Article

The effect of wave-induced pore water circulation on the transport of reactive solutes below a rippled sediment bed

January 1993
Journal of Geophysical Research Atmospheres 981(C6):10289-10302

January 1993
981(C6):10289-10302

Authors:

The role of pore water flow in solute transport below a rippled sediment bed in a coastal environment is investigated. Based on the velocity field induced by the passage of gravity waves, a steady-state transport equation is solved numerically over a range of field conditions. A consumption term governed by first-order kinetics, a constant diffusivity and a constant boundary concentration slightly above the sediment-water interface are assumed in the two-dimensional continuum model. Significant spatial variations in concentration are found when there is an appreciable pore water flow. The concentration below a ripple trough can be many times higher than that at the same depth below a crest, and the concentration gradient in the horizontal can be of the same order of magnitude as that in the vertical. At high advective transport, a region of uniform concentration is found immediately below the ripple surface away from the crests. The rate of consumption over the sediment layer is compared with the corresponding rate in the absence of advective transport. Their ratio increases monotonously with the ripple slope and the ratio of the magnitudes of advective to diffusive transport and can reach up to 5 even for a relatively mild ripple slope of 0.1. As the magnitude of advective transport depends on the prevalent wave conditions, the rate of consumption and the distribution of solute concentration can vary appreciably with time. These results suggest that hydrodynamic conditions, sediment characteristics, and surface topography should be integral parts of field measurements of the distribution and flux of solutes in coastal sediments.

Wave-driven porewater and solute circulation through rippled elastic sediment under highly transient forcing

Article

May 2011

Waves induce porewater flow and solute transport through permeable marine sediment. However, past studies have ignored high-frequency pressure pulses, under the assumption that the porewater flow field is adequately represented by a time-averaged one or that the saturated sediment is incompressible. We modeled porewater flow and solute transport inside ripples, forced by instantaneous pressure profiles along the sediment-water interface (SWI) with 0.1-s temporal resolution. The transient pressure profiles were taken from a field data–driven large-eddy simulation model of wave-driven oscillatory flow. The simulations suggest that in elastic, permeable, and saturated sediment, a time-averaged representation of the flow field may be inadequate and that this also leads to shortcomings in how transport is modeled. Bursts in fluid flushing occur when high-frequency pressure fluctuations were considered, leading to larger long-term average fluid fluxes compared to a steady flow field driven by a time-averaged pressure profile. The pressure perturbations along the SWI propagate within a few milliseconds to meter depths within the sediment leading to strongly transient porewater velocity fields. This leads to enhanced dispersion of solutes and larger time-averaged solute fluxes. However, enhanced solute flux across the SWI diminished through time with increasing permeability. The high-frequency transient pressures and sediment elastic properties we considered have been largely ignored and unrecognized. Future observational and modeling studies should consider these processes, especially since they mediate timing-sensitive biogeochemical reactions.

Fluid circulation and seepage in lake sediment due to propagating and trapped internal waves

Article

Nov 2012

It has long been known that surface gravity waves induce significant seepage through the porous layer found at the lake bottom. Away from coastal regions, however, the pressure signature of surface waves at the lake bottom is weak. We consider fully nonlinear internal gravity waves, whose long wavelength and slow motion implies a sustained and strong pressure perturbation even in the deep regions of the lake. We argue that internal waves can induce significant seepage through the sediment layer, in regions where surface gravity waves have negligible impact. The pressure profile at the fluid-porous layer interface is computed from the "exact" Dubreil-Jacotin-Long theory, giving a reliable profile even for large waves. This profile is used in conjunction with Darcy's law to compute the seepage within the porous region. We find that the geometric distribution of seepage is strongly controlled by both the ratio of porous media thickness to the horizontal length of the pressure perturbation, and the bottom topography, when it is present. Based on work on the interaction of internal solitary waves with the bottom boundary layer, we develop a model to account for the changes in permeability due to wave-induced instabilities in the bottom boundary layer and enhanced benthic turbulence. This turbulence acts to unplug the pores near the surface by lifting the detritus that clogs them. The resulting changes in permeability significantly enhance exchange between the free fluid and the porous medium on the downstream side of the wave.

Oxygen dynamics in permeable sediments with wave-driven pore water exchange

Article

May 2004

The effects of advective pore water exchange driven by shallow water waves on the oxygen distribution in a permeable (k = 3.3 × 10-12 to 4.9 ∓ 10-11 m2) natural sediment were studied with a planar oxygen optode in a wave tank. Our experiments demonstrate that pore water flow driven by the interaction of sediment topography and oscillating boundary flow changes the spatial and temporal oxygen distribution in the upper sediment layer. Oxygenated water intruding in the ripple troughs and deep anoxic pore water drawn to the surface under the ripple crests create an undulating oxic-anoxic boundary within the upper sediment layer, mirroring the topographical features of the sediment bed. Anoxic upwelling zones under ripple crests can separate the oxic sediment areas of neighboring ripple troughs with steep horizontal oxygen concentration gradients. The optode showed that migrating wave ripples are trailed by their pore water flow field, alternately exposing sediment volumes to oxic and anoxic pore water, which can be a mechanism for remobilizing particulate oxidized metal precipitates and for promoting coupled nitrification-denitrification. More rapid ripple migration (experimental threshold ∼20 cm h-1) produces a continuous oxic surface layer that inhibits the release of reduced substances from the bed, which under slowly moving ripples is possible through the anoxic vertical upwelling zones. Swift, dramatic changes in oxygen concentration in the upper layers of permeable seabeds because of surface gravity waves require that sediment-dwelling organisms are tolerant to anoxia or highly mobile and enhance organic matter mineralization.

Enhanced Submarine Ground Water Discharge from Mixing of Pore Water and Estuarine Water

Article

Dec 2004
GROUND WATER

Submarine ground water discharge is suggested to be an important pathway for contaminants from continents to coastal zones, but its significance depends on the volume of water and concentrations of contaminants that originate in continental aquifers. Ground water discharge to the Banana River Lagoon, Florida, was estimated by analyzing the temporal and spatial variations of Cl− concentration profiles in the upper 230 cm of pore waters and was measured directly by seepage meters. Total submarine ground water discharge consists of slow discharge at depths > ∼70 cm below seafloor (cmbsf) of largely marine water combined with rapid discharge of mixed pore water and estuarine water above ∼70 cmbsf. Cl− profiles indicate average linear velocities of ∼0.014 cm/d at depths > ∼70 cmbsf. In contrast, seepage meters indicate water discharges across the sediment-water interface at rates between 3.6 and 6.9 cm/d. The discrepancy appears to be caused by mixing in the shallow sediment, which may result from a combination of bioirrigation, wave and tidal pumping, and convection. Wave and tidal pumping and convection would be minor because the tidal range is small, the short fetch of the lagoon limits wave heights, and large density contacts are lacking between lagoon and pore water. Mixing occurs to ∼70 cmbsf, which represents depths greater than previously reported. Mixing of oxygenated water to these depths could be important for remineralization of organic matter.

Variability in Benthic Exchange Rate, Depth, and Residence Time Beneath A Shallow Coastal Estuary

Article

Feb 2018

Hydrodynamically driven benthic exchange of water between the water column and shallow seabed aquifer is a significant and dynamic component of coastal and estuarine fluid budgets. Associated exchange of solutes promotes ecologically important chemical reactions, so quantifying benthic exchange rates, depths, and residence times constrains coastal chemical cycling estimates. We present the first combined field, numerical, and analytical modeling investigation of wave-induced exchange. Temporal variability of exchange was calculated with data collected by instruments deployed in a shallow estuary for 11 days. Differential pressure sensors recorded pressure gradients across the seabed, and up- and down-looking ADCPs recorded currents and pressures to determine wave parameters, surface-water currents, and water depth. Wave-induced exchange was calculated (1) directly from differential pressure measurements, and indirectly with an analytical model based on wave parameters from (2) ADCP and (3) wind data. Wave-induced exchange from pressure measurements and ADCP-measured wave parameters matched well, but both exceeded wind-based values. Exchange induced by tidal pumping and current-bed form interaction—the other primary drivers in shallow coastal waters were calculated from tidal stage variation and ADCP-measured currents. Exchange from waves (mean = 20.0 cm/d; range = 1.75–92.3 cm/d) greatly exceeded exchange due to tides (mean = 3.7 cm/d) and current-bed form interaction (mean = 6.5 × 10−2 cm/d). Groundwater flow models showed aquifer properties affect wave-driven benthic exchange: residence time and depth increased and exchange rates decreased with increasing hydraulic diffusivity (ratio of aquifer permeability to compressibility). This new understanding of benthic exchange will help managers assess its control over chemical fluxes to marine systems.

Porewater CO2–carbonic acid system chemistry in permeable carbonate reef sands

Article

Apr 2016
MAR CHEM

N° d'ordre 4075 THÈSE

Thesis

Full-text available

Oct 2010

R. Chassagne

For a better understanding of the complexity of the biogeochemical processes in coastal re- gion, a 2D model has been developed. This model couples hydrodynamic forcing generated by the tide and the transport-reaction processes of biogeochemical species. Di- scharge of the tide into the variably-saturated porous media is modelled by Richards equation. Some numerical me- thods are required for solving this kind of complex problem, as Streamline Upwind Petrov Galerkin (SUPG) method and shock capturing method. The highly dynamical site of the Truc-Vert beach has been chosen as reference field, mainly because of avalaible biogeochemical and hydrodynamical studies. The input parameters of the model come from these field data (ANR PROTIDAL and MO- BISEA projects) and from the bibliography. The validation of the model was made in regards of two case studies from published hydrodynamic simu- lations under tidal forcing and from avaliable transport-reaction solutions. A first version of the model has been declined to describe silicic acid evolution into porous media under tidal forcing. The flux of the silicic acid to the ocean and the residence time of silicic acid into permeable sandy sediments were estimated. After some days, we observe the formation of a lens of low silicic acid concen- tration in the upper part of the intertidal zone. This lens is the main imprint of the tidal forcing. We studied also variations of the lens geo- metry and the residence time under influence of model parameters, such as the beach slope, the tide amplitude and the dispersion coefficient. A second version of the model describes the organic mat- ter degradation , and simulates the concentrations of oxygen, nitrates and phosphates. The model reproduces the spatial (2D) and temporal distribution of the concentration of these different che- mical species into the sediment. Nowadays environmental problems are fondamental for our society and the understanding of the sediment-ocean interactions is a crucial step. The new model allows us a better understanding of the tidal impact on biogeochemical processes in permeable sediments and offers a quantitative approach on biogeochemical processes that occur into variably-saturated sandy sediments. The model also offers a useful tool to optimize sampling strategy for field studies. Keywords : early diagenesis, intertidal zone, hydrodynamic, transport-reaction equation, Richards equation, permeable sediment, variably-saturated porous media.

Impact of microforms on nitrate transport at the groundwater –surface water interface in gaining streams

Article

Nov 2014
ADV WATER RESOUR

The wave-induced solute flux from submerged sediment

Article

Full-text available

Feb 2014

The issue of the transport of dissolved nutrients and contaminants between the sediment in the bottom of a lake or reservoir and the body of water above it is an important one for many reasons. In particular the biological and chemical condition of the body of water is intricately linked to these mass transport processes. As the review by Boudreau (Rev Geophys 38(3):389–416, 2000) clearly demonstrates those transport processes are very complex involving mechanisms as diverse as the wave-induced flux between the sediment and the overlying water and the effect of burrowing animals on the transport within the sediment as well as basic diffusion mechanisms. The present paper focuses on one facet of these transport processes; we re-examine the balance of diffusion and wave-induced advection and demonstrate that the wave-induced flux of a solute from submerged sediment is not necessarily purely diffusive as suggested by Harrison et al. (J Geophys Res 88:7617–7622, 1983) but can be dominated by a mean or time-averaged flux induced by the advective fluid motion into and out of the sediment caused by the fluctuating pressure waves associated with wave motion. Indeed along the subtidal shoreline where the fluctuating bottom pressures are greatest, wave-induced advection will dominate the mean, time-averaged transport of solute into or out of the sediment as suggested in the work of Riedl et al. (Mar Biol 13:210–221, 1972). However, the present calculations also indicate that this advective flux decreases rapidly with increasing depth so that further away from the shoreline the advective flux becomes negligible relative to the diffusive flux and therefore the latter dominates in deeper water.

Thermal and chemical evidence for rapid water exchange across the sediment-water interface by bioirrigation in the Indian River Lagoon, Florida

Article

May 2006

Time-series measurements of chloride (Cl-) concentrations in lagoon and pore waters of an estuary on the east coast of Florida (Indian River Lagoon) demonstrate exchange of lagoon surface water to depths of ∼40 cm in the sediment in less than 46 h. The exchange rate may be as fast as 150 cm d-1 based on models of the decay in the amplitude of diurnal temperature variations and the time lag of maxima and minima of the temperature variations at depths of 15 and 30 cm below the sediment-water interface. These flow rates indicate a minimum residence time of 0.33 d for the pore water. Considering the small tides and waves, rate of the exchange, and large number of bioturbating organisms in the Indian River Lagoon, the exchange of water is driven largely by bioirrigation. The exchange provides a greater flux of excess radon-222 from the sediment to the lagoon than would occur from diffusion alone. The exchange also pumps oxygenated water into the sediments, thereby enhancing organic carbon remineralization and the flux of nitrogen from sediments to the lagoon water. High rates of exchange across the sediment-water interface indicate that marine sources are volumetrically more important than terrestrial sources to submarine groundwater discharge in the permeable sediments of this estuary. © 2006, by the American Society of Limnology and Oceanography, Inc.

Surface and Pore Water Mixing in Estuarine Sediments: Implications for Nutrient and Si Cycling

Article

Full-text available

Jan 2005
J COASTAL RES

Dissolved constituent fluxes from the sediment to the water column are important in estuarine environments. Benthic nutrient source depends on the mechanisms driving advection, the advective transport rate, and the concentration of nutrients in the discharged water, all of which depend on the source of discharged water and water-solid reactions along its flow. Pore water advection has been measured at rates of 3 to 6 cm/d using seepage meters in the Banana River Lagoon, Florida. Diffusion, advection, and reaction modeling of SiO2 profiles in pore water indicate that advection and reactions are more important than diffusion in the upper I in of the sediments. Advection results from the recirculation of the overlying water column through the bottom sediments, oxygenating the lagoon water, while pore water lacks oxygen. As lagoon water recirculates through sediments, the subsequent loss of oxygen enhances regeneration of buried organic matter. Using measured seepage rates and average pore water concentrations of nutrients, N and P fluxes from the sediment are estimated to be 33 to 38 mu g/cm(2)/y and 3 to 5 mu g/cm(2)/y, respectively. On the basis of sedimentation rates and the average concentrations of N and P in the sediment, the fluxes of N and P to the sediment are estimated to be 9 to 38 mu g/cm(2)/y and 2 to 6 mu g/cm(2)/y, respectively. These values suggest that 100% more N and 30% more P may discharge with recirculating lagoon water than is deposited in the sediment. Because the source of most pore water is surface water, the excess nutrients appear to originate from organic matter regeneration at or near the sediment-water interface, thereby elevating their concentrations in pore waters and depleting their concentration in the buried sediment. This regeneration of nutrients appears to limit their burial rate in the lagoon.

Simultaneous velocity and concentration measurements over a rippled boundary subjected to oscillating fluid forcing

Article

Full-text available

Jun 2024
EXP FLUIDS

We describe an oscillating boundary layer apparatus (OBLA) to investigate mass and momentum transfer in the wave bottom boundary layer. The facility is designed such that near-bed shallow water orbital velocities are physically modeled in full scale. A PIV/PLIF system allows for simultaneously resolving the intra-ripple velocity and dye concentration fields. We examine two cases by injecting dye at the trough and crest of the rippled boundary. The extent of the plume is the largest near the zero-crossing of the free-stream velocity and 40∘\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^\circ$$\end{document} later for the trough and crest case, respectively. Both cases showed periodic turbulent vortical structures influencing the phase-averaged concentration plumes. For normalized concentrations greater than 0.01, the plumes remained within the boundary layer and traveled half a ripple length for both cases. Dye spread vertically upward about 2 and 1.5 ripple heights from the crest and trough sources, respectively. Stronger advection was observed over the crests, along with a clear dependence on bedform asymmetry.

A case for addressing the unresolved role of permeable shelf sediments in ocean denitrification

Article

Full-text available

Oct 2021

The marine nitrogen cycle is a main driver of ocean productivity and affects global climate. Despite decades of study, we still have an incomplete understanding of the role of the marine nitrogen cycle in the Earth system. While marine sediments play a major role in nitrogen cycling in the ocean, magnitudes and mechanisms are largely unconstrained. Recent research suggests that permeable sandy sediments on the highly energetic and dynamic continental shelf are key components of the marine nitrogen cycle, but data to quantify their contribution are lacking. Here, we use insights from measurements and modeling studies to substantiate the hypothesis that shelf sediments are an overlooked driver of the marine nitrogen cycle. Specifically, we propose that permeable shelf sediments are sites for intense nitrogen conversions and suggest a three‐pronged approach to address unresolved controversies.

Geochemical fluxes in sandy beach aquifers: Modulation due to major physical stressors, geologic heterogeneity, and nearshore morphology

Article

Sep 2021
EARTH-SCI REV

Coastal beach aquifers are biogeochemically active systems that mediate chemical and material fluxes across the land-sea interface. This paper provides a review of major physical stressors and geologic features that influence flow and solute fate and transport in coastal beach aquifers. We outline current understanding of the interactions between these factors and their associated impacts on water and geochemical fluxes within and across these aquifers. The physical processes that control flow, transport, and the formation and distribution of reactive zones in beach aquifers (e.g., tides, waves, density gradients, precipitation, episodic ocean events, and evaporation) operate across overlapping temporal and spatial scales, and present challenges for measuring and modeling physical flow and biogeochemical processes in coastal groundwater systems. Geologic heterogeneity introduces further complexity by modifying flowpaths, mixing patterns, and rates of biotransformation. Interactions between these physical stressors and geological controls are likely to evolve with changes in sea level, climate variability, human settlement, coastal erosion, and other natural and anthropogenic stresses, providing avenues for scientific exploration into the future role of beach aquifers as chemical mediators between the land and ocean.

Instantaneous Effects of Sediment Resuspension on Inorganic and Organic Benthic Nutrient Fluxes at a Shallow Water Coastal Site in the Gulf of Finland, Baltic Sea

Article

Full-text available

Oct 2019

Climate change is leading to harsher resuspension events in shallow coastal environments influencing benthic nutrient fluxes. However, we lack information on the quantitative connection between these fluxes and the physical forces. Two identical experiments that were carried out both in May and August provided novel knowledge on the instantaneous effects of resuspension with known intensity on the benthic dissolved inorganic (phosphate: DIP, ammonium: NH4⁺, nitrite+nitrate: NOx, silicate, DSi) and organic nutrient (phosphorus: DOP, nitrogen: DON, carbon: DOC) fluxes in the shallow soft bottoms of the archipelago of Gulf of Finland (GoF), Baltic Sea. Resuspension treatments, as 2 times the critical shear stress, induced effluxes of one to two orders of magnitude higher than the diffusive fluxes from the studied oxic bottoms. The presence of oxygen resulted in newly formed iron oxyhydroxides and the subsequent precipitation/adsorption of the redox-dependent nutrients (DIP, DSi, organic nutrients) affecting their fluxes. Resuspension-induced NH4⁺ and NOx fluxes were associated with the organic content of sediments showing the highest values at the organic rich sites. NH4⁺ showed the strongest responses to resuspension treatments in August, but NOx at the time of high oxygen concentrations in near-bottom water in May. Foreseen increases in the frequency and intensity of resuspension events due to climate change will most likely enhance the internal nutrient loading of the studied coastal areas. The fluxes presented here, connected to known current velocities, can be utilized in modeling work and to assess and predict the internal nutrient loading following climate change.

WAVE IMPACT AND DUNE EROSION DURING HURRICANE MATTHEW

Article

Full-text available

Dec 2018

Extreme storms can cause rapid morphological changes that pose high risk to society (Sallenger 2000). Semiempirical and process-based models often are used to simulate storm-induced coastal processes (Roelvink et al. 2009, Palmsten & Holman 2012, Stockdon et al. 2014, Overbeck et al. 2017). However, there are few observations of surfzone waves and currents during extreme storms. Therefore, parameterizations often are calibrated by minimizing model-data errors for pre- to post-storm bathymetric and topographic changes, and the accuracy of the simulated processes during the storm is unknown. Here, surf, swash, and dune observations collected near Duck, NC, USA, will be used to investigate wave processes and dune erosion during the passage of recent (2015-2017) Hurricanes.

Wave Ripple Development on Mixed Clay-Sand Substrates: Effects of Clay Winnowing and Armoring

Article

Full-text available

Oct 2018

Based on bed form experiments in a large-scale flume, we demonstrate that the rate of development of wave ripples on a mixed sand-clay bed under regular waves is significantly lower than on a pure-sand bed, even at clay fractions as low as 4.2%, and that this rate of development decreases exponentially from 4.2% to 7.4% clay. These experiments also showed that, despite the slow growth of the bed forms in the mixed sand-clay, the equilibrium length and height of the wave ripples were independent of the initial bed clay fraction. Given that the ripple crests were composed of pure sand at the end of all the experiments that started with well-mixed sand-clay, it is inferred that the clay was removed from the bed during the development of the wave ripples through winnowing into the water column, and possibly also by sieving into the subsurface, where the final clay fractions were found to be higher than the initial clay fractions. These clay removal processes are interpreted to have facilitated the wave ripples to reach equilibrium lengths and heights that are similar to those in pure sand. Clay-carrying pore flow initiated by pressure gradients between the wave ripple troughs and crests might also have contributed to the accumulation of clay in the sediment below the wave ripples. The formation of the clay-enriched “armoring” layer in the substrate is likely to further reduce erosion rates and could influence the dispersion of nutrients and pollutants in coastal seas.

From soil to sea: the role of groundwater in coastal critical zone processes: From soil to sea

Article

May 2016

Near coasts, surface water–groundwater interactions control many biogeochemical processes associated with the critical zone, which extends from shallow aquifer to vegetative canopy. For example, submarine groundwater discharge delivers a significant fraction of weathering products such as silica and calcium to the world's oceans. Owing to changing fertilizer and land use practices, submarine groundwater discharge is also responsible for high nitrogen loads that drive eutrophication in marine waters. Submarine groundwater discharge is generally unmonitored due to its heterogeneous and diffuse spatial patterns and complex temporal dynamics. Here, we review the physical processes that drive submarine groundwater discharge at various spatial and temporal scales and highlight examples of interdependent critical zone processes. Like the inland critical zone, the coastal critical zone is undergoing rapid change in the Anthropocene. Disturbances include warming air and sea temperatures, sea‐level rise, increasing storm severity, increasing nutrient and contaminant inputs, and ocean acidification. In a changing world, it is more important than ever to understand complex feedbacks between dynamic surface water‐groundwater interaction, rocks, and life through long‐term monitoring efforts that extend beyond inland rivers to coastal groundwater. WIREs Water 2016, 3:706–726. doi: 10.1002/wat2.1157 This article is categorized under: Water and Life > Nature of Freshwater Ecosystems Science of Water > Water and Environmental Change Science of Water > Water Quality

Application of Porous Media Theories in Marine Biological Modeling

Chapter

Aug 2010

Passive Diffusion in Ecosystems

Chapter

Full-text available

Jan 2002

In this chapter we consider several aspects of passive diffusion in the environment. In this book, passive diffusion is defined to be the diffusion of chiefly abiotic objects that are not capable of performing random motion without the help of environmental turbulence. Likewise, active diffusion is defined to be the diffusion of objects, chiefly animals, that perform motion by themselves.

Determination of Tetracyclines Residues in the Gulf of Gdańsk (Southern Baltic Sea) Sediments Using a Tandem Solid-Phase Extraction with Liquid Chromatography Coupled with Tandem Mass Spectrometry

Chapter

Jan 2014

Pharmaceuticals are biologically active and relatively persistent substances which have been recently recognized as a continuing threat to the aquatic environment. Numerous adverse effects may arise for aquatic non-target organisms from the presence of pharmaceutical residues. One of the main pharmaceutical contaminants are antibiotics which are in the environment in considerable concentrations and can have negative biological effects to marine microorganisms. Despite of this, the knowledge on antibiotics occurrence, behaviour and fate in the Baltic Sea is very limited. This study presents the preliminary results on tetracycline and oxytetracycline residues occurrence in the sediments collected from the Gulf of Gdańsk (southern Baltic Sea). Among the antibiotics, tetracyclines are one of the most popular class used for human and animal therapy and for animal breeding. The method for tetracyclines residues determination was optimized, using two types of Baltic sediments (sandy and muddy) spiked at nine concentration levels (from 1 to 1,000 ng g−1 d.w.) to achieve the best validation parameters. Acetonitrile and EDTA-McIlvaine extraction buffer solution (pH 4) (1:1) for extraction and tandem SPE technique with Discovery SAX and Oasis HLB cartridges for extracts clean-up were applied. Quantitative and qualitative determination of analysed antibiotics was performed with the use of liquid chromatography tandem mass spectrometry (LC-MS/MS) with electrospray ionization source in the positive mode. The recoveries for target antibiotics ranged from 78.8 to 131.1 % for tetracycline and 75.5–114.8 % for oxytetracycline. The LOD values ranged from 1.5 to 3.5 ng g−1 d.w. for tetracycline and from 1.2 to 2.2 ng g−1 d.w. for oxytetracycline. The developed method was applied in the determination of tetracyclines in six sediments collected in 2012 during r/v “Oceania” cruises. Oxytetracycline was identified in four of analyzed samples at concentrations from 21 to 625 ng g−1 d.w. Tetracycline was determined only in one sediment sample collected close to WWTP “Gdańsk Wschód” outlet (13.8 ng g−1 d.w.). None of the target compounds was identified in the accumulation area—the Gdańsk Deep.

CHEMICAL TRACING AND ANALYTICAL AND MASS-BALANCE MODES OF PORE WATER CIRCULATION IN THE BANANA RIVER LAGOON, FLORIDA

Article

Full-text available

Jehangir Bhadha

Hyporheic flow and transport processes

Data

Oct 2014

10.1002 2012RG000417

Data

Oct 2014

Exonerating Bernoulli? On evaluating the physical and biological process affecting marine seepage meter measurements

Article

Jun 2006
LIMNOL OCEANOGR-METH

Seepage meters, like most benthic flux chamber techniques, come with inherent concerns about how their presence may alter the environment and flow regimen of the benthic boundary layer and underlying sediments. Flow due to wave and current movement across topographic features induces a downward and upward flow field within the sediments surrounding the feature. We found this Bernoulli-induced flow is a real, but maybe minor, component of measured advection using seepage meters. This study was conducted in a Florida coastal lagoon to test the physical forcing mechanisms that may influence seepage measurements from sediments. Calculated Bernoulli seepage was within the measured background (∼1 to 2 cm day-1) expected from seepage meters when a plastic barrier beneath the device is used to inhibit natural seepage contributions. Nearby seepage measurements made with Lee-type seepage meters placed directly in the sediments ranged from 1 to 12 cm day -1. Thus, when seepage flow is very slow from sediments, Bernoulli-induced seepage may obscure the measurement. However, this study demonstrates that seepage in the Indian River Lagoon must be driven by forces other than Bernoulli-induced (pumped) flow. Suggestions for these forcing mechanisms highlight the uncertainty of the water source(s) in seepage measurements. In these Florida lagoon sediments, bioirrigation and terrestrial groundwater inputs are the most likely drivers, depending on distance from shore, benthic community composition, and continental recharge. Seepage measurements can be an excellent measure of advection in shallow-water marine sediments if Bernoulli-induced seepage is taken into account either experimentally or calculated based on local hydrographic and meteorological data. © 2006, by the American Society of Limnology and Oceanography, Inc.

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.

Submarine Groundwater Discharge: Its Measurement and Implications For Nutrient Inputs and Biogeochemical Processes in the Nearshore Coastal Zone

Article

Jan 2010

B.M. Mwashote

Submarine groundwater discharge (SGD) assessments conducted both in the laboratory and at a field site in the northeastern Gulf of Mexico, using a continuous-heat type automated seepage meter (seepmeter) have shown that the device has the potential of providing long-term, high-resolution measurements of SGD. The improvements on the device using a simple inexpensive laboratory set up, have shown that: (1) connecting an extension cable to the seepmeter has a negligible effect on its measuring capability and, (2) influence of very low temperature (≤ 3 ºC) on seepmeter measurements can be accounted for by conducting calibrations at such temperatures prior to field deployments and, (3) salinity had no significant effect on the performance of the seepmeter. Calibration results from fresh water and sea water showed close agreement at a 95% confidence level significance between the data sets from the two media (R2 = 0.98). The observed artifacts on seepmeter measurements associated with Bernoulli-induced flow, the vertically directed flow arising due to water movement across topographic features can significantly be reduced by burying (or submerging) the seepmeter to nearly the same level as the sediment topography. While the study revealed that in general wind speeds > 6 m/s were associated with enhanced SGD measurements in seepmeters with buried and unburied benthic chambers, the influence was greater in the unburied meters, and more pronounced for SGD rates < 2 cm/day. Comparatively, the seepmeter SGD measurements provided data that are comparable to manually-operated seepage meters, the radon geochemical tracer approach, and an electromagnetic (EM) seepage meter. Study of the Sarasota Bay (SB) system revealed SGD advection rates ranging from 0.7 to 24.0 cm/day, except for rare isolated hot spot occurrences where higher rates were observed. In general, SGD estimates were relatively higher in the middle and south regions (5.9 – 24.0 cm/day) compared to the north region (0.7 – 5.9 cm/day). Although no obvious seawater nutrient concentration trend was revealed, the average N/P ratio was higher in the north compared to the middle and south regions of the SB system. The importance of SGD was evident in that about 40% of the regional nutrient fluxes were observed in the north while ~ 60% occurred in the middle and south regions combined. The latter two regions also had the highest overall nutrient flux per water volume ratio, compared to the north region, thus making them potentially more vulnerable to eutrophic conditions. On average, we estimate about 27% of total dissolved N in the SB system was derived via SGD.

Coupling between hydrology in unsaturated porous media and geochemical models

Article

Full-text available

Apr 2009

Along the beach we can observe tidal variations who lead to a runoff of seawater into sand beach, this infiltration infer some geochemical reactions that we want to quantify. We have developed a 2D numerical model that couples the non-linear Richards'equation for describe runoff into beach permeable sediment with transport-diffusion equations submitted to tides influence. The flow into the sediment is described by the Richards' equation which generalizes the Darcy's law for variably-saturated porous media. The velocity field and the watertable location, deduced from the numerical resolution of the Richards' equation, are introduced into the transport diffusion-equation. Tidal oscillations are modeled as a sinusoidal pressure boundary condition along the beach slope. Both flow characteristics and concentration are solved by finite element method. Numerical results have been compared with concentration measured in the Truc Vert beach located along the Atlantic coast.

Tracer dispersal in sandy sediment porewater under enhanced physical forcing

Article

Oct 2007
CONT SHELF RES

This research (1) integrated a fluorescent dye injection and monitoring system for measuring the mixing of a fluorescent dye tracer (fluorescein) in permeable (sandy) sediments with a cabled ocean observatory, Kilo Nalu, Oahu, Hawaii, and (2) used this system to conduct remotely controlled in situ measurements of wave-enhanced porewater mixing in a physically well-characterized wave-dominated setting. Laboratory results indicated that the fiber-optic sensor is effective at measuring fluorescence-traced enhanced mixing in sandy sediments. Observed dye mixing, driven by paddle-generated surface waves in a laboratory wave channel was 2 3 orders of magnitude greater than molecular diffusion, and decreased with depth in the sediments. Similarly, field experiments used a remotely controlled submersible syringe pump for fluorescent tracer injection into sediment that was monitored with a fiber-optic sensor. These experiments were carried out at 10 m water depth, with mean wave heights of 0.3 1.5 m and periods of ˜15 s. At 15 cm sediment depth, transport rates of 0 5 cm h-1 were measured, with maximum dispersion coefficients 2 3 orders of magnitude faster than sedimentary molecular diffusion. Hydrodynamic measurements collected simultaneously via Kilo Nalu correlated with porewater transport, with significant wave height and diffusion having the strongest covariation.

Oxygen exchange flux between sediment and water in an intertidal sandflat, measured in situ by the eddy-correlation method

Article

Full-text available

Jan 2006

High temporal–resolution fluctuations in oxygen concentration and vertical velocity were measured over an intertidal sandflat (water depth <69 cm) using an oxygen microelectrode and an acoustic Doppler velocimeter, in order to estimate oxygen flux across the sediment–water interface using the eddy-correlation method. The effect of flux estimate procedures, including noise removal and extraction of fluctuating components, was investigated. The estimated oxygen effluxes from the sediment ranged from –3.2 to 6.6 mmol O2 m–2 h–1 in the light and from –14.5 to –6.6 mmol O2 m–2 h–1 in the dark. The oxygen-uptake fluxes in the dark were markedly higher than those measured by a conventional enclosure technique. High-frequency turbulence and/or noise (>5 Hz) observed in the vertical velocity and oxygen concentration data made little contribution to the total oxygen flux (0 to 7%). However, trends (steady change over a longer time scale) caused significant artifacts in the estimated fluxes for several cases. Thus, removal of trends from raw time-series data is recommended. The co-spectrum of the fluctuating components of vertical velocity and oxygen concentration revealed that the oxygen flux at a frequency band between 0.3 and 1.4 Hz (at a period from 0.7 to 3.3 s) was a major contributor to the total oxygen flux. This frequency was consistent with the dominant frequency of vertical velocity, indicating that transport and exchange of porewater and overlying water by wind-induced waves may be crucial processes to dissolved oxygen flux between permeable sandy sediments and shallow waters.

Long-term phytoplankton-nutrient interactions in a shallow coastal sea: Algal community structure, nutrient budgets, and denitrification potential

Article

Full-text available

Jan 2000

In the eutrophic Marsdiep, the westernmost tidal inlet of the Wadden Sea, phytoplankton biomass, and production almost doubled at the end of the 1970s and remained high ever since. Principal component analysis of 21-yr (19741994) high-resolution time series of the 32 most numerous marine algal species revealed that the phytoplankton community changed drastically both between 1976 and 1978 and again between 1987 and 1988, and that it was relatively stable in-between (1974-1976, 1978-1987) and thereafter (1988-1994). These major changes in phytoplankton biomass and species composition coincided with changes in absolute and relative (TN:TP) nutrient concentrations. During the summer of 1977, the Marsdiep shifted from a rich, but phosphorus-controlled system to an even more eutrophic but nitrogen-controlled environment. The system reshifted towards P-control between 1987 and 1988. The coincidence of the shifts in relative nutrient concentrations and phytoplankton species composition implies a strong causal relationship between TN:TP ratios and phytoplankton community structure. Among diatoms, the observed increase in phytoplankton biomass under eutrophic N-controlled conditions was particularly due to an increase of the abundance of larger algae. Our results indicate that the N budget of the area is correlated with the community structure, suggesting enhanced loss of nitrogen to the sediment through increased deposition of larger algal cells.

Sandy sediments as active biogeochemical reactors: Compound cycling in the fast lane

Article

Full-text available

Sep 2008

Carlos Rocha

The concept of assessing benthic biogeochemical standing stocks as an ecologically relevant parameter has been challenged by one of a dynamic nature: in sands, low standing stocks may mean low carbon burial efficiency due to rapid turnover aided by advective interfacial flows. This concept suggests that a large, diverse and very adaptable population of microbes is present in sands, and that these have a previously unforeseen biogeochemical importance. This view has profound consequences for the scientific outlook on the ecological role of permeable sediments and on the methodological strategies used in the study of coastal ecosystems. Based on a review of the current literature and results gathered at a coastal setting, progress within this new paradigm is examined and underlying questions are speculated upon. The evidence so far shows that, in permeable sediments and at timescales of seconds to a few hours, the dynamics of advective flow to a large extent control microbial diversity, the rates of microbial processes, the size of organic and inorganic pools, and even their respective changes. The importance of obtaining further information on the microbial diversity, the structure of different communities present in sands and their link to biogeochemical function arises from recent field studies. What is also clear from the available evidence is that only a combination of different techniques and approaches, some of which are under development on the fringes of previously almost water-tight research areas, will further understanding of the important functional role of microbial populations in benthic ecosystems.

Silicic acid flux to the ocean from tidal permeable sediments: A modeling study

Article

Jun 2012
COMPUT GEOSCI-UK

Attenuation of groundwater pressure due to surface waves.

Article

Full-text available

Apr 2010

Anna Przyborska

For tideless seas, the groundwater flow in shallow water is governed entirely by the surface wave dynamics on the beach. As waves propagate towards the shore, they become steeper owing to the decreasing water depth and at some depth, the waves lose their stability and start to break. When waves break, waves energy is dissipated and the spatial changes of the radiation stress give rise to changes in the mean sea level, known as the set_up. Longuet-Higgins demonstrated that the mean on-shore pressure gradient due to wave set_up driver a groundwater circulation within the beach zone. Water infiltrates into the coastal aquifer on the upper part of the beach near the maximum run_up, and exfiltration occurs on the lower part of the beach face near the breaking point. The velocity of the flow as well as the amount of water circulation within the permeable beach is important for the biological status of the organisms inhabiting the beach sand, transporting organic matter and dissolved oxygen to beach body , influence on sediment transport at shallow waters and stability of engineering structures. The paper is organized in two main parts. The first part of the paper is dedicated to the formulation of the mathematical model for attenuation of pore pressure in shallow water zone when wave breaking is present. Solution of system of nonlinear equations for wave propagation on permeable beach is compared with experimental data. The main purpose of the experimental part of the paper is dealing with the analysis of sets of good quality data on pore pressure data which will serve for comparison with theoretical results. In particular, two set of data are discussed, namely data obtained during measurements in the shallow water at the Coastal Station Lubiatowo (Poland) in Southern Baltic Sea and data from the large scale laboratory experiments in the Grossen Wallenkanal in Hannover (Germany). In the first case, the set of transmittance functions between the surface waves and pore pressure in the soil at various levels and transmittance functions between the pressures recorded at different levels are compared with the developed theory. During the laboratory experiment in Hannover two components of pore pressure were clearly distinguished i.e. in the zone of non-breaking waves only so called phase resolving component induced by surface waves is observed and in the surf zone two types of pore pressure are present : phase resolving and so called phase averaged , induced by set-up phenomena (mean water level rising). The total pressure recorded by the pressure gauges is a summation of the phase-averaged and the phase-resolving components. The pore pressure gradients provide also valuable information on the kinematics of groundwater flow in the beach body. In the experiment we are not able to measure the flow velocity in a straightforward manner, but the flow velocity can be estimated from the recorded pressure gradients using the formulas resulting from the theoretical solution.

Magnitudes of submarine groundwater discharge from marine and terrestrial sources: Indian River Lagoon, Florida

Article

Full-text available

May 2007

1] Magnitudes of terrestrial (fresh) and marine (saline) sources of submarine groundwater discharge (SGD) are estimated for a transect across Indian River Lagoon, Florida. Two independent techniques (seepage meters and pore water Cl À concentrations) show terrestrial SGD decreases linearly to around 22 m offshore, and these techniques, together with a model based on the width of the outflow face, indicate a cumulative discharge of between 0.02 and 0.9 m 3 /d per meter of shoreline. Seepage meters and models of the deficiencies in 222 Rn activity in shallow sediments indicate marine SGD discharges of roughly 117 m 3 /d per meter of shoreline across the entire 1800-m-wide transect. Two surface streams nearest the transect have an average discharge of about 28 m 3 /d per meter of shoreline. Marine SGD is thus 4 times greater then surface water discharge and more than 2 orders of magnitude greater than terrestrial SGD. The magnitude of the terrestrial SGD is limited by the amount of regional precipitation, evaporation, recharge, and groundwater usage, while marine SGD is limited only by processes circulating marine water into and out of the sediments. The large magnitude of marine SGD means that it could be important for estuarine cycling of reactive components such as nutrients and metals with only slight modification from estuarine water compositions. The small magnitude of terrestrial SGD means that large differences from estuarine water composition would be required to affect chemical cycling.

Aquatic sediments

Article

Jun 1994

References

Chapter

Jan 1997

Bernard P. Boudreau

Spatial and Temporal Distribution Patterns and Their Underlying Causes

Chapter

Jan 1999

Temporal developments of dissolved nutrients, phytoplankton and zooplankton in the water column were investigated on different time scales from 1994 to 1996. Measurements of single tidal cycles allowed to assess the fate of some species in the Wadden Sea. Dissolved nutrients showed some regularly recurring patterns, which could be explained by the tidal mixing of water bodies with different nutrient concentrations. Seasonal cycles of the nutrients, phytoplankton and zooplankton showed a large interannual variability. Timing and extension of algal blooms influenced the occurrence of many meroplanktonic larvae. Most of the algal species in the backbarrier system of Spiekeroog were part of the phytobenthos and the phytoplankton.

Okubo-et-al-2002-Passive-Diffusion

Data

Dec 2015

Attenuation of Groundwater Flow Due to Irregular Waves in Permeable Sea Bottom

Article

Full-text available

Jan 2014

Anna Przyborska

The disappearance of wave energy in porous medium, expressed by the decreasing pore pressure, depends on the properties of the soil matrix and compressibility of water in the matrix pores. On the other hand, the compressibility of pore water depends to a large extent on the content of air/gas in water. Thus, it seems that the problem of water circulation resulting from surface waves is in fact a problem from the theory of multiphase media. Transmittance functions between surface waves and pore pressure at different depths as a result of the linear wave theory well reproduce the experimental data at frequencies close to the spectrum of peak energy.

In situ determination of porewater gases by underwater flow- through membrane inlet mass spectrometry

Article

Mar 2012
LIMNOL OCEANOGR-METH

An underwater membrane introduction mass spectrometer was deployed in permeable sandy sediment on the Georgia continental shelf (depth = 27 m) to measure in situ dissolved gas concentrations in sediment porewaters. Over a 54-h period, 30 profiles (up to 18 cm deep) were sampled using an automated sediment probe coupled with an underwater positive displacement syringe pump. Porewater was analyzed with a flow-through membrane assembly at constant sample flow rate (0.35 mL/min) and membrane temperature (45°C). Calibration was performed on-site using ambient seawater equilibrated with gas standards. Measurements of methane, nitrogen, argon, oxygen, and carbon dioxide concentrations were used to produce depth-time contours and demonstrate the dynamics of dissolved gases in the porewater. Profiles indicated a well-oxygenated surface layer (1 to 2 cm depth) and anoxia below -3 to -5 cm. Elevated concentrations of methane below the oxycline reveal active methanogenesis in shelf sands despite low (0.05%) organic carbon content. Chemocline depth and sediment ripple height were correlated, suggesting that the porewater environment is controlled by advection-driven interactions between boundary-layer flow and bottom topography. By coupling in situ concentration profiles to independent estimates of sediment-water exchange, it was estimated that maximal oxygen consumption at this site occurs > 2 cm below the interface. Oxygen consumption at this site is estimated as 2.3 mmol m-2 d-1 based on combined dissolved profiles and advection estimates. Raw data and data analysis scripts (Matlab) are available electronically in a Web Appendix. © 2012, by the American Society of Limnology and Oceanography, Inc.

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.

The Influence of Ambient Groundwater Discharge on Exchange Zones Induced by Current-Bedform Interaction

Article

Nov 2006
J HYDROL

We use numerical modeling to investigate the interactions between laminar flow in a water column, current bedform induced flow in underlying permeable sediments, and discharge of deep groundwater to the water column. Groundwater discharge reduces the spatial extent of current-induced exchange zones within the sediments and may completely prevent the development of current bedform induced flow at high groundwater discharge. Although exchange zone size is reduced by groundwater discharge, the reduction in exchange flux is not significant, suggesting reduced residence times of materials cycled through these smaller zones. Fluid exchange zones tend to be centered around the reattachment points of eddies that also mark the location of flow divides within the sediments; while deep groundwater is discharged preferentially along troughs. Exchange zone depth and the water column Reynolds number are functionally related through a Morgan Mercer Flodin model. The relationship between the current-induced flux through the sediments and the Reynolds number is described by low-order monotonic polynomial functions.

Transfer of Nonsorbing Solutes to a Streambed With Bed Forms: Theory

Article

Jan 1997
WATER RESOUR RES

A theoretical analysis of the exchange of nonsorbing solutes between a permeable streambed and the overlying water of a stream or river is presented in this paper. In a companion paper [Elliott and Brooks, this issue] the results of experimental studies of such exchange are presented. The analysis focuses on water movements into, within, and out of the bed which result from the presence of bed forms (ripples and dunes) and highlights the mechanisms of “pumping” and “turnover.” Pumping is the movement of fluid through the bed induced by steady dynamic pressure variations over bed forms. Turnover occurs as moving bed forms trap and release interstitial fluid. The detailed analysis was used to generate the distribution of the residence time of solute within the bed, which can be used to calculate the net mass exchange due to pumping and turnover.

Evidence that increased nitrogen efflux from wave-influenced marine sediment enhances pelagic phytoplankton production on the inner continental shelf of Western Australia

Article

Jan 2010

Jim Greenwood

Increased biological and chemical reaction rates within permeable continental-shelf sediment can result from the action of passing surface waves, especially when the seabed is rippled. The effect of this on the exchange of nitrogen between the sediment and water column is the focus of the present paper. The continental shelf of Western Australia is used as an example. A time series of chlorophyll a is compared with surface-wave height revealing seasonal and sub-seasonal correlation between the two variables. At the same time, results from a coupled pelagicbenthic biogeochemical model show that temperature-controlled changes in sedimentary nitrogen efflux cannot account for the observed seasonal changes in chlorophyll a in the overlying water column. It is proposed that enhanced pore-water circulation within the sediment, caused by the action of passing surface waves, results in an increase in the efflux of nitrogen from the sediment during winter, supporting higher pelagic phytoplankton production. The parameterisation of sedimentary nitrogen mineralisation as a function of the square of wave height is suggested for the inclusion of this effect in regional-scale continental shelf models.

Hydraulic control of pore water geochemistry within the oxic-suboxic zone of a permeable sediment

Article

May 2000

The geochemical behavior of the top 70 cm of permeable reef flat sediments on Checker Reef, Oahu, Hawai'i was examined using spatial and temporal changes in pore water O2, NO , NO , NH , and N2O concentrations. 221 324 The shallow depth of the sediment-water interface relative to the height of waves propagating across the reef creates an environment in which pore water mixing and transport are dominated by wave-induced mechanisms. Dissolved oxygen penetrated 15-50 cm in the four sites studied. The depth-integrated concentration of dissolved oxygen was greater under larger wave conditions and in more permeable sediments, which suggests that dissolved oxygen may be a good indicator of the relationship between sediment metabolism and the physical nature of the sediment environment. The absence of any subsurface N2O maxima, limited temporal variability in pore water N2O concen- trations, and vertically coherent profiles of O2, NO , NO , and N2O suggest that suboxic processes are typically 22 32 restricted to spatial scales that are smaller than for oxic processes in these sediments. The geochemical response of the sediments to a natural perturbation was monitored by examining the composition of pore waters immediately following a large wave event and for several weeks thereafter. Significant changes in the depth-integrated concen- trations of O2, NO , NO , and NH were observed following the perturbation; however, only changes in NO 22 1

Free Surface Flow over Permeable Wavy Bed

Article

Sep 1998

Kazumasa Mizumura

A theoretical analysis based on potential flow theory over a permeable wavy bed and a linear Darcy equation shows that the perturbations of the flow field have a phase shift relative to the wavy bed form. This phase shift results in a net form resistance on the permeable wavy surface of the bed form. The experimental results indicate that there is a phase shift between the permeable wavy bed form and the water surface profile. The theoretical results show that the phase shift and friction factors increase with the Reynolds number of the porous media flow; however, the experimental results show that the phase shift decreases with the Reynolds number of the porous media flow. This is caused by the phenomenon of flow separation on the wavy bed. The water surface profiles over the permeable wavy bed are compared with those over the impermeable wavy bed in the case of the same Froude numbers. The velocity distributions over the permeable and the impermeable wavy bed are also investigated experimentally and theoretically. The boundary wavy profiles of the permeable and the impermeable bed form vary sinusoidally.

The role of adsorption-desorption surface reactions in controlling interstitial Si(OH)4 concentrations and enhancing Si(OH)4 turn-over in shallow shelf seas

Article

Jun 2002

We report on rapid adsorption–desorption of Si(OH) 4 on continental shelf sediments. Experiments were carried out with sandy, low-porosity sediments poor in biogenic silica collected at five stations in the southeastern North Sea and with one silty sediment sampled in the same area. Sorption isotherms for sediments from six stations and three depth layers revealed differences in the capacity of sediments to sorb Si(OH) 4 . The equilibrium concentration, the concentration for which neither adsorption nor desorption occurs, determined for core top sediments correlates with the HCl-extractable Fe content. Our results point to amorphous Fe-oxides as the main sorbing mineral phase. The comparison between Si(OH) 4 equilibrium concentrations derived from isotherms and in situ pore water levels suggests a buffering of interstitial Si(OH) 4 below 300 μM by surface reactions. Repeated exposure of particles to rapid variations of Si(OH) 4 associated with biological and physical stirring of surface sediments, an ubiquitous process in shallow shelf seas, was simulated by resuspension experiments progressing from low to high (0.15–225 μM), respectively, high to low (225–0.15 μM) Si(OH) 4 . Time-series of release or uptake of Si(OH) 4 by suspended sediments were adequately described by a model combining a kinetically controlled, reversible surface reaction, the equilibrium of which was derived from the sorption isotherms, with a first-order dissolution term for Si. Apparent saturation concentrations were below the solubility of biogenic silica. They ranged from 516 to 106 μM. Rate constants for both reactions were quantified and yielded characteristic time-scales extrapolated to in situ pore water conditions in the order of minutes (sorption) to several months (dissolution). Sorption thus appears as the rapid first response to drastic changes in Si(OH) 4 concentrations typically associated with physical or biological reworking of surface sediments and percolation of bottom waters due to wave and current activity. We estimated the amount of Si(OH) 4 added to bottom waters by a resuspension event. Desorption of Si from sediment particles following their resuspension from the seabed into bottom waters has the potential to enhance Si(OH) 4 recycling in shallow shelf seas.

A numerical study of the wave-induced solute transport above a rippled bed

Article

Sep 1995
J FLUID MECH

K.T. Shum

The role of wave-induced separated flow in solute transport above a rippled bed is studied from numerical solutions to the two-dimensional Navier–Strokes equations and the advection-diffusion equation. A horizontal ambient flow that varies sinusoidally in time is imposed far above the bed, and a constant concentration difference between the upper and lower boundaries of computation is assumed. The computed flow field is the sum of an oscillatory rectilinear flow and a vortical flow which is periodic both in time and in the horizontal. Poincaré sections of this flow suggest chaotic mixing. Vertical lines of fluid particles above the crest and above the trough deform into whorls and tendrils, respectively, in just one wave period. Horizontal lines near the bottom deform into Smale horseshoe patterns. The combination of high shear and vortex-induced normal velocity close to the sediment surface results in large net displacements of fluid particles in a period. The resulting advective transport normal to the bed can be higher than molecular diffusion from well within the viscous boundary layer up to a few ripple heights above the bed. When this flow field is applied to the transport equation of a passive scalar, two distinct features – regular temporal oscillations in concentration and a linear time-averaged vertical concentration profile – are found immediately above the bed. These features have also been observed previously in field measurements on oxygen concentration. Advective transport is shown to be dominant even in the region where the time-averaged concentration profile is linear, a region where vertical solute transport has often been estimated using diffusion-type models in many field studies.

The mathematics of early diagenesis: From worms to waves

Article

Full-text available

Aug 2000

Bernard P. Boudreau

The changes that sediments undergo after deposition are collectively known as diagenesis. Diagen-esis is not widely recognized as a source for mathemat-ical ideas; however, the myriad processes responsible for these changes lead to a wide variety of mathematical models. In fact, most of the classical models and meth-ods of applied mathematics emerge naturally from quan-tification of diagenesis. For example, small-scale sedi-ment mixing by bottom-dwelling animals can be described by the diffusion equation; the dissolution of biogenic opal in sediments leads to sets of coupled, nonlinear, ordinary differential equations; and modeling organisms that eat at depth in the sediment and defecate at the surface suggests the one-dimensional wave equa-tion, while the effect of waves on pore waters is governed by the two-or three-dimensional wave equation. Diage-netic modeling, however, is not restricted to classical methods. Diagenetic problems of concern to modern mathematics exist in abundance; these include free-boundary problems that predict the depth of biological mixing or the penetration of O 2 into sediments, algebra-ic-differential equations that result from the fast-revers-ible reactions that regulate pH in pore waters, inverse calculations of input functions (histories), and the deter-mination of the optimum choice in a hierarchy of pos-sible diagenetic models. This review highlights and ex-plores these topics with the hope of encouraging further modeling and analysis of diagenetic phenomena.

Early Diagenesis: A Theoretical Approach

Book

Dec 1980

Robert A. Berner

Small amplitude wave theory

Article

Jan 1966

Porous Media Fluid Transport and Pore Structure

Article

Jan 1979

F. A. L. Dullien

Loss of wave energy due to percolation in a permeable sea bottom

Article

Jan 1949

J.A. Putnam

A theory is developed for the loss in wave energy as a result of currents induced in a permeable sea bottom by wave action. This loss in energy is reflected as a reduction in the wave height as the waves move shoreward. It is shown by numerical examples that this reduction may be on the order of ten per cent on very flat beaches for wave periods and bottom materials commonly found in the ocean.

1. Pore Structure

Article

Dec 1992

F. A. L. Dullien

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

Erratum: ``Wave-induced advective transport below a rippled water-sediment interface'' [Journal of Geophysical Research, 97, 789-808 (1992)

Article

Sep 1992

K. T. Shum

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.

Comparison of oxygen microgradients, oxygen flux rates and electron transport system activity in coastal marine sediments

Article

May 1987

A Wave-Induced Transport Process in Marine Sediments

Article

Jan 1983

We show how surface wave action can increase the rate of transport of solutes into a sandy seabed by orders of magnitude via a mechanism known as mechanical dispersion. It is most effective for large sediment permeability and thickness, high surface wave amplitude, and shallow water. A method for setting up the appropriate transport equation, valid when dispersion is well developed, is given. Its dispersion term contains two mechanical dispersion parameters that can be estimated roughly from existing data when the sediments are well sorted. The dispersion can be inhomogeneous and anisotropic in homogeneous, isotropic sediments. The effect of surface wave action on transport into sediments on the eastern U.S. shelf is shown to be significant under certain conditions. The effect on thawing of subsea permafrost beneath Prudhoe Bay, Alaska, seems negligible.

Article

Mar 1991

Whole‐sediment molecular difusion coefficients ( D s ) for tritiated water in pore waters of various lakes were determined experimentally by adding ³ H 2 O to the overlying water of asphyxiated (without bioirrigation) and unasphyxiated cores and measuring the resulting pore‐water profiles after a period of time. Our objectives were to determine the relationship between D s and D o (the diffusion coefficient in pure water) in sediments with a wide range of porosities and organic contents and to examine the influence of bioirrigation on solute transport and on the predictability of D s . We found that D o /D s did not change as much as expected with increasing porosity, i.e. in low‐porosity sediments the average D o /D s was 1.8±0.1 and in high‐porosity sediments it was 1.5±0.2. We also found that the effect of faunal activity on the predictability of D s was only significant in sediments with high (14,000 ind. m ⁻² ) invertebrate populations. This result means that in most freshwater sediments, the sediment diffusion coefficient can be predicted reliably from the molecular diffusion coefficient at in situ temperature.

Benthic oxygen fluxes on the Washington shelf and slope: A comparison of in situ microelectrode and chamber flux measurements

Article

May 1992

Benthic oxygen fluxes calculated from in situ microelectrode profiles arc compared with benthic flux chamber O2 uptake measurements on a transect of eight stations across the continental shelf and three stations on the slope of Washington State. Station depths ranged from 40 to 630 m and bottom-water oxygen concentrations were 127-38 FM. The fluxes measured by the two methods were similar on the slope, but on the shelf, the chamber flux exceeded the microelectrode flux by as much as a factor of 3-4. We attribute this difference to pore-water irrigation, a process which apparently accounts for the oxidation of a significant amount of organic C in the continental shelf sediments. Combining our diffusive flux data with other data demonstrates clearly that the bottom- water oxygen concentration must play some significant role in determining the sedimentary oxygen consumption rate. Numerical simulation of the microelectrode 0, profiles suggests that roughly half the diffusive 0, flux must be consumed within - 1 mm of the sediment surface. If this conclusion is correct, then the magnitude of the diffusive flux depends both on the bottom-water oxygen concentration and on the supply rate of labile C to the sediment surf'ace.

Bedform-Generated Convective Transport in Bottom Sediment

Article

Jan 1987

Convective flow within bed sediment is an important mechanism enhancing the mobility of chemicals, both natural and anthropogenic, and thermal energy in this region of aquatic environments1,2. Experimental observations indicated that significant in-bed convection currents can be generated by water flowing over small obstructions on the surface of a porous bed. Significant porewater flow is induced by imbalances in pressure over distance, generated by differences in temperature, density and hydrostatic head3. We demonstrate here by laboratory simulation and a vignette model that flow over bedforms induces additional pressure imbalances which generate significant and complex convection currents within porous bed sediment. A model is proposed for estimating Peclet numbers for this effect/The results have particular application to chemical transport in the upper sediment layer that is often the recipient of high levels of chemical contamination. Although our analysis reflects river conditions, the results may have wider applications and include submarine currents moving over dune-like mega ripples on the ocean floor.

Density-Driven Interstitial Water Motion in Sediments

Article

Sep 1982

Sediment depth distributions and fluxes of dissolved chemical substances have been interpreted as being a result of reaction, diffusion, bioturbation and irrigation1,2. However, several studies suggest that density-driven convection3 can alter the depth distribution and increase the fluxes of dissolved substances when density decreases below the sediment surface4–7. We present here temperature–time series measurements for a freshwater lake undergoing autumn cooling. These are the first in situ observations of heat transport due to motion of interstitial waters over periods of less than 1 hour. Density, calculated from temperature, decreases with depth at the time and place that this motion occurs.

Microstructure of diffusive boundary layers and oxygen uptake of the sea floor

Article

Jun 1990

THE diffusive boundary layer (DBL) is a thin (≲1 mm) film of water that covers the sea floor, and through which molecular diffusion is the dominant transport mechanism for dissolved material. The diffusive fluxes are a measure of the rate of remineralization of organic matter in the sea bed, and of the dissolution or precipitation of minerals such as carbonates or metal oxides. Here we report detailed in situ analyses of chemical microgradients within the DBL, using a microelectrode profiling instrument with a spatial resolution of 2550 µm. Over a Danish coastal sediment at 15 m water depth, the DBL was 0.5-0.7 mm thick and showed both stochastic fluctuations of oxygen distribution owing to boundary-layer turbulence and harmonic oscillations resulting from surface waves. A three-dimensional mapping of the DBL and the corresponding sediment surface showed that the DBL was spatially well defined and followed surface contours, but smoothed out sediment microtopographic features smaller than 100 µm. The three-dimensional oxygen diffusive flux across the sediment/water interface was about 2.5 times higher than that calculated from a simple one-dimensional diffusion model. These results indicate that benthic oxygen consumption and other fluxes can be studied by direct measurement of DBL microgradients at the undisturbed sea floor.

Irrigation of Submerged Marine Sands through Wave Action

Article

Nov 1968

PRELIMINARY tests carried out on the permanently submerged amphioxus sands at Le Racou (Roussillon) on the Mediterranean coast of France on August 27, 1968, have shown that turbulence and differences in hydrostatic pressure at the seabed caused by the passage of surface waves produce water movements in permeable deposits. These chiefly quartz sands were at a depth of 3 m and had the approximate grain size distribution, determined from samples taken from the area at this depth, shown in Table 1. Sand ripples ran north to south across the line of approach of the waves of maximum fetch from the east and were approximately parallel to the shore. The crests of the ripples were 75 cm apart and separated by a more or less flat trough 30 cm broad. The height from trough to crest of the ripple was 12–15 cm. The hand could be thrust into the sand without excessive resistance to a depth of about 7.5 cm on the crest of the ripple and 5 cm in the trough, indicating unconsolidated conditions to these depths. It was relatively calm at the time of the test, the wave direction being from the east, that is across the sand ripples, and the wave height from trough to crest about 50 cm.

Wave-induced circulation in submerged sands

Article

Nov 1972
J MAR BIOL ASSOC UK

Pressure waves moving across the sand beds at 3 m depth at Le Racou caused circulation of water in the unconsolidated sand layer. The pattern of circulation apparently depended on the depth of the unconsolidated layer and the form of the sand ripples. The rate of flow was determined by the height of the surface waves and the permeability of the deposit. The physical properties of the sands at the crests and in the troughs of the ripples were in part determined by the organic component which in turn would have been governed largely by the patterns and flow rates of the interstitial currents supplying organic material and nutrients to the sand. Differences in the physical properties of the sediment at the crest and in the trough of a ripple due to the organic component were such as would have affected substantially the prevalence of Branchiostoma lanceolatum and harpacticoid copepods. Sand properties dependent on the organic component would clearly be subject to change in accordance with the prevailing weather conditions and hence the supply of organic material. The role of irrigated sands in the removal of organic material from shallow seas is discussed.

Saffman, P.G.: A theory of dispersion in a porous medium. J. Fluid Mech. 6, 321-325

Article

Oct 1959
J FLUID MECH

P. G. Saffman

This paper is concerned with the dispersion of a dynamically neutral material quantity in a fluid flowing throuh a porous medium. The medium is regarded as an assemblage of randomly orientated straight pores, and it is assumed that the path of a marked element of the material quantity consists of a sequence of statistically independent steps whose direction and duration vary in some random manner. The probability density function for the displacement of a single marked element is calculated and values for the dispersion of a cloud of marked elements then follow.

The Subtidal Pump: A Mechanism of Interstitial Water Exchange by Wave Action

Article

Apr 1972

Sandy subtidal sediments are part of the earth's largest filter system. Water flow through bottom sediments is driven by wave action on the sea surface. The mechanisms involved are described, including a theoretical deduction and field measurements. As an example, the total water exchange through part of the West Atlantic shelf is computed and the influence of the phenomenon is discussed from a biological point of view and with regard to its importance for the world's oceans.

Modelling early diagenesis of silica in non-mixed sediments

Article

Feb 1990
Deep Sea Res

Bernard P. Boudreau

The diagenesis of dissolvable opal in non-mixed sediments of anoxic and dysaerobic basins is described by a set of couple differential conservation equations. A semi-analytical solution is developed for the special case of the steady-state constant-porosity diagenesis. This approach reduces the problem of predicting opal accumulation to one of solving a single nonlinear algebraic equation and provides and explicit equation for the critical opal flux below which no opal would be preserved. It also allows the derivation of a simple asymptotic model valid when a small fraction of incoming opal dissolves, i.e. <50%.

A theory of dispersion in a porous medium

Saffman

The effect of wave-induced pore water circulation on the transport of reactive solutes below a rippled sediment bed

Abstract

No full-text available

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