Article

Solute Exchange by Convection Within Estuarine Sediments

February 1996
Estuarine Coastal and Shelf Science 42(2):171-183

February 1996
42(2):171-183

DOI:10.1006/ecss.1996.0013

Authors:

The rates of material transport through sediments are central to determining the biochemical status of sediments. Transport within sediments can occur by molecular diffusion through porewaters and by the activities of burrowing biological organisms, but a number of physical processes can enhance transport within porewaters. Here, convection is investigated as a mechanism for solute transport within sediments. Convection will occur as the gravitational adjustment to the occurrence of porewater which is of lower salinity and density than the water at the sediment surface. This process could be important for exchange between sediments and the water column in regions subject to significant temporal variations in salinity such as estuaries

On the Stability of Density Stratified Flow Below a Ponded Surface

Article

Full-text available

Apr 2019
TRANSPORT POROUS MED

Flooding of coastal areas with seawater often leads to density stratification. The stability of the density-depth profile in a porous medium initially saturated with a fluid of density $\rho _\mathrm{f}$ after flooding with a salt solution of higher density $\rho _\mathrm{s}$ is analyzed. The standard convection/diffusion equation subject to the so-called Boussinesq approximation is used. The depth of the porous medium is assumed to be infinite in the analytical approaches and finite in the numerical simulations. Two cases are distinguished: the laterally unbounded ${{{\mathbf {{\small {\uppercase {case~A}}}}}}}$ and the laterally bounded ${{{\mathbf {{\small {\uppercase {case~B}}}}}}}$. The ratio of the diffusivity and the density difference $(\rho _\mathrm{s} - \rho _\mathrm{f})$ induced gravitational shear flow is an intrinsic length scale of the problem. In the unbounded ${{{\mathbf {{\small {\uppercase {case~A}}}}}}}$, this geometric length scale is the only length scale and using it to write the problem in dimensionless form results in a formulation with Rayleigh number $R = 1$. In the bounded ${{{\mathbf {{\small {\uppercase {case~B}}}}}}}$, the lateral geometry provides another length scale. Using this geometrical length scale to write the problem in dimensionless form results in a formulation with a Rayleigh number R given by the ratio of the geometric and intrinsic length scales. For both ${{{\mathbf {{\small {\uppercase {case~A}}}}}}}$ and ${{{\mathbf {{\small {\uppercase {case~B}}}}}}}$, the well-known Boltzmann similarity solution provides the ground state. Three analytical approaches are used to study the stability of this ground state, the first two based on the linearized perturbation equation for the concentration and the third based on the full nonlinear equation. For the first linear approach, the surface spatial density gradient is used as an approximation of the entire background density profile. This results in a crude estimate of the $L^2$-norm of the concentration showing that the perturbation at first grows, but eventually decays in time. For the other two approaches, the full ground-state solution is used, although for the second linear approach subject to the restriction that the ground state slowly evolves in time (the so-called frozen profile approximation). Just like the ground state, the resulting eigenvalue problems can be written in terms of the Boltzmann variable. The linearized stability approach holds only for infinitesimal small perturbations, whereas the nonlinear, variational energy approach is not subject to such a restriction. The results for all three approaches can be expressed in terms of Boltzmann $\sqrt{t}$ transformed relationships between the system Rayleigh number and perturbation wave number. The results of the linear and nonlinear approaches are surprisingly close to each other. Based on the system Rayleigh number, this allows delineation of systems that are unconditionally stable, marginally stable, or transiently unstable. These analytical predictions are confirmed by direct two-dimensional numerical simulations, which also show the details of the transient instabilities as function of the wave number for ${{{\mathbf {{\small {\uppercase {case~A}}}}}}}$ and the wave number and Rayleigh number for ${{{\mathbf {{\small {\uppercase {case~B}}}}}}}$. A numerical example of the effect of a layer with low permeability is also shown. Using typical values of the physical parameters, the analytical and numerical results are interpreted in terms of dimensional length and time scales. In particular, an explicit stability criterion is given for vertical column experiments.

On the stability of density stratified flow below a ponded surface: comprehensive version

Preprint

Full-text available

May 2018

Density stratified flow; Linear stability analysis; Energy method; Transient instabilities; Stability criteria

Stochastic Nature of Salt Mass Transport in Porous Media Under Unstable Conditions

Article

Full-text available

Jan 2017

ABSTRACT The two main transport mechanisms that occur simultaneously under unstable flow conditions are transport of saltwater from an overlying salt source to the porous media, and transport of salt through the porous media. These mechanisms were simultaneously studied through two fixed mass experiments conducted over 15 days. The transport through the porous media was also studied via three continuous injection experiments lasting between 5 to 29 days. There was no hydraulic gradient across the porous media in any of the experiments. Experiments were conducted in a 1-cm thick plexiglass rectangular sand column (1.70 m * 0.61 m * 0.61 m). The saline source concentration was 36 g/l, and the source heights were 4.5 cm. The sand porosity and hydraulic conductivity were 32% and 9.0 m/d, respectively. The rate of mass transport from the source to the porous media was observed by measuring the salt concentration within the source, while the salt transport through the porous media was documented by measuring breakthrough curves at five locations within the sand column. Fixed mass experiment results, using mass analysis, showed that the salt transport from the source to the porous media was deterministic since both experiments produced identical rates of mass transport from the source to the porous media, the salt transport through the porous media was stochastic since the observed breakthrough curves at the five locations were considerably different. The breakthrough curves measured in three identical continuous injection experiments were also very different supporting the results of the fixed mass experiments. The implications of these findings is that, under unstable conditions, one can predict the salt mass that would enter from a salt source into the underlying porous media with certainty, one cannot predict the rate or pattern of salt transport through the porous media itself.

Numerical Heat Transfer, Part A: Applications An International Journal of Computation and Methodology ISSN: (Print) ( Magnetized flow of naturally convective viscous fluid in permeable rhombus-shaped annulus by executing FEM simulations Magnetized flow of naturally convective viscous fluid in permeable rhombus-shaped annulus by executing FEM simulations

Article

May 2023

The present work is deliberated to investigate the flow and thermal behavior of viscous fluid flow in permeable rhombic-shaped enclosure. Annular region lying below inner and outer rhombuses is the area of concentration during this study. Magnetic field is employed in horizontal direction to envision aspects of Lorentz field. Governing equations are developed in dimensional form of partial differential setup. Numerical experiments via finite element approach are capitalized to find solution. Grid convergence test is executed to show domain distribution along with decision about mesh level at which computations are to conducted. Streamlines and isothermal maps are sketched to show comprehensive change in associated flow fields against governing parameters. Kinetic energy and average heat flux coefficient are evaluated in comparative manner for magnetic and hydrodynamic situations. Through attained outcomes, it is concluded that kinetic energy and heat flux coefficient enhances reduction against Hartmann number.

Thermosolutal natural convection across an inclined square enclosure partially filled with a porous medium

Article

Full-text available

Jan 2021

The present study is a numerical inspection of the natural convection with double diffusion in a tilted square cavity filled with a vertical bi-layered composed porous and an adjacent fluid. Isotropic and homogeneous porous layer is considered in this case study. The porous layer is saturated by an aqueous solution with a Prandtl number Pr = 7. The horizontal walls are considered impermeable and adiabatic, while uniform concentrations and temperatures are set on the vertical walls. The Thomas algorithm is employed to find a solution for the system of the governing equations. The divergence of the non-linear system is avoided by introducing under–relaxation factors. The effects of the various non-dimensional governing parameters namely the thickness of porous layer, Rayleigh number, buoyancy ratio, Lewis number, cavity inclination angle, and the thermal conductivity ratio on the thermo-solutal natural convection and the rate of mass and thermal exchange are highlighted and discussed. The predicted findings are given in terms of flow patterns, thermal fields, and iso-concentration lines. The variation of the velocity field, stream function, Sherwood number, and Nusselt number is also inspected. The numerical results show complex flow structure modifying the local transfer (Nusselt and Sherwood numbers).

Daily inundation induced seasonal variation in the vertical distribution of soil water salinity in an estuarine mangrove forest under a tropical monsoon climate

Article

Jun 2020

Previously, we revealed opposing patterns of the vertical distribution of soil water salinity between the dry and rainy seasons in an estuarine mangrove forest under a tropical monsoon climate. This study clarifies the causes of such seasonal variation in soil water salinity distribution. We investigated the salinity of inundating water, and diurnal changes in soil water salinity and water table levels. Freshwater inundated the study plot in the rainy season, while saline water inundated it in the dry season. At the Sonneratia , Avicennia and Rhizophora sampling sites, soil water salinity measured in experimental pits fluctuated diurnally. Mean salinity significantly decreased in the dry season but tended to increase in the rainy season. The effect of evaporation on salinity was negligible. Water table levels at all sites primarily fluctuated within 0–10 cm from the surface and were generally not synchronized with tidal movements. We divided the soil profile into a thin tide‐sensitive layer and an underlying aquifer layer based on soil water movement pattern. The inundating water saturated the tide‐sensitive layer and then gradually infiltrated into the aquifer layer. The water in the tide‐sensitive layer was partially replaced by subsequent inundation. The water in the aquifer layer was gradually desalinized or salinized, depending on the salinity level of the inundating water, although soil water from the former season remained still in deep soil horizons. Thus, seasonal variation of the vertical distribution of soil water salinity was induced by daily inundation and subsequent infiltration processes in the mangrove soil.

Occurrence of seasonal water replacement in mangrove soil and the trunk growth response of Avicennia alba related to salinity changes in a tropical monsoon climate

Article

May 2019

Salinity is a crucial factor regulating mangrove growth. We evaluated seasonal variations in soil water salinity and the water replacement process in a tropical monsoon mangrove forest of eastern Thailand during 2015–2018. Trunk growth of Avicennia alba was monitored monthly using dendrometer bands and was analyzed in relation to water replacement. Soil water salinity showed remarkable seasonal variation that was influenced by the infiltration of inundated water from the river, with a salinity level similar to that of seawater during the middle of the dry season and to that of fresh water during the middle of the rainy season. Patterns of soil water salinity shifted seasonally in both horizontal and vertical distributions, highlighting soil water replacement between the two seasons. In the middle of the rainy season, soil water salinity across the horizontal distribution was nearly fresh at most sampling points on the river side but gradually increased landward along a 120‐m transect. The vertical distribution of soil water salinity showed relatively low salinity at the surface horizon, which gradually increased downward to a 100‐cm depth. In the dry season, the soil salinity gradient in both distributions was opposite to that in the rainy season. This seasonal change in soil water salinity was significantly related to the trunk growth of A. alba, which increased greatly during the rainy season. Seasonal water replacement causes fluctuations in soil salinity and probably nutrient availability. The combination of low soil water salinity and large nutrient influx might enhance trunk growth during the rainy season. Tropical monsoon mangrove forest shows seasonal variations in river and soil salinity. Soil water salinity allocated differently in the horizontal and vertical distributions. Increases in A. alba trunk growth were influenced by seasonal water replacement.

Physical and Numerical Modeling of Unstable Flow Due to Heavier Saltwater Overlying Freshwater

Conference Paper

Full-text available

May 2016

It is well-known that unstable flows occur when denser saltwater overlies lighter freshwater. One such situation occurs in the Indian River Lagoon (IRL), a coastal estuary, located on the east coast of Florida. The IRL water is brackish as it is a mixture of the saltwater from the Atlantic Ocean flowing through inlets and the freshwater flowing from canals, rivers and groundwater. This brackish water overlies the freshwater that flows in the unconfined aquifer below the IRL. All cases of unstable density stratification result in a rapid and erratic redistribution of the salt. The rate of salt movement is much different than predicted by ordinary molecular diffusion alone. The dynamics of this unstable flow was investigated using a physical model made of Plexiglas which is 1.83m high and has a base of 0.61m by 0.61m. The model was filled with 1.64m of sand and the part above the sand, the source area, was used to add seawater or brackish water. The model consisting of five ports located at 0.255, 0.560, 0.865, 1.170, and 1.474m from the source was filled with 40F sand which was saturated with freshwater. This paper shows breakthrough curves from two experiments, conducted over five days and over 28 days in which the source concentration was kept constant at 36,000 mg/L at a depth of 4.5cm. Results indicate that the flow was unstable in the beginning (did not yield smooth breakthrough curves) but stabilized after approximately 23 days. Measured breakthrough curves were compared with numerically simulated curves using SEAWAT. It was found that the salt concentrations, under unstable conditions, can be predicted with reasonable accuracy if (1) the selected molecular diffusion coefficients are several orders of magnitude higher than values obtained through traditional experiments and (2) the selected molecular diffusion coefficients are time-dependent.

Sadat-Noori, 2016, Intermittently Closed and Open Lakes and Lagoons (ICOLLs) as groundwater-dominated coastal systems

Data

Feb 2016

Intermittently Closed and Open Lakes and/or Lagoons (ICOLLs) as groundwater-dominated coastal systems: Evidence from seasonal radon observations

Article

Feb 2016
J HYDROL

High-resolution permeability determination and two-dimensional pore water flow in sandy sediment

Article

Full-text available

Jan 2005
LIMNOL OCEANOGR-METH

A new, inexpensive method is proposed to measure permeability in natural sandy sediment with high spatial resolution. This methodology allows for a reconstruction of the vertical permeability anisotropy in natural sediments, with a depth resolution of a few millimeters. Thus, the possible intrusion depth of advective flow over the water-sediment interface of sandy sediments can be deduced. Shipboard measurements on five natural sandy sediment cores taken from North Sea sediments are used to demonstrate that both the direction and magnitude of the second-order permeability tensor can be calculated from direct measurements using this method. This presents a major improvement over previous methods particularly in the context of quantifying flow and reaction in permeable sediments. © 2005, by the American Society of Limnology and Oceanography, Inc.

Origin and becoming of organic matter in mangrove sediments along the coastline of French Guiana - Precursors, Deposition environment, Decay processes and Relationships with heavy metals –

Article

Dec 2003

Cyril Marchand

- Mme E. Lallier-Vergès, Directeur de recherche CNRS - M. R. Aller, Professeur, Stony Brook University (NY, USA) - M. D. Cossa, Directeur de recherche IFREMER - M. F. Fromard, Chargé de Recherche CNRS - M. T. Hevor, Professeur, Université d'Orléans - M. C. Largeau, Directeur de recherche CNRS

Rhythmic ammonium regeneration and flushing in intertidal sediments of Sado estuary

Article

Full-text available

Jul 1998

Carlos Rocha

The effects of exposure to the atmosphere on ammonium cycling in intertidal sediments were examined at a single site in the Sado estuary (Portugal) during a tidal cycle in November 1994. During a 9-h period, covering pre-ebb to postflood, 11 high-resolution vertical profiles of dissolved and sorbed ammonia and water content were collected. In addition, the vertical distribution of temperature was measured, primarily at the beginning and the end of the exposure period. The most dramatic changes occurred at the end of the exposed period when water flooded the sediment; -75% of the dissolved and sorbed NH;' pool (44.2 mmol m-*) was flushed into the water column by buoyancy-driven Porewater exchange. Some 64% of the flushed inventory (28.2 mmol m-*) was produced during the exposure period, at an average rate of 4.9 mmol NH, t m-* h I. The build-up of ammonium in the sediment was faster in the sorbed pool than in the dissolved pool. An average first-order carbon mineralization rate of 85.2 mmol C m * h-l during exposure was estimated from the ammonification rate. Residence times for the organic carbon and nitrogen pools were also calculated (37.5 and 43.4 d, respectively), giving indication of the rapidity of the turnover of organic matter possible in estuary intertidal sediments.

Pockmarks and associated fresh submarine groundwater discharge in the seafloor of Puck Bay, southern Baltic Sea

Article

Full-text available

Sep 2024
SCI TOTAL ENVIRON

This is the first well-documented report on the occurrence of pockmarks in Puck Bay. Pockmarks in the seafloor of Puck Bay were discovered during a hydroacoustic survey carried out in 2020. They are located at a depth of 25–27 m in the southwestern part of the bay. Significant depletion of chloride (Cl−) concentrations in sediment pore water was found within the depressions. Most likely, the formation of pockmarks was due to groundwater flow through the Miocene–Pleistocene system of aquifers, which extends from land to the bay area. One-dimensional modeling of vertical Cl− concentration profiles in pore water revealed the upward flow of freshened groundwater within the pockmarks. The magnitude of submarine groundwater discharge (SGD) was estimated to vary from 1.53·10−2 to 18·10−2 L·m−2·h−1. The effect of groundwater seepage was also observed at 3 cm above the seafloor within the pockmarks, which was identified as a decrease in salinity of approximately 0.12 PSU compared to reference sites. Furthermore, due to the effect of water advection, SGD can be detected even several meters above the seafloor as a decrease in salinity values within the thermocline layer.

Mapping the vulnerability of groundwater to saltwater intrusion from estuarine rivers under sea level rise

Article

Full-text available

Nov 2023
J HYDROL

Surface water bodies connected to the ocean such as estuarine rivers can act as pathways for saltwater intrusion (SI, i.e., the displacement of fresh groundwater by saltwater in an aquifer) far inland of the coast, presenting one of the earliest risks associated with relative sea level rise (SLR). However, SI vulnerability mapping approaches have largely focused on SI from the coast and do not consider estuarine surface water bodies, except for GALDIT SUSI (Kazakis et al., 2019); a weighted indexing approach designed for regional-scale mapping applications using a Geographic Information Systems (GIS) framework. However, GALDIT-SUSI is subjective in ranking the importance of factors that can contribute to SI and combining these into a vulnerability index. A less subjective approach to assessing SI vulnerability than weighted indexing methods involves the use of physically-based analytic solutions such as Strack (1976), but these have not been applied in a GIS framework or along estuaries and rivers previously. Here, these analytic solutions, surface water salinity, and surface water-groundwater freshwater head gradients are used in the development of a new SI vulnerability tier system. This new approach was applied in the low-lying coastal city of Ōtautahi Christchurch, Aotearoa New Zealand, along 70 km of coastal, estuary, and river margins under current sea level and SLR using GIS, then compared to GALDIT SUSI. The main advantage of the SI vulnerability tiers method is that it considers surface water behaviour, e.g., increased saltwater encroachment up rivers under SLR, which exposed new areas of the aquifer to saltwater and increased SI vulnerability upstream. In contrast, GALDIT-SUSI does not consider surface water conditions and accounts for topography and groundwater level as the main SI vulnerability drivers in this application. The SI vulnerability tiers proposed here are more theoretically robust than GALDIT-SUSI and provide a physically-based, large-scale, relatively low-budget and rapid screening tool to highlight areas most vulnerable to SI under current and future conditions for further monitoring and management; a gap of national and international relevance.

FEM analysis of the impact of surface undulations on the natural convective flow of viscous fluid in a permeable trapezoidal enclosure

Article

Full-text available

May 2023

Examination of the transport mechanism in a permeable trapezoidal enclosure with an undulation effect is commenced. A formulation describing naturally convective flow in a permeable domain is conceded by employing Boussinesq and Darcy approximations. Uniform temperature is provided at the circular cylinder and base wall of the enclosure, whereas non-parallel side extremities are kept cold. No heat flux condition is applied at a wavy surface (upper) to maintain the potential difference in temperature for generation of convection. A finite element scheme is opted to resolve the governing system for accounted physical problems. The grid sensitivity test is also executed to assure the credibility of the code and results. A wide range of physical parameters is selected to comprehend their impact on streamlines and isotherm patterns. Results are revealed comparatively for zero undulation (upper solid straight wall) and with undulations (wavy wall). Heat flux and kinetic energy are also enumerated as key quantities against concerning parameters. It is depicted that the average Nusselt number and kinetic energy are more in the absence of undulations than when it is present. Additionally, it is manifested that the placement of a heated cylinder helps transfer heat in the domain and the production of thermal convective potential.

Saltwater intrusion from an estuarine river: A field investigation

Article

Dec 2022
J HYDROL

Estuarine rivers provide critical pathways for seawater to travel upstream of the coast and salinize adjacent aquifers. However, this salinization mechanism (forthwith termed riparian saltwater intrusion) has received relatively little attention compared to saltwater intrusion (SI) at the coast. Time series measurements of river and groundwater freshwater head and specific conductance (SC), as well as horizontal river-aquifer hydraulic gradient were collected at transects of piezometers perpendicular to an estuarine river in Ōtautahi Christchurch, Aotearoa New Zealand. The uncertainties of freshwater head and hydraulic gradient were estimated using error propagation methods. Discrete Fourier Transforms were applied to river and groundwater freshwater head and SC time series data, which confirmed the tidal influence in both systems. Cross–correlation analyses of river and groundwater freshwater heads showed very strong relationships with varying time lags. Hydraulic gradients and river SC fluctuated with tides, resulting in the alternation of SI (increase in groundwater SC) and saltwater retreat (decrease in groundwater SC) with various time delays that may be driven by cyclic flow processes. While riparian SI occurred in most piezometers, the hydraulic gradient on the outside of the river meander was steeper than on the inside of the river meander resulting in less SI. Although positive hydraulic gradient (aquifer to river flow direction) occurred most of the time at all sites, land subsidence and climate change conditions of sea–level rise, increased drought, and decreased river flows could increase the occurrence of negative hydraulic gradient, which may result in increased groundwater salinization.

Measure and evaluate the hydrothermal flow of a Newtonian fluid in homogeneous permeable media equipped with a fin: A numerical approach

Article

Full-text available

Nov 2022

This study envisions the hydrothermal characteristics of a viscous fluid in a homogenously permeable hexagonal enclosure. Permeability aspects in the flow domain are described by employing the Brinkman-extended Darcy law. A corrugated hexagonal enclosure along with the placement of a star-shaped fin is taken into account. Heated rectangular blocks at horizontal extremities are installed, and sliding sides of the enclosure are considered to be cold to provide convective potential to the flow. In addition, adjoining portions of the heated rectangular blocks are supposed to be adiabatic. The dimensionless governing equations of the resultant problem are derived initially and then solved numerically by implementing the Galerkin finite element approach, and COMSOL is obliged. For this purpose, first, domain discretization is demonstrated in view of 2D elements by performing hybridized meshing. Then, the system of non-linear equations is resolved by a non-linear solver (PARADISO). The grid convergence test is performed to confirm the credibility of the carried out simulations by calculating the average Nusselt number at different refinement levels. A change in associated distributions against the involved physical parameters (Darcy number (Da), Rayleigh number (Ra), and Prandtl number (Pr)) for a wide range is revealed through graphs and tables. Quantities like kinetic energy and heat flux (local and average) are also evaluated through concerned parameters. The results clearly demonstrate that the Darcy number tends to reduce the heat transfer rate. In particular, it is depicted that by increasing the Rayleigh number (Ra), strengthening in the temperature potential arises in the system, thereby magnifying the heat transfer rate. Moreover, it is disclosed that by reducing the Darcy number, kinetic energy shows a decreasing trend.

Behavior of Li, S and Sr isotopes in the subterranean estuary and seafloor pockmarks of the Hanko submarine groundwater discharge site in Finland, northern Baltic Sea

Article

Full-text available

Sep 2022
APPL GEOCHEM

This study investigated the behavior of 87Sr/86Sr, δ7Li and δ34S in the STE and three seafloor pockmarks with different degrees of groundwater influence, as constrained based on δ2H and δ18O, at the Hanko SGD site in Finland, in the northern Baltic Sea. These data were supplemented by groundwater and seawater measurements.

Mixed-Convective Processes Within Seafloor Sediments Arising From Fresh Groundwater Discharge

Article

Full-text available

Apr 2021

The dependence of near-shore ecosystems on the freshwater component of submarine groundwater discharge (SFGD) is well recognized. Previous studies of SFGD have typically assumed that SFGD occurs through aquitards that are in direct contact with seawater. These studies provide no guidance on the distribution of freshwater discharge to the seafloor where SFGD occurs through sandy sediments, even though in most situations, seabed sediments are permeable. We find that SFGD may occur in unconfined, seafloor sediments as density-driven flow in the form of fingers, or otherwise, diffusive freshwater discharge is also possible. Unstable, buoyancy-driven flow within seabed sediments follows similar patterns (except inverted) to the downward free convection of unstable (dense over less-dense groundwater) situations. Consequently, the same theoretical controlling factors as those developed for downward mixed-convective flow are expected to apply. Although, there are important differences, in particular the boundary conditions, between subsea freshwater-seawater interactions and previous mixed-convective problems. Simplified numerical experiments in SEAWAT indicate that the behavior of fresh buoyant plumes depends on the aquifer lower boundary, which in turn controls the rate and pattern of SFGD to the seafloor. This article provides an important initial step in the understanding of SFGD behavior in regions of sandy seafloor sediments and analyses for the first time the mixed-convective processes that occur when freshwater rises into an otherwise saline groundwater body.

Estimation of Diffusion Coefficients for Salt Transport in Porous Media under Unstable Conditions Using Numerical Modeling

Conference Paper

May 2020

Missing links in the study of solute and particle exchange between the sea floor and water column

Article

Full-text available

Mar 2020

factors, which are identified in this review and may be moderated by others, such as currents, granulometry, nutrient and matter inputs, as well as living organisms. In turn, the occurrence of exchanges can influence adjacent environments and organisms. Major gaps in the present knowledge include the temporal and spatial variation in many of the processes driving benthic/pelagic exchange processes and the variability in the relative importance of individual processes caused by this variation. Furthermore the accurate assessment of some anthropogenic impacts is deemed questionable due to a lack of baseline data, and long-term effects of anthropogenic actions are often unknown. It is suggested that future research should be transdisciplinary and at ecosystem level wherever possible, and that baseline surveys should be implemented and long-term observatories established in order to fill the current knowledge gaps.

Effects of Tidally Varying Salinity on Groundwater Flow and Solute Transport: Insights From Modelling an Idealized Creek Marsh Aquifer

Article

Full-text available

Nov 2019
WATER RESOUR RES

Most existing numerical research on tide‐induced groundwater dynamics assume a constant surface water salinity on the seaward boundary (constant salinity case). Few studies have investigated the influence of tidally‐varying salinity on shallow groundwater dynamics in coastal aquifers (tidal salinity case). We compiled field observations of tidally‐varying salinity in multiple estuaries across the eastern coast of China and a tidal creek in North inlet‐Winyah Bay, the USA. Numerical simulations were then conducted to explore the effect of tidally‐varying salinity on groundwater flow and salt transport in an idealized creek‐marsh aquifer. Results showed that the upper saline plume and classical saltwater wedge appeared in all cases, but the salinity in the saltwater wedge was diluted in the tidal salinity cases. Notably, groundwater transit times were shorter in the tidal salinity case than in the constant salinity case, especially under the creek bottom. Quantitative analyses indicated that tidally‐varying salinity significantly enhanced surface water‐groundwater exchange, increasing submarine groundwater discharge by 10% and the total inflow of surface water across the water‐sediment interface by 7%. As the density of groundwater differs from that of the overlying surface water, fingered saltwater flow formed in sediments under the creek bottom, leading to some small local water circulation cells. These small cells reduced groundwater transit times, and almost doubled the water exchange rate. Coupling the density‐dependent flow to a simplified nitrogen reaction network revealed that the tidally‐varying salinity may have the potential to influence nitrogen biogeochemical transformations that modify nitrogen loads prior to discharge.

Analysis of Submarine Groundwater Discharge to Manila Bay: 3D Density Dependent Hydrogeological model of the SE coastal zone of Bataan, Philippines

Thesis

Full-text available

Aug 2010

Aileen Robert

A regional scale modular variable-density groundwater flow model (MOCDENS3D) has been used to estimate the magnitude of submarine groundwater discharge to Manila Bay. The model area is located at the south-eastern flank of Mt. Mariveles volcano in Bataan Peninsula, Philippines. The area stretches on 12 km of coastline where preliminary SGD flux measurements were made earlier. The hydrogeologic properties of these sediments are estimated using information from pumping tests, borehole data, seismic sections and geological maps. Key features of the groundwater system include high relief in the upper slopes and narrow low relief coastal plain. The model domain incorporates both the terrestrial recharge and also the re-circulated water in the coastal sediments. Different scenarios were run to evaluate the sensitivity of SGD to changes in different input parameters. SGD rates peak during the rainy season and there’s a one month delay to the peak rainfall. SGD shows major influence of rainfall (recharge), geology and topography. Model calculations suggest that there is substantial contribution of freshwater from terrestrial origin which discharges to the bay. The presence of confining layers greatly affects the offshore extent of SGD, and therefore its potential impact on the regional environment. Model results are consistent with previously measured SGD rates using seepage meters and geochemical tracers by Taniguchi et al. (2008).

Geophysical characterisation of the groundwater-surface water interface

Article

Sep 2017
ADV WATER RESOUR

Interactions between groundwater (GW) and surface water (SW) have important implications for water quantity, water quality, and ecological health. The subsurface region proximal to SW bodies, the GW–SW interface, is crucial as it actively regulates the transfer of nutrients, contaminants, and water between GW systems and SW environments. However, geological, hydrological, and biogeochemical heterogeneity in the GW–SW interface makes it difficult to characterise with direct observations. Over the past two decades geophysics has been increasingly used to characterise spatial and temporal variability throughout the GW–SW interface. Geophysics is a powerful tool in evaluating structural heterogeneity, revealing zones of GW discharge, and monitoring hydrological processes. Geophysics should be used alongside traditional hydrological and biogeochemical methods to provide additional information about the subsurface. Further integration of commonly used geophysical techniques, and adoption of emerging techniques, has the potential to improve understanding of the properties and processes of the GW–SW interface, and ultimately the implications for water quality and environmental health.

Natural Convection Flow of a Nanofluid in an Inclined Square Enclosure Partially Filled with a Porous Medium

Article

Full-text available

May 2017

This work analyses free convection flow of a nanofluid in an inclined square enclosure consisting of a porous layer and a nanofluid layer using the finite difference methodology. Sinusoidal temperature boundary conditions are imposed on the two opposing vertical walls. Nanofluids with water as base and Ag or Cu or Al2O3 or TiO2 nanoparticles are considered for the problem. The related parameters of this study are the Darcy number, nanoparticle volume fraction, phase deviation, amplitude ratio, porous layer thickness and the inclination angle of the cavity. A comparison with previously published work is performed and the results are in good agreement. Detailed numerical data for the fluid flow and thermal distributions inside the square enclosure, and the Nusselt numbers are presented. The obtained results show that the heat transfer is considerably affected by the porous layer increment. Several nanoparticles depicted a diversity improvement on the convection heat transfer.

Darcian Natural Convection in Inclined Square Cavity Partially Filled Between the Central Square Hole Filled with a Fluid and Inside a Square Porous Cavity Filled with Nanofluid

Article

Full-text available

Jun 2016

The problem of darcian natural convection in inclined square cavity partially filled between the centralsquare hole filled with fluid and inside a square porous cavity filled with nanofluid is numericallystudied using the finite element m ethod. The left vertical wall is maintained at a constant hot temperatureTh and the right vertical wall is maintained at a constant cold temperature Tc, while the horizontalwalls are adiabatic. The governing equations are obtained by applying the Darcy model andBoussinesq approximation. COMSOL's finite element method is used to solve the non-dimensionalgoverning equations together with the specified boundary c onditions. The governing parameters ofthis study are the Rayleigh number (103≤Ra≤107), the Darcy number (10-5≤Da≤10-3), thefluid layer thickness (0.4≤S≤0.8) and the inclination angle of the cavity (0°≤ω≤6 0°). Theresults for the values of the governing parameters in terms of the streamlines, isotherms and averageNusselt number will be presented. The convection is shown to be inhibited by the presence of thehole insert. The thermal property of the insert and the size have opposite influence on the convection.The results have possible applications in heat-removal and heat-storage fluid-saturated poroussystems.

Using a Thermal Proxy to Examine Sediment–Water Exchange in Mid-Continental Shelf Sandy Sediments

Article

Full-text available

Dec 2016
AQUAT GEOCHEM

Fluid exchange across the sediment–water interface in a sandy open continental shelf setting was studied using heat as a tracer. Summertime tidal oscillation of cross-shelf thermal fronts on the South Atlantic Bight provided a sufficient signal at the sediment–water interface to trace the advective and conductive transport of heat into and out of the seabed, indicating rapid flushing of ocean water through the upper 10–40 cm of the sandy seafloor. A newly developed transport model was applied to the in situ temperature data set to estimate the extent to which heat was transported by advection rather than conduction. Heat transported by shallow 3-D porewater flow processes was accounted for in the model by using a dispersion term, the depth and intensity of which reflected the depth and intensity of shallow flushing. Similar to the results of past studies in shallower and more energetic nearshore settings, transport of heat was greater when higher near-bed velocities and shear stresses occurred over a rippled bed. However, boundary layer processes by themselves were insufficient to promote non-conductive heat transport. Advective heat transport only occurred when both larger boundary layer stresses and thermal instabilities within the porespace were present. The latter process is dependent on shelf-scale heating and cooling of bottom water associated with upwelling events that are not coupled to local-scale boundary layer processes.

Dynamics of nutrient cycling and related benthic nutrient and oxygen fluxes during a spring phytoplankton bloom in South San Francisco Bay (USA)

Article

Full-text available

May 2000

Benthic oxygen uptake and nutrient releases of N, P and Si were measured weekly at 2 sites in South San Francisco Bay around the 1996 spring bloom. Exchanges across the sediment-water interface were estimated from whole, care incubations performed in the laboratory at in situ temperature and in dark. Fluxes changed significantly on a weekly time scale. Over a period of 15 wk the fluxes of dissolved inorganic N, P and Si ranged from -40 to +200, 0 to 13 and from 30 to 400 mu mol m(-2) h(-1) respectively. Sediment oxygen demand increased from 10 before to 64 mg O-2 m(-2) h(-1) just after the bloom period. During the bloom, nutrient flutes represented about 20, 16 and 9% of the Si, P and N requirements for primary production. Before and after the bloom period, Si fluxes contributed up to 30 and >100% of this requirement and P and N fluxes up to 15 and 50% respectively. Simple empirical models explain most of the spatial-temporal variability of benthic fluxes of Si, P and NH4 (but not NO3) from 3 predictor variables: sediment porosity, nutrient concentration in bottom waters and chlorophyll content of surficial sediments. These models show that algal blooms influence benthic-pelagic nutrient exchange through 2 processes: (1) depletion of nutrients from the water column (which enhances gradient-driven transports across the sediment-water interface) and (2) sedimentation of labile phytodetritus (which promotes remineralization in or on the surficial sediments). Rates and patterns of nutrient cycling were very different at the shallow and deep study sites, illustrating the challenge of extrapolating measurements of coupled algae-nutrient dynamics to whole ecosystems.

Evaluating Submarine Groundwater Discharge to the Mediterranean Sea by using Radium isotopes

Thesis

Full-text available

Nov 2014

Valentí Rodellas

Submarine groundwater discharge (SGD) is defined as any flow of water across the continental margin from the seabed to the coastal ocean, including fresh meteoric groundwater and seawater recirculating through coastal aquifers. SGD has been recognized as a major component of the hydrological cycle and a significant source of various dissolved terrestrial compounds (e.g. nutrients, trace metal, carbon, contaminants) to the coastal ocean. These fluxes of chemical elements via SGD may have a profound impact on the biogeochemical cycles of the receiving water bodies. This can be especially relevant in oligotrophic and semi-arid regions, such as the Mediterranean Sea. However, and despite the potential importance of SGD in regulating coastal biogeochemical cycles of the Mediterranean Sea, there is still a lack of detailed assessments on the relevance of SGD as a source of chemical constituents into this basin. Indeed, the magnitude of SGD to the entire Mediterranean basin and its associated fluxes of dissolved compounds have never been evaluated. The main objective of this PhD Thesis is to evaluate the importance of SGD in the Mediterranean Sea by using radium (Ra) isotopes, paying attention to the role that SGD plays as a source of dissolved chemical compounds to the sea and to the use of Ra isotopes as SGD tracers. To this aim, contrasting Mediterranean coastal environments were selected, including: i) a coastal wetland nourished by groundwater inflowing from several aquifers (Peníscola marsh, Castelló); ii) a semi-enclosed embayment highly influenced by bottom sediments (Port of Maó, Minorca, Balearic Islands); and iii) a detrital bay open to the sea (Palma Bay, Majorca, Balearic Islands). Aside from these three specific sites, the first appraisal of the magnitude of SGD into the entire Mediterranean Sea was also conducted, demonstrating its significance as a source of dissolved compounds in a basin-wide scale. Results from these studies provide new insights into the use of Ra isotopes as tracers to quantify SGD and underline their suitability in a wide range of Mediterranean hydrogeological settings. We successfully applied them to estimate SGD-driven fluxes of dissolved nutrients and, for the first time, trace metals to a coastal Mediterranean area, stressing the role SGD may play as a source of these constituents to the marine environment. We show that SGD is a volumetrically important process in the Mediterranean Sea, contributing up to (0.2–4.3)·10^12 m3/yr, a magnitude that is significantly larger than riverine discharge. SGD also represents a major source of dissolved nutrients to the basin, rivaling the conventional external sources (i.e. atmospheric deposition and river discharge). This new understanding of the magnitude of SGD and its associated chemical fluxes demonstrates the profound implications of SGD in the biogeochemical cycles of the Mediterranean Sea, emphazising the need for its consideration in coastal and basin-wide studies.

The salt wedge pump: Convection-driven pore-water exchange as a source of dissolved organic and inorganic carbon and nitrogen to an estuary

Article

Sep 2012

Hypoxia and anoxia in coastal waters have typically been explained by the respiration of sinking organic matter associated with nutrient over-enrichment and phytoplankton blooms. Here, we assess whether submarine groundwater discharge and seawater recirculation in sediments can explain widespread chemical anomalies, including low dissolved oxygen, in salt wedge estuaries. We rely on high-resolution radon (a natural groundwater and pore-water tracer), and dissolved carbon concentrations and stable isotope observations in the Yarra River estuary in Melbourne, Australia. Radon was highly enriched within the salt wedge, demonstrating enhanced pore-water exchange at this area. We use the term "salt wedge pump" to describe convection-driven advective pore-water exchange at the sediment-water interface during the upstream propagation of the salt wedge. Radon-derived convection-driven pore-water exchange rates within the salt wedge were estimated at 2.8 cm d(-1), a value equivalent to 2.4% of the total river freshwater runoff to the estuary. Pore-water exchange led to pulsed dissolved inorganic carbon (DIC) and ammonium fluxes similar to 10-fold higher than measured diffusive fluxes. In contrast, diffusive sediment oxygen uptake was 5-fold higher than oxygen uptake related to advective pore-water exchange. Estimated fluxes, associated with the nonconservative DIC, delta C-13-DIC, and ammonium behavior within the estuary support convective pore-water exchange as a major source of DIC and ammonium to the estuary, but not of dissolved organic carbon, nitrate, dissolved organic nitrogen, and anoxia. Accounting for seawater recirculation in sediments may help reconcile unbalanced carbon and nitrogen budgets in several coastal systems.

Geochemistry of Redox-Sensitive Trace Elements in a Shallow Subterranean Estuary

Article

Mar 2015
MAR CHEM

Unsteady Natural Convection in a Square Cavity Partially Filled with Porous Media Using a Thermal Non-Equilibrium Model

Conference Paper

Full-text available

Jan 2015

Sulfate reduction and sulfide oxidation in extremely steep salinity gradients formed by freshwater springs emerging into the Dead Sea

Article

Oct 2014
FEMS MICROBIOL ECOL

Abundant microbial mats, recently discovered in underwater freshwater springs in the hypersaline Dead Sea, are mostly dominated by sulfur oxidizing bacteria. We investigated the source of sulfide and the activity of these communities. Isotopic analysis of sulfide and sulfate in the spring water showed a fractionation of 39-50 ‰ indicative of active sulfate reduction. Sulfate reduction rates (SRR) in the spring sediment (<2.8 nmol cm−3 d−1) are too low to account for the measured sulfide flux. Thus, sulfide from the springs, locally reduced salinity and O2 from the Dead Sea water are responsible for the abundant microbial biomass around the springs. The springs flow is highly variable and accordingly the local salinities. We speculate that the development of microbial mats dominated by either Sulfurimonas/Sulfurovum-like or Thiobacillus/Acidithibacillus-like sulfide oxidizing bacteria, results from different mean salinities in the microenvironment of the mats. SRR of up to 10 nmol cm−3 d−1 detected in the Dead Sea sediment are surprisingly higher than in the less saline springs. While this shows the presence of an extremely halophilic sulfate reducing bacteria community in the Dead Sea sediments, it also suggests that extensive salinity fluctuations limit these communities in the springs due to increased energetic demands for osmoregulation.This article is protected by copyright. All rights reserved.

Hyporheic flow and transport processes

Data

Oct 2014

10.1002 2012RG000417

Data

Oct 2014

Variations in turbulent kinetic energy at a point source submarine groundwater discharge in a reef lagoon

Article

Full-text available

Sep 2014

The influence of sea level variations due to tides and wave setup on turbulent kinetic energy (TKE) was observed at a point source submarine groundwater discharge in a fringing coral reef lagoon. Tidal and wave setup variations modulated speed, TKE, TKE dissipation, and water temperature and salinity at the buoyant jet. The primary driver of jet TKE and speed variations was tides, while wave setup was a minor contributor. An inverse relationship between surface elevation and TKE was explained with an exponential equation based on sea level variations. During low tides, peak jet speeds (up to 0.3 m s(-1)) and TKE per unit mass (up to 0.4 m(2) s(-2)) were observed. As high tide approached, the jet produced minimum TKE of similar to 0.003 m(2) s(-2) and TKE dissipation ranged from 2 to 8x10(-4) m(2) s(-3). This demonstrated the sensitivity of the jet discharge to tides despite the small tidal range (< 20 cm). Jet temperatures and salinities displayed semidiurnal oscillations with minimum salinity and temperature values during maximum discharge. Jet salinities increased throughout low tides while temperatures decreased. This pattern suggested the jet conduit was connected to a stratified cavity within the aquifer containing cool fresh water over cool salty water. As low tides progressed, jet outflow increased in salinity because of the mixing within the conduit, while lower jet temperatures suggested water coming from further or deeper in the aquifer. The presence of such a cavity has been recently confirmed by divers.

Hyporheic flow and transport processes: Mechanisms, models, and biogeochemical implications

Article

Aug 2014
REV GEOPHYS

Fifty years of hyporheic zone research have shown the important role played by the hyporheic zone as an interface between groundwater and surface waters. However, it is only in the last two decades that what began as an empirical science has become a mechanistic science devoted to modeling studies of the complex fluid dynamical and biogeochemical mechanisms occurring in the hyporheic zone. These efforts have led to the picture of surface-subsurface water interactions as regulators of the form and function of fluvial ecosystems. Rather than being isolated systems, surface water bodies continuously interact with the subsurface. Exploration of hyporheic zone processes has led to a new appreciation of their wide reaching consequences for water quality and stream ecology. Modern research aims toward a unified approach, in which processes occurring in the hyporheic zone are key elements for the appreciation, management, and restoration of the whole river environment. In this unifying context, this review summarizes results from modeling studies and field observations about flow and transport processes in the hyporheic zone and describes the theories proposed in hydrology and fluid dynamics developed to quantitatively model and predict the hyporheic transport of water, heat, and dissolved and suspended compounds from sediment grain scale up to the watershed scale. The implications of these processes for stream biogeochemistry and ecology are also discussed.

Influence of porewater exchange on nutrient dynamic in two New Zealand estuarine intertidal flats

Article

May 2014
MAR CHEM

Submarine groundwater discharge revealed by radium isotopes (Ra-223 and Ra-224) near a paleochannel on the Southern Brazilian continental shelf

Article

Full-text available

Jul 2013
BRAZ J OCEANOGR

Submarine Groundwater Discharge (SGD) has been recognized as an important component of the ocean-continent interface. The few previous studiesin Brazil have focused on nearshore areas. This paper explores SGD on the Southern Brazilian Continental Shelf using multiple lines of evidence that include radium isotopes, dissolved nutrients, and water mass observations. The results indicated that SGD may be occurring on the Continental Shelf in the Albardão region, near a paleochannel located 50 km offshore. This paleochannel may thus be a preferential pathway for the delivery of nutrient- and metal-enriched groundwater and porewater into continental shelf waters.

Groundwater‐surface water interactions in a river estuary: insights from hydraulic, thermal and geophysical observations

Article

Sep 2021

This study presents the groundwater flow and salinity dynamics along a river estuary, the Werribee River in Victoria, Australia, at local and regional scales. Along a single reach, salinity across a transverse section of the channel (~80 m long) with a point bar was monitored using time-lapse electrical resistivity (ER) through a tidal cycle. Groundwater fluxes were concurrently estimated by monitoring groundwater levels and temperature profiles. Regional porewater salinity distribution was mapped using 6-km long longitudinal ER surveys during summer and winter. The time-lapse ER across the channel revealed a static electrically resistive zone on the side of the channel with a pronounced cut bank. Upward groundwater flux and steep vertical temperature gradients with colder temperatures deeper within the sediment suggested a stable zone of fresh groundwater discharge along this cut bank area. Generally less resistive zones were observed at the shallow portion of the inner meander bank and at the channel center. Subsurface temperatures close to surface water values, vertical head gradients indicating both upward and downward groundwater flux, and higher porewater salinity closer to that of estuary water suggest strong hyporheic circulation in these zones. The longitudinal surveys revealed higher ER values along deep and sinuous segments and low ER values in shallow and straighter reaches in both summer and winter; these patterns are consistent with the local channel-scale observations. This study highlights the interacting effects of channel morphology, broad groundwater-surface water interaction, and hyporheic exchange on porewater salinity dynamics underneath and adjacent to a river estuary. This article is protected by copyright. All rights reserved.

Experimental and Numerical Study of Salt Transport under Unstable Conditions

Conference Paper

Jun 2021

Nitrogen and Phosphorus Export After Flooding of Agricultural Land by Coastal Managed Realignment

Article

Jun 2020

Climate-driven sea level rise has severe consequences for drained agricultural areas near coasts. The least productive of these can be restored into marine wetlands of high ecological quality by managed realignment. This study assessed the nitrogen (N) and phosphorus (P) balance in a 214 ha coastal lagoon formed after flooding of agricultural land by managed realignment. N and P loss from the soils was monitored over a 5-year period after flooding using three independent approaches: (1) temporal changes in N and P inventories of the soil; (2) flux of dissolved inorganic N and P from the flooded soil; and (3) tidal N and P exchange across the outer boundary in the form of particulate and dissolved nutrients. All three approaches showed similar initial release and tidal export of N and P the first year(s) after flooding followed by decreasing rates. The annual loss ranged from 157 to 299 kg N ha−1 yr−1 and 29 to 63 kg P ha−1 yr−1 during the first year. N loss decreased rapidly after the first 2 years and reached a level of 28–65 kg N ha−1 yr−1, while P loss declined after the first year and remained stable and relatively high at 18–32 kg P ha−1 yr−1 thereafter. High N and P export after implementing managed realignment of agricultural land may deteriorate environmental conditions in the adjacent marine recipients for at least 5 years. Particularly small and stagnant water bodies vulnerable to eutrophication should be avoided as recipients.

Numerical model on the flow dynamics around the sediment-water interface in the tidal coastal area

Article

Jun 2017

Investigating flow dynamics around the sediment-water interface is one of difficult issues in marine environment. It has important significance to study the physical, chemical and biological activities between the overlying water and sedimentary layer with tidal forcing. In this study, we established a numerical model to investigate the flow dynamics around the sediment-water interface in the tidal coastal area. The model reflected the hydrodynamics of periodic reciprocating unsteady flow in the overlying water layer and the fully coupled simulation of flow movement between the overlying water layer and sedimentary layer. A sedimentary layer, an overlying water layer, and an air layer were all treated as the fluid zone. The unsteady Reynolds-averaged Navier-Stokes equations, and the Reynolds stress model with porosity, were solved by the finite volume method. Moreover, the drag source term in the momentum equation was modified as the Darcy-Forchheimer extended form. The model showed a strong performance in simulating the hydrodynamics of coupled flow around the sediment-water interface with tidal forcing in a coastal area. With considering the sedimentary layer, the benthic boundary layer velocity distributions were more accurate than those obtained by a wall function model. Around the sediment-water interface, there was inertial loss in the flow. Furthermore, velocity increased with increasing porosity, and velocity gradient became larger. Compared with models with Darcy's Law, the numerical model in this study had better performance in the turbulent characteristics of sediment-water interface layer. The model can lead to better understanding of the exchange mechanisms of oxygen, nitrogen and nutrient between overlying water and sediment.

Physical and Chemical Factors to Consider when Studying Historical Contamination and Pollution in Estuaries

Chapter

Full-text available

Feb 2017

Understanding the history of contamination at a site may provide useful information to interpret past conditions. While organic compounds, such as pesticides, may behave quite differently in the environment compared to inorganic substances, such as metals, one common feature is that for different reasons, sediments often act as a common sink. In this sense sites with a history of deposition and little reworking are of interest to both the palaeo-environmental scientists and pollution scientists. Estuaries are often areas of significant deposition and are attractive to the historical study of anthropogenic inputs, however, sediments are also subject to a wide range of physicochemical conditions (from fresh to marine water) that fluctuate both in space and time. Changing water and sediment geochemistry influences metal binding capacity and flocculation of fine particles and for these reasons estuaries are challenging for the pollution scientist. However, methods of sediment characterization, sample preparation, and analysis have been developed over time to help understand the geochemistry that influences sources and sinks of contaminants, and their pathways through the environment. This chapter provides an extensive review of sediment contaminant characterization, including the sources, pathways and fate of contaminants in estuaries. It details analytical procedures and explores considerations when interpreting results. It has a focus on Australian estuaries but is relevant to estuaries around the world. The main aims of this review were to provide multidisciplinary researchers with a tool to further their inquiry and to encourage further studies of pollution/contamination in estuaries.

Resuspension and estuarine nutrient cycling: insights from the Neuse River Estuary

Article

Full-text available

Apr 2010
BGD

D. Reide Corbett

For at least the past several decades, North Carolina's Neuse River Estuary (NRE) has been subject to water quality problems relating to increased eutrophication. Research studies initiated in the past several years have addressed the complex nutrient cycles in this system. Most of this research, however, is concerned with the nutrient processes of the water column and the passive diffusion processes of the benthic sedimentary environment. Resuspension of bottom sediments, by bioturbation, tides, or wind-generated waves, may have a significant effect on the flux of nutrients in an estuarine system These processes can result in the advective transport of sediment porewater, rich with nitrogen, phosphorus and carbon, into the water column. Thus, estimates of nutrient and carbon inputs from the sediments may be too low. This study focused on the potential change in porewater and bottom water nutrient concentrations associated with measured resuspension events. Previous research used short-lived radionuclides and meteorological data to characterize the sediment dynamics of the benthic system of the estuary. These techniques in conjunction with the presented porewater inventories allowed evaluation of the depth to which sediments have been disturbed and the advective flux of nutrients to the water column. The largest removal episode occurred in the lower NRE as the result of a wind event and was estimated that the top 2.2 cm of sediment and corresponding porewater were removed. NH4+ advective flux (resuspended) was 2 to 6 times greater than simply diffusion. Phosphate fluxes were estimated to be 15 times greater than the benthic diffusive flux. Bottom water conditions with elevated NH4+ and PO43− indicate that nutrients stored in the sediments continue to play an important role in overall water quality and this study suggests that the advective flux of nutrients to the water column is critical to understand estuarine nutrient cycling.

Driving while under the influence: Pumping-driven circulation under the influence of regional groundwater flow

Article

Jan 2007

Regional submarine or ambient groundwater discharge (AGD) along sea coasts interacts with current-driven interfacial pumping of seawater through shallow sediments. We use numerical simulations to investigate this interaction for a turbulent current flowing over dune topography. AGD reduces the extent of the current-topography driven interfacial exchange zone (IEZ) and may prevent its development when AGD overpowers interfacial exchange. Under upwelling-AGD conditions (upward flux of deep ground-water into the water-column), the IEZ is centred on the stoss face of bedforms and AGD occurs near the crest. Under downwelling-AGD conditions (downward flux of seawater deep into the aquifer), the IEZ forms around the crest and water infiltrating along the stoss face into the sediments does not return to the sediment-water interface. The IEZ depth, flux and residence time are functionally related to current Reynolds number (Re). For example, the IEZ water residence time, which follows a power-law distribution, is larger at lower current Re.

Seasonal Pattern of the Biogeochemical Properties of Mangrove Sediments Receiving Shrimp Farm Effluents (New Caledonia)

Article

Full-text available

Jul 2014

Coastal tropical shrimp farming may impact the adjacent ecosystems through the release of large quantities of effluents rich in nutrients. In New Caledonia, mangroves are considered as a natural biofilter to reduce impacts on the surrounding World Heritage listed lagoon. Our main objective was to understand the influence of effluent discharge on the biogeochemistry of mangrove sediments. A monitoring of the physico-chemical parameters of mangrove sediments was carried out during a whole year, including active and non active periods of the farm. The parameters studied were: i) benthic primary production (Chl-a concentrations), ii) physico-chemical parameters of sediments (redox potential, pH, salinity, TOC, TN, TS, δ13C and δ15N), iii) concentrations of dissolved nitrogen, iron and phosphorus. A mangrove developing in the same physiographic conditions, presenting the same zonation, and free of anthropogenic input was used as reference. The concentration of benthic Chl-a measured at sediment surface in the effluent receiving mangrove was twice to three times that measured in the control zone whatever the season. We thus suggest that nutrients inputs significantly increased the phytobenthic production in the effluent receiving mangrove during the whole year, even after the cessation of discharges and because of natural seasonal dynamic of phytobenthos. Although the flow of surface OM was increased, the OM content at depth was not higher than in the control mangrove. However, the contribution of mangrove detritus to the sedimentary organic pool was higher probably as a result of higher density and much greater individual size of the mangrove trees. Unlike the control mangrove sediment, the effluent receiving mangrove sediment was not stratified, redox potential values were high and presence of Fe3+ was detected down to 50 cm depth, probably as a result of a larger root system, allowing a better sediment oxygenation and accentuated OM decomposition processes, and thus limiting ecosystem saturation.

Benthic Exchange and Biogeochemical Cycling in Permeable Sediments

Article

Full-text available

Aug 2013

The sandy sediments that blanket the inner shelf are situated in a zone where nutrient input from land and strong mixing produce maximum primary production and tight coupling between water column and sedimentary processes. The high permeability of the shelf sands renders them susceptible to pressure gradients generated by hydrodynamic and biological forces that modulate spatial and temporal patterns of water circulation through these sediments. The resulting dynamic three-dimensional patterns of particle and solute distribution generate a broad spectrum of biogeochemical reaction zones that facilitate effective decomposition of the pelagic and benthic primary production products. The intricate coupling between the water column and sediment makes it challenging to quantify the production and decomposition processes and the resultant fluxes in permeable shelf sands. Recent technical developments have led to insights into the high biogeochemical and biological activity of these permeable sediments and their role in the global cycles of matter. Expected final online publication date for the Annual Review of Marine Science Volume 6 is January 03, 2014. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.

Advective Port-Water Exchange Driven by Surface Gravity Waves and Its Ecological Implications

Article

Jul 2003

The effects of surface gravity waves on pore-water release from permeable sediment (k = 1.3-1.8 × $10^{-11}\ \text{m}^{2}$ ) in shallow water were studied in a wave tank. Our tracer experiments demonstrated that shallow-water waves can increase fluid exchange between sandy sediment and overlying water 50-fold, relative to the exchange by molecular diffusion. The main driving force for this increased exchange are the pressure gradients generated by the interaction of oscillating boundary flows and sediment wave ripples. These gradients produce a pore-water flow field, with a regular pattern of intrusion and release zones, that migrates with ripple propagation. The ensuing topography-related filtering rates in the wave tank ranged from 60 to 590 $\text{L m}^{-2}\ \text{d}^{-1}$ and exceeded the solute exchange rates caused by hydrostatic wave pumping (38 L m-2 d-1 and initial molecular diffusion (corresponding to 10-12 L m-2 d-1). Wave-induced filtration is ecologically relevant because permeable sandy sediments are very abundant on the continental margins and can be converted into effective filter systems, which suggests that these sediments are sites for rapid mineralization and recycling. We propose that the wave influenced continental shelf may be subdivided into two zones: a shallow zone (water depth < wavelength/2), where wave orbital motion at the sea floor creates ripples and causes topography related advective filtering; and a deeper zone (wavelength/2 < water depth < wavelength), where wave pumping enhances interfacial exchange by hydrostatic pressure oscillations.

Convection in groundwater below an evaporating Salt Lake: 2. Evolution of fingers or plumes

Article

Full-text available

Jun 1997
WATER RESOUR RES

The downward convection of salt fingers or plumes developed from the unstable boundary layer of an evaporating "dry" salt lake is examined using a numerical model and Hele-Shaw cell experiments. In the convecting layer the early small waves evolve into fingerlike or plumelike formations, the number of fingers or plumes decreasing with time owing to differential growth and/or coalescence. Comparison of intermediate formational stages of this pattern with the pattern generated by a two-dimensional numerical simulation shows good qualitative agreement. However, there is a significant mismatch of the growth rates at long times. In the computer simulation the plume length develops approximately twice as rapidly as it does in the experimental case. A simple numerical experiment independent of the salt-lake boundary conditions is compared to previously published laboratory-scale measures of plume development in Hele-Shaw cells which confirms the retardation of Hele-Shaw plumes by a factor of approximately 50%. This departure is attributed to the differences in dimensionality between the Hele-Shaw flow domain and the model domain. The data indicated that leading plumes develop isolated behavior at long times and may not be adequately represented in Hele-Shaw analog models, and numerical simulation provides a more accurate simulation of field-scale behavior.

Convection in groundwater below an evaporating salt lake: 1. Onset of instability

Article

Jan 1997

R. A. Wooding

Secondary convection in a Hele Shaw cell

Article

Oct 1975
J FLUID MECH

A convective instability is produced by salt water diffusing onto the surface of a fresh-water layer in a Hele Shaw cell. Although the horizontal wavelength of the initial instability is small, an increase in the horizontal wavelength of the convective flow with time and depth is observed as the resulting two-dimensional convection develops. The phenomenon of wavelength variation is confirmed numerically, but quantitative observational and theoretical comparison is limited to small Rayleigh numbers. It is shown that perturbations in the density field cause horizontal pressure gradients, which in turn cause convective elements to combine.

Wave-Induced Advective Transport Below a Rippled Water-Sediment Interface

Article

Jan 1992

K. T. Shum

The total exchange across the water-sediment interface, averaged over one wave period, is significantly higher across a rippled interface than across a flat bed. This difference increases with increasing ripple slope and the strength of the wave motion, and it decreases with increasing thickness of the sediment layer relative to the length of the gravity wave. Since rippled bed forms are commonly found in coastal waters, the increase in the total exchange across a rippled water-sediment boundary can enhance the exchange of solutes due to "wave pumping'. Immediately below the water-sediment interface, circulation cells with net advective transport over a wave period are found. -from Author

Rotational dispersion in porous media due to fluctuating flow

Article

Jan 1992
WATER RESOUR RES

We suggest that dispersion in a saturated porous medium subject to a fluctuating pressure gradient can occur as a result of the processes of shear (Taylor) dispersion and what we will label rotational dispersion. In contrast to shear dispersion, rotational dispersion does not rely on molecular diffusion to be effective but requires that the direction of the pressure gradient rotates with time. Such rotational gradients are ubiquitous in nature, occurring whenever a pressure wave propagates across the surface of a porous medium such as soil or marine sediments. The efficiency and character of rotational dispersion is investigated using Monte Carlo simulations of the dispersal of clouds of particles through a highly idealized porous medium. These simulations demonstrate that rotational dispersion behaves as a diffusive process and that it can be many times more effective than molecular diffusion or shear dispersion as a transport mechanism. The results of the theory were tested experimentally using a wave tank with a bed of sand as the porous medium. These experiments demonstrate that passing waves can greatly enhance solute transfer between the bed and the overlying water. Furthermore, the measured increases in solute transfer rates are quantitatively consistent with the predictions obtained from the theory of rotational dispersion presented herein.

Convective Transport Within Stable River Sediments

Article

Sep 1987

Local pressure variations of the order of 100-1000 N/m2 can be observed between the upstream and downstream faces of the typically triangular-shaped dunelike sediment structures that form at the sediment-water interface of rivers. Laboratory experiments were conducted examining the influence of this localized pressure variation on contaminant transport processes within the sediment. Numerical modeling of the in-bed flow via boundary element methods was also undertaken in order to predict convective transport under typical field conditions. The laboratory experiments and numerical simulation of the in-bed flow in several rivers verified that the pressure distribution observed on the sediment surface and the resulting interstitial fluid convection can control transport of chemically inert, nonsorbed contaminants in stable sediments. In-bed Peclet numbers were of the order of 100-1000, indicating the negligible influence of diffusion under the conditions examined.

An Introduction To Fluid Dynamics

Article

Jan 1967

G. K. Batchelor

Dual flow-injection analysis system for determining bromide and reactive phosphorus in natural waters

Article

Oct 1993
ANAL CHIM ACTA

An inexpensive, dual flow-injection instrument for simultaneously determining low concentrations of both bromide and dissolved reactive phosphorus in natural waters is reported. An in-line C18 resin column was needed to remove interference in the bromide determination from dissolved organic matter present in most natural waters. The bromide method had a detection limit of 4 υg Br 1−1 and was linear over the range 0–2 mg Br 1−1 with good precision [C.V. 5.3% at 0.1 mg Br 1−1 (n = 10) and 0.5% at 2.0 mg Br 1−1 (n = 10)]. The dissolved reactive phosphorus method had a detection limit of 0.6 υg P 1−1 and was linear over the range 0–50 υg P 1−1 with good precision [C.V. 2.9% at 2.0 υg P −1 (n = 3) and 0.5% at 50 υg P 1−1 (n = 3)]. The dual method can analyse 30 samples per hour when analysing P and Br sequentially (35 samples per hour in simultaneous analysis mode) and has been used in field experiments to determine the spiralling of phosphorus in Myrtle Creek, Australia.

Diagenetic Processes Near the Sediment-Water Interface of Long Island Sound.II. Fe and Mn

Article

Dec 1980
ADV GEOPHYS

Robert Curwood Aller

This chapter analyzes diagenetic processes for Fe and Mn near the sediment–water interface of Long Island Sound (LIS). The results obtained in this study show that pore-water profiles of Fe2+ and Mn2+ from three stations in LIS have general depth-dependent concentration distributions similar to those reported from other sedimentary basins: concentrations rise above seawater values to a maximum below the interface and then decrease again or remain constant deeper in the deposit. It is found that the production of Mn2+ in pore waters is directly related to the rate of reduction of Mn oxides during the decomposition of organic matter, both as a function of depth in the sediment as well as seasonally. Fe2+ is produced both by the reduction of Fe oxides and by abiogenic or biogenic oxidation of Fe sulfides. The temporal changes in both Mn2+ and Fe2+ profiles are repeatable from year to year. During the summer, pore-water Mn2+ in the top few centimeters reaches the highest concentration of the year. In the fall, Mn2+ concentrations are lowered in magnitude throughout the sediment column as a result of both decreased production.

The Effects of Macrobenthos on Chemical Properties of Marine Sediment and Overlying Water

Article

Jan 1982

Robert Curwood Aller

The composition of any environment or object is determined by a particular balance between material transport processes and chemical reactions within and around it. In the case of marine sedimentary deposits, the dominant agents of mass transport are often large bottom-dwelling animals that move particles and fluids during feeding, burrowing, tube construction, and irrigation. Such biogenic material transport has major direct and indirect effects on the composition of sediments and their overlying waters. In this chapter I review some of what is presently known about these effects, their implications for both chemical and biological properties of a deposit, and how they can be conceptualized in quantitative models.

Effect of microinvertebrates on the exchange of solutes between sediments and freshwater

Article

Mar 1985

Infaunal macrobenthos (tubificid oligochaetes, chironomid larvae, and unionid bivalves) were studied in laboratory microcosms to determine their effects on freshwater sediment diagnesis and the exchange of solutes between sediments and water. Tubificids enhanced the flux of ammonium, bicarbonate, and silica from sediments. After the onset of anoxia, they decreased the flux of iron and phosphate. Chironomids increased the flux of nitrate, bicarbonate, and silica, but did not affect the flux of phosphate. Pore water concentrations were low within the irrigated burrowed zone for chemical species normally high in reduced sediments. Concentration gradients were less steep in the actively irrigated burrow zone, but radial diffusion to and from burrows, increased rates of organic decomposition, and enhanced diatom frustule dissolution rates result in enhanced mass transport from sediments. Data from an experiment with unionid clams demonstrated the presence of radial diffusion gradients. These clams enhanced the chloride and nitrate flux from sediments, decreased the bicarbonate flux, but did not affect the flux of either phosphate or silicate. Although the clams did not actively irrigate their burrows, their effect on sediments was similar to that of chironomids. Comparison of direct and indirect flux estimates showed that both types of estimates could be highly variable. In general, indirect flux estimates were higher than direct flux estimates.

The plankton tower. II. Release of nutrients from sediments due to changes in the density of bottom water

Article

Mar 1976

Results of an enclosure experiment carried out in Kiel Bight are presented. A water column of about 30 m3, extending from the surface to the bottom over a 3 m2 patch of sediment (coarse sand), was isolated and observed over a period of 33 days. Considerable water exchange took place with the surroundings partly because of near-surface openings that appeared in the enclosure but mainly because of density changes due to salinity fluctuations in the surrounding water. This denser water entered the enclosure through the sediments, displacing the lighter water through the near-surface openings. Very high nutrient and low oxygen values were measured in the bottom water immediately following this higher-density water influx. The same effect was observed outside the enclosure, but this phenomenon was not marked here, probably due to greater turbulent mixing obscuring this effect. It is postulated that interstitial, water is flushed out of coarse-grained sediments by gravity displacement due to changes in the density of bottom water. In certain areas this mechanism of nutrient release from the sediments is presumably of great ecological importance, both for the phytoplankton and the benthos.

The effect of oxygen on release and uptake of cobalt, manganese, iron and phosphate at the sediment-water interface

Article

Jun 1986
GEOCHIM COSMOCHIM AC

The porewater of a sediment core taken at 6 m depth in Gullmarsfjorden, Sweden, was enriched in Fe, Mn, Co, and phosphate compared to the overlying bottom water. Yet, in situ measurements with a benthic flux-chamber, in which dissolved oxygen and pH were maintained near ambient values (regulated flux-chamber), showed that the sediment did not release any of these ions but instead removed Co, Mn, and Fe from the overlying water. In a parallel experiment, where dissolved oxygen and pH were not maintained but allowed to decrease as a result of benthic respiration, Co, Mn, Fe, and PO4 were released from the sediment. An accidental interruption of the stirring in the regulated chamber caused a pulse of dissolved Co, Mn, Fe, and PO4 to be released from the sediment. When the stirring was resumed, all four ions were again removed. The kinetics of the removal process was apparent first order with half-removal times of 3–5 days, similar to the removal kinetics of the radioactive tracers 59Fe and 54Mn from the water in a smaller chamber, run in parallel.The critical variable which controls the reactions at the sediment-water interface is the flux of oxygen from the water column into the sediment. When benthic chambers are used to measure fluxes of redox-sensitive ions, the oxygen flux must be maintained as close as possible to the actual in situ flux. If not, the measured fluxes may vary greatly in magnitude and even change direction.

On the distribution of iron and manganese at the sediment/water interface: thermodynamic versus kinetic control

Article

Jul 1982
GEOCHIM COSMOCHIM AC

Wolfgang Balzer

Iron and manganese solubility at the sediment/water interface has been studied at a water depth of 20 m in Kiel Bight, Western Baltic. By means of an in situ bell jar system enclosing 3.14 m2 sediment surface and 2094 l water a complete redox turn-over in the bottom water was simulated in an experiment lasting 99 days. The concentration of dissolved Fe in the bell jar water never exceeded 0.041 μmol · dm−3during the first 50 days of the experiment and then rose abruptly as the Eh fell from +600 to −200 mV. The concentration of dissolved Fe under oxic and anoxic conditions seems to be limited by equilibria with solid Fe-phases (hydroxides and amorphous sulphide, respectively). In contrast to Fe, manganese was released continuously from the bottom during the first 50 days of the experiment leading to exponentially increasing manganese concentrations in the bell jar water. During this time dissolved O2 had become ready depleted and pH had dropped from 8.3 to 7.5. Contrary to iron, manganese being solubilized in reduced sediment layers can penetrate oxic strata in metastable form due to slow oxidation kinetics; when the redoxcline moves upwards Mn2+ is enriched in bottom waters. The maximum concentration of dissolved Mn under anoxic conditions is controlled by a solid phase with solubility properties similar to MnCO3 (rhodochrosite). Bottom water enrichment in dissolved Mn2+ could be traced to originate from excess solid manganese within the top 3 cm of the sediment.

Computational Design for Long-Term Numerical Integration of the Equations of Fluid Motion: Two-Dimensional Incompressible Flow. Part I

Article

Aug 1966

Akio Arakawa

The integral constraints on quadratic quantities of physical importance, such as conservation of mean kinetic energy and mean square vorticity, will not be maintained in finite difference analogues of the equation of motion for two-dimensional incompressible flow, unless the finite difference Jacobian expression for the advection term is restricted to a form which properly represents the interaction between grid points, as derived in this paper. It is shown that the derived form of the finite difference Jacobian prevents nonlinear computational instability and thereby permits long-term numerical integrations.

Algorithm 541: Efficient Fortran Subprograms for the Solution of Separable Elliptic Partial Differential Equations [D3]

Article

Sep 1979

Solute Exchange by Convection Within Estuarine Sediments

Abstract

No full-text available

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