Sediment-pore Water System Research Articles

Occurrence and phase distribution of lipophilic marine algal toxins in the bottom boundary layer and sediment-porewater system of two mariculture sites

To date, understanding the fate of lipophilic marine algal toxins (LMATs) in benthic environments on which cultivated shellfish depend is still limited. In this work, the occurrence, concentration levels, and phase distributions of LMATs in the benthic environments of two mariculture sites (Sishili and Rongcheng Bays) in China were investigated for the first time. Five LMATs: okadaic acid (OA), pectenotoxin-2 (PTX2), gymnodimine, 13-desmethyl spirolide C, and azaspiracid-2 (AZA2) and three derivatives: dinophysistoxin-1 isomer (DTX1-iso), pectenotoxin-2 seco acid, and 7-epi- pectenotoxin-2 seco acid were detected in different environmental samples. OA and PTX2 were the dominant LMATs in the bottom boundary layer (BBL) and sediment, whereas AZA2 was present in the sediment only. Notably, DTX1-iso was found for the first time to be widely distributed in the benthic environments of the bays. In BBL, the average proportion of LMATs in the dissolved phase (99.20%) was much higher than in the particulate phase (0.80%). Partition of LMATs was more balanced between sediment porewater (57.80% average proportion) and sediment (42.20%). The concentrations of ∑LMATs in the BBL seawater ranged from 19.09 ng/L to 41.57 ng/L (mean of 32.67 ng/L), and the spatial distribution trend was higher in offshore than nearshore. ∑LMATs concentrations in the sediment and porewater of the two bays ranged from 17.04 ng/kg to 150.13 ng/kg (mean of 53.58 ng/kg) and from 8.29 ng/L to 120.58 ng/L (mean of 46.63 ng/L), respectively. Their spatial distributions differed from those in BBL, showing a trend of high concentrations in areas with heavy land-based inputs. ∑LMATs concentrations in porewater were significantly higher than those in BBL seawaters, suggesting that the potential hazards of LMATs to benthic organisms may be underestimated.

Sewage discharge and organic matter affect the partitioning behaviors of different polycyclic aromatic hydrocarbons in a river surface sediment-pore water system

The partitioning behaviors of persistent organic pollutants (POPs) between sediments and water are significant for their transport and fate in aquatic environments; however, an in-depth understanding is still required. Herein, we studied partitioning coefficients, fugacity fractions (ff), and influencing factors of parent, nitrated, and oxygenated polycyclic aromatic hydrocarbons (PPAHs, NPAHs, and OPAHs, respectively), which are typical POPs composed of multiple aromatic rings, in the surface sediment-pore water system of the Haihe River. The logarithmic organic carbon normalized partitioning coefficient (lgKOC) of PPAHs (4.04 ± 0.80) was evidently greater than those of NPAHs (2.64 ± 1.30) and OPAHs (2.69 ± 0.77). Owing to the effects of the polar groups in NPAHs and OPAHs, their partitioning behaviors were different from that of PPAHs. ff values showed that most PAH species approached dynamic equilibrium. As an important source of riverine PAHs, sewage discharge affected PAHs partitioning, whereas dam interception exerted less impact in this study. lgKOC and ff were negatively correlated with sediment organic matter, represented by particulate organic carbon (POC) and n-alkanes, and positively correlated with the sand fraction in surface sediments. In addition, positive correlations of lgKOC and ff with pH and salinity were observed in pore waters. The results indicate appreciable impacts of sewage discharge and organic matter on the partitioning processes of different PAHs.

Sedimentary cycling of zinc and nickel and their isotopes on an upwelling margin: Implications for oceanic budgets and paleoenvironment proxies

Zinc and Ni are essential micronutrients whose stable isotope systematics in marine sediments represent promising, but still developing, tracers of past ocean chemistry and biology. Sediments from upwelling continental margins have been identified as important sinks for Zn and Ni, driven by high productivity, incorporation of the metals into cells and organic carbon burial. At present, however, our understanding of how Zn and Ni isotope composition of upwelling sediments reflects specific processes in the water column is incomplete. Here, we present coupled Zn and Ni abundance and isotope data for both solid phase and porewater in a series of sediment cores collected on a shelf-to-slope transect across an oxygen minimum zone (OMZ) from the southwest African margin off Namibia.Zinc/Al and Ni/Al ratios in Namibian margin sediments are elevated relative to the lithogenic background, by factors of 1.4–2.4 and 1.7–5.7, respectively. Systematic relationships between solid phase Zn and Ni concentrations and organic carbon accumulation corroborate the view that authigenic Zn and Ni are mainly delivered to these sediments via export production from the photic zone. Corrections for detrital Ni are sometimes substantial, but authigenic δ60Ni values are within uncertainty of the modern deep ocean, at 1.38 ± 0.18 ‰ (mean and 1SD, n = 42). Authigenic δ66Zn is more variable, at −0.09 to 0.50 ‰, at or below the value for the deep ocean average. This latter observation can be attributed to Zn isotope fractionation during sequestration into authigenic ZnS phases in the sediments, either quantitatively, or partially.Porewater systematics are more complex and the profiles do not necessarily reflect the redox state of the sediment-porewater system at the time of sampling, but rather characteristics inherited from more reducing conditions in a temporally heterogeneous system. Porewater zinc concentrations (30–369 nM) are much higher than the deep ocean and increase with depth beneath the sediment–water interface. Despite this prominent increase in Zn concentrations, δ66Zn is rather constant and lighter than the deep ocean average, at 0.18 ± 0.06 ‰ (mean and 1SD, n = 33). These observations are most consistent with the buffering of the small porewater pool by the dissolution of a previously formed sulfide phase. Porewater Ni concentrations are closer to the deep ocean, mostly at 5–15 nM. The lightest porewater isotope compositions (δ60Ni as low as 0.15 ‰) are consistent with a Mn-oxide source of Ni, whereas the heaviest isotope compositions (up to 1.76 ‰) clearly reflect uptake of light Ni into sulfide at depth within the sediment.Our data, for the largest upwelling site in the modern ocean, add to sparse data from the eastern Pacific, and confirm that upwelling margin sediments bury Ni that is close to the deep ocean in isotope composition, but Zn that is generally lighter. The data thus confirm the importance of such margins in setting the heavy isotope composition of oceanic Zn. Sediments from this kind of setting hold significant potential for the construction of records of past oceanic Ni isotopes.

Source identification and ecological risks of parent and substituted polycyclic aromatic hydrocarbons in river surface sediment-pore water systems: Effects of multiple factors

Substituted polycyclic aromatic hydrocarbons (SPAHs) have shown higher health and ecological risks than the corresponding parent PAHs (PPAHs) from laboratory studies, their variations in freshwater system, especially in surface sediment and pore water, remain inadequate understanding. In this study, we revealed the coexistence, ecological risk, and multiple factors affecting variations and sources of PPAHs and SPAHs (nitrated PAHs (NPAHs), oxygenated PAHs (OPAHs)) in the surface sediment-pore water system from a typical urban river in Northern China. The concentration ranges of Σ26PPAHs, Σ10NPAHs, and Σ4OPAHs in the surface sediments were 153.0–5367.4, not detected (N.D.)–105.4, and 42.2–1177.0 ng·g−1 dry weight, and fell within 0.6–38.8, N.D.–297.9, and N.D.–212.6 ng·mL−1 in the pore waters. The t-distributed stochastic neighbor embedding (SNE) coupled with the partitioning around medoids (PAM) elucidated spatiotemporal the variations in PAHs, emphasizing the impacts of industrial activities and sewage discharges. Besides the geochemical and hydrochemical conditions, SPAHs were affected by the potential secondary formation, especially during the wet season. The method comparisons indicated the advantages of principal component analysis-multivariate linear regression (PCA-MLR) and n-alkanes model on source identification. PAHs mainly originated from fossil fuel combustion and vehicular exhaust. The top risk quotient (RQ) values for PAHs occurred in the urban and industrial sections. A majority of the surface sediment samples emerged with low to moderate exposure risks, while all the pore water samples showed high exposure risks. The RQs of OPAHs were significantly higher (p < 0.01) than those of PPAHs. The results suggested the secondary formation of SPAHs as an important role in ecological risks of PAHs in the urban river system.

Polycyclic aromatic hydrocarbons in sediment–porewater system from the East China Sea: Occurrence, partitioning, and diffusion

The distribution, partitioning behavior, and diffusion of polycyclic aromatic hydrocarbons (PAHs) within sediment–porewater system were determined in two cores obtained from the Min-Zhe coastal mud of the East China Sea (ECS). Depth profiles of apparently dissolved PAH levels exhibited greater variabilities, with their elevated levels at depth and a high abundance of two-to three-ring PAHs observed. These distribution and composition patterns were inconsistent with the corresponding sediment PAHs, indicating differences in controlling factors for PAHs present in the system. In addition to compound's hydrophobicity, low detection of heavier PAHs in porewater was possibly correlated with the sediment transport process, as indicated by a relatively high weathering ratio in southern Min-Zhe coastal mud. PAH sorption affinity to the collected core sediments exhibited a generally decreasing trend downcore, as expressed by sediment–porewater partition coefficients. This was consistent with the higher content of porewater PAHs in deep core sediment. The established sediment total organic carbon (TOC)–porewater partitioning profiles in cores were predicted with amorphous organic carbon (AOC)-, coal tar-, and TOC-based distribution models, suggesting a dominant nonlinear sorption of PAHs to AOC. Through activity determinations, PAH diffusion within porewater was elucidated, with significant upward and downward mass transfer for PAHs occurring in both cores. The upward diffusion in the core collected from northern Min-Zhe coastal mud was in significant association with sediment TOC. This suggests that sediment TOC (especially AOC)-desorption of lighter PAHs into porewater, and therefore the possibility of their participation in environmental cycling. Baseline toxicity potential and toxic unit calculations indicated a relatively low exposure risk for benthic organisms to porewater PAHs.

Temporally and spatially dynamic redox conditions on an upwelling margin: The impact on coupled sedimentary Mo and U isotope systematics, and implications for the Mo-U paleoredox proxy

The abundances and isotope compositions of molybdenum (Mo) and uranium (U) in ancient sediments are promising tracers of the redox state of the past ocean, whose basis lies in the environmentally dependent Mo and U isotope signatures in modern oceanic settings. Despite their dominance in oceanic budgets, the controls on the Mo-U systematics of upwelling margin sediments remain to be fully understood. Here we present a comprehensive sediment-porewater Mo and U isotope study in the Benguela upwelling system off Namibia, including the first dataset incorporating coupled Mo-U abundance and isotope analysis of both solid authigenic phases and porewaters.The investigated stations represent shelf-to-slope settings, which lie on the upper edge, within and below the oxygen minimum zone (OMZ) of the South Atlantic. The sediments across all stations share similar characteristics: both Mo and U show increasing authigenic enrichment with depth, coupled to an overall decrease in δ98Moauth (from ~2.0‰ to 1.3‰ and from ~2.0‰ to 0.3‰) and increase in δ238Uauth (from −0.18‰ to 0.05‰ and from −0.34‰ to −0.21‰). Nevertheless, the extents of Mo and U enrichment and associated isotopic fractionations display spatial variability across the OMZ, reflecting variations in local sedimentary redox conditions. Porewater Mo and U concentration patterns are more complex, exhibiting peaks in Mo and U abundance well in excess of seawater (up to 8 times seawater for Mo) associated with correlated shifts in isotope composition. As a result, porewaters exhibit a wide range in isotope compositions, between 0.90‰ and 2.79‰ for δ98Mo and between −1.74‰ and 0.26‰ for δ238U.Porewater gradients at the time of sampling are inconsistent with diffusion downwards across the sediment–seawater interface as a means of enrichment of the sediment-porewater system. Though these sampled conditions may represent only a snapshot, so that periodically more reducing conditions could lead to concentration gradients that do permit downward diffusion, the data are also readily explained by addition of Mo and U to the sediment-porewater system in particulate form, also under more reducing conditions than at the time of sampling. For example, sequestration of Mo and U to particulate matter as a result of the presence of intermittent sulfide, either in bottom water or in porewater right at the sediment–water interface, explains much of the geochemical and isotope data.The data thus suggest that the early diagenetic enrichment of Mo and U in sediments of upwelling margins is strongly governed by temporal redox fluctuations. Early diagenesis under these dynamic redox conditions on the Namibian upwelling margin are strongly reflected in Mo-U co-variation patterns, as well as anti-correlations between authigenic δ98Mo and δ238U in sediments. Overall, our new data demonstrate that early diagenetic processes on open-marine continental margins reproduce patterns previously observed for coupled Mo-U isotope systematics in restricted and semi-restricted basins, but via a different set of processes and with important implications for the use of such a coupled approach in the study of ancient marine anoxia.

Distribution, partitioning behavior, and ecological risk assessment of phthalate esters in sediment particle-pore water systems from the main stream of the Haihe River, Northern China

The distribution, partitioning behavior and risk assessment of phthalate esters (PAEs) in the surface sediment-pore water system of the Haihe River were investigated. The total cumulative concentrations of 21 PAE species (Σ21PAEs) in the surface sediment ranged from 45.9 to 1474.1 ng·g−1 dry weight (dw) and were from 17.9 to 2628.8 ng·mL−1 in the pore water. Di (2-ethylhexyl) phthalate (DEHP), dibutyl phthalate (DBP), and diisobutyl phthalate (DIBP) were the dominant components, and their sum accounted, on average, for 88.4% and 72.0% of Σ21PAEs in the surface sediment and pore water, respectively. The spatial distributions of Σ21PAEs in the surface sediment and pore water indicated that large amounts of the consumed products contained plasticizers in the urban and nearshore areas and increased the discharge of PAEs into the Haihe River. The river dam also affected PAEs distributions. The organic carbon normalized partitioning coefficient (logKOC) followed a sequence as dry season (2.47 ± 0.35 mL·g−1) > wet season (2.02 ± 0.45 mL·g−1) > normal season (1.98 ± 0.42 mL·g−1). The risk quotient (RQ) method was employed to assess the potential ecological risk from specific species. High ecological risks of DEHP to the sensitive algae, crustacean, and fish species along with high ecological risks of DIBP to sensitive fish species were found in the surface sediment and pore water for all sampling seasons. In addition, DBP in the surface sediment and pore water exhibited moderate and high ecological risks to sensitive aquatic species. The highest RQ values for PAEs were found in the surface sediment and pore water in suburban and urban areas, respectively, and indicated that anthropogenic activities may cause severe river pollution and high risk to the local aquatic ecosystem. CapsuleHigh levels and ecological risks from PAEs were found in the urban river, and the partitioning behaviors of PAEs between the surface sediment and pore water were not significantly affected by their hydrophobicity, especially for species with low KOW.

Pharmaceuticals and personal care products in a drinking water resource of Yangtze River Delta Ecology and Greenery Integration Development Demonstration Zone in China: Occurrence and human health risk assessment

The occurrence, partition, and human health risk of thirteen pharmaceuticals and personal care products (PPCPs) have been investigated in surface water, overlying water, pore water and sediment samples from Dianshan Lake of Yangtze River Delta Ecology and Greenery Integration Development Demonstration Zone in China. PPCPs were ubiquitous in aqueous phase and sediments from Dianshan Lake. Sulfamethazine (SMZ) was dominated in surface water and overlying water, while ketoprofen (KPF) was rich in sediment. The total concentration of PPCPs ranged from 0.38–85.27 ng/L, 24.26–130.03 ng/L and 5.39–149.84 μg/kg in surface water, overlying water and sediment, respectively, which were in middle levels compared with these reported in other aquatic environment in China. Naproxen (NPX), sulfadimethoxine (SDM), sulfamethoxazole (SMX) and sulfamethazine (SMZ) in surface water showed a relatively higher level in lake side than those in lake center suggesting that a mixed containment source of human- and animal-derived from the areas around lake. The significant season variations of most PPCPs were mainly attributed to their usage, water temperature and dilution effect. The partition behaviors of PPCPs in sediment-overlying water and sediment-pore water system were mainly affected by their logKow values, and showed weak correlation with total organic carbon (TOC) content in sediment and molecular weights of PPCPs. Preliminary results indicated that PPCPs in Dianshan Lake have not posed a high risk to human health by exposure to drinking water for all age groups. Nevertheless, their potential to cause the mixture toxicity and resistance genes cannot be neglected. This work will contribute to the clear picture of PPCPs contamination in drinking water source in the Demonstration Zone, and provide reliable and simple-to-use information to regulators on the exposure and risk levels of PPCPs, as well as recommendations for future research.

Genesis of mud volcano fluids in the Gulf of Cadiz using a novel basin-scale model approach

Mud Volcanism and fluid seepage are widespread phenomena in the Gulf of Cadiz (SW Iberian Margin). In this seismically active region located at the boundary between the African and Eurasian plates, fluid flow is typically focused on deeply rooted active strike-slip faults. The geochemical signature of emanating fluids from various mud volcanoes (MVs) has been interpreted as being largely affected by clay mineral dehydration and recrystallization of Upper Jurassic carbonates. Here we present the results of a novel, fully-coupled 1D basin-scale reactive-transport model capable of simulating major fluid forming processes and related geochemical signatures by considering the growth of the sediment column over time, compaction of sediments, diffusion and advection of fluids, as well as convective and conductive heat flow. The outcome of the model is a realistic approximation to the development of the sediment pore water system over geological time scales in the Gulf of Cadiz. Combined with a geochemical reaction transport model for clay mineral dehydration and calcium carbonate recrystallization, we were able to reproduce measured concentrations of Cl, strontium and 87Sr/86Sr of emanating mud volcano fluids. These results support previously made qualitative interpretations and add further constraints on fluid forming processes, reaction rates and source depths. The geochemical signature at Porto MV posed a specific problem, because of insufficient constraints on non-radiogenic 87Sr/86Sr sources at this location. We favour a scenario of basement-derived fluid injection into basal Upper Jurassic carbonate deposits (Hensen et al., 2015). Although the mechanism behind such basement-derived flow, e.g. along permeable faults, remains speculative at this stage, it provides an additional source of low 87Sr/86Sr fluids and offers an idea on how formation water from the deepest sedimentary strata above the basement can be mobilized and eventually initiate the advection of fluids feeding MVs at the seafloor.The dynamic reactive-transport model presented in this study provides a new tool addressing the combined simulation of complex physical-geochemical processes in sedimentary systems. The model can easily be extended and applied to similar geological settings, and thus help us to provide a fundamental understanding of fluid dynamics and element recycling in sedimentary basins.

Distribution of n-alkanes in the ferromanganese nodule–sediment–pore water system (Clarion–Clipperton Fracture Zone)

The paper presents the first results of a comparative study of the composition and distribution of organic matter (OM) (Corg, n-alkanes, Pr, Phy) in samples of the ferromanganese nodule–sediment–pore water system taken at a single site in the Clarion–Clipperton Fracture Zone, Pacific Ocean. Samples were taken during Cruise 120 of R/V James Cook in 2015. The results of the comparison of organogeochemical parameters in the lipid fraction of OM in ferromanganese nodules of different size and morphology with the underlying sediment (0–1 cm) and pore water revealed both principle genetic affinity of OM therein and significant discrepancies between the processes of diagenesis during the formation and growth of each individual ore sample.

Different partitioning behaviors of molybdenum and tungsten in a sediment–water system under various redox conditions

Molybdenum (Mo) and tungsten (W) are redox-sensitive elements that exhibit contrasting geochemical behaviors under different redox and/or sulfidic conditions despite belonging to the same group in the periodic table. In this study, the geochemistry of Mo and W in a sediment–porewater system was investigated using a core sample collected from sediment containing hydrothermal minerals (Izena Hole, Okinawa, Japan). The Mo contents in the sediment and the porewater were quantified to calculate the partition coefficient Kd (concentration ratio of the solid phase to the aqueous phase) for Mo. However, the W concentration in the porewater was exceedingly low to be detected. Thus, we also conducted laboratory experiments simulating the sediment–porewater system to clarify the partitioning behaviors of Mo and W under redox conditions. X-ray absorption near-edge structure (XANES) spectra were obtained to determine the chemical species of Mo and W in sediment at different depths to understand the chemical processes of Mo and W. In particular, high-sensitivity XANES spectroscopy using wavelength-dispersive fluorescence mode was applied to W L3 edge XANES spectra to reduce the interference signals from coexistent elements (Zn and Ni). This step thereby facilitating the identification of oxygen- and sulfur-coordinated species for W in the sediments.The collected core sample covered a broad range of redox conditions under various hydrogen sulfide (H2S) concentrations depending on the depth. The presence of iron oxides in the upper layer suggested an oxic condition above within 4cm depth below the seafloor (cmbsf). Iron and Zn sulfide minerals were found in deeper layers (16–24 cmbsf) and indicated the reductive conditions formed in such layer. Hydrogen sulfide was also observed in the porewater of the deeper layer. Analyses for Mo in the natural sediments, as well as XANES analyses, revealed that the Kd was higher in the deeper layer, with high pyrite and H2S contents, than in the upper layer. These results implied that Mo was removed from the porewater under reductive and/or sulfidic conditions. The chemical species of Mo was also an oxygen-coordinated species in the upper layer (0–8 cmbsf). Meanwhile, the detection of sulfide in the deeper layer suggests that Mo sulfidation is an important reaction in Mo enrichment in sediments.On the other hand, at all the depths, W formed oxygen-coordinated species in the sediment. The subtle change of Kd for W with depth suggested that H2S did not affect the W adsorption in our samples. Hence, the Mo/W ratio in the sediments increased with the development of reducing conditions and vice versa in the coexistent porewater. Speciation analysis revealed that the high stability of oxygen-coordinated species of W was responsible for the variations of the Mo/W ratio under various redox conditions.

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

Porewater was collected from highly permeable, carbonate-rich, sandy sediments at two locations on Oahu, Hawai'i, CRIMP-2 on the Kaneohe Bay barrier reef and Ala Wai on a fringing coral reef nearshore to the Ala Wai canal and urban Honolulu. Samples were collected at the sediment–water interface and from porewater wells installed at sediment depths of 2, 4, 6, 8, 12, 16, 20, 30, 40, and 60cm. Total alkalinity and dissolved inorganic carbon were enriched, relative to the overlying water column, and ratios of TA:DIC at the two sites (0.80 and 0.93) suggest that aerobic respiration and sulfate reduction – both coupled with carbonate mineral dissolution – in the oxic and anoxic layers, respectively, are the major controls on the biogeochemistry of the sediment–porewater system. The porewater was approaching thermodynamic saturation with respect to aragonite with increasing depth (Ω=1.2–3.5) and was found to be undersaturated with respect to all phases of magnesian calcite (Ω=0.3–0.9) containing greater than 12mol% MgCO3. In addition to microbial controls on porewater diagenesis, transient physical events in the water column, such as swells and changing bottom current speeds, appear to exert a strong influence on the porewater chemistry due to the highly permeable and porous nature of the sediments. Profiles collected before and after swell events at each location show an apparent flushing of the porewater system, replacing low pH, high DIC interstitial waters with seawater from the overlying water column. Based partially on the results of our porewater analysis, we hypothesize that future changes in surface water DIC and pH resulting from ocean acidification (OA) could have a very significant impact on the dissolution rates of metastable skeletal and abiotic carbonate phases of varying magnesian calcite compositions (Mg-calcite) and aragonite. This is especially important in sandy reef sediments like those of this study, which are greatly influenced by the overlying water column. As the carbonate mineral saturation state of the overlying water column continues to decrease due to OA, an increase in carbonate mineral dissolution is expected and the high advective rate of water exchange between the porewater of sandy sediments and the overlying water column, as observed in this study and others, along with increased rates of dissolution of metastable carbonate phases, could lead to significantly higher future rates of mass transfer of TA and DIC between the sediments and the overlying water column. This may result in a deficit of the CaCO3 balance in some reef ecosystems and a decrease in accretion rates. Analysis of our porewater work in conjuction with previous studies of the porewater chemistry of Kaneohe Bay, Oahu in both siliciclastic-rich and siliciclastic-poor carbonate sediments leads to the conclusion that the porewaters of the former are more strongly buffered with respect to pH than those of the latter due to reverse weathering reactions. Thus carbonate-rich sandy sediments of reefs with little terrestrial influence and aluminosilicate detritus may become more susceptible to calcium carbonate loss due to the enhanced environmental and microbial dissolution of carbonate substrates expected due to OA.

Equilibrium passive sampling as a tool to study polycyclic aromatic hydrocarbons in Baltic Sea sediment pore-water systems

Solid Phase Microextraction (SPME) was applied to provide the first large scale dataset of freely dissolved concentrations for 9 polycyclic aromatic hydrocarbons (PAHs) in Baltic Sea sediment cores. Polydimethylsiloxane (PDMS) coated glass fibers were used for ex-situ equilibrium sampling followed by automated thermal desorption and GC–MS analysis. From the PAH concentrations in the fiber coating we examined (i) spatially resolved freely dissolved PAH concentrations (Cfree); (ii) baseline toxicity potential on the basis of chemical activities (a); (iii) site specific mixture compositions; (iv) diffusion gradients at the sediment water interface and within the sediment cores and (v) site specific distribution ratios. Contamination levels were low in the northern Baltic Sea, moderate to elevated in the Baltic Proper and highest in the Gulf of Finland. Chemical activities were well below levels expected to cause narcosis to benthos organisms. The SPME method is a very sensitive tool that opens new possibilities for studying the PAHs at trace levels in marine environments.

Biogeochemical processes controlling the mobility of major ions and trace metals in aquitard sediments beneath an oil sand tailing pond: Laboratory studies and reactive transport modeling

Increased production and expansion of the oil sand industry in Alberta are of great benefit to the economy, but they carry major environmental challenges. The volume of fluid fine tailings requiring storage is 840×10(6) m(3) and growing, making it imperative that we better understand the fate and transport of oil sand process-affected water (OSPW) seepage from these facilities. Accordingly, the current study seeks to characterize both a) the potential for major ion and trace element release, and b) the principal biogeochemical processes involved, as tailing pond OSPW infiltrates into, and interacts with, underlying glacial till sediments prior to reaching down gradient aquifers or surface waters. Objectives were addressed through a series of aqueous and solid phase experiments, including radial diffusion cells, an isotope analysis, X-ray diffraction, and sequential extractions. The diffusion cells were also simulated in a reactive transport framework to elucidate key reaction processes. The experiments indicate that the ingress and interaction of OSPW with the glacial till sediment-pore water system will result in: a mitigation of ingressing Na (retardation), displacement and then limited precipitation of exchangeable Ca and Mg (as carbonates), sulfate reduction and subsequent precipitation of the produced sulfides, as well as biodegradation of organic carbon. High concentrations of ingressing Cl (~375 mg L(-1)) and Na (~575 mg L(-1)) (even though the latter is delayed, or retarded) are expected to migrate through the till and into the underlying sand channel. Trace element mobility was influenced by ion exchange, oxidation-reduction, and mineral phase reactions including reductive dissolution of metal oxyhydroxides - in accordance with previous observations within sandy aquifer settings. Furthermore, although several trace elements showed the potential for release (Al, B, Ba, Cd, Mn, Pb, Si, Sr), large-scale mobilization is not supported. Thus, the present results suggest that in addition to the commonly cited naphthenic acids, remediation of OSPW-impacted groundwater will need to address high concentrations of major ions contributing to salinization.

Using solid-phase microextraction to evaluate the role of different carbon matrices in the distribution of PAHs in sediment-porewater systems of the Baltic Sea

Purpose The speciation of polycyclic aromatic hydrocarbons (PAHs) in sediment-porewater systems affects both the chemical fate and bioavailability of these compounds. PAHs may be dissolved or sorbed to sediment particles or dissolved organic carbon (DOC). Furthermore, soot carbon has been shown to control the sorption of PAHs onto particles in natural waters. The present study investigates the distribution of individual PAHs among these three phases by examining sediments from the western Baltic Sea, focusing on a highly contaminated former dumping area and evaluating the importance of soot-carbon partitioning.

Benthic phosphorus and iron budgets for three NW African slope sediments: a balance approach

Abstract. Despite intensive research on the different domains of the marine phosphorus (P) cycle during the last decades, frequently discussed open questions still exist especially on controlling factors for the benthic behaviour of P and its general distribution in sediment-pore water systems. Steady state or the internal balance of all relevant physical and (bio)geochemical processes are amongst the key issues. In this study we present and discuss an extended data set from surface sediments recovered from three locations on the NW African continental slope. Pore water data and results from sequential sediment extractions give clear evidence to the well-known close relationship between the benthic cycles of P and iron. Accordingly, most of the dissolved phosphate must have been released by microbially catalyzed reductive dissolution of iron (oxhydr)oxides. However, rates of release and association of P and iron, respectively, are not directly represented in profiles of element specific sediment compositions. Results from steady-state based transport-reaction modelling suggest that particle mixing due to active bioturbation, or rather a physical net downward transport of P associated to iron (oxyhydr)oxides, is an essential process for the balance of the inspected benthic cycles. This study emphasizes the importance of balancing analytical data for a comprehensive understanding of all processes involved in biogeochemical cycles.

Oxygen transfer from flowing water to microbes in an organic sediment bed

Mass transfer of dissolved oxygen (DO) across a sediment–water interface was investigated using laboratory experiments and a numerical simulation model. DO concentration profiles and velocity profiles were measured with high resolution in a recirculating flume with water flowing at cross-sectional average velocities from 3.5 to 11.5 cm/s over a flat and hydrodynamically smooth organic sediment bed. Parameters extracted from the measurements included (1) the DO penetration depth, (2) the effective diffusion coefficient in the sediment layer, (3) the thickness of the turbulent diffusive boundary layer, and (4) the diffusion coefficients in the diffusive boundary layer. DO penetration depths were less than 1 mm, and diffusive boundary layer thicknesses were less than 8 mm. Diffusion in the sediment porewater system was shown to be essentially molecular. The laboratory data were compared to results from a deterministic simulation model. The model included explicit descriptions of (1) mass transfer through the diffusive boundary layer above the sediment/water interface, (2) the boundary layer development over the sediment bed of finite length, (3) diffusive transfer in the sediment porewater system, and (4) microbial uptake of DO in the sediment. The model included both water-side and sediment-side mass transport limitations. The control of DO flux could alternate between water-side and sediment-side without discontinuity. Monod-type kinetics was adopted for DO uptake in the sediment. Organic substrate availability in the sediment did not vary over the course of an experiment. A kinetic limitation for organic matter (substrate) was not considered, and microbial activity in the sediment was parameterized by biomass density. Measured and simulated DO concentration profiles showed satisfactory agreement, with some discrepancies at the interface caused by roughness and porosity effects of the sediment surface.

Controls on the δ13C of dissolved inorganic carbon in marine pore waters: An integrated case study of isotope exchange during syndepositional recrystallization of biogenic carbonate sediments (South Florida Platform, USA)

Abstract The carbon isotope systematics of marine carbonates, organic matter and dissolved inorganic carbon (DIC) play a critical role in quantifying carbonate dissolution fluxes from modern deep-ocean sediments to paleoocean–atmospheric modeling. However, there is a growing body of evidence that C mass and isotope balances in marine pore waters appear incompatible, suggesting that some processes other than mass transport, carbonate dissolution, and organic matter decomposition have significantly increased the value of δ13C(DIC). We present a comprehensive data set of pore water and sediment geochemistries in biogenic carbonates from well-characterized depositional environments of the South Florida platform. Pore water elemental and δ13C(DIC) values are integrated with δ13C values of carbon sources (seawater, organic and inorganic carbon), sediment mixing rates (210Pb profiles), microbial sulfate reduction rates (SRR) (radiotracer 35SO42−), and incubation experiments spiked with low δ13C(DIC) to estimate the rate and extent of C isotope exchange. Together, these data indicate that biogenic carbonates undergo extensive syndepositional recrystallization at rates comparable to net dissolution rates, permitting significant exchange between isotopically depleted organic C and isotopically enriched inorganic C pools. Significant amounts of net carbonate dissolution are common in the pore waters of these low-Fe sediments, as manifested by Ca2+/Cl− ratios increased by up to 25% relative to overlying seawater. Despite rapid microbial SRR, degrees of pore water SO42− reduction usually are maintained below 5% by H2S oxidation, the main acid source for dissolution. These processes increase pore water DIC concentrations by more than 6 mM, over a 5-fold increase relative to overlying seawater values. Pore water δ13C(DIC) values are usually greater than −5‰, and sometimes as high as +2‰, despite decomposition of organic matter with low δ13C values (−9‰ to −15‰ VPDB). Pore water δ13C(DIC) values thus require inputs from inorganic C that exceed those from organic C, even though less than 25% of the DIC can be accounted for by the amount of net carbonate dissolution indicated by Ca2+/Cl− ratios. Incubation experiments attain DIC concentrations in excess of 20 mM during closed system SO42− reduction, but δ13C(DIC) values remain buffered between –4‰ and –6‰. Exchange of 12C for 13C during carbonate recrystallization in pore waters is required to reconcile the significant imbalance between mass and isotope budgets. Past work and data presented here for Sr2+/Ca2+ in pore waters show that recrystallization of biogenic aragonites and high-Mg calcites from South Florida platform sediments can occur without significant mineralogic change. Although these phases are metastable relative to lower Mg-calcite, the drive for recrystallization does not appear to be mineralogic stabilization. Instead, the high surface areas of many biogenic carbonates from algal and foraminiferal sources (about 2 m2 g−1) contribute excess surface free energy that is released by recrystallization to larger crystallites of the same mineralogy. Studies of carbonate dissolution and precipitation kinetics have firmly established that high ratios of solid surface area: solution volume causes reaction of CO2 with grain surfaces to become the rate-limiting step. Thus, in sediment-pore water systems containing carbonates with high specific surface areas, pore water δ13C(DIC) may be buffered either by the value of the bulk CaCO3, producing δ13C(DIC) values approaching +2‰, or by the relatively large fractionation factor between CO2 and solid CaCO3 (Δ∼−8‰), producing δ13C(DIC) values that approach −6‰ VPBD. The observed magnitude of isotopic exchange between organic and inorganic carbon pools during the earliest stages of sedimentation has ramifications for the interpretation of the C isotopic record of marine carbonate rocks and contributes significant uncertainty to reconstructions of paleoocean and atmospheric chemistries.

Simultaneous Microbial Reduction of Iron(III) and Arsenic(V) in Suspensions of Hydrous Ferric Oxide

Bacterial reduction of arsenic(V) and iron(III) oxides influences the redox cycling and partitioning of arsenic (As) between solid and aqueous phases in sediment-porewater systems. Two types of anaerobic bacterial incubations were designed to probe the relative order of As(V) and Fe(III) oxide reduction and to measure the effect of adsorbed As species on the rate of iron reduction, using hydrous ferric oxide (HFO) as the iron substrate. In one set of experiments, HFO was pre-equilibrated with As(V) and inoculated with fresh sediment from Haiwee Reservoir (Olancha, CA), an As-impacted field site. The second set of incubations consisted of HFO (without As) and As(III)- and As(V)- equilibrated HFO incubated with Shewanella sp. ANA-3 wild-type (WT) and ANA-3deltaarrA, a mutant unable to produce the respiratory As(V) reductase. Of the two pathways for microbial As(V) reduction (respiration and detoxification), the respiratory pathway was dominant under these experimental conditions. In addition, As(III) adsorbed onto the surface of HFO enhanced the rate of microbial Fe(III) reduction. In the sediment and ANA-3 incubations, As(V) was reduced simultaneously or prior to Fe(III), consistent with thermodynamic calculations based on the chemical conditions of the ANA-3 WT incubations.

Experimental and modeling investigation of metal release from metal‐spiked sediments

In sediments that contain iron monosulfide, cadmium, nickel, lead, zinc, and silver(I) form insoluble metal sulfides that lower the metal ion activity in the sediment-pore water system, thereby reducing toxicity. However, metal sulfides are susceptible to oxidation by molecular oxygen resulting in metal solubilization. To better understand the sources and sinks of metal sulfides in sediments, iron monsulfide-rich freshwater sediments were spiked with cadmium, nickel, lead, zinc, or silver(I) and placed into cylindrical cores with an overlying layer of oxygen-saturated water. Measurements of the dissolved metal concentration in the overlying water were made as a function of time and the vertical profiles of acid-volatile sulfide (AVS) and simultaneously extracted metal (SEM) were measured after 150 d. A one-dimensional reactive and transport model has been employed to help elucidate processes controlling the fate of metals in sediments. The model incorporates metal-sulfide formation, metal-sulfide oxidation, and metal partitioning onto sediment organic carbon and iron oxyhydroxide to simulate the vertical transport of metals throughout the sediment core.