Microbial nitrogen cycling at the saltwater–freshwater interface

Microbial communities inhabiting “subterranean estuaries” along the subsurface freshwater–saltwater continuum determine the fate of nitrogen discharged to coastal waters. Little is known about the microbes that comprise these communities, or what their ecological and biogeochemical responses will be to increased salinity resulting from saltwater intrusion and aquifer salinization. This review covers basic aspects of the nitrogen cycle relevant to the coastal subsurface and provides a framework for predicting the types of microbes and nitrogen transformations that exist in different subterranean estuary systems. Literature concerning the freshwater–saltwater mixing zones of surficial estuaries, where microbial communities are better characterized, is also reviewed to explore what is known about the impact of increasing salinity on both the community composition and biogeochemical function of the microbial assemblage. Collectively, these studies suggest that salinization will alter microbial community composition for all functional groups involved in nitrogen cycling, and may lead to decreases in nitrification and coupled nitrification-denitrification, and increases in dissimilatory nitrate reduction to ammonium (DNRA). Future collaboration between hydrogeologists and microbial ecologists is needed to fully predict the impact of saltwater intrusion on subsurface microbial communities.     

Factors causing dynamic variations in the saltwater–freshwater transition zone in a beach aquifer, Mangsang, South Korea

Dynamic variation in the saltwater–freshwater transition zone below a seafront beach in South Korea was investigated with long-term monitoring of the groundwater in relation to the precipitation, wave height, and tide. Correlation, spectral analysis, and regression analysis of monitoring data were performed to deduce the relationships between these factors. The general shape of the transition zone was affected by the seasonal groundwater levels, but temporary fluctuations were predominantly affected by local rising-groundwater-level events. The distinct increases in the groundwater level were closely related to the wave height. Different patterns of electrical conductivity (EC) change were detected in the shallow and deep zones, and these differences indicated that the transition zone was highly dynamic. The EC values at shallow depths were temporarily increased by the wave setup and tidal fluctuations during the rising-groundwater events, but the EC at greater depths was reduced by the seaward or downward movement of the relative freshwater. In exceptional cases, during extreme increases in the groundwater level resulting from seawater flooding, the rapid downward flow of the flooding saltwater through the well bore caused synchronous EC fluctuations at all depths.     

Geochemical processes in the saltwater–freshwater transition zone: comparing results of a sand tank experiment with field data

Geochemical processes, occurring in a stable transition zone between saltwater and freshwater, were simulated in a 2D, multi-layer flow chamber experiment. Mixing, calcite dissolution, and oxidative degradation of organic matter were identified as the main controlling factors. The results of the chamber experiment were compared to field data and verified by thermodynamic modeling. Similarity in most ion distributions suggests the general applicability of the experimental method. Differences in the redox conditions between field and experiment were reflected by the oxidants involved in the mineralization of organic carbon; while field data show evidence of sulfate reduction, the presence of oxygen in the laboratory experiment resulted in the reoxidation of sulfides.     

Soil–structure cyclic direct shear tests: a new interpretation of the direct shear experiment and its application to a series of cyclic tests

This paper presents a series of cyclic 2D direct shear tests on sand–rough material interfaces under constant normal load (CNL) and constant normal stiffness (CNS) conditions. The aim of these tests is to describe the behavior of the soil–pile contact subjected to a large number of cycles due to environmental or anthropic loadings. These cycles (typically 10⁴ or less due to an early rupture) are small (10, 20 and 40 kPa in terms of shear stress). A new interpretation of the direct shear tests is proposed. The sample of soil is schematically composed of a sheared interface and of a buffer under oedometric load. The problem of sand leakage between the shear box and the rough plate, classical phenomenon in this type of test, is focused. The effect of initial density, position of “center of cycles” in stress plane (mean cyclic variables) and cyclic amplitude is investigated. The cycles are defined by the initial mean cyclic normal stress, the level of initial mean cyclic stress ratio and the normalized cyclic amplitude. Under CNL condition, either dilation or contraction is exhibited, in agreement with the characteristic state developed by Luong (International symposium on soils under cyclic and transient loading, Swansea, 7–11 January, pp 315–324, 1980). The influence of a prescribed normal stiffness is especially considered. It can be highlighted that CNS cyclic paths are always contractive. This contraction results in a drop of mean cyclic normal stress often called degradation of friction.     

Can we use tracer tests to obtain data for performance assessment of repositories for nuclear waste?

With regard to the long-term safety of repositories for spent nuclear fuel, tracer tests have commonly been used in site characterisation (SC) for finding information that can be later used for performance assessment (PA). The question arises as to whether data obtained in tracer tests performed over a time scale of weeks or months are relevant for PA calculations. As part of a study overseen by the Äspö Task Force, the mechanisms that determine the radionuclide residence time under SC and PA conditions are addressed, given that they influence the validity of data transference from SC to PA. The results show that radionuclide transport in SC and PA, although governed by the same retardation mechanisms, are dominated by different sub-processes. In a practical sense this means that the parameter values (typically fracture apertures and “in situ” apparent retention data) determined in tracer tests, performed as a part of the SC program, should not be used directly for making PA predictions. The emphasis in SC should focus more on the determination of other parameters of relevance at PA timescales. The PA-specific flow rate, flow connectivity, and flow-wetted surface to flow ratio are given here as examples.     

Numerical modeling of soil–structure interface of a concrete-faced rockfill dam

Concrete-faced rockfill dams (CFRD) are widely used in large-scale hydraulic projects. The face slab, the key seepage-proof structure of great concern, has a strong interaction with the neighboring gravel cushion layer due to a significant difference in their stiffness. An elasto-plasticity damage interface element, a numerical format of the EPDI model, is described for numerical analysis of a CFRD that can trace the separation and re-contact between the face slab and the cushion layer at the interface. As verified by simulating slide block and direct shear interface tests, this element was confirmed to capture effectively the primary monotonic and cyclic behaviors of the interface. This element can easily be extended to the finite element method (FEM) programs that involve the Goodman interface element. The analysis of a typical CFRD showed that the interface model describes a significant effect on the stress response of the face slab under different conditions, including dam construction, water storage, and earthquake. Treatments of the cushion layer, such as an asphalt layer, changed the behavior of the interface between it and the face slab, which resulted in a significant effect on the stress response of the face slab. The top of the face slab exhibited a significant separation from the cushion layer during construction, induced mainly by construction of the neighboring dam body.     

Development of a Darcy-Brinkman model to simulate water flow and tracer transport in a heterogeneous karstic aquifer (Val d’Orléans, France)

Darcy’s law is the equation of reference widely used to model aquifer flows. However, its use to model karstic aquifers functioning with large pores is problematic. The physics occurring within the karstic conduits requires the use of a more representative macroscopic equation. A hydrodynamic model is presented which is adapted to the karstic aquifer of the Val d’Orléans (France) using two flow equations: (1) Darcy’s law, used to describe water flow within the massive limestone, and (2) the Brinkman equation, used to model water flow within the conduits. The flow equations coupled with the transport equation allow the prediction of the karst transfer properties. The model was tested by using six dye tracer tests and compared to a model that uses Darcy’s law to describe the flow in karstic conduits. The simulations show that the conduit permeability ranges from 5?×?10^?6 to 5.5?×?10^?5?m² and the limestone permeability ranges from 8?×?10^?11 to 6?×?10^?10?m². The dispersivity coefficient ranges from 23 to 53 m in the conduits and from 1 to 5 m in the limestone. The results of the simulations carried out using Darcy’s law in the conduits show that the dispersion towards the fractures is underestimated.     

Coupled deformation–seepage analysis of dynamic capacity tests on open-ended piles in saturated sand

Open-ended piles such as tubular piles or I-beams are used as foundations for offshore and nearshore construction. After the pile installation a load test to estimate the bearing capacity of these open-ended piles is necessary. Due to the offshore conditions and the high bearing capacity of the installed piles a static load test is not normally feasible. Therefore, dynamic load tests are carried out where the wave propagation due to an dynamic impact at the pile head is measured. The methods to estimate the bearing capacity from the measured signal of the dynamic tests were derived for solid pile profiles. It is questionable whether these evaluation techniques are applicable for open-ended piles. Hence, the influence of various important system parameters as well as the differences between static and dynamic load tests on open-ended piles is investigated in this paper.     

Mechanical behaviors of conjugate-flawed rocks subjected to coupled static–dynamic compression

Conjugate flaws widely exist in rock masses and play a significant role in their deformation and strength properties. Understanding the mechanical behaviors of rock masses containing conjugate flaws is conducive to rock engineering stability assessment and the related supporting design. This study experimentally investigates the mechanical properties of conjugate-flawed sandstone specimens under coupled static–dynamic compression, thereby providing insight into how conjugate fractures interact to produce tracing tensional joints. Results indicate that the coupled compressive strength and the dynamic elastic modulus of conjugate-flawed rock specimens show remarkable loading rate dependence. For a fixed strain rate, the specimen with a static pre-stress equal to 60% of its uniaxial compressive strength has the highest coupled strength. Besides, both higher static pre-stress and strain rate can induce smaller mean fragment size and greater fractal dimension of the specimen, corresponding to a more uniform distribution of the broken fragments with smaller sizes. When the static pre-stress is lower than 80%UCS, the flawed specimen under a higher strain rate is characterized by higher absorbed energy. However, when the pre-stress equals 80%UCS, the value of the energy absorbed by the specimen in the dynamic loading process is negative due to the release of the preexisting considerable elastic strain energy input from the static pre-loading. As for the failure modes, cracks always penetrate the preexisting ipsilateral flaw tips to form anti-wing cracks. Under dynamic loading, the conjugate-flawed specimen generally shows tensile failure at a low strain rate, while the shear failure dominates at a high strain rate. In addition, based on progressive failure processes of the conjugate-flawed rock specimens, the evolution of tracing tensional joints in the field is discussed.

     

Enhancement of a hypoplastic model for granular soil–structure interface behaviour

Modelling of interfaces in geotechnical engineering is an important issue. Interfaces between structural elements (e.g., anchors, piles, tunnel linings) and soils are widely used in geotechnical engineering. The objective of this article is to propose an enhanced hypoplastic interface model that incorporates the in-plane stresses at the interface. To this aim, we develop a general approach to convert the existing hypoplastic model with a predefined limit state surface for sands into an interface model. This is achieved by adopting reduced stress and stretching vectors and redefining tensorial operations which can be used in the existing continuum model with few modifications. The enhanced interface model and the previous model are compared under constant-load, stiffness and volume conditions. The comparison is followed by a verification of two the approaches for modelling the different surface roughness. Subsequently, a validation between available experimental data from the literature versus simulations is presented. The new enhanced model gives improved predictions by the incorporation of in-plane stresses into the model formulation.     

A modification to dense sand dynamic simulation capability of Pastor–Zienkiewicz–Chan model

A modification to the nonlinear Pastor–Zienkiewicz–Chan (PZC) constitutive model without any change in the number of model parameters is introduced in order to simulate stiffness degradation of dense sands at dynamic loading. The PZC model is based on generalized plasticity and was verified by good prediction of liquefaction and undrained behavior of saturated sand. The PZC is a robust model that can predict drained dynamic behavior of sands, especially stiffness increase in loose sand at reloading of dynamic loading. Yet, this model does not show stiffness degradation of dense sand at reloading. The modification is made through modifying the stress memory factor, H _DM, which is multiplied by the plastic modulus, H _L. This modification does not influence reloading behavior of loose sand. The modified PZC model is verified via results of drained cyclic tests. Two cyclic triaxial tests on loose and dense specimens, along with two cyclic plane strain tests on dense sand are utilized for validation. The model simulation shows that the modified PZC model is able to predict the stiffness degradation of dense sand at reloading well.     

Selective separation of very fine particles at a planar air–water interface

Selective fine particle separation is a key unit operation in the mineral and related industries. In flotation, the capture of fine particles by bubbles is inefficient due to their low mass and momentum, which result in low particle–bubble collision efficiency. We demonstrate that it is possible to selectively separate a mixture of very fine hydrophobic graphite and hydrophilic quartz particles by direct contact with an air–water interface without a particle–bubble collision step involved. We demonstrate that it is possible to scale-up the process from a simple batch to a continuous process. Good selective separation of graphite from quartz gangue could be obtained under continuous conditions.     

The dilatant behaviour of sand–pile interface subjected to loading and stress relief

Property and behaviour of sand–pile interface are crucial to shaft resistance of piles. Dilation or contraction of the interface soil induces change in normal stress, which in turn influences the shear stress mobilised at the interface. Although previous studies have demonstrated this mechanism by laboratory tests and numerical simulations, the interface responses are not analysed systematically in terms of soil state (i.e. density and stress level). The objective of this study is to understand and quantify any increase in normal stress of different pile–soil interfaces when they are subjected to loading and stress relief. Distinct element modelling was carried out. Input parameters and modelling procedure were verified by experimental data from laboratory element tests. Parametric simulations of shearbox tests were conducted under the constant normal stiffness, constant normal load and constant volume boundary conditions. Key parameters including initial normal stress ( $ \sigma_{{{\text{n}}0}}^{\prime } $ ), initial void ratio (e ₀), normal stiffness constraining the interface and loading–unloading stress history were investigated. It is shown that mobilised stress ratio ( $ \tau /\sigma_{\text{n}}^{\prime } $ ) and normal stress increment ( $ \Updelta \sigma_{\text{n}}^{\prime } $ ) on a given interface are governed by $ \sigma_{{{\text{n}}0}}^{\prime } $ and e ₀. An increase in $ \sigma_{{{\text{n}}0}}^{\prime } $ from 100 to 400 kPa leads to a 30 % reduction in $ \Updelta \sigma_{\text{n}}^{\prime } $ . An increase in e ₀ from 0.18 to 0.30 reduces $ \Updelta \sigma_{\text{n}}^{\prime } $ by more than 90 %, and therefore, shaft resistance is much lower for piles in loose sands. A unique relationship between $ \Updelta \sigma_{\text{n}}^{\prime } $ and normal stiffness is established for different soil states. It can be applied to assess the shaft resistance of piles in soils with different densities and subjected to loading and stress relief. Fairly good agreement is obtained between the calculated shaft resistance based on the proposed relationship and the measured results in centrifuge model tests.     

Effects of the planned ephesus recreational canal on freshwater–seawater interface in the Selçuk sub-basin, İzmir-Turkey

An artificial water canal opening is planned between the Agean Sea and the historical Ephesus site for the sake of tourism in the Selçuk sub-basin. In order to predict the effects of the planned canal on freshwater–seawater interface and related contamination in the aquifer, 3-D numerical density dependent flow and solute transport simulations were carried out. The simulations included the pre-pumping and pumping periods without a canal and the prediction period in the presence of the canal. Chloride concentration comparisons of the results obtained from the pre-pumping period and the pumping period indicate that the freshwater-seawater interface in the aquifer has progressed inland due to artificial discharge in the sub-basin. Drawdown during the pumping period is about 15 cm. The planned canal opening could further lower the groundwater levels in the area and would change the groundwater flow directions in the first 4 years. Then the levels and flow directions will nearly recover. However, the canal opening could cause further seawater intrusion into the aquifer to the extent that groundwater would be unfit to use for irrigation after the seventh year of the canal opening in the irrigation cooperative II wells area and would be unfit to use for drinking purposes after the tenth year in the municipality wells area located at the south of the cooperative II wells. On the other hand, the cooperative I wells would not be effected by the opening of the canal.     

Properties of a diffuse interface model based on a porous medium theory for solid–liquid dissolution problems

In this paper, a local non-equilibrium diffuse interface model is introduced for describing solid–liquid dissolution problems. The model is developed based on the analysis of Golfier et al. (J Fluid Mech 457:213–254, 2002) upon the dissolution of a porous domain, with the additional requirement that density variations with the mass fraction are taken into account. The control equations are generated by the upscaling of the balance equations for a solid–liquid dissolution using a volume averaging theory. This results into a diffuse interface model (DIM) that does not require an explicit treatment of the dissolving interface, e.g., the use of arbitrary Lagrangian–Eulerian (ALE) methods, for instance. Test cases were performed to study the features and influences of the effective coefficients inside the DIM. In particular, an optimum expression for the solid–liquid exchange coefficient is obtained from a comparison with the referenced solution by ALE simulations. Finally, a Ra–Pe diagram illustrates the interaction of natural convection and forced convection in the dissolution problem.     

Is 222Rn a suitable tracer of stream–groundwater interactions? A case study in central Italy

River water infiltration into an unconfined porous aquifer (∼73% gravels, ∼12% sands, ∼15% silts and clays) in the Petrignano d’Assisi plain, central Italy, was traced combining isotopic techniques (²²²Rn) with hydrochemical and hydrogeologic techniques in order to characterize the system under study. The ²²²Rn gave information about the river water residence times within the aquifer and hydrochemical data, in a two-component mixing model, which allowed estimating the extent of mixing between surface waters and groundwater in wells at increasing distances from the river. The mixing measured in the well closer to the riverbank indicated a higher contribution of river water (up to 99%) during the groundwater recession phase and a moderate contribution (up to 64%) during the recharge phase. A model describing ²²²Rn concentrations in groundwater as the result of both parent/daughter nuclide equilibrium and mixing process (²²²Rn mixing/saturation model) was used to describe observed Rn concentrations and mixing index trends with the aim of evaluating water mean infiltration velocities along the transect. The stream bank infiltration velocities obtained by the model ranged from 1 m day⁻¹ during groundwater recharge periods, when river water infiltration is lower, to 39 m day⁻¹ during recession phases, when river water infiltration is larger.     

Transfer of litter-derived N to soil mineral–organic associations: Evidence from decadal 15N tracer experiments

Mineral–organic associations act as mediators of litter-derived N flow to the mineral soil, but the time scales and pathways involved are not well known. To close that gap, we took advantage of decade old ¹⁵N litter labeling experiments conducted in two European forests. We fractionated surface soils by density with limited disaggregating treatment and investigated organic matter (OM) characteristics using δ¹³C, δ¹⁵N and the C/N ratio. Mineral properties were studied by X-ray diffraction and selective dissolution of pedogenic oxides.Three types of associations were isolated: plant debris with few trapped minerals (<1.65 g/cm³), aggregates dominated by phyllosilicates (1.65–2.4 g/cm³), and single mineral grains and pedogenic oxides with little OM (>2.4 g/cm³). A small proportion of ¹⁵N tracer was rapidly attached to single mineral grains, while most of it moved from plant debris to aggregates of low density and progressively to aggregates of higher density that contain a more microbially processed OM. After a decade, 60% of the ¹⁵N tracer found in the investigated horizon was retained in aggregates, while plant debris still contained 40% of the tracer.We present a conceptual model of OM and N flow through soil mineral–organic associations, which accounts for changes in density, dynamics and chemistry of the isolated structures. It suggests that microbial reworking of OM entrapped within aggregates (1.65–2.4 g/cm³) causes the gradient of aggregate packing and, further on, controls the flow of litter-derived N through aggregates. For associations with denser material (>2.4 g/cm³), mineralogy determines the density of the association, the type of patchy OM attached to mineral surfaces and controls the extent of litter-derived N incorporation.     

Influence of calcium leaching on the mechanical behavior of a rock–mortar interface: A DEM analysis

Being a potential preferential way for water to flow, interfaces between host rock and engineered barriers are critical in the design of deep radioactive waste repositories. In case of cementitious materials, presence of water may lead to long term degradation by leaching. Such a phenomenon could impede the integrity of the confinement by its effect on the hydro-mechanical properties of the interface. Recent experimental results from Buzzi et al. [8] have evidenced some effects of leaching on the hydro-mechanical behavior of rock–concrete interfaces for one leaching time. This paper intends to investigate the influence of leaching on the mechanical behavior of rock–mortar interfaces by means of numerical simulations. These latter will be run for several leaching times to produce a better understanding of the phenomenon. For this purpose, a DEM approach has been developed to simulate the increase of the macro-porosity resulting from the leaching process. The implementation of the approach is first discussed. Then direct shear tests under constant normal stress are performed on a simple interface geometry and on a natural interface geometry. The results after Buzzi et al. [8] are corroborated by this research.