows. The analysis of the results shows the adequacy of the proposed model to solve this complicated geophysical problem.

Salinity evolution of aquitard porewater associated with transgression and regression in the coastal plain of Eastern China

Aquitards can preserve paleo-saltwater and have a long and profound influence on the aquifer groundwater, yet they have received less attention. Here, five one-dimensional vertical profiles of the natural tracers, Cl−, Br−, δ2H, and δ18O of aquitard porewater were measured and combined with eight existing profiles collected from previous studies in three Eastern China coastal plains, Bohai bay, Laizhou bay, and Jiangsu coastal plain, to show the salinity origin and the timescale of the transport of porewater across aquitards since the mid-Holocene. Evidence of paleo-seawater impacted by Holocene transgression is noticed as peak Cl− concentration, enriched water isotopes, and lower Cl/Br ratios are observed in the porewater at about the bottom depth of Holocene of 25 m. The shape of Cl− profiles and associated 1-D transport models confirms Holocene seawater's presence in the sediment and the alternating transgression and regression control the salinity evolution of coastal aquitard porewater. Diffusion is the dominant transport across aquitard with an upward velocity between 0.03 and 0.6 mm/yr. Reverse advective flow has affected the solute transport in areas of extensive pumping. Anthropogenic activities enhance the leaching of salty porewater into aquifers in coastal areas, necessitating the need for research focused on marine aquitard salty water.

Hydrochemical evolution of pore water in riverbed sedimentation zone during riverbank infiltration

Abstract The riverbed sedimentation zone is an important zone of hydrochemistry, and the biogeochemical action in this zone has a significant impact on groundwater quality. As the main area where hydrochemistry occurs, studying the law of hydrochemical evolution within 1 m below the riverbed is of great significance for understanding the migration and removal of river pollutants. In this study, a combination of onsite monitoring and indoor experiments was used to analyze the variation characteristics of the hydrochemical composition of pore water during riverbank infiltration, as well as the main hydrochemical effects and influencing factors. The results show that in the process of river water infiltration, a series of redox reactions occur in the riverbed sedimentation zone, and there are differences in different infiltration depths. From 0 to 20 cm below the riverbed, strong respiration and denitrification mainly occurred. Reductive dissolution of manganese minerals mainly occurred from 20 to 60 cm, and reductive dissolution of iron minerals mainly occurred from 60 to 90 cm. River water level, dissolved organic carbon content and microbial activity had varying degrees of influence on these redox effects. The recharge of river water infiltration ensures the exploitation amount of the pumping wells, but it also leads to the increase of some components in groundwater, and the extracted water cannot be directly drunk.

A rate law for sepiolite growth at ambient temperatures and its implications for early lacustrine diagenesis

The magnesium silicate mineral sepiolite is an important component of both alkaline lake and siliceous marine sediments. However, apart from historical constraints on its equilibrium solubility and effective solubility in surface waters, little is known about the rate at which it grows from pore waters during diagenesis. To place constraints on the early evolution of these pore waters, we have performed a series of sepiolite-seeded batch experiments at room temperature with varying concentrations of Mg and SiO2(aq) at pH = 9.2 and 10.1. In general, the slopes of time-series measurements of Mg and Si concentrations are consistent with heterogeneous growth of a phase with sepiolite stoichiometry during the experiments within calculated uncertainties, although some slopes are more consistent with the growth of a phase with kerolite stoichiometry. In total, thirty-six individual rates were derived from the pH = 9.2 experiments and nine individual rates were determined from the pH = 10.1 experiments. In the case of the pH = 9.2 data set, fitting a simplified Transition State Theory rate law to the Si-based sepiolite precipitation rates yields:Rate(molessepiolite/m2/s)=10-16.0±0.3Ω-10.44±0.04 and, for the pH = 10.1 experiments:Rate(molessepiolite/m2/s)=10-15.0±0.2Ω-10.23±0.02where Ω is the saturation index for crystalline, stoichiometric sepiolite. These results are consistent with a strong dependence of growth kinetics on pH and saturation state. The saturation state dependence is significantly stronger at pH = 9.2 than it is at pH = 10.1, such that, at extreme supersaturations approaching the critical supersaturation for homogeneous sepiolite nucleation, the calculated pH = 9.2 rates are faster than the pH = 10.1 rates. Inferences based on the uncertainties on the calculated rate parameters and the predictive capacity of the rate law suggest that these parameters can also predict the growth rates of kerolite without introducing significant additional uncertainty. Using these experimental constraints, we present a conceptual model for the evolution of pore waters during the early stages of diagenesis in sepiolite-bearing sediments. This conceptual model, which implies that exceptional sepiolite supersaturations observed in alkaline lake waters rapidly diminish as porewaters are separated during sediment burial, is in agreement with field measurements of pore waters obtained from Mg-silicate-bearing sediments.

Modelling of Fluidised Geomaterials: The Case of the Aberfan and the Gypsum Tailings Impoundment Flowslides.

The choice of a pure cohesive or a pure frictional viscoplastic model to represent the rheological behaviour of a flowslide is of paramount importance in order to obtain accurate results for real cases. The principal goal of the present work is to clarify the influence of the type of viscous model—pure cohesive versus pure frictional—with the numerical reproduction of two different real flowslides that occurred in 1966: the Aberfan flowslide and the Gypsum tailings impoundment flowslide. In the present work, a depth-integrated model based on the Biot–Zienkiewicz formulation, enhanced with a diffusion-like equation to account for the pore pressure evolution within the soil mass, is applied to both 1966 cases. For the Aberfan flowslide, a frictional viscous model based on Perzyna viscoplasticity is considered, while a pure cohesive viscous model (Bingham model) is considered for the case of the Gypsum flowslide. The numerical approach followed is the SPH method, which has been enriched by adding a 1D finite difference grid to each SPH node in order to improve the description of the pore water evolution in the propagating mixture. The results obtained by the performed simulations are in agreement with the documentation obtained through the UK National Archive (Aberfan flowslide) and the International Commission of large Dams (Gypsum flowslide).

Pore-water evolution and solute-transport mechanisms in Opalinus Clay at Mont Terri and Mont Russelin (Canton Jura, Switzerland)

Data pertinent to pore-water composition in Opalinus Clay in the Mont Terri and Mont Russelin anticlines have been collected over the last 20 years from long-term in situ pore-water sampling in dedicated boreholes, from laboratory analyses on drillcores and from the geochemical characteristics of vein infills. Together with independent knowledge on regional geology, an attempt is made here to constrain the geochemical evolution of the pore-waters. Following basin inversion and the establishement of continental conditions in the late Cretaceous, the Malm limestones acted as a fresh-water upper boundary leading to progressive out-diffusion of salinity from the originally marine pore-waters of the Jurassic low-permeability sequence. Model calculations suggest that at the end of the Palaeogene, pore-water salinity in Opalinus Clay was about half the original value. In the Chattian/Aquitanian, partial evaporation of sea-water occurred. It is postulated that brines diffused into the underlying sequence over a period of several Myr, resulting in an increase of salinity in Opalinus Clay to levels observed today. This hypothesis is further supported by the isotopic signatures of SO42− and 87Sr/86Sr in current pore-waters. These are not simple binary mixtures of sea and meteoric water, but their Cl− and stable water-isotope signatures can be potentially explained by a component of partially evaporated sea-water. After the re-establishment of fresh-water conditions on the surface and the formation of the Jura Fold and Thrust Belt, erosion caused the activation of aquifers embedding the low-permeability sequence, leading to the curved profiles of various pore-water tracers that are observed today. Fluid flow triggered by deformation events during thrusting and folding of the anticlines occurred and is documented by infrequent vein infills in major fault structures. However, this flow was spatially focussed and of limited duration and so did not markedly affect the bulk pore-water.

Multiphysics Modelling of the Mandel- Cryer Effect

In porous medium studies the Mandel-Cryer effect is known, describing non-monotonic pore-water pressure evolution in response to loading or to changed stress conditions. In a 2D poro-elastic model we couple the pore water hydraulics with mechanics (HM). The Mandel-Cryer effect is identified in parts of the model region that are far from the drainage boundary. At parts of the loaded boundary an even more complex pressure evolution is revealed. Variations of the Biot-parameter as the coupling parameter clearly indicate the relevance of the two-way coupling between the involved physical regimes. Hence the Mandel-Cryer effect is a typical result of multi-physical coupling.

Feldspar dissolution, authigenic clays, and quartz cements in open and closed sandstone geochemical systems during diagenesis: Typical examples from two sags in Bohai Bay Basin, East China

Feldspar dissolution and precipitation of clays and quartz cements are important diagenetic reactions affecting reservoir quality evolution in sandstones with detrital feldspars. We examined two sets of sandstone reservoirs to determine whether the sandstone diagenetic systems were open or closed to the mass transfer of products from feldspar dissolution and its impact on reservoir quality. One of the reservoirs is the Eocene fan delta sandstone buried 2.5–4.0 km (1.5–2.5 mi) below sea level (BSL) in the Gaoliu (GL) area of the Nanpu sag, and the other is the Eocene subaqueous fan sandstone buried 1.5–4.5 km (1–2.8 mi) BSL in the Shengtuo (ST) area of the Dongying sag. Both sandstones consist mainly of lithic arkoses and feldspathic litharenites, and have secondary porosity formed by dissolution of feldspars. In the GL sandstones, the absolute amounts of authigenic clays and quartz cements (generally 125°C [257°F]). The low abundance of authigenic clays and quartz cements, and low pore-water salinity indicate that much of the , , and released from leached K-feldspars were exported from the GL sandstone system. And the extensive feldspar dissolution enhanced much porosity and permeability. In contrast, the ST sandstones with secondary pores formed by feldspar dissolution generally contain authigenic clays (kaolinite and illite) and quartz cements with almost identical volume of secondary pores. Kaolinite dominates in the ST sandstones at shallower depth (3.1 km [2 mi] BSL) where temperature exceeds 125°C (257°F). The presence of abundant clays and quartz cements indicates that and released from leached feldspars were retained in the ST sandstone system. The dominance of authigenic illite at greater depth indicates that sufficient should have been retained within the sandstones for occurrence of illitization of kaolinite and feldspars. Secondary porosity in thin sections can be up to 3%, but little porosity ( The diagenetic difference between the GL and the ST sandstones can be interpreted by assessing pore-water evolution in these two areas. The current pore waters with low salinity and negative hydrogen isotopic compositions in the GL sandstone system indicate the significant impact of meteoric water, whereas the current pore waters with high salinity and the paleofluids with positive oxygen isotopic compositions in the ST sandstone system indicate little trace of meteoric water. Access of meteoric freshwater to the GL area probably occurred during the late Oligocene to Neogene through widely developed faults in the Paleogene and Neogene strata. The low-salinity water could have been responsible for flushing of solutes derived from feldspar dissolution. As such, diagenesis in the GL sandstones is considered to have occurred in an open geochemical system, whereas with limited faults and high water salinity, the ST sandstones acted as a closed geochemical system where precipitation of kaolinite, illite, and quartz cements occurred following dissolution of feldspars.

Carbon and oxygen isotope analysis of hydrate-associated Oligocene authigenic carbonates using NanoSIMS and IRMS

Petrography and stable C and O isotope compositions of methane-derived carbonates from the Polish Outer Carpathians were analyzed. Two methods were employed in isotopic determinations. Conventional IRMS measurements on powdered bulk samples were performed with high precision (errors ~0.06‰ for δ13C and ~0.08‰ for δ18O, 1σ), whilst in situ measurements were made using NanoSIMS with a high spatial resolution (5μm), but with low precision (errors ~3.0‰ for δ13C and ~2.3‰ for δ18O, 2σ). The NanoSIMS study proved very useful in identifying the sources of the fluids precipitating the carbonates. They were formed predominantly by the anaerobic oxidation of biogenic methane (AOM). The very fine spatial resolution of the analyses revealed highly variable δ13C values, even in individual crystals, from extremely negative related to AOM to positive related to methanogenesis (Me). Other methods, including laser ablation and standard SIMS, would fail to detect this variability and would not indicate the true sources of the fluids, because they would show average values. Therefore, any straightforward interpretation of the sources of authigenic carbonates based on these methods may be attempted only if the δ13C values are extreme, i.e. resulting either from AOM or Me.Unusual clast-like druses from the carbonates were analyzed in detail with NanoSIMS. They are filled with strongly 13C-depleted (down to −54‰) and 18O-enriched (up to 6‰) fringe calcite which precipitated on the walls of the druses from heavy-oxygen-containing water, released from the dissociation of gas hydrates. The rocks can thus be termed clathrites. The bulk rock isotope analyses of powdered samples of these druses show no evidence of hydrates and alone would not point to the true origin of these rocks. Precipitation of the carbonate cements started when hydrates were still present, so they are a type of melt-and-seal fabric. The recognition of such features may be crucial evidence of the former occurrence of hydrates in fossil methane-derived carbonates.The isotopic results have demonstrated that NanoSIMS can be very useful in reconstructing the diagenetic evolution of pore water by analyzing the stable isotope compositions of even microcrystalline cements. This provides a basis for the application of NanoSIMS to tracing diagenetic pathways from various, even complexly formed, authigenic precipitates.

Geochemical evolution and residence time of porewater in low-permeability rocks of the Michigan Basin, Southwest Ontario

Drill cores collected from an 860-m-thick Paleozoic (Cambrian to Devonian) sedimentary sequence of the Michigan Basin in southwest Ontario, Canada, were used to develop vertical profiles of natural tracers in porewater from the sedimentary sequence and build a conceptual model regarding processes and timing of solute migration. In this paper, the conceptual model is quantitatively assessed with numerical simulations of diffusion-dominated solute transport (Cl− and 18O). The conceptual model suggests that the tracer profiles result from mixing between a deep shield brine with isotopic characteristics consistent with water–rock interaction over a long period of geologic time, Ordovician seawater at intermediate depth, and an overlying hypersaline brine formed by evaporation of seawater in a restricted basin during the Silurian. Between the peak of basin consolidation at 300MaBP and the Pleistocene (2.6MaBP), salinity and isotopic concentration gradients are thought to have driven mixing, dominantly by diffusion. Water isotope data indicate that in the Pleistocene, melt water infiltrated to a maximum depth of 330m in the Silurian Salina Formation, likely in response to transient glacial loading and elevated sub-glacial hydraulic pressures. Also in the Pleistocene, over-pressured conditions in deep regions of the basin drove high-salinity brine eastward in the basal Cambrian aquifer, interrupting pre-existing tracer-diffusion profiles.A base case set of boundary and initial conditions provides a good fit between simulated and measured tracer profiles, but requires effective diffusion coefficients (De) in the Ordovician limestones that are lower than the average values obtained in laboratory measurements by as much as a factor of 10. The base-case values are still within the overall range of measured values. At the formation scale, relatively low in-situ De values are attributed to the intermittent occurrence of gas and liquid hydrocarbons in the Ordovician carbonate units and the effect of lithostatic confining pressure; neither of which are accounted for in the laboratory measurements but both would have the effect of decreasing De for aqueous solutes. The numerical model is most sensitive to the formation-specific De values and geologically reasonable variations in initial and boundary conditions have a relatively small influence on the results. Additional simulations indicate that hydraulic disturbance and ingress of meltwater during the Pleistocene can explain second-order features observed in the measured tracer profiles. This work provides supporting evidence for previous research indicating that the porewater residence time in the Ordovician sequence approaches the age of the rocks.

Micro porosity evolution in compacted swelling clays — A chemical approach

This paper presents a new approach to investigate the variation of microscopic porosity/pore water interactions in compacted swelling clays. The aim of the research has been to develop a theoretical formulation for the prediction of micro-porosity variations with suction and temperature in compacted smectite. The model developed is based on a geochemical formulation of interlayer hydration/dehydration of smectite.An established theoretical approach based on regular solid-solution theory is adopted to describe the water adsorption/desorption in the interlayer of smectite. The thermodynamic parameters of the model for the case of two bentonite clays, namely MX-80 and FEBEX are presented. Thermodynamic parameters of the hydration model including Margules parameter (Ws ) and logarithm of the equilibrium constant (log Keq ) at ambient temperature were found to be −2420 cal/mol and −1.42, for compacted MX-80 and −3330 cal/mol and −2.79, for compacted FEBEX, respectively.Micro/macro-porosity evolutions with dry density and relative humidity are presented. The results are compared with alternative approximations reported in the literature which show a close correlation. Variations of the micro and macro-porosity in compacted bentonite with hydration processes are also studied through the application of the model under restrained swelling and isothermal conditions. The results provide an insight into the evolution of pore water in compacted bentonite during saturation and quantify the distribution of water in micro and macro pores.

Paleoenvironmental reconstruction of the Ediacaran Araras platform (Western Brazil) from the sedimentary and trace metals record

Ocean and atmosphere oxidation in the Ediacaran Period paved the way for the dawn of animals. In an attempt to better document the record of seawater redox state at the onset of the Ediacaran, we performed a paleoenvironmental study of post-Marinoan (635Ma) carbonate strata from the southeastern margin of the Amazon craton, western Brazil. Five sections were sampled along the Araras carbonate platform. Outcrop-based facies analysis, complemented by petrographic description of representative samples, was performed on these sections. Seven facies associations (FA) were recognized. Four FA are encountered in the inner-shelf, from the basal glacio-marine deposits of the Puga Formation to shallow and moderately deep platform facies, which are systematically covered by deeper CaCO3 over-saturated facies. Five FA occur on the outer-shelf, including storm-wave influenced facies, below storm-wave base facies and slope intraformational breccia. These facies associations indicate a transgressive systems tract over the carbonate platform. Trace metals (U, Mo, Zn, Pb, Cd, Cu, Ni, V) and Al concentrations, pyrite abundance and total organic carbon (TOC) contents in these sections are generally low. They nonetheless present, from base to top of the sections, significant stratigraphic variations which can be traced along the platform, with successive enrichments of (i) Pb and Zn, (ii) U, and (iii) both U and Mo in the thin marl levels containing the highest amount of organic carbon and pyrite (0.4% and 1.9%, respectively). We interpret this succession to record the progressive evolution of sediment pore-waters from oxic to more reducing conditions, driven by an increase in sedimentary organic matter accumulation. The lack of evidence for persistent sulphide mineralization and associated enrichments in Mo suggests that sulfidic conditions were only achieved in sediment pore waters. In the aftermath of the Marinoan glaciation the water column must then have been essentially oxic on the Araras platform, with anoxia only sporadically reaching the sediment–water column interface in the deepest parts of the platform.

Microtextures, microchemistry, and mineralogy of basaltic glass alteration, Jeju Island, Korea, with implications for elemental behavior

The meteoric alteration of basaltic glass in hyaloclastite (0.55 to 0.2 Ma) from Jeju Island, Korea, was examined using high-resolution electron microscopy, in situ sample preparation using focused ion beam, and microchemical analysis. The glass particles showed a sequential development of alteration textures toward the glass interior as follows: oscillatory zoning of smectite and amorphous bands/hemispheres, congruent dissolution of glass with smectite infilling in interparticle pore, and the replacement of nanogranular amorphous aggregate (NAA) for porous leached glass (LG) followed by a partial crystallization of smectite. Chemical compositions of LG widely varied from partial to complete leaching of almost all kinds of cations, to a preferred leaching of Na only. The order of leaching in LG followed Na + >Mg 2+ >Ca 2+ = Fe 2+ >Al 3+ >Ti 4+ . Leached cations were removed from the hyaloclastite-water system or incorporated into secondary phases. Sodium was all but removed from the system. Magnesium was largely lost with partial incorporation into smectite. Calcium was significantly retained in NAA, partly incorporated into the smectite interlayer, and partly lost. Iron and Al were partitioned between NAA and smectite. Titanium was mostly retained in NAA and Ti-rich silicates. The molar ratio of major cations to Si was reversed from Ca 2+ >Mg 2+ >Na + in basaltic glass to Na + >Mg 2+ >Ca 2+ in groundwater. The geochemical evolution of pore water was influenced by the microtextural and microchemical evolution of glass particles, and vice versa. Systematic microtextural, microchemical, and mineralogical analyses of natural glass alteration are critical to understanding the long-term behavior of glass alteration.

Testing geochemical models of bentonite pore water evolution against laboratory experimental data

The determination of a bentonite pore water composition and understanding its evolution with time underpins many radioactive waste disposal issues, such as buffer erosion, canister corrosion, and radionuclide solubility, sorption, and diffusion, inter alia. Previous modelling approaches have tended to ignore clay dissolution–precipitation reactions, a consequence of which is that montmorillonite is theoretically preserved indefinitely in the repository system. Here, we investigate the applicability of an alternative clay pore fluid evolution model, that incorporates clay dissolution–precipitation reactions as an integral component and test it against well-characterised laboratory experimental data, where key geochemical parameters, Eh and pH, have been measured directly in compacted bentonite. Simulations have been conducted using different computer codes (Geochemist’s Workbench, PHREEQC, and QPAC) to test the applicability of this model. Thermodynamic data for the Gibb’s free energy of formation of MX-80 smectite used in the calculations were estimated using two different methods (‘Polymer’ and ‘Vieillard’ Models). Simulations of ‘end-point’ pH measurements in batch bentonite–water slurry experiments showed different pH values according to the complexity of the system studied. The most complete system investigated revealed pH values were a strong function of partial pressure of carbon dioxide, with pH increasing with decreasing PCO 2 (with log PCO 2 values ranging from −3.5 to −7.5 bars produced pH values ranging from 7.9 to 9.6). A second set of calculations investigated disequilibrium between clay and pore fluid in laboratory squeezing cell tests involving pure water (pH = 9.0) or a 1 M NaOH solution (pH = 12.1). Simulations carried out for 100 days (the same timescale as the experiments) showed that smectite remained far from equilibrium throughout, and that the lowering of pH due to smectite hydrolysis was trivial. However, extending the duration of the simulations to that required for clay–fluid equilibrium, necessitated timescales of 7 and 65 years for pure water and 1 M NaOH, respectively, but again produced relatively minor reduction in pH (in the order of 0.1–0.2 pH units). If the (equilibrium) precipitation of secondary minerals was included in the simulations, then not only was the clay–fluid equilibration period extended dramatically (from 7 to 360 years for pure water, and from 65 to 2600 years for 1 M NaOH), but concomitant changes in pH were significant, decreasing from 9.0 to 8.6 (pure water) and from 12.1 to 9.0 (1 M NaOH). Repetition of these latter calculations using an alternative method for Δ G f 0 smectite produced an increase in equilibration time for reaction with 1 M NaOH from 2600 to 5000 years, highlighting the potential effects of the uncertainty in thermodynamic data for smectite. A final set of calculations was carried out to investigate both the time- and space-dependent variations in pore fluid composition in laboratory in-diffusion experiments conducted for over 1200 days, initially with pure water and ‘spiked’ after 271 days with a Na–Ca–OH–Cl solution (pH = 11.7). Here, the sensitivity of the results to both variations in a number of parameters/conditions (porosity, reaction rate of secondary minerals, the degree of mixing of the external fluid reservoirs in the experiments, the effective diffusion coefficient) and the inclusion/exclusion of key processes (clay hydrolysis, secondary mineral precipitation, ion exchange, clay edge protonation–deprotonation reactions) was investigated. These calculations confirmed that smectite dissolution–precipitation reactions alone have an insignificant impact upon pH buffering over laboratory timescales and that the pH buffering observed is most likely controlled by clay protonation–deprotonation reactions, and kinetic secondary mineral (brucite + tobermorite) precipitation. Ion exchange reactions were found to have little effect on pH. Alternative data for the kinetic dissolution of smectite produced no observable differences, and the adoption of a reduced diffusion coefficient produced a poorer fit to experiment results. In conclusion, modelling predicts that the effects of smectite dissolution on the chemistry of bentonite pore waters would be essentially undetectable over experimental time scales, but when the model is combined with plausible constraints on the precipitation of secondary minerals, significant changes in solution chemistry and mineralogy are predicted to occur over time scales that are relevant to repository near-field evolution (hundreds to thousands of years). There are remaining fundamental uncertainties related to the variable chemistry of the smectite clays, the nature of porosity in highly compacted buffer materials, the reactive surface area of smectite, and the thermodynamic properties of these clay minerals.