Pore Water Discharge Research Articles

Dynamic Response Analysis and Liquefaction Potential Evaluation of Riverbed Induced by Tidal Bore

Tidal bores, defined by sudden upstream surges of tidal water in estuaries, exert significant hydrodynamic forces on riverbeds, leading to complex sedimentary responses. This study examines the dynamic response and liquefaction potential of riverbeds subjected to tidal bores in macro-tidal estuaries. An analytical model, developed using the generalized Biot theory and integral transform methods, evaluates the dynamic behavior of riverbed sediments. Key factors such as permeability, saturation, and sediment properties are analyzed for their influence on momentary liquefaction. The results indicate that fine sand reduces liquefaction risk by facilitating pore water discharge, while silt soil increases sediment instability. Additionally, the study reveals that pressure gradients induced by tidal bores can trigger momentary liquefaction, with the maximum liquefaction depth predicted based on horizontal pressure gradients being five times that predicted based on vertical pressure gradients. This research highlights the critical role of sediment characteristics in riverbed stability, providing a comprehensive understanding of the interactions between tidal bores and riverbed dynamics. The findings contribute to the development of predictive models and guidelines for managing the risks of tidal bore-induced liquefaction in coastal and estuarine environments.

Rivers and tidal flats as sources of dissolved organic matter and trace metals in the German Bight (North Sea)

The German Bight in the southern North Sea receives nutrients, dissolved organic matter (DOM), and trace metals from rivers, porewater reflux, and porewater outwelling. We studied the marine, riverine, and porewater sources analyzing molecular transformations of solid-phase extracted (SPE) DOM in the German Bight. We applied a combination of ultrahigh-resolution mass spectrometry (FT-ICR-MS) with quantitative data of dissolved organic sulfur (DOS), dissolved black carbon (DBC), dissolved trace metals (Ba, Co, Gd, Mo, Mn, W), and nutrients (nitrite, nitrate, phosphate, silicate). While aromatic DOM and DBC mainly originated from the rivers, nitrogen-containing, more saturated DOM was enriched offshore suggesting greater contributions of marine (algal) sources. Except for dissolved Mo, rivers were the primary source of trace metals and nutrients. However, tidal flats contributed to dissolved nutrient (e.g., dissolved phosphate), trace metal and DOS inventories of the southern North Sea. The input of DOS from intertidal flats was identified by the molecular index derived from sulfidic porewaters (ISuP), non-conservative behavior of elemental sulfur-to-carbon ratio and sulfur content of molecular formulae (from FT-ICR-MS). Dissolved Mn and Si were removed in the German Bight, likely due to precipitation as Mn(hydr)oxides and biological uptake, respectively. Preliminary estimates suggest that DOS from porewater outwelling is approximately four times higher than DOS discharged by the three main rivers in the region. Our study therefore highlights the need to consider porewater discharge in addition to riverine sources to comprehensively assess elemental budgets within the complex interplay and transformations of DOM, nutrients, and trace metals in coastal ecosystems.

Seismic Performance of Drained Piles in Layered Soils.

The provision of drains to geotechnical elements subjected to strong ground motion can reduce the magnitude of shaking-induced excess pore pressure and the corresponding loss of soil stiffness and strength. A series of shaking table tests were conducted within layered soil models to investigate the effectiveness of drained piles to reduce the liquefaction hazard in and near pile-improved ground. The effect of the number of drains per pile and the orientation of the drains relative to the direction of shaking were evaluated in consideration of the volume of porewater discharged, the magnitude of excess pore pressure generated, and the amount of de-amplification in the ground's motion. The following main conclusions can be drawn from this study. Single, isolated piles and a group of drained piles were tested in three series of shake table tests. Relative to conventional piles, the drained piles exhibited improved performance with regard to the generation and dissipation of excess pore pressure and stiffness of the surrounding soil, with increases in performance correlated with increases in the discharge capacity of the drained pile. The acceleration time histories observed within the pile-improved soil indicated a coupling of the rate and magnitude of porewater discharge, excess pore pressure generated, and de-amplification of strong ground motion. The amount of de-amplification reduced with increases in the number of drains per pile and corresponding reductions in excess pore pressure. The improved performance should prove helpful in the presence of sloping ground characterized with low-permeability soil layers that inhibit the dissipation of pore pressure and have demonstrated the significant potential for post-shaking slope deformation.

Construction and Calculation Analysis of Mechanical Model of Crushed Rock Pile Treatment of Foundation Earthquake

In recent years, earthquakes have occurred frequently at home and abroad, causing huge losses to the local society. Therefore, it is important to study the mechanical response of foundations under earthquakes. In this paper, firstly, gravel piles are selected as soft ground improvement materials, and A single bulk material pile section and a peripile soil section within the influence of a single pile are considered as a study unit, and the pore water discharge from the consolidation process is equal to the volume reduction of the unit, considering the consolidation deformation of the pile body, the composite foundation consolidation equation is derived, and the consolidation equation is solved by the separation variable method through the isostrain assumption and initial boundary conditions, and the average superporous water pressure of the soil between the body and the pile is obtained. Then, a more realistic non-uniform ground motion input method is adopted, and the seismic fluctuations are converted into equivalent loads on artificial boundary nodes through the Matlab program, and the equivalent loads are applied to the established viscoelastic artificial boundary by OpenSees software to realize ground motion input. Finally, a mechanical analysis model is established by hypothesis, and with the help of mode orthogonal theory and Hilbert yellow transform, the calculation method of ground seismic mechanical response based on non-uniform ground motion input is obtained. The results show that the effect of gravel pile encryption can reduce the accumulation rate and peak of super-static pore pressure during vibration, and the surface settlement of the site is reduced by 40–50% and the surface lateral shift is reduced by 30–50% after encryption; the drainage effect has a significant effect on reducing the accumulation rate of super-static pore pressure and the post-earthquake dissipation time of the soil during vibration, and the surface settlement is reduced by about 10–20% and the lateral shift is reduced by about 30–40% after increasing the drainage effect.

Benthic jellyfish act as suction pumps to facilitate release of interstitial porewater

Upside-down jellyfish, genus Cassiopea (Péron and Lesueur, 1809), are found in shallow coastal habitats in tropical and subtropical regions circumglobally. These animals have previously been demonstrated to produce flow both in the water column as a feeding current, and in the interstitial porewater, where they liberate porewater at rates averaging 2.46 mL h−1. Since porewater in Cassiopea habitat can be nutrient-rich, this is a potential source of nutrient enrichment in these ecosystems. This study experimentally determines that porewater release by Cassiopea sp. jellyfish is due to suction pumping, and not the Bernoulli effect. This suggests porewater release is directly coupled to bell pulsation rate, and unlike vertical jet flux, should be unaffected by population density. In addition, we show that bell pulsation rate is positively correlated with temperature, and negatively correlated with animal size. As such, we would predict an increase in the release of nutrient-rich porewater during the warm summer months. Furthermore, we show that, at our field site in Lido Key, Florida, at the northernmost limit of Cassiopea range, population densities decline during the winter, increasing seasonal differences in porewater release.

Analysis of One-Dimensional Consolidation for Double-Layered Soil with Non-Darcian Flow Based on Continuous Drainage Boundary

The boundary drainage performance controls the rate of pore water discharge in the soil and plays an important role in the prediction of soil consolidation and settlement. Based on a continuous drainage boundary that can reflect the change of boundary drainage performance with time, a one-dimensional consolidation model of double-layered soil considering non-Darcian flow is established. The finite-difference method and semianalytical method are used to solve the consolidation equation, and the reliability of the two methods is verified by comparing with existing solutions. Based on the proposed solution, the consolidation behaviors of the double-layered soil are explored in depth through a systematic parametric study. The results show that, if the time effect of drainage boundary and the influence of non-Darcian flow are ignored, the estimated consolidation rate is relatively fast in the whole consolidation stage. The non-Darcian flow has a greater influence on soil consolidation under the continuous drainage boundary condition compared with that under the traditional drainage boundary condition. The consolidation rate of the foundation can be improved by appropriately increasing the permeability of the underlying soil layer or decreasing the compressibility of the underlying soil layer.

Salt Transport Under Tide and Evaporation in a Subtropical Wetland: Field Monitoring and Numerical Simulation

AbstractTidal wetland ecosystems are sensitive to porewater salinity dynamics. However, it is unclear how salts move and distribute in these wetlands, particularly how the salts accumulated by evaporation get removed from the wetland soil, so that the salinity levels may stabilize to accommodate vegetation. We conducted a combined field and modeling study to identify the porewater flow and salinity patterns in a subtropical wetland subjected to tidal inundation and evaporation. Measured and simulated salinity contours indicated the formation of hypersaline porewater plumes in the upper intertidal zone and the fresher porewater zones with salinity close to that of seawater near the creek and in the supratidal zone. Simulations indicated the discharge of the hypersaline upper intertidal porewater to the creek with a discharge pathway developed under the fresher near‐creek porewater zone, and this was further confirmed by the field‐observed significant salinity gradient under the creek. Our model suggested that both water and salt discharge from the wetland soil occurred predominantly at the creek bank and creek bed. The porewater discharge is more intensive through the creek bank than the creek bed, while the salt discharge across both the creek bank and creek bed was comparable due to the much higher salinity level under the creek bed. Salt discharge driven by density gradients and tidal‐induced porewater circulation provides a mechanism for removing salts accumulated in the upper intertidal zone due to evaporation and could prevent salt flat formation and marsh plants dieback.

Variations in the mineral structures dominating solute mobilization during clay compaction

Microbial-mediated redox reactions typically control the solute mobilization in groundwater systems, in which the clay aquitard is usually a triggering factor of the release of the dissolved organic carbon, ammonium, and iron into the aquifer during pore water discharge and clay compaction. However, solute mobilization inside the aquitard during clay compaction is generally mistaken for a similar water–rock interaction process in aquifers controlled by microbial-mediated redox reactions. Through a simulation experiment on argillaceous sediment compaction, we tested the currently accepted solute mobilization mechanism and determined that the variation in the mineral structure dominates it. The variation in the mineral structure occurs in low symmetry minerals, such as clay minerals, and is controlled by the sediment moisture content, the liquid-plastic limit, and the effective stress. When the sediment moisture content decreases to below the plastic limit through pore water discharge and compaction, sudden changes in the mineral structure occur, releasing iron and capturing ammonium through variations in the relative position of the Si-O tetrahedrons and isomorphism of similar atoms without the participation of microbial-mediated redox reactions. These results suggest that the biochemical reactivities of organic carbon, iron, and ammonium are sometimes overestimated, i.e., when the role of the physical processes is ignored, in solute mobilization during clay compaction, which warrants more attention and investigation.

Exploring the suitability of using Foam concrete as pore pressure dissipation measure for slope Stability: A state of art review

Landslides are one of the major factors leading to disruption of transportation and urbanization of north-eastern India (NEI). The primary reasons are continuous precipitation and alarming seismicity at frequent intervals. Among various factors which can trigger landslide, most common is the increase of pore water pressure (PWP). This in turn increases the weight of the backfill and subsequently contribute to the overturning moment. Hence, measures to extract the pore water (PW) from the backfill through provision of drainage path is an efficient and widely followed approach to mitigate landslides. This paper presents a detailed overview on various landslides which have happened in the NEI during rainfalls and earthquake events, confirming these as two main triggering factors. Later, the limitations of various conventional landslide prevention techniques is also discussed. Though RWs are commonly adopted slope stabilization techniques, many case studies exist in where even RWs had failed to stabilize the slopes particularly where the slopes with higher groundwater table were subjected to heavy precipitation. The primary reason of failure was development of excess PWP behind RWs. While weep-holes in the RW play a critical role in dissipating PWP of the backfill, these are clogged resulting in the failure of the RWs. The fact cannot be denied that the effectiveness of the weep-hole depends upon its design, construction as well as its maintenance.Recently few researchers have shown that the Foam concrete (FC), a type of lightweight concrete, act as an excellent draining material to facilitate PW discharge for slope stabilization owing to its high permeability. Further, as the density of air voids in FC is more, rate of clogging is expected to be lesser, thus demanding lesser maintenance when compared to conventional weep-holes. This way, the need of regular maintenance of weep-holes can be minimized by use of FC. It should be highlighted here that though FC has lot of potential to be use in aforementioned application, there is not much of research exist in the literature on use of FC for PWP dissipation application. In this light, the present work also presents an in-depth review on permeation characteristics of FC, enumerating the PWP dissipation mechanism with the help of case studies from the literature.

Comparative analysis of soft soil consolidation time due to improvement using Prefabricated Vertical Drain

Construction built on soft soils will cause several geotechnical problems, one of which is a consolidation settlement that lasts for a long time due to the low permeability of soft soils. This study aims to compare the time of soft soil consolidation settlement before and after soil improvement using Prefabricated Vertical Drain (PVD). PVD is a plastic sheet made from a composite, consisting of a drain core and filter jacket with a function to accelerate the process of pore water discharge to the ground surface by shortening the horizontal waterway. The method used using analysis, where the time of consolidation settlement reaches a degree of consolidation of 90%. The PVD pattern used is a triangle pattern and a rectangular pattern with a distance variation of 0.6 m; 0.7 m; 0.8 m; 0.9 m; 1.0 m; 1.1 m; and 1.2 m. The results obtained, based on the PVD pattern, the time of consolidation settlement is faster if using a triangle pattern compared to the rectangular pattern. Whereas, based on distance, the smaller the distance between PVDs, the faster the time of consolidation will decrease.

Laboratory Model Tests on Consolidation Performance of Soil Column with Drained‐Timber Rod

Timber pile is an ancient technology applied in soft ground improvement for more than 1000 years. With the rise of many high‐rise buildings, many types of modern mechanized‐construction piles are widely developed and applied; for example, steel pile, precast concrete pile, sand pile, and gravel pile are widely used instead of timber piles. Yet, in some special conditions, timber piles have certain advantages due to their environment‐friendly characteristics, which result in obvious economic benefits and suitability. To overcome the weakness of the traditional timber pile technology and expand its application in engineering practice, a drained‐timber pile technique was put forward. This technology is to wrap the permeable filter‐type drainage geotextile around the timber pile, so that the timber pile not only has the replacement function to strengthen the foundation but also has the drainage function, can accelerate the pore water discharge, and speeds up the soft soil foundation consolidation. The reduced scale soil‐column consolidation model was designed to perform the consolidation tests for the soil column with a drained‐timber rod. In total, eight types of soil‐column consolidation tests were carried out to verify the feasibility and effectiveness of the drained‐timber pile technique. The results revealed that, under the same loading and consolidation time, the drained‐timber rod can obviously increase the degree of consolidation when compared with traditional timber rod. It can be expected that the drained‐timber pile technique has a good application prospect for the construction of medium‐small hydraulic structures and for the treatment of super soft clay.

Manganese dynamics in tidal basins of the Wadden Sea: Spatial/seasonal patterns and budget estimates

The concentrations of dissolved and particulate manganese (Mndiss and Mnpart) were studied in the Wadden Sea and adjacent estuaries to gain insights into spatial and regional differences, but also to assess the source function of the tidal basins for the open North Sea compared to river discharge. Ten transects across tidal basins of the southern and northern Wadden Sea as well as the estuaries of the rivers Elbe, Weser, and Ems were sampled. A case study in the tidal basin of Spiekeroog Island (southern Wadden Sea) was further used to evaluate seasonal and inter-annual Mn dynamics and the potential importance of deeper, groundwater-associated (up to 20 m sediment depth) Mn sources.In the estuaries and the Jade Bay, Mnpart is the predominant Mn fraction, whereas Mndiss is the major faction in the tidal basins. Higher Mndiss levels in the tidal basins of the southern compared to the northern Wadden Sea result from differences in tidal basin characteristics such as intertidal area and tidal asymmetry, influencing the extent of benthic-pelagic interaction and material import. Offshore-directed Mndiss fluxes exhibit less regional variability than Mndiss levels. Overall, the net export fluxes of total Mn from the tidal basins are variable but clearly exceed the river input for the considered summer situation. Annual Mndiss export fluxes, exemplified for the Spiekeroog tidal basin, exhibit high seasonal variations with highest fluxes during spring and late summer. Along with onshore material transport, budget calculations for the Spiekeroog tidal basin indicate that offshore-directed Mndiss fluxes are mainly controlled by surface and deep pore water discharge. Groundwater-associated Mn-rich pore waters in the deep sediments of the intertidal area are likely counteracting sedimentary Mn exhaustion on longer time scales.

Experimental test of concrete plate deflection on soft soil improved by prefabricated vertical drain

The low permeability of soft soils results in slow water flows from the consolidation and the low bearing capacity. The soil improvement that can be used is Prefabricated Vertical Drain (PVD). The use of PVD significantly reduces the length of the soil pore drainage pathway, so that it can increase the bearing capacity of the soil due to the compaction process from the release of pore water from the soil. Experimental testing in the laboratory was carried out using a steel box measuring 1000 mm × 1000 mm × 1000 mm. There are three samples, soil without PVD and soil with PVD triangular patterns of distances of 100 mm and 150 mm. Preloading is carried out before loading on the concrete plate. Concrete plates measuring 700 mm × 100 mm × 30 mm and load media are proving rings with a capacity of 10 kN. The load placed in the center, the end, and the edge of the plate. The loading test results on a concrete plate on soil with PVD can reduce deflection that occurs compared to soil without PVD. Deflection reduction is more effective for PVD with a distance of 100 mm.

Porewater exchange drives trace metal, dissolved organic carbon and total dissolved nitrogen export from a temperate mangrove wetland

Porewater exchange is usually the least quantified process in delivering dissolved material from wetlands to coastal waters, although it has been recognised as an important pathway for the transport of trace metal, carbon and nutrient to the ocean. Here, surface water fluxes of dissolved manganese (Mn), iron (Fe), dissolved organic/inorganic carbon (DOC/DIC), total dissolved nitrogen (TDN) and phosphorous (TDP) were estimated from a temperate mangrove wetland (Kooragang Island, Newcastle, Australia). Radon (222Rn, a natural groundwater tracer) was used to develop a mass balance model to quantify porewater exchange rates and evaluate the contribution of porewater-derived dissolved material to the overall wetland surface water export. A 25-h time series dataset depicted a clear peak of Mn, Fe, TDN, DOC and radon during ebb tides which related to porewater discharge. Porewater exchange rates were estimated to be 14.0 ± 6.3 cm/d (0.18 ± 0.08 m3/s), mainly driven by tidal pumping, and facilitated by a large number of crab burrows at the site. Results showed that the wetland was a source of Mn, Fe, TDN and DOC to the adjacent river system and a sink for TDP and DIC. Surface water Mn, Fe, TDN and DOC exports were 4.0 ± 0.6, 6.6 ± 1.6, 23.9 ± 3.6 and 197.7 ± 29.7 mmol/m2 wetland/d, respectively. Porewater-derived Mn, Fe, TDN and DOC accounted for ~95, 100, 89 and 54% of the wetland surface water exports demonstrating its significant contribution. Our study indicates that temperate mangrove wetlands can be a major source of dissolved metal, carbon and nutrient delivery to coastal waters and that mangrove porewater exchange significantly contributes to this process.

Assessing nutrient dynamics in mangrove porewater and adjacent tidal creek using nitrate dual-stable isotopes: A new approach to challenge the Outwelling Hypothesis?

The importance of mangrove-derived material in sustaining coastal food webs (i.e. the Outwelling Hypothesis) is often invoked in support of mangroves conservation. Biogeochemical cycling, particularly nitrogen (N) and phosphorus (P) in mangrove ecosystems, however, is poorly understood because of high spatial heterogeneity and temporal variability of sources, sinks, and transformation pathways. Here we show that the distribution of N and P are intimately related to vegetation distribution, tidal cycles, and seasonality. We examined the dynamics of N and P in sediments and in a tidal creek of the Can Gio Mangrove Forest, Vietnam. Our objectives were to (1) determine the spatial distribution of dissolved inorganic nitrogen and phosphorus in the mangrove forest along a Rhizophora-Avicennia-mudflat transect; and (2) identify the respective inputs and transformation pathways of N and P in the water column via 24-h time series measurements in a tidal creek. Sediment porewater had N-NH4+ and N-NO3− concentrations <11 μM, except in the mudflat where N-NH4+ was as high as 162 μM. This difference was likely due to N-NH4+ uptake by trees in the vegetated areas and suggests that mangrove sediments can be a zone of NH4+ production via ammonification of organic nitrogen. In all stands, P-PO43− concentrations were three-fold higher during the wet season, with a maximum of 34.4 μM. This can be explained by enhanced microbial activity during the rainy season. The phosphorus seasonal trend was also observed in the creek water but with a maximum P-PO43− value of 4.3 μM only. In the tidal creek, N-NH4+ was highly variable (0 to 51 μM), with the higher values measured at low tide and related to porewater discharge from the mudflat. Our data suggest that mangroves act both as a sink of dissolved inorganic nutrients via vegetation uptake and a source of ammonium from unvegetated mudflat porewater towards the tidal creek. The dual stable isotopes approach (δ15NNO3 & δ18ONO3) revealed that this ammonium was later nitrified within the water column. Moreover, the approach showed that some nitrate originated from the river-estuarine system during rising tides. The export of ammonium from mangrove porewater is presumably entirely consumed before exiting the tidal creek, thereby limiting the spatial extent of mangrove Outwelling. Nevertheless, our multi-isotope approach leads us to conclude that nutrients recycling via mangrove-derived organic matter mineralization may play a fundamental role in sustaining coastal food web.

Porewater-derived nutrient fluxes in a coastal aquifer (Shengsi Island, China) and its implication

As an important component of the hydrological and biogeochemical cycle, porewater discharge represents a significant pathway for releasing chemical solutes into coastal zones, particularly in highly permeable aquifers. In this study, a 222Rn advection-diffusion model was used to estimate the porewater discharge in a coastal aquifer (Shengsi Island, East China Sea) during November 2015. Porewater discharge was estimated to range from 7.4 to 25.8 (mean: 12.9 ± 5.8) cm d−1. Furthermore, the estimated porewater-derived nutrient fluxes (dissolved inorganic nitrogen (DIN), phosphorus (DIP) and silicon (DSi)) (mol m−2 d−1) were (1.7 ± 1.4) × 10−2, (2.1 ± 1.1) × 10−4 and (1.5 ± 1.3) × 10−2, respectively. The Si/N ratio of coastal seawater at Shengsi Island was ∼0.83, which is close to that of porewater along the coastal aquifers of Shengsi Island (∼0.92) but higher than that of the Yangtze River Estuary (∼0.68). Thus, porewater-derived Si flux with a higher Si/N ratio may mitigate the outbreak of non-siliceous algae (i.e., Prorocentrum dentatum) in adjacent waters of Shengsi Island. By comparing the SGD-derived nutrient fluxes worldwide, this study suggests that Si flux with a higher Si/N ratio through porewater discharge (or SGD) may strongly influence the Si budget and cycling because such porewater/SGD-derived Si can compensate for the dwindling Si flux from riverine sources due to human activity (i.e., dam construction, reservoirs). Our results are expected to increase our understanding of not only biogenic elements of cycling processes but also eco-environment processes such as the occurrences of harmful algal blooms alone river-influenced coasts.

Radium isotopes–suspended sediment relationships in a muddy river

Radium isotopes are known to be excellent geochemical tracers for study of oceanographic processes. We show here that radium isotopes can also be used to assess adsorption/desorption and transport processes in rivers. The Yellow River (Huanghe), one of the longest, most turbid and heavily regulated rivers in the world, is used as an example. We first investigated the temporal and spatial behavior of radium isotopes (224Ra and 226Ra) in the lower reaches of the river, and found that this zone displayed some of the highest known riverine radium concentrations and fluxes in the world. Suspended particulate matter (SPM) is shown to be the dominant factor controlling radium activities. Laboratory simulation experiments showed that radium desorption from SPM obeys an exponential relationship in fresh water (S = 0). When salinities are >10, the increase in radium concentration follows a linear increase with respect to the amounts of SPM added. Significantly higher radium concentrations (3–5 times), especially for short-lived 224Ra, were observed during the “Water-Sediment Regulation Scheme” (WSRS), an annual management event when ∼15%–55% of the annual water discharge and ∼30%–75% of the annual sediment load are released from a reservoir to control sedimentation in the Yellow River. The radium fluxes during WSRS periods (∼2 weeks long) accounted for more than half of the entire annual load during the periods studied. Sediment erosion and pore water release are also thought to be important processes supplying radium to the river. After a WSRS, Ra desorption from SPM increases and becomes the prevailing process.

Assessing the contribution of porewater discharge in carbon export and CO2 evasion in a mangrove tidal creek (Can Gio, Vietnam)

Although mangrove forests are efficient natural carbon sinks, most of the atmospheric carbon dioxide (CO2) fixed by its vegetation is believed to be exported via tidal exchange, rather than stored in the vegetative biomass and sediment. However, the magnitude of tidal export is largely unknown because direct measurements are scarce. We deployed a novel experimental design that combined automated high-resolution measurements of hydrodynamic, hydrogeochemical and biogeochemical parameters during the dry season in a mangrove tidal creek in the Can Gio Mangrove Forest in Vietnam. The objective was to quantify the tide-controlled water, porewater, DIC and DOC exchange, and estimate the CO2 evasion throughout tidal cycles contrasted by amplitude. Data from three 25-h time series showed a clear peak of DIC, DOC, pCO2, and 222Rn at low tide, particularly during tidal cycles of large amplitude, which directly relate to porewater discharge. Our mass balance models revealed that the tidal creek was a net exporter of dissolved carbon to coastal waters, with an important contribution (38%) coming from DIC in porewater discharge. Porewater exchange varied from 3.1 ± 1.6 to 7.1 ± 2.4 cm day−1. DIC exchange ranged from 352 ± 34 to 678 ± 79 mmolC m−2 day−1; DOC exchange, 20.6 ± 1.9 to 67.7 ± 7.9 mmol C m−2 day−1; and CO2 evasion, 69.9 ± 10.5 to 173.7 ± 26.1 mmolC m−2 day−1. These estimates were in the high range of previous carbon assessments and were explained by (i) the monitoring station being located at equal distance from the head and the mouth of the creek, which minimized carbon degradation and losses associated to transport in water; and (ii) the site being a highly productive mangrove within South East Asia.

Thallium dynamics in the southern North Sea

In open ocean waters thallium (Tl) belongs to the group of conservative elements, even though deviations from this trend have been observed in NW German coastal waters. Here, we report on tidal, seasonal and spatial dynamics of Tl along with Mo and Mn in the water column of a backbarrier tidal flat close to the island of Spiekeroog, the Jade system (Inner Jade and Jade Bay) and the adjacent offshore region. Dissolved thallium (Tldiss) displays strong tidal and seasonal variations (∼25–60 pM) unrelated to salinity. In all study areas, Tldiss clearly deviates from conservative behavior. In general, Tldiss is low during low tide (with a loss of up to 50%) and inversely related to Mndiss, except in summer. The tidal Tl variations as well as the loss of Tl in the water column may be due to Tl removal from pore waters in reducing sediments and drainage of Tl-free but Mn-rich pore waters into the water column during low tide. The negative Tl anomaly can be traced offshore for more than 40 km to the island of Helgoland.The redox chemistry of Tl is not well studied, and Tl removal from pore waters was previously suggested to only occur under anoxic/sulfidic conditions. By contrast, our preliminary pore water results suggest that Tl could be removed already under slightly reducing (suboxic) conditions, likely along with microbially induced Mn reduction in the sediments. Therefore, this study supports the biological involvement in the aqueous cycling of Tl. We propose the use of Tldiss next to Mndiss as valuable indicator of suboxic or anoxic pore water discharge to the coastal realm.

Trace metals partitioning between particulate and dissolved phases along a tropical mangrove estuary (Can Gio, Vietnam)

Mangroves can be considered as biogeochemical reactors along (sub)tropical coastlines, acting both as sinks or sources for trace metals depending on environmental factors. In this study, we characterized the role of a mangrove estuary, developing downstream a densely populated megacity (Ho Chi Minh City, Vietnam), on the fate and partitioning of trace metals. Surface water and suspended particulate matter were collected at four sites along the estuarine salinity gradient during 24 h cycling in dry and rainy seasons. Salinity, pH, DO, TSS, POC, DOC, dissolved and particulate Fe, Mn, Cr, As, Cu, Ni, Co and Pb were measured. TSS was the main trace metals carrier during their transit in the estuary. However, TSS variations did not explain the whole variability of metals distribution. Mn, Cr and As were highly reactive metals while the other metals (Fe, Ni, Cu, Co and Pb) presented stable log KD values along the estuary. Organic matter dynamic appeared to play a key role in metals fractioning. Its decomposition during water transit in the estuary induced metal desorption, especially for Cr and As. Conversely, dissolved Mn concentrations decreased along the estuary, which was suggested to result from Mn oxidative precipitation onto solid phase due to oxidation and pH changes. Extra sources as pore-water release, runoff from adjacent soils, or aquaculture effluents were suggested to be involved in trace metal dynamic in this estuary. In addition, the monsoon increased metal loads, notably dissolved and particulate Fe, Cr, Ni and Pb.