Assessing the Effects of Bioturbation on Metal Bioavailability in Contaminated Sediments by Diffusive Gradients in Thin Films (DGT)

Diffusive Gradients in Thin Films Can Predict the Toxicity of Metal Mixtures to Two Microalgae: Validation for Environmental Monitoring in Antarctic Marine Conditions

Anthropogenic contamination in the Antarctic near-shore marine environment is a challenge for environmental managers because of its isolation, high costs associated with monitoring and remediation activities, and the current lack of Antarctic-specific ecotoxicological data. The present study investigated the application of diffusive gradients in thin films (DGT) with a Chelex-100 binding resin for metal contaminant assessment in Antarctic marine conditions. Diffusion coefficients for cadmium (Cd), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn), determined at 1 °C, ranged between 2.1 and 2.6×10^-6  cm² /s and were up to 32% lower than those derived by theoretical calculations. Competition of metals on the DGT binding resin was observed at subsaturation concentrations, reducing the effective capacity for metal uptake by approximately 60%. The lability of the dissolved (0.45 µm filterable) Cd, Cu, Pb, and Zn metal fraction to DGT was generally >90% and unaffected by the presence of the Antarctic marine microalga Phaeocystis antarctica. Both DGT and dissolved metal concentrations gave equivalent mixture toxicity predictions in independent action and concentration addition models to P. antarctica and Cryothecomonas armigera; that is, predictions using DGT-labile concentrations also showed antagonism to P. antarctica, which agrees with previously determined mixture interactivity. The benefits of DGT over traditional sampling techniques (i.e., discrete water sampling) include lower method detection limits (MDLs), in situ assessment, and time-averaged concentrations which capture pulses of contamination typical of the Antarctic near-shore marine environment. The present study provides MDLs and recommended minimum deployment times to guide field deployments in Antarctica. Environ Toxicol Chem 2019;38:1323-1333. © 2019 SETAC.

Assessment of heavy metal pollution, distribution and quantitative source apportionment in surface sediments along a partially mixed estuary (Modaomen, China)

The aim of this study is to investigate the distribution, ecological risk and quantitative source apportionment of seven heavy metals in surface sediments along a partially mixed estuary (Modaomen, China). The study area was divided into three regions based on salt-freshwater mixing: a tidal river channel, weakly mixed zone and strongly mixed zone. The concentrations of Cu, Zn, Pb, Cd, Cr and Ni were significantly higher in the weakly mixed zone, while a higher Co content was found in the tidal river channel. From enrichment factor (EF) analyses, Cd was moderate to extreme severely enriched at most stations, and other heavy metals were minor enrichments. Adverse effects on aquatic biota may occur occasionally for all seven heavy metals based on the sediment quality guidelines (SQGs). The correlations between Al and heavy metals suggested that metals in sediments were mainly attributed to anthropogenic sources. Then Positive matrix factorization (PMF) and Geostatistic method were used to quantify the heavy metal sources and determine impacted regions. A mixed source from the inner Lingding Bay and West River contributed approximately 50% of the Cu, Zn, Pb, Cr and Ni. The electroplating industry accounted for about 57% of the Cd content, and the major contaminated area was concentrated at the confluence of the West River and its tributaries. The alloy industry contributed approximately 65% of the Co, most of which was near an industrial park.

Benthic community status and mobilization of Ni, Cu and Co at abandoned sea deposits for mine tailings in SW Norway

During 1960-94 tailings from an ilmenite mine in southwest Norway were placed in sea deposits in a sheltered fjord and a more exposed coastal basin. In 2015 both deposit sites were sampled to assess the state of metal contamination and macrobenthic communities 20-30 years after deposition was ended. The results showed that nickel and copper still exceeded environmental quality standards in sediment and pore water from the 0-1 cm layer, and fluxes of nickel, copper and cobalt to the overlying water was high compared to adjacent reference stations. Fauna communities were classified as good, but moderate disturbance was recorded along an environmental gradient defined by depth and tailings-induced parameters such as particle size and copper. The results were interpreted in terms of current discharges, biological sediment reworking and near-surface leaching of metal sulphides. No evidence was found for recycling of metals from tailings buried below the bioturbated surface layer.

In situ simulation of thin-layer dredging effects on sediment metal release across the sediment-water interface

Dredging is widely applied to remediate contaminated sediments in aquatic ecosystems. However, the efficiency of thin-layer dredging for metal pollution control remains uncertain and even controversial. This study conducted an in-situ simulation experiment in Lake Taihu to investigate dredging effects on sediment metal release based on metal fractions, diffusion flux and kinetics parameters of metal resupply, using diffusive gradient in thin films (DGT), multi-microelectrode, and European Community Bureau of Reference (BCR) sequential extraction scheme. Results indicated that the exchange fluxes of metals did not necessarily correspond to total sediment metal concentrations or the contents of different sequentially-extracted metal fractions; there were appreciable decreases in Ni, Cd, Cu and Zn in terms of total sediment metal concentrations and metal fractions, whereas the bioavailability and release fluxes of labile Ni, Cu and Zn (but not Cd) were all notably promoted (by 136, 128 and 149%, respectively) in dredged area compared to those in un-dredged sediments. Further analysis on the kinetics of metal resupply by DGT technique and DGT-induced fluxes in sediments model (DIFS) showed higher concentrations of labile metals, with a larger resupply ability from sediments after dredging. Therefore, thin-layer dredging had the possibility to increase metal release from sediments to the water column. This was attributed to the remobilization of metal sulfides in anoxic deep sediments, as oxidation increased after dredging due to the introduction of oxygenated water, causing subsequent dissolution of sulfide-bound metals. In conclusion, dredging may not mitigate metal contamination, although it can reduce the total pollution load. Our findings indicated dual effects of dredging and provided new insights into the remobilization mechanism of metal release induced by dredging.

Evaluation of the DGT technique for predicting uptake of metal mixtures by fathead minnow (Pimephales promelas) and yellow lampmussel (Lampsilis cariosa)

Diffusive gradients in thin films (DGT) were assessed for their predictive capability of fathead minnow and yellow lampmussel bioaccumulation in copper (Cu) and lead (Pb) mixed metal exposures. Nine treatments with a matrix of 3 Cu and 3 Pb concentrations were utilized. Exposures were coupled, with organisms and DGT exposed in tanks for 6 days. The Cu measured in fish, mussel, and DGT was found not to be influenced by Pb treatment, whereas Pb accumulation was impacted by the interaction of Cu and Pb treatment. The Pb accumulation increased with increasing Cu concentration, which was attributed to the different speciation of Cu and Pb in the water where Cu binds preferentially to ligands, decreasing its bioavailability and concomitantly displacing Pb from complexing sites. The DGT values were significantly correlated with accumulated Cu and Pb in the fish, but not with Pb in the mussel. In addition, DGT was determined to better predict aquatic organism bioaccumulation of Cu than the inorganic Cu fraction calculated by the speciation model, because DGT accumulated not only inorganic metal fractions but also complexes of metal and organic matter. The present study provides insights into metal speciation in polluted environments, extends the understanding of using DGT as a tool for estimating metal bioavailability, and provides implications for the selection of geochemical modeling, biological sampling, and passive sampling techniques for monitoring trace metal contamination. Environ Toxicol Chem 2019;38:61-70. © 2018 SETAC.

Relationship between heavy metals and dissolved organic matter released from sediment by bioturbation/bioirrigation

Organic matter (OM) is an important component of sediment. Bioturbation/bioirrigation can remobilize OM and heavy metals that were previously buried in the sediment. The remobilization of buried organic matter, thallium (Tl), cadmium (Cd), copper (Cu) and zinc (Zn) from sediment was studied in a laboratory experiment with three organisms: tubificid, chironomid larvae and loach. Results showed that bioturbation/bioirrigation promoted the release of dissolved organic matter (DOM) and dissolved Tl, Cd, Cu and Zn, but only dissolved Zn concentrations decreased with exposure time in overlying water. The presence of organisms altered the compositions of DOM released from sediment, considerably increasing the percentage of fulvic acid-like materials (FA) and humic acid-like materials (HA). In addition, bioturbation/bioirrigation accelerated the growth and reproduction of bacteria to enhance the proportion of soluble microbial byproduct-like materials (SMP). The DOM was divided into five regions in the three-dimensional excitation emission matrix (3D-EEM), and each part had different correlation with the dissolved heavy metal concentrations. Dissolved Cu had the best correlation with each of the DOM compositions, indicating that Cu in the sediment was in the organic-bound form. Furthermore, the organism type and heavy metal characteristics both played a role in influencing the remobilization of heavy metal.

Field and laboratory evaluation of DGT for predicting metal bioaccumulation and toxicity in the freshwater bivalve Hyridella australis exposed to contaminated sediments

The diffusive gradients in thin films (DGT) technique has shown to be a useful tool for predicting metal bioavailability and toxicity in sediments, however, links between DGT measurements and biological responses have often relied on laboratory-based exposures and further field evaluations are required. In this study, DGT probes were deployed in metal-contaminated (Cd, Pb, Zn) sediments to evaluate relationships between bioaccumulation by the freshwater bivalve Hyridella australis and DGT-metal fluxes under both laboratory and field conditions. The DGT-metal flux measured across the sediment/water interface (±1 cm) was useful for predicting significant cadmium and zinc bioaccumulation, irrespective of the type of sediment and exposure. A greater DGT-Zn flux measured in the field was consistent with significantly higher zinc bioaccumulation, highlighting the importance of performing metal bioavailability assessments in situ. In addition, DGT fluxes were useful for predicting the potential risk of sub-lethal toxicity (i.e., lipid peroxidation and lysosomal membrane damage). Due to its ability to account for multiple metal exposures, DGT better predicted bioaccumulation and toxicity than particulate metal concentrations in sediments. These results provide further evidence supporting the applicability of the DGT technique as a monitoring tool for sediment quality assessment.

Contamination, potential mobility, and origins of lead in sediment cores from the Shima River, south China

Identifying contamination sources of environmental media and revealing their changing trends over time is useful for regional contamination control and environmental improvements. Four sediment cores (S1-S4) were collected from the Shima River to determine lead (Pb) concentrations, geochemical fractions and isotopic compositions, as well as the geochronology of core S3. The results show that Pb concentrations decreased from the upper and middle reach sites (means: 57.6, 95.9, and 97.6 mg kg^-1, respectively) to the lower reach site (43.8 mg kg^-1), resulting in a minimal to moderate enrichment in the sediments; enrichment increased due to anthropogenic Pb inputs at the river middle reach site since the 1990s. Sediment Pb in the geochemical fractions followed a decreasing order of reducible (47.3%) > residual (37.8%) > oxidizable (11.2%) > acid-soluble fraction (3.68%), exhibiting high mobility, further verifying the anthropogenic inputs. A descending trend in the ²⁰⁶Pb/²⁰⁷Pb ratio of the top sediments was the result of anthropogenic activities. In the present study, coal combustion, which was the major anthropogenic Pb source determined by its isotopic composition, contributed significantly (means: 18.4-60.6%) to sediment Pb based on a three end-members model. Less of a contribution (0-10.6%) was derived from vehicle exhaust. The increasing trend in the coal contribution was in accordance with that of the coal consumption in the study area. These results suggest that Pb contamination resulting from coal combustion has grown to become a major environmental issue in the study area.

Evaluation of diffusive gradients in thin films for prediction of copper bioaccumulation by yellow lampmussel (Lampsilis cariosa) and fathead minnow (Pimephales promelas)

Using a coupled method of diffusive gradients in thin films (DGT) exposure with aquatic organism bioassays, we assessed the use of DGT as a tool for estimating copper (Cu) bioavailability in contaminated waters. The DGT-accumulated Cu fraction could possibly be used as a surrogate for other assessments of metal bioavailability. The Cu concentrations in fathead minnow (Pimephales promelas) and yellow lampmussel (Lampsilis cariosa) soft tissue were compared with DGT-accumulated Cu after 2, 4, and 6 d of exposure to a Cu concentration series in static, water-only assays. The DGT-accumulated Cu was found to include free Cu ions, labile inorganic Cu complexes, and labile dissolved organic matter Cu complexes, compared with Cu speciation output from the biotic ligand model. Regressions of Cu concentrations between DGT and fathead minnow at 4 and 6 d of exposure demonstrated linear relationships. The Cu bioaccumulated in yellow lampmussel was overpredicted by DGT at Cu concentrations greater than 10 µg L^-1 , which may be caused by internal regulation of Cu. The speciation component of the biotic ligand model predicted relationships between inorganic Cu and animal-accumulated Cu that were similar to predicted relationships between DGT-indicated Cu and animal-accumulated Cu at all deployment durations. Environ Toxicol Chem 2018;37:1535-1544. © 2018 SETAC.

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.

River sediment quality assessment using sediment quality indices for the Sydney basin, Australia affected by coal and coal seam gas mining

Coal mining activities in the Sydney basin have been historically associated with significant environmental impacts. The region is facing more recent coal seam gas extraction activities and the synergetic environmental impacts of the new mining activities are still largely unknown. The aim of this study was to provide environmental assessment of river sediments comparing upstream to downstream areas relative to industrial-discharge sites associated with coal and coal-seam-gas extraction within the Sydney basin. Various contaminants were measured to determine the sediment quality according to the Australian and New Zealand Environment and Conservation Council (ANZECC) guidelines. Arsenic, nickel and zinc were the main sediment contaminants in downstream samples exceeding the ANZECC guidelines. Degree of contamination (C_d), geoaccumulation index (I_geo), enrichment factor (EF), pollution load index (PLI) and sediment environmental toxicity quotients' increment in downstream sediment were estimated for the studied areas. Toxicology indices of metals present in the sediments near industrial discharge sites were used as an additional tool to compare the level of environmental effects with their increment. The study revealed that the sediments from coal mining sites were highly affected by increased concentrations of manganese, zinc, cobalt, nickel and barium. The sediments associated with coal mining activities were found to be substantially more affected than the sediments near coal seam gas production sites, mainly attributed to the different wastewater discharge licencing requirements. The approach applied in this study can be used as an additional model to assess the contribution of industrial and mining activities on aquatic environments.

Evaluating the diffusive gradients in thin films technique for the prediction of metal bioaccumulation in plants grown in river sediments

The diffusive gradients in thin films (DGT) technique is a useful tool for assessing metal bioavailability in sediments. However, the DGT technique has not been used to predict metal bioaccumulation in plants grown in sediments in river systems. In this study, the DGT technique was evaluated for predicting metal bioaccumulation in Phragmites australis growing in contaminated sediments. In sediments with high levels of contamination, release of DGT-labile Cr, Zn, Cu, and Cd occurred, which resulted in high bioaccumulation of these metals in P. australis. Bioaccumulation of Cr, Cu, Zn, and Cd was strongly correlated with the metal concentrations in the sediments measured by the DGT technique. By contrast, the correlation between sediment content and bioaccumulation for As was weak. There were significant negative correlations between the content of Ni in the plant tissues and the contents of the other metals. Overall, the DGT technique provided predictions of metal bioaccumulation similar to those obtained using total metal measurements in multiple polluted sediment samples. Therefore, DGT analysis could be used for assessing heavy metal bioavailability, and metal bioaccumulation in P. australis was not all significantly correlated with the bioavailability concentrations of metals in river sediments.

Interplay between flow and bioturbation enhances metal efflux from low-permeability sediments

Understanding the interplay effects between processes such as hydrodynamic forcing, sediment resuspension, and bioturbation is key to assessment of contaminated sediments. In the current study, effects of hydrodynamic forcing, sediment resuspension, and bioturbation by the marine polychaete Nereis virens were evaluated both independently and together in a six-month flume experiment. The results show that hydrodynamic forcing without resuspension or worm action slightly enhanced efflux of dissolved Cu to the water column, sediment resuspension released considerable amounts of dissolved Cu, and interactions between hydrodynamics and worm burrowing further enhanced Cu efflux. In non-bioturbated sediments, fine particles were only resuspended to the overlying water under the highest imposed shear stress, 0.58Pa. However, bioturbated sediments were resuspended under all shear stresses tested (0.11-0.58Pa), indicating that bioturbation destabilized the sediment bed. Further, increases in fluid shear following bioturbation caused rapid releases of dissolved Cu to the overlying water within a few hours. Cu efflux under fluid shears of 0.47Pa and 0.58Pa were 360× and 15× greater after the introduction of worms compared with the same flow conditions without their presence. Overall, our results indicate that the release of metals from low-permeability sediments is greatly enhanced by interactions between flow and bioturbation.

Assessing potential release tendency of As, Mo and W in the tributary sediments of the Three Gorges Reservoir, China

As the largest man-made reservoir in China, the Three Gorges Reservoir (TGR) has significant influence on national drinking water safety. The geochemical behavior of trace elements at the sediment-water interface (SWI) is still unknown. The mobilization characteristics of trace elements (As, Mo and W)-determined by diffusive gradients in thin films (DGT)-were studied to quantitatively calculate the release trends in the SWI in three typical tributaries and the mainstream of the TGR in the summer. The results showed that concentrations of DGT-labile As, Mo and W in the overlying water and sediment cores showed significant variations in the ranges of 0.05-50.90, 0.30-1.63 and 0.01-0.42μgL^-1, respectively. The apparent net diffusive fluxes were significantly positive in most sampling sites (77.8% for As, 88.8% for Mo and 66.6% for W), suggesting that the sediment was the source of these three elements. It was noteworthy that the maximum net diffusive fluxes of As and W were found in the upstream of Meixi tributary, which may be attributed to anthropogenic activities. In addition, As, Mo and W may be incorporated in Fe and Mn oxyhydroxides and these three elements simultaneously remobilized with Fe and Mn.

Assessment of potential bioavailability of heavy metals in the sediments of land-freshwater interfaces by diffusive gradients in thin films

It is important to understand the potential bioavailability of heavy metals in the sediments of land-freshwater interfaces (between terrestrial and aquatic ecosystems). Therefore, we evaluated the pollution of Cr, Ni, Cu, Zn, As, and Cd in land-freshwater interface sediments/soils according to total concentrations, and used sequential extraction method to measure different fractions of Cd. Then, the diffusive gradients in thin films (DGT) technique was employed to study the potential bioavailability of metals. Results showed that the concentrations of exchangeable and weak acid soluble fraction and oxidizable fraction were higher in ecotone area, and the values of reducible fraction and residual fraction were higher in deep water area. There existed significant positive correlations between [Cd]-DGT (concentration of Cd measured by DGT) and EXC-Cd (exchangeable and weak acid soluble fraction of Cd) (r² = 0.65), but the significant negative correlation was found between [Cd]-DGT and RES-Cd (Residual fraction of Cd) (r² = 0.52). DGT technique is a feasible method to measure potential bioavailability of heavy metals for risk assessment in the sediments/soils of land-freshwater interfaces.

Sediment Zn-release during post-drought re-flooding: Assessing environmental risk to Hyalella azteca and Daphnia magna

Hydrologic variability exacerbated by climate change affects biogeochemical cycling in sediments through changes in pH, redox, and microbial activity. These alterations affect the lability and speciation of metals, such that toxicity may be observed in otherwise non-toxic sediments. In this study, we investigate the effects of drought and reflooding on metal bioavailability in sediments with low to moderate concentrations of Zn (18-270 mg kg^-1). Sediments were collected from coastal wetlands in Michigan, dried (36-days) and re-inundated in lab microcosms. We investigated the relationships between key parameters, for surface/porewater (dissolved and particulate metals, dissolved oxygen, redox (Eh), reduced iron, and temperature) and sediment (simultaneously extracted metals (SEM), acid volatile sulfide (AVS), Fe/Mn-oxyhydroxide, organic carbon, water content analyses, and diffusive gradient in thin films (DGTs) metal concentrations). Porewater Zn increased with inundation of dried sediments for all sediment types, exceeding United States Environmental Protection Agency (U.S. EPA) chronic criteria for freshwater organisms, and decreased as sediments became reduced. Effects on Hyalella azteca (7-day exposure) and Daphnia magna (10-day exposure) were quantified. Results show decreased growth of H. azteca for sites with elevated Zn and increased Zn-body concentration (BC_Zn) in the most contaminated sediment type. Further, BC_Zn was negatively correlated with H. azteca growth. D. magna survival, growth, and reproduction were not affected. DGT metal concentrations were more reflective of porewater than organism bioaccumulation. Outcomes of predictive toxicology methods are compared to toxicity test results and suggestions are provided for model improvements. This study demonstrates that post-drought re-flooding of sediments affects Zn biogeochemical cycling with potentially adverse effects on benthic organisms, even in sediments with only moderately elevated concentrations (>150 mg kg^-1).

Effects-based spatial assessment of contaminated estuarine sediments from Bear Creek, Baltimore Harbor, MD, USA

Estuarine sediments in regions with prolonged histories of industrial activity are often laden to significant depths with complex contaminant mixtures, including trace metals and persistent organic pollutants. Given the complexity of assessing risks from multi-contaminant exposures, the direct measurement of impacts to biological receptors is central to characterizing contaminated sediment sites. Though biological consequences are less commonly assessed at depth, laboratory-based toxicity testing of subsurface sediments can be used to delineate the scope of contamination at impacted sites. The extent and depth of sediment toxicity in Bear Creek, near Baltimore, Maryland, USA, was delineated using 10-day acute toxicity tests with the estuarine amphipod Leptocheirus plumulosus, and chemical analysis of trace metals and persistent organic pollutants. A gradient of toxicity was demonstrated in surface sediments with 21 of 22 tested sites differing significantly from controls. Effects were most pronounced (100% lethality) at sites proximate to a historic industrial complex. Sediments from eight of nine core samples to depths of 80 cm were particularly impacted (i.e., caused significant lethality to L. plumulosus) even in locations overlain with relatively non-toxic surface sediments, supporting a conclusion that toxicity observed at the surface (top 2 cm) does not adequately predict toxicity at depth. In seven of nine sites, toxicity of surface sediments differed from toxicity at levels beneath by 28 to 69%, in five instances underestimating toxicity (28 to 69%), and in two instances overestimating toxicity (44 to 56%). Multiple contaminants exceeded sediment quality guidelines and correlated positively with toxic responses within surface sediments (e.g., chromium, nickel, polycyclic aromatic hydrocarbon (PAH), total petroleum hydrocarbon). Use of an antibody-based PAH biosensor revealed that porewater PAH concentrations also increased with depth at most sites. This study informs future management decisions concerning the extent of impact to Bear Creek sediments, and demonstrates the benefits of a spatial approach, relying primarily on toxicity testing to assess sediment quality in a system with complex contaminant mixtures.

Investigation of heavy metals release from sediment with bioturbation/bioirrigation

Bioturbation/bioirrigation can affect the remobilization of metals from sediments. In this study, experiments were performed to examine the effect of bioturbation/bioirrigation by different organisms on cadmium (Cd), copper (Cu), zinc (Zn) and lead (Pb) releasing from the spiked sediment. The diffusive gradient in thin films technique (DGT) revealed that at the end of exposure time, the labile heavy metals concentrations in the pore water for all metal and organisms combinations except Cu and chironomid larvae were much lower than that in the control group. However, the concentrations of heavy metals detected by the DGT were virtually indistinguishable among the treatments with tubificid, chironomid larvae and loach. The correlation analysis of heavy metals with iron (Fe) and manganese (Mn) suggested that Cd, Zn and Pb were most likely bound as Fe-Mn oxidation form in the pore water, but Cu was in other forms. After 28 d of exposure, bioturbation/bioirrigation produced a significant release of particulate heavy metals into the overlying water, especially in the treatment with loach. The bioturbation/bioirrigation impact on the Pb remobilization was less than the other three heavy metals. The effects of bioturbaiton/bioirrigation on the heavy metals remobilization in the sediment were complex that with studying the heavy metals remobilization in the sediment and water interface, the biological indicators should be recommended.

A scheme to scientifically and accurately assess cadmium pollution of river sediments, through consideration of bioavailability when assessing ecological risk

Evaluating heavy metal pollution status and ecological risk in river sediments is a complex task, requiring consideration of contaminant pollution levels, as well as effects of biological processes within the river system. There are currently no simple or low-cost approaches to heavy metal assessment in river sediments. Here, we introduce a system of assessment for pollution status of heavy metals in river sediments, using measurements of Cd in the Shaocun River sediments as a case study. This system can be used to identify high-risk zones of the river that should be given more attention. First, we evaluated the pollution status of Cd in the river sediments based on their total Cd content, and calculated a risk assessment, using local geochemical background values at various sites along the river. Using both acetic acid and ethylenediaminetetraacetic acid to extracted the fractions of Cd in sediments, and used DGT to evaluate the bioavailability of Cd. Thus, DGT provided a measure of potentially bioavailable concentrations of Cd concentrations in the sediments. Last, we measured Cd contents in plant tissue collected at the same site to compare with our other measures. A Pearson's correlation analysis showed that Cd-Plant correlated significantly with Cd-HAc, (r = 0.788, P < 0.01), Cd-EDTA (r = 0.925, P < 0.01), Cd-DGT (r = 0.976, P < 0.01), and Cd-Total (r = 0.635, P < 0.05). We demonstrate that this system of assessment is a useful means of assessing heavy metal pollution status and ecological risk in river sediments.

Effects of enhanced bioturbation intensities on the toxicity assessment of legacy-contaminated sediments

Many benthic communities within estuarine ecosystems are highly degraded due to the close proximity of urban and industrial contamination sources. The maintenance of recolonised, healthy ecosystems following remediation is a challenge, and better techniques are required for monitoring their progressive recovery. Rates of ecosystem recovery are influenced by the changes in the concentrations and forms of contaminants, the sensitivity of recolonising organisms to bioavailable contaminants, and a range of abiotic and biotic factors influencing the exposure of organisms to the contamination. Here we investigate the influence of bioturbation by an active amphipod (Victoriopisa australiensis) on the bioavailability of metals and hydrocarbons in highly contaminated sediments. Changes in contaminant bioavailability were evaluated by assessing sublethal effects to a smaller cohabiting amphipod (Melita plumulosa). For predominantly metal-contaminated sediments, the presence of V. australiensis generally increased survival and reproduction of M. plumulosa when compared to treatments with only M. plumulosa present (from 42 to 93% survival and 3-61% reproduction). The decrease in toxic effects to M. plumulosa corresponded with lower dissolved copper and zinc concentrations in the overlying waters (14-9 μg Cu L^-1, and 14 to 6 μg Zn L^-1 for absence to presence of V. australiensis). For sediments contaminated with both hydrocarbons and metals, the increased bioturbation intensity by V. australiensis resulted in decreased reproduction of M. plumulosa, despite lower dissolved metal exposure, and indicated increased bioavailability of the hydrocarbon contaminants. Thus, the presence of a secondary active bioturbator can enhance or suppress toxicity to co-inhabiting organisms, and may depend on the contaminant class and form. The results highlight the need to consider both abiotic and biotic interactions when using laboratory studies to evaluate the ability of organisms to recolonise and reproduce within benthic environments degraded by contamination, or for more general extrapolation for sediment quality assessment purposes.

Assessing remobilization characteristics of arsenic (As) in tributary sediment cores in the largest reservoir, China

The environmental impact of the Three Gorges Reservoir (TGR) in China has raised widespread concern especially in relation to metal pollution. The diffusive gradient in thin films (DGT) technology was applied to investigate arsenic (As) remobilization in sediment cores collected from the main stream and a tributary in the TGR during July 2015. The results showed that the horizontal and vertical distributions of C_DGT-As varied among the four sampling sites. For the same DGT probe, the horizontal distributions of C_DGT-As (0-6mm, 6-12mm, 12-18mm) exhibited similarity in the overlying water and different tendencies in the sediment profiles; the vertical variations of C_DGT-As showed different mobilization tendencies. Moreover, the mobility patterns of As in the sediment profile showed the diffusion potential of As from the deep sediment to the surface sediment and overlying water were in the order of MX-2< MX-1< CJ < MX-3. In addition, similar distribution characteristics and correlation analysis showed that the mechanisms of As mobility were associated with Fe and Mn. The results also showed that sulphide and As were simultaneous remobilization in the tributary sediment core in the TGR.

Comparison of in situ DGT measurement with ex situ methods for predicting cadmium bioavailability in soils with combined pollution to biotas

To assess the capabilities of the different techniques in predicting Cadmium (Cd) bioavailability in Cd-contaminated soils with the addition of Zn, one in situ technique (diffusive gradients in thin films; DGT) was compared with soil solution concentration and four widely used single-step extraction methods (acetic acid, EDTA, sodium acetate and CaCl₂). Wheat and maize were selected as tested species. The results demonstrated that single Cd-polluted soils inhibited the growth of wheat and maize significantly compared with control plants; the shoot and root biomasses of the plants both dropped significantly (P < 0.05). The addition of Zn exhibited a strong antagonism to the physiological toxicity induced by Cd. The Pearson correlation coefficient presented positive correlations (P < 0.01, R > 0.9) between Cd concentrations in two plants and Cd bioavailability indicated by each method in soils. Consequently, the results indicated that the DGT technique could be regarded as a good predictor of Cd bioavailability to plants, comparable to soil solution concentration and the four single-step extraction methods. Because the DGT technique can offer in situ data, it is expected to be widely used in more areas.

The Daphnia magna role to predict the cadmium toxicity of sediment: Bioaccumlation and biomarker response

To evaluate Daphnia magna role to predict the Cd toxicity in contaminated sediment, the Cd accumulation, metallothionein (MT), and mortality of D. magna exposed to overlying water system or water-sediment coexistence system were measured. The mortality, Cd accumulation, and MT in D. magna increased with the increasing Cd content in sediment. The Cd accumulation and MT in D. magna exposed to the coexistence system were significantly higher than those exposed to the overlying water system because of the ingestion of Cd-containing sediments by D. magna. However, the mortality did not significantly differ in the two systems, suggesting that mortality was less sensitive than accumulation and MT. The Cd accumulation/MT index can explain why the two systems had the similar mortality but different Cd accumulation and MT. Not all the percentage composition of nonresidual fractions (e.g., exchangeable, carbonate bound, and organic bound phases) significantly correlated with the difference values of Cd accumulation and MT, as well as Cd accumulation/MT. However, these indexes increased with the percentage composition of the nonresidual fractions, indicating that the distribution of Cd chemical fractions is crucial for its bioavailability and biotoxicity.