Interstitial Water Concentrations Research Articles

Migration of various ions based on pH shifts triggered by the application of sediment microbial fuel cells.

Sediment microbial fuel cells (SMFCs) represent a technology that can enhance sediment quality through processes such as nutrient suppression while simultaneously generating electricity from microorganisms. Despite its importance in elucidating the principles of nutrient suppression, the complex behavior of various ions within this context has been rarely explored. Herein, we applied an SMFC and systematically evaluated alterations in ion concentrations in interstitial and overlying waters. The SMFC deployment substantially decreased Na+ concentrations and increased Cl- levels in the interstitial water. This intriguing phenomenon was attributed to reactions driven by the electrodes. These reactions induced remarkable shifts in pH. Consequently, this pH shift triggered the leaching of heavy metals, particularly Fe, and decreased HCO3- concentrations within the interstitial water, thereby inducing the migration of other ions, including Na+ and Cl-, as compensation. Moreover, the PO43- concentration in interstitial water showed an increasing trend upon SMFC application, which contradicts the results of several previous reports. This increase was primarily attributed to the release of PO43-caused by the leaching of Fe salts, which was triggered by the pH shift. These findings provide new insights into sediment improvement research through SMFCs, enhancing our understanding of the fundamental principles and broadening the potential applications of this technology.

Advantages of using a carbon-rich substrate in a constructed wetland for agricultural water treatment: Carbon availability and biota development

Proper bed substrate selection is essential for the growth of microorganisms and plants, which are key elements in constructed wetlands (CWs) performance. This is even more important in CWs that treat irrigated agricultural drainage water characterised by a high nitrogen (N), but low carbon (C) and phosphorus (P) concentration. This nutrient imbalance compromises biota activity and limits water treatment processes. Three substrates were tested in a CW field-scale pilot plant for 2 years: gravel (100%) and two mixed substrates, gravel + 30% natural wetland soil and gravel + 10% biochar. While gravel is one of the most commonly used substrates in subsurface flow CWs, soil has hardly been employed, and field-scale biochar application studies are scarce. We analysed the effect of adding a C-rich substrate to gravel on both nutrient imbalance correction and the biotic performance of CWs. Adding natural soil or biochar increased P availability in beds. However, the dissolved organic C concentration in interstitial water was only enhanced by soil addition. Unlike gravel, microbial density and activity, and Phragmites australis plant growth, were higher in beds with soil, followed by those with biochar. A similar pattern was observed for plant tissue quality. Although biochar proved positive for biota, it had a short-term effect as a source of C and P. Therefore, employing a suitable gravel-soil mixture provides notable advantages for the irrigated agricultural water treatment challenge. Even after considering some limitations, our results represent an important step to design CWs that treat this wastewater type and for eutrophication control in sustainable agroecosystems.

Toxicity of sediments in eight urban stormwater management ponds: bioassessment by oligochaete community metrics used in the sediment quality triad.

Implemented for decades as part of the 'best management practices (BMPs)' for controlling urban runoff impacts on receiving waters, stormwater management ponds (SMPs) have been increasingly viewed as potential habitats for urban wildlife. However, since SMPs are subject to a lot of environmental constraints, research toward assessing their ecological quality and their actual benefits as habitats for biota is needed. In this study, the sediment toxicity of eight SMPs located in Southern Ontario, Canada was assessed using the sediment quality triad (SQT) approach. Sediment samples were collected for chemical, ecotoxicological and biological analyses. An oligochaete-based index approach (Oligochaete Index of Lake Bioindication and percentage of pollution-sensitive species) was used as the biological endpoint and integrated into a weight-of-evidence approach to assessing the general sediment quality of the ponds. Our results showed that (i) heavy metals in the sediment and (ii) chloride concentrations in the sediment interstitial water caused detrimental effects on the ecological quality of the sediments in the ponds studied. The oligochaete indices applied in this study showed value as biological endpoints to be integrated into the SQT and used for setting up sediment ecological quality goals.

Metal bioaccumulation in stygophilous amphipod Synurella ambulans in the hyporheic zone: The influence of environmental factors

Given the increasing need to protect vulnerable freshwater ecosystems and make them more resilient to human use and climate change, biomonitoring of the hyporheic zone (HZ), which plays a critical role in pollution attenuation, is essential. The aim of the present study was to assess the potential of the amphipod species Synurella ambulans as a bioindicator of metal contamination in the HZ of the Sava River (Croatia). Amphipods were collected during the four seasons at two sampling sites (average sampling depth 55 cm) differing in type (agricultural and urban) and intensity (diffuse and point source contamination) of anthropogenic influence, one located upstream (Medsave), and the other downstream (Jarun) of the wastewater treatment plant discharge. Concentrations of Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Rb, Sn, Zn, Ca, K, Mg and Na were measured in the interstitial water, sediments and specimens of S. ambulans by HR ICP-MS. Physicochemical parameters (temperature, DO, O2 saturation, pH, conductivity, alkalinity, total water hardness, CODKMnO4, nutrients) were measured in the interstitial water, while organic carbon was measured in the interstitial water and sediments. Metal concentrations in interstitial water and sediments were below thresholds set by environmental quality standards. Metal concentrations in S. ambulans were classified as follows: higher at the Jarun site (Al, Cr, Fe, Ni, Pb, Sn), higher at the Medsave site (Cd, Cu, Rb) and mostly comparable at both sites (Co, Mn, Zn). Bioaccumulation factors were generally higher at Jarun, with average values ranging from 322 to 143,278 L kg−1. Bioaccumulation of metals in S. ambulans depended on various environmental factors, with metal exposure level and dissolved macro elements showing the strongest association with metals accumulated in S. ambulans. The findings provided the first evidence on the suitability of S. ambulans as a good bioindicator of chronic metal contamination in the HZ.

Sediment diffusion is feasible to simultaneously reduce nitrate discharge from recirculating aquaculture system and ammonium release from sediments in receiving intensive aquaculture pond

Nitrogen accumulation has become one of the greatest unresolved challenges restricting the development of aquaculture worldwide. In recirculating aquaculture system (RAS), lack of organic matter (OM) and sensitive organisms makes it difficult to apply efficient denitrifying technology, thus leading to a high nitrate‑nitrogen (NO3−-N) accumulation. In contrast, excess OM accumulation in intensive aquaculture pond sediments is associated with dissolved oxygen depletion and ammonium‑nitrogen (NH4+-N) accumulation in the sediments. Based on the opposing effects of OM on the nitrogen accumulation in RAS and intensive aquaculture ponds, this study assessed the feasibility of simultaneously reducing NO3−-N discharge from RAS and controlling NH4+-N accumulation in intensive aquaculture ponds by in situ diffusing RAS tailwater containing NO3−-N into intensive aquaculture pond sediments. The results showed that NO3−-N diffusion strategy improved the native sediment denitrification capacity, thus increasing NO3−-N removal efficiency from RAS tailwater and significantly decreasing the NH4+-N concentration in interstitial water and the total organic carbon content in intensive aquaculture pond sediments. High-throughput sequencing and quantitative real-time polymerase chain reaction (qPCR) results revealed that NO3−-N addition significantly increased both nitrifying bacteria and denitrifying bacteria abundance. These results implied that NO3−-N diffusion strategy could effectively stimulate microbial decomposition of OM, thus relieving the hypoxia limitation of sediment nitrification. Overall, this study offers a feasible method for simultaneous reduction of NO3−-N from RAS tailwater and NH4+-N in intensive aquaculture ponds with low cost and high efficiency.

Physi-chemical mechanism and control effect of CaO2 inhibiting phosphorus release from sediments under different dosing modes

CaO2 is known as an outstanding restoration agent to control phosphorus (P) release from sediments, and its mechanism is believed to depend on chemical passivation. However, we found that the physical actions might also be involved in inhibiting endogenous P release induced by CaO2. To further explore the mechanism of CaO2 controlling P release and optimize the dosing method, a 94-day incubation experiment was conducted under different CaO2 dosing modes. The results showed that CaO2 could form a dense passivation layer near its dosing position by reducing the median diameter of sediments, thereby inhibiting P release through physical obstruction. At the same time, the increase in the specific surface area and Ca content of sediments induced by CaO2 could synchronously enhance the physical and chemical adsorption properties of sediments to P. In addition, CaO2 could significantly reduce the P concentration in sediment interstitial water and the mobile−P and BAP contents in sediments through chemical oxidation and chemical precipitation. Under the combined actions of physical obstruction, physi-chemical adsorption, chemical oxidation, and chemical precipitation, CaO2 effectively inhibited endogenous P release. Finally, the P release flux in each reactor showed that multiple coverage and shallow injection had the optimal effect on inhibiting P release, and the former is recommended for the water systems with shallow sediments, and the latter is suitable for the water systems with deep sediments. In general, this experiment proposed the physi-chemical mechanism of P immobilization mediated by CaO2, studied the formation characteristics of the passivation layer, and optimized the dosing mode, which can provide valuable reference for the research and application of CaO2 controlling P release.

Scale-dependent effects of vegetation on flow velocity and biogeochemical conditions in aquatic systems

In rivers, scale-dependent feedbacks resulting from physical habitat modifications control the lateral expansion of submerged plant patches, while the mechanisms that limit patch expansion on a longitudinal dimension remain unknown. Our objective was to investigate the effects of patch length on physical habitat modification (i.e., flow velocity, sediment grain size distribution), the consequences for biogeochemical conditions (i.e., accumulation/depletion of nutrients, microbial respiration), and for individual plants (i.e., shoot length). We measured all of these parameters along natural patches of increasing length. These measurements were performed at two sites that differed in mean flow velocity, sediment grain size, and trophic level. The results showed a significant effect of patch length on organic matter content and nutrient concentrations in interstitial water. For the shortest patches sampled, all of these parameters had similar values to those measured at the upstream control position. For longer patches, organic matter content and orthophosphate and ammonium concentrations increased within the patch compared to the upstream bare sediment, whereas nitrate concentrations decreased, suggesting changes in vertical water exchanges and an increase in anaerobic microbial activities. Furthermore, plant height was related to patch length by a quadratic pattern, probably due reduced hydrodynamic stress occurring for increasing patch length, combined with conditions that are less favourable for plants over a threshold length, possibly due to the light limitation or to the high concentration of ammonium that in the concentration range we measured may be toxic for plants. The threshold lengths over which patches influence the nutrient concentrations were reduced for the site with higher nutrient levels. We demonstrated that the plant-induced modifications of the physical habitat exert important effects on biogeochemical conditions, with possible consequences for patch dynamics and ecosystem functioning.

Suppression of phosphorus release from eutrophic lake sediments by sediment microbial fuel cells

ABSTRACT Sediment microbial fuel cells (SMFCs) have served as an alternative technique to suppress phosphorus release from lake sediments to water bodies and thus mitigate eutrophication. However, the phosphorus regulation mechanism remains unclear. The purpose of this research was to understand the electrochemical influence of an SMFC on the phosphorus concentration in interstitial water. In this study, a lab-scale SMFC was applied to acetate-spiked sediments (ace+) and unspiked sediments (sed) with closed-circuit (CC)/open-circuit (OC) columns, and the circuitry was switched to investigate the relationship between electron transfer and phosphorus concentration. The dissolved total phosphorus (DTP) concentration in the sediment interstitial water in CC columns significantly decreased to below 0.1 mg/L, whereas the DTP in OC columns remained high for nine weeks. After switching the circuit, the DTP in OC→CC columns dropped but that in CC→OC columns increased within one week. At the end of the experimental period, the DTP concentrations in CC/sed, CC/ace+, OC/sed, and OC/ace+ columns were 0.10 ± 0.02, 0.03 ± 0.00, 0.82 ± 0.01, and 1.66 ± 0.12 mg/L, respectively. The respective estimated anode capacitances of those columns were 2.05 ± 0.49, 5.15 ± 0.14, 0.72 ± 0.19, and 0.71 ± 0.12 nF. We concluded that the phosphorus may have been electrochemically attracted and retained on the anode in the sediment because the adsorbed DTP contents and the increased anode capacitances were strongly correlated. Thus, SMFCs can be used for suppressing phosphorus release from eutrophic lake sediments.

Mechanism of ammonium sharp increase during sediments odor control by calcium nitrate addition and an alternative control approach by subsurface injection

Nitrate-driven sulfide/ferrous oxidation has been proved a cost-effective approach for river sediments in-situ odor control. However, calcium nitrate addition would sharply increase ammonium concentration in interstitial water and the mechanism was not yet clear. In this work, though sulfide and ferrous iron were efficiently oxidized, about 102% of NH4+ concentration increased in interstitial water on the first day of calcium nitrate injection (30 mg kg dwt−1), and about 31% more NH4+ increase at the 21st days was observed. To discover the mechanism of ammonium sharp release, desorption kinetics experiment was conducted and the results suggested that the short-time sharp releases of ammonium when calcium nitrate was added could be attributed to the chemical extraction of exchangeable ammonium by calcium ion. Furthermore, at the end of treatment, many genus such as Thiobacillus, Sulfurimonas, Thermomonas, and Clostridium, which were closely related to sulfide and ferrous-driven denitrification and dissimilatory nitrate reduction to ammonium (DNRA), were identified by 16S rRNA Illumina sequencing method. These findings indicated the long-time increase of ammonium might be determined by the biochemical processes (e.g. DNRA) driven by nitrate reduction. Therefore, to avoid the impact of ammonium release, an alternative subsurface injection method was introduced in this work, and the results showed that ammonium releases could be well controlled when the injection position was beneath 10 cm of the sediment surface.

Influence of sediment organic carbon on toxicity depends on organism’s trophic ecology

Studies which showed the influence of organic carbon on the toxicity of sediment-associated contaminants on benthic invertebrates suggest this was primarily due to its influence on the interstitial water concentrations of the contaminant. A higher organic content offers more binding sites for organic contaminants, which means lower toxicity for organisms whose exposure route is mainly through contaminated interstitial water. However, a higher organic content in the sediment could mean a higher toxicity for deposit-feeding organisms, which can assimilate the contaminant by ingestion of contaminated particles. To investigate the influence of sedimentary organic carbon content on the toxicity of an organic contaminant on a benthic community, a microcosm experiment was carried out where natural nematode assemblages were exposed to three concentrations of Irgarol in sediments with two different levels of organic carbon for 7 and 35 days. The response of the nematode assemblage to sediment contamination by Irgarol differed between organically “Lower organic carbon” and “Higher organic carbon” sediments. Responses were genus specific and although community composition was the same in both sediments in the beginning of the assay, contamination by Irgarol affected different genera at each sediment type. Also, the differential amount of organic carbon promoted responses of different functional groups. In Lower organic carbon sediments, contaminated treatments showed lower abundances of the genus Viscosia and the group of predacious nematodes, which were probably affected by an increased availability of Irgarol in the interstitial water in this treatment. In Higher organic carbon sediments, the group of deposit-feeders were mainly affected, suggesting the ingestion of contaminated food as the main route of contamination in this condition. These results indicate that the bioavailability of toxic substances in sediments is not only determined by their partitioning between the different phases of the sediment but also by the organism’s trophic ecology.

Arsenic distribution and speciation in multiphase media of a lake basin, Tibet: The influences of environmental factors on arsenic biogeochemical behavior in the cold arid plateau lake

Widespread arsenic (As) has been found in surface media on the Tibetan Plateau, but few studies of As species have been performed because of the difficult sampling conditions. In this study, As distribution and speciation in multiphase media (including surface water, interstitial water, and sediment) in Lake Yamdrok Basin in Tibet in wet and dry seasons were investigated to allow the biogeochemical behavior of As in the cold arid plateau lake to be understood. The total As (TAs) (mainly containing arsenate (As(V)) and arsenite (As(III))) concentrations were generally higher in surface and interstitial water in the lake zones than in Inflowing rivers. Among the four lake zones, significantly higher As concentrations were found in Chen Co, Yamdrok Tso, and Kongmo Co than in Bajiu Co, and surface sediments from the former three lake zones contained relatively high concentrations of the labile As. Redox potential (Eh) in sediments and HCO3 concentration in surface water primarily controlled labile As mobilization through reductive dissolution of As-bearing iron (hydr)oxides and oxyanion competition for As adsorption sites, and therefore affected the As distributions in aqueous phases. As(III) concentrations in interstitial water accounted for 41% ± 33% of TAs, and positively correlated with the arsenate-reducing microbe population in sediments. In contrast, As(V) was predominant in surface water (accounting for 95% ± 8% of TAs), and even trace amounts of dimethylarsinic acid (DMA) was found in wet season. Notably, lower Eh values in dry season triggered a marked increase in the As concentrations in interstitial water, and this probably increased the risk of As contamination of surface water.

Assessment of potential mercury toxicity to native invertebrates in a high-gradient stream.

This study evaluated potential effects of mercury (Hg) on benthic macroinvertebrates in the South River, Virginia, USA. The study used a multiple lines of evidence approach, including spatially and temporally matched sediment chemistry, biological, and toxicological information (Sediment Quality Triad), exposure and effect analysis in bulk and interstitial sediment and interstitial water, and critical body residue analysis. Ten-day Chironomus dilutus and Hyalella azteca toxicity tests established site-specific no-effect concentrations (NOEC) at 18.9 μg/g total Hg (THg) and 102 ng/g methylmercury (MeHg). However, the benthic community at these locations was impaired, with lower mayfly and caddisfly composition. Few locations had concentrations of THg and MeHg that exceeded the NOEC in bulk or interstitial sediment. The THg concentrations in interstitial water were far below concentrations expected to reduce survival in benthic invertebrates, and only a low percentage of samples exceeded sublethal (growth) low-effect concentrations (LOEC) for THg or MeHg. The THg concentrations in invertebrate tissue did not exceed the NOEC or LOEC in the South River, and MeHg concentrations exceeded the LOEC at all locations, including those with no evidence of benthic impairment, illustrating the uncertainty associated with this line of evidence. Finally, statistical modeling that evaluated diversity of sensitive invertebrate species as a function of Hg exposure, geomorphological parameters, and physicochemical variables indicated that physicochemical and geomorphological parameters were most predictive of benthic community; where Hg was indicated, we were unable to distinguish between models with or without interstitial water Hg. Overall, the lines of evidence indicate that Hg, while clearly toxic to invertebrates at sufficiently high exposure concentrations, is not negatively impacting invertebrate communities in the South River. This study illustrates the difficulty of assessing risks to invertebrates using traditional tools of risk assessment and identifies critical gaps in knowledge that complicate the management of Hg risk. Integr Environ Assess Manag 2019;00:000-000. © 2019 SETAC.

Vertical profiles of arsenic and arsenic species transformations in deep-sea sediment, Nankai Trough, offshore Japan

Concentrations of arsenic (As) and its chemical forms were determined on deep-sea sediments drilled at three sites of Nankai Trough, off the Kii Peninsula, Japan. Those sediments were analyzed to document the behavior of As in relation to methane hydrate formation and the deep biosphere.The analytical results showed the total As concentration of interstitial water (IW) and squeezed cake (SC) ranged from 0.9 to 380 ppb and from 3 to 14 ppm (average, 6.4 ppm), respectively. The sediments from Site C0002, of which sediment column was the longest down to 2200 m below the seafloor (mbsf) among the studied three drilling sites, were analyzed for the host phase transformation of As. The total concentration of As of IW and SC from 200 to 500 mbsf, where methane hydrate zone was included, was higher than those from the uppermost 200 m. Concentration of As was ultimately controlled by pH. Also, organoarsenicals, such as methylarsonic acid (MMA) and arsenobetaine (AsB), were detected in the sediment column, implying that these organoarsenicals appeared in relation to the in situ microbial activities. These observations suggest that As becomes mobilized directly or indirectly as a result of microbial activity in deep-sea sediments.

Distribuição espacial da concentração de nutrientes em sedimentos na Laguna hipersalina de Araruama – RJ, Brasil

The present study was carried out in Laguna de Araruama, Rio de Janeiro. Pollution in the lagoon has hampered the ecosystem structure and caused the eutrophication process to intensify. In order to improve water quality, it is proposed to open channels of the lagoon with the sea, but may result in the remobilization of the nutrients in the sediment to the water column. Considering the importance of sediment as a nutrient storage compartment, the objective of this study was to determine nutrient concentrations in sediments and interstitial water and their distribution in the sediments. Measurements of pH and Eh were made in the sediment, conductivity and temperature along the water column. The granulometry was determined and the concentrations of ammonium, nitrite, nitrate and phosphate in the interstitial water were determined, and the total concentrations of phosphorus, nitrogen and carbon in the sediments. Considering the granulometric results, most of the lagoon has sandy formations. The sediment presented temperatures between 22 °C and 25 °C, basic pH (7.1 - 8.2) and reducing characteristics (-400mV). In the stations near Sao Pedro da Aldeia and Araruama, there are high levels of total nitrogen in the sediment due to the high amount of sewage discharged into the rivers that flow into the lagoon. High concentrations of total phosphorus were also found in the region of the Itajuru Channel, closer to the sea, which due to its lower salinity has higher primary productivity. It is concluded that the lagoon presents a high degree of pollution by nutrients, in specific points, due to the contributions caused by the discharge of sewage

Operation of sediment microbial fuel cells in Tokyo Bay, an extremely eutrophicated coastal sea

In this study, over the summer of 2015, a set of five SMFCs was operated on the seafloor in Tokyo Bay, an extremely eutrophicated coastal estuary, to evaluate their electrochemical characteristics and effects on sediment near the anode. The power density of the SMFCs was reached 11.5 ± 0.89 mW/m2 but decreased during cloudy or rainy weather, likely because dissolved oxygen concentrations were reduced as a photosynthetic activity of phytoplankton declined at shallow depths and because electric conductivity decreased when freshwater inflow decreased salinity. The SMFCs significantly increased oxidation-reduction potential in sediment and reduced sulfide concentration in interstitial water, but no sediment-amending effect was apparent regarding the reduced organic matter in the sediment, as measured by loss on ignition.

Appraising the effect of in-situ remediation of heavy metal contaminated sediment by biochar and activated carbon on Cu immobilization and microbial community

The immobilization of heavy metal Cu with rice husk biochar (RHB) and activated carbon (AC) addition were compared by in-situ remediation of sediment in this research. The RHB and AC were characterized using SEM, FTIR, XRD, XPS, Raman and BET surface area measurements. Heavy-metal contaminated sediments were immobilized by RHB and AC with the addition amount of 2%, 5% and 10% (w/w). The sequential extraction experiment demonstrated that the acid-soluble fractions of Cu were reduced by 48%–93% and 18%–31% with the treatment of AC and RHB, respectively. With the decrease of acid-soluble fractions of Cu, the results showed that the surface water concentration of Cu decreased by 32%–68% and 8%–60% with the treatment by AC and RHB, respectively. The Cu concentration of interstitial water decreased by 14.8%–63% and 11.1%–48.1% with the treatment by AC and RHB, respectively. Meanwhile, hierarchical cluster analysis which was used for the detection of microbial community was conducted. The results showed that the microbial community was destroyed in the sediment treatment with high amount of AC. However, new microbial community appeared in the sediment treatment with three addition amount of RHB. Based on the data analysis, AC was more effective than RHB on Cu immobilization in the sediment. Unfortunately, AC was more expensive than RHB and had potential negative effect on microbial community. By comparing these aspects comprehensively, our results suggest that the RHB would be a low-cost and environmentally friendly material for heavy-metal contaminated site remediation.

Biostabilization of cadmium contaminated sediments using indigenous sulfate reducing bacteria: Efficiency and process

Sulfate reducing bacteria (SRB) was used to stabilize cadmium (Cd) in sediments spiked with Cd. The study found that the Cd in sediments (≤600 mg kg−1) was successfully stabilized after 166 d SRB bio-treatment. This was verified by directly and indirectly examining Cd speciation in sediments, mobilization index, and Cd content in interstitial water. After 166 d bio-treatment, compared with control groups, Cd concentrations in interstitial water of Cd-spiked sediments were reduced by 77.6–96.4%. The bioavailable fractions of Cd (e.g., exchangeable and carbonate bound phases) were reduced, while more stable fractions of Cd (e.g., Fe-Mn oxide, organic bound, and residual phases) were increased. However, Cd mobilization in sediment was observed during the first part of bio-treatment (32 d), leading to an increase of Cd concentrations in the overlying water. Bacterial community composition (e.g., richness, diversity, and typical SRB) played an important role in Cd mobilization, dissolution, and stabilization. Bacterial community richness and diversity, including the typical SRB (e.g., Desulfobacteraceae and Desulfobulbaceae), were enhanced. However, bacterial communities were also influenced by Cd content and its speciations (especially the exchangeable and carbonate bound phases) in sediments, as well as total organic carbon in overlying water.

Numerical modeling of early diagenetic processes in Haiyang 4 Area in the northern slope of the South China Sea

Early diagenesis affects the distribution of solutes and minerals in unconsolidated sediments. The investigation of diagenesis is critical to understanding the geochemical transformation and benthic fluxes of elements. During the cruise mission SO-177 in 2004, gravity coring samples were recovered in the Haiyang 4 Area of the northern slope of the South China Sea (SCS). The geochemical concentrations in interstitial water were determined onboard. The 1D C.CANDI reactive transport software was used to model the early diagenesis processes at four sites: 56-GC-3, 70-GC-9, 94-GC-11, and 118-GC-13. All of the simulations reproduced concentration profiles that matched the measurements with the implemented geochemical reactions. The degradation of organic carbon and anaerobic oxidation of methane (AOM) primarily determine the distribution of solutes in the working area. The degradation is active in the top 150 cm, and AOM is vigorous at depths below 200 cm. The local advective flux, sediment rate, and kinetic reaction constants of organic matter, methane and sulfate were calibrated based on the existing concentrations of pore water solutes. Geochemical reactions in this area occur in considerably deeper layers compared to depths cited in the literature. The model results provide evidence for the existence of deep hydrocarbon reservoirs that provide methane to the upper sediments.

Sources and conservative mixing of uranium in the Taiwan Strait

Seawater samples are collected in the spring of 2013 from the Taiwan Strait for the analysis of uranium (U) concentrations and isotopic compositions using MC-ICP-MS, and the geochemical behavior patterns of U in the Taiwan Strait are then investigated. Average concentrations of individual U isotopes are (3.23±0.14) μg/kg for 238U, (2.34±0.09)×10–2 μg/kg for 235U and (2.05±0.07)×10–4 μg/kg for 234U. Correspondingly, the U isotopic compositions are 155±18 for δ234U and 138±2 for 238U:235U. The U concentrations and isotopic ratios in the Taiwan Strait are similar to those of open ocean seawater, suggesting the dominance of the open ocean input to the strait’s U pool. However, river input, as suggested by the slightly lower salinity than that of the open ocean, also affected the U concentrations and isotopic compositions in the strait. From a compilation of U concentrations in the Taiwan Strait and adjacent areas, including the Jiulong Estuary and Zhujiang Estuary, the Xiamen Bay and the northern South China Sea, a strong and significant relationship between U concentration and salinity [U:S; U=(0.093 4±0.002 4)S+(0.092 0±0.061 5)] is revealed, suggesting conservative mixing of U in the Taiwan Strait. To better understand the U geochemistry in the Taiwan Strait, a multiple endmembers mixing model is applied to estimate the contributions of potential sources. The open ocean seawater contributed 69%–95% of U in the Taiwan Strait, with river water approximately 2%, and dust deposition only around 0.13%. Therefore, the model results supported the open ocean input source and the conservative mixing behavior of U derived from the observation of U concentrations and isotopic ratios and U:S ratios. The sediment interstitial water may be an important source of U to the Taiwan Strait with a possible contribution of 3%–29%, consistent with previous investigations based on radium isotopes. However, further investigations are warranted to examine the U concentration in the sediment interstitial water and its input to the overlying seawater in the Taiwan Strait.

Impact of an alien invasive plant Amaranthus retroflexus on wetland sediment properties under two growth stages

ABSTRACTIt is meaningful to investigate the impact of alien invasive plants on wetland sediment. Field experiments were conducted to study the impact of the alien invasive plant, Amaranthus retroflexus, on wetland sediment properties under two growth stages. The results showed that growth of A. retroflexus tended to increase moisture content, porosity, and could significantly decrease total nitrogen (TN) and nitrate (N-NO3ˉ) concentrations in sediment and ammonium (N-NH4+) concentration in interstitial water (p < 0.05). However, no obvious impact was observed on total phosphorus (TP) nor on N-NH4+ concentrations in sediment. There was a difference in impact of A. retroflexus on sediment properties under two growth stages. The plants with a longer growth stage significantly decreased loss on ignition in sediment and P-PO43ˉ concentration in interstitial water. There were also significant differences in N-NH4+ concentration in both sediment and interstitial water over the vertical profile of the plant. In the plant treatment with a longer growth stage, P-PO43ˉ concentration also showed significant differences over the vertical profile. Therefore, the growth of A. retroflexus can improve sediment physical and chemical properties, reducing N-NH4+ and P-PO43ˉ release from sediment to overlying water to some extent, thereby decreasing sediment nutrient loading.