The spatial distribution, accumulation and potential source of seldom monitored trace elements in sediments of Three Gorges Reservoir, China

Vertical distributions and potential contamination assessment of seldom monitored trace elements in three different land use types of Yellow River Delta

The Yellow River Delta (YRD) is the second largest petrochemical base in China and the impact of human activities has been continuously increasing in recent decades, however, the contamination status of seldom monitored trace elements (SMTEs) in YRD has rarely been reported. This study evaluated the levels, vertical distributions, contamination status and sources of SMTEs in soil samples of three different land use types in YRD. The results indicated that the vertical distributions of SMTEs contents showed a gradually upward increasing trend for the soil profiles of black locust forest, while the SMTEs contents displayed a gradually upward decreasing trend for the soil profiles of cotton field. However, the SMTEs contents in the oil field area showed no significant difference among different depths. The vertical distributions of SMTEs were very likely related to the anthropogenic disturbance in the later stage. The environmental pollution status assessment of SMTEs showed obvious enrichment of Cs, Sn, and U in the soils of YRD. Moreover, the potential source analysis based on multivariate statistical methods indicated that Ga, Rb, Cs, Sc, Sn, Tl, Be, Bi, Ca and Mo were clustered together and positively correlated with Al, Fe, Mg and K, and may be mainly associated with geochemical weathering process, while the Ce, La, Th, U, Nb, Ta, and Hf may be impacted by both natural process and human activities. Though the SMTEs pollution status was not very serious, our results highlighted the non-negligible influence of anthropogenic activities on vertical distributions of SMTEs in three different land use types from YRD. Our results provide valuable information for understanding the vertical distribution and pollution status of SMTEs in YRD.

Comprehensive assessment of seldom monitored trace elements contamination and its anthropogenic impact record in a sediment core from the North Yellow Sea

The environmental status of seldom monitored trace elements (SMTEs) has rarely been reported in the North Yellow Sea (NYS). This study investigated the levels, sources and ecological risks of 18 SMTEs in a 209-cm-long sediment core from NYS. The concentrations of SMTEs exhibited a gradual increasing trend in the upper 70 cm. Based on the assessment results of enrichment factor (EF), geo-accumulation index (I_geo) and contamination factor (CF), obvious enrichment of Cs, Li, and U was observed for the NYS sediments, indicating possible anthropogenic sources, which are consistent with the geochemical background normalized patterns. Moreover, the pollution load index (PLI) values ranged from 0.93 to 1.24 and showed a steadily increasing trend in the upper 70 cm part, indicating gradual deterioration of environment in NYS. Combined with the multivariate statistical analysis results and PLI variations, the first principal component (PC1) with high positive loading on Be, Cs, Ga, Hf, In, Li, Nb, Rb, Sc, Ta and Tl was very likely an "anthropogenic factor". Therefore, the historical anthropogenic impact record in the NYS was reconstructed based on the PC1 scores, which indicated significant anthropogenic influence over the past 300 years. This study provides valuable information for understanding the pollution history of SMTEs and historical record of anthropogenic impact in the NYS.

Distribution, sources, and pollution levels of toxic metal(loid)s in an urban river (Ichamati), Bangladesh using SOM and PMF modeling with GIS tool

Indexical assessment coupled with a self-organizing map (SOM) and positive matrix factorization (PMF) modeling of toxic metal(loid)s in sediment and water of the aquatic environment provides valuable information from the environmental management perspective. However, in northwest Bangladesh, indexical and modeling assessments of toxic metal(loid)s in surface water and sediment are still rare. Toxic metal(loid)s were measured in sediment and surface water from an urban polluted river (Ichamati) in northwest Bangladesh using an atomic absorption spectrophotometer to assess distribution, pollution levels, sources, and potential environmental risks to the aquatic environment. The mean concentrations (mg/kg) of metal(loid)s in water are as follows: Fe (871) > Mn (382) > Cr (72.4) > Zn (34.2) > Co (20.8) > Pb (17.6) > Ni (16.7) > Ag (14.9) > As (9.0) > Cu (5.63) > Cd (2.65), while in sediment, the concentration follows the order, Fe (18,725) > Mn (551) > Zn (213) > Cu (47.6) > Cr (30.2) > Ni (24.2) > Pb (23.8) > Co (9.61) > As (8.23) > Cd (0.80) > Ag (0.60). All metal concentrations were within standard guideline values except for Cr and Pb for water and Cd, Zn, Cu, Pb, and As for sediment. The outcomes of eco-environmental indices, including contamination and enrichment factors and geo-accumulation index, differed spatially, indicating that most of the sediment sites were moderately to highly polluted by Cd, Zn, and As. Cd and Zn content can trigger ecological risks. The positive matrix factorization (PMF) model recognized three probable sources of sediment, i.e., natural source (49.39%), industrial pollution (19.72%), and agricultural source (30.92%), and three possible sources of water, i.e., geogenic source (45.41%), industrial pollution (22.88%), and industrial point source (31.72%), respectively. SOM analysis identified four spatial patterns, e.g., Fe-Mn-Ag, Cd-Cu, Cr-Pb-As-Ni, and Zn-Co in water and three patterns, e.g., Mn-Co-Ni-Cr, Cd-Cu-Pb-Zn, and As-Fe-Ag in sediment. The spatial distribution of entropy water quality index values shows that the southwestern area possesses "poor" quality water. Overall, the levels of metal(loid) pollution in the investigated river surpassed a critical threshold, which might have serious consequences for the river's aquatic biota and human health in the long run.

Pollution levels and risk assessment of thallium in Chinese surface water and sediments

Thallium (Tl) is one of the most toxic metals and can cause chronic and acute damage to humans. Due to occurrences of incidents involving Tl pollution in China, its potential environmental impacts are receiving increased attention. However, there is still limited information on Tl concentrations in the environment and their risks to human health and wildlife. This paper provides an overview of the contamination of surface water and sediments by Tl across China and assesses the potential risks using several methods. The acute and chronic aquatic life criteria for Tl were determined to be 13.25 and 1.65 μg/L, respectively. The acute and chronic risk quotients (RQs) of Tl in surface water near mining areas were 0.01-41.51 and 0.20-666.67, respectively, indicating medium to high ecological risks to aquatic organisms. Tl in sediments of Pearl and Gaofeng rivers pose a high risk based on the higher geo-accumulation index (Igeo) and potential ecological risk index (EI) values. Exposure parameters for the Chinese population were used to derive health criteria and assess non-carcinogenic risk posed by Tl in centralized drinking water sources. Tl criteria for protection of human health were calculated to be 0.18 μg/L for water+organisms and 0.30 μg/L for organisms only. The non-carcinogenic risk posed by Tl was acceptable. The human health criteria of Tl for children were the lowest among all age groups. The risks posed by Tl to health of children are greater than those for adults. Therefore, emphasis should be placed on protecting children from exposure to Tl. For the Chinese population, the drinking water guidance value to ensure protection of human health was determined to be 0.44 μg/L. The availability of multiple Tl guidance values for designated water uses will improve the environmental regulation and surveillance of Tl pollution in China and other countries.

Streambed pollution: A comprehensive review of its sources, eco-hydro-geo-chemical impacts, assessment, and mitigation strategies

Streambeds are an integral part of the river ecosystem. They provide habitat to a vast array of aquatic and benthic organisms as well as facilitate the bio-degradation and transformation of organic matter and vital nutrients. Increasing anthropogenic influence introduces multiple stressors to the stream networks resulting in pollution of streambeds, which in turn, have detrimental effects on the overall stream ecosystem health. There is a huge gap in the current understanding of streambed pollution and its impacts, and the widely practiced streambed pollution mitigation strategies lack a holistic approach. In this comprehensive review, we first synthesize the state-of-the-art knowledge of conventional and emerging forms of contaminants, their overall impacts on stream ecosystem functions, and present future directions to comprehend the problem of streambed pollution. We highlight that fine sediments and plastics (found especially in urban streambeds) are among the major physical pollutants causing streambed pollution and the chemical pollutants generally comprise hydrophobic compounds including various legacy contaminants such as polychlorinated biphenyl (PCB), dichlorodiphenyltrichloroethane (DDT), a wide range of pesticides and a variety of heavy metals. Moreover, in recent years, highly polar and hydrophilic emerging contaminants such as micro-plastics, pharmaceutical waste and personal care products have been identified in riverbeds and streambeds across the world. We stress that the impacts of streambed pollution have been largely studied with discipline-driven perspectives amongst which the ecological impacts have received a lot of attention in the past. To present a comprehensive outlook, this review also synthesizes and discusses most of the understudied hydrological, geomorphological and biochemical impacts of different forms of streambed pollution. Subsequently, we also present a global inventory by compiling information from the published literature to highlight the status of streambed pollution around the globe. In the end, we endorse the positive and negative aspects of the current impact assessment methodologies and also highlight various physical, chemical and biological remediation measures that could be undertaken to alleviate streambed pollution.

Partitioning behavior and ecological risk of arsenic and antimony in the sediment-porewater profile system in the Three Gorges Reservoir, China

Arsenic and antimony are widely distributed toxic metalloids in aquatic environments. However, their partitioning behaviors in the sediment profile remain not well understood. Here, partitioning behaviors, diffusive fluxes, as well as the ecological risks of As and Sb in the sediment-porewater profile system in the tributaries of the Three Gorges Reservoir (TGR) were investigated. As and Sb showed markedly different spatial variations in the longitudinal profiles of both porewater and sediment samples. Specifically, the concentration of As showed an accumulation trend with depth, while that of Sb showed a relatively complicated trend. Further, As showed lower sediment-porewater partitioning coefficient (K_d) values, suggesting that it had a relatively lower sediment affinity and a higher mobility than Sb. Its residual fraction (30%-60%) was also lower than that of Sb. This phenomenon could be attributed to the chemical fractions of the trace metals and the pH value of the sediments. Furthermore, the K_d values corresponding to As were influenced by both the residual fraction (r = 0.338, p < 0.05) and the exchangeable fraction (r = -0.643, p < 0.01), while those corresponding to Sb were only influenced by pH. Additionally, even though these two trace metals showed low ecological and mobility risks, the diffusive fluxes at the sediment-water interface suggested that the sediment acted as a source of As and a sink for Sb relative to the overlying water. This study indicated that As and Sb had different partitioning behaviors and release risks in the sediment-porewater profile system, enhanced the understanding the transport and fate of As and Sb in the aquatic environment.

Spatiotemporal distribution of vanadium in the flooding soils mediated by entrained-sediment flow and altitude in the Three Gorges Reservoir

The re-emergence of vanadium (V) as a toxic metal has been highlighted recently due to its long-standing environmental and health hazard. This work targeted the world largest reservoir-Three Gorges Reservoir (TGR) to explore the spatial variation of V in the flooding soils from 2014 to 2018; meanwhile, the typical riparian zones with different altitudes and land-uses at the middle reach of the TGR were selected to decipher the key drivers on the V distribution. The results showed that the concentrations of soil V in the mainstream markedly exceeded local background, but they did not vary significantly with time except a marked increase at the upper-middle reaches. Spatially, the concentrations of soil V increased towards the dam, and the increase trend became increasingly significant with time. At the typical riparian zone, the concentrations of soil V decreased strikingly with altitude despite the difference in the land-uses, and a marked change-point occurred at 160-165 m. The soil V dominated by residual fraction, followed by oxidizable and reducible fractions, and then the minimal acid-soluble fraction. The contamination and eco-risk of V in the soils were low with similar spatiotemporal variation to its concentrations. Entrained-sediment flow and particle size rather than pH and organic matters led to the spatiotemporal variation in the distribution of soil V in the mainstream, and the driving effects tended to be more predominant with time. Altitude-regulated alteration of soil properties including particle sizes and iron/manganese (hydr)oxides with different flooding duration dominated the vertical distribution of V over the local land-uses at the riparian zone. Our results reveal the hotspots of V contamination in the riparian soils of the TGR and highlight unceasing focus on the variation in the distribution and dynamic migration of soil V due to its levels out of limits and changing soil conditions.

Spatial variations and periodic changes in heavy metals in surface water and sediments of the Three Gorges Reservoir, China

Substantial changes have occurred in hydrological situation of the Three Gorges Reservoir (TGR) after the operation of the Three Gorges project, as have the heavy metals (HMs) pollution characteristics. In this study, concentrations of Cu, Zn, Pb, Cr, and Cd in surface water and sediments of the TGR were determined during the water impoundment period (December 2015) and water drawdown period (June 2016). The index of geoaccumulation, principal component analysis (PCA), and correlation analysis were used to analyze HMs pollution characteristics. Results showed that HMs concentrations in surface water were much lower than the quality standards for drinking water and surface water of China. The pollution levels of HMs in sediments were nonpolluted for Cr, nonpolluted to moderately polluted for Cu, Zn, Pb, and moderately polluted for Cd. In the fluctuating backflow zone, HMs concentrations in sediments during the water drawdown period were lower than those during the water impoundment period, which was attributed to that faster flow velocity during the water drawdown period resulted in less deposition of suspended solids and faster release of HMs pollutants from sediments to water. HMs concentrations of sediments at sites M14 and M17 showed similar periodic changes to those at the fluctuating backflow zone, which might be attributed to the density-stratified flow in the adjacent upstream tributaries (Meixi River and Qinggan River, respectively). Correlation analysis and PCA analysis results showed that for the sediments, Cr came from natural sources, while Cu, Zn, Pb, and Cd mainly came from anthropogenic sources.

Effects of water impoundment and water-level manipulation on the bioaccumulation pattern, trophic transfer and health risk of heavy metals in the food web of Three Gorges Reservoir (China)

The Three Gorges Reservoir (TGR) of China, the largest hydropower project over the world, has attracted much attention to the water impoundment and water-level manipulation. In this study, we evaluated potential effects of water impoundment and seasonal water-level manipulation on the bioaccumulation, trophic transfer and health risk of HMs (Cu, Fe, Zn, Hg, Cd and Pb) in food web components (seston, aquatic invertebrate and fish) in TGR. Our results show that, after the impoundment for eight years (2003-2010), all of the six metal concentrations in aquatic biota fell within the criteria of safety quality guidelines. The concentrations of Cu, Fe, Zn and Hg in fish and aquatic invertebrates were higher than those before impoundment, whereas Cd and Pb were lower than those before impoundment. Nonetheless, Hg, Cd and Pb in aquatic consumers underwent an increasing trend during the entire impoundment, implying potential reservoir effect in the future. Only the concentrations of Hg, Cd and Pb in aquatic consumers exhibited a declining trend towards the dam, showing consistent with the background level at the three reaches. Seasonal variations in HM concentrations of fish and aquatic invertebrates were ascribed to the water-level manipulation associated with reservoir management. Our findings show that Hg or Cd biomagnified through aquatic food web during different hydrological periods, whereas Pb, Cu, Fe and Zn exhibited weak biomagnification power. Overall, Hg, Cd and Pb showed a higher risk than that of Cu, Fe and Zn.

Composition and physico-chemical properties of bottom sediments in the southern part of the Bratsk Reservoir (Russia)

The paper presents the results of studies of bottom sediments taken from the southern part of the Bratsk Reservoir. The following analyses have been conducted: trace element analysis, particle-size analysis as well as chemical analysis of water, hydrochloric acid and alkaline extracts for 18 samples of the bottom sediments. The granulometric analysis has identified the predominance of fine silt and silty-clayed sediments. The data on the content of trace elements in the bottom sediments of the Bratsk Reservoir is presented in comparison with their content in the natural environment of the Baikal region; the anomaly ratio was used to characterize the excess for trace elements. The chemical analysis of extracts has showed "saline contamination" of mud (silt), high concentration of carbonates in it, as well as the presence of mobile (free) forms of aluminum oxide. In this research, an attempt was made by using a correlation analysis to evaluate the impact of various physical and chemical characteristics of the bottom sediments, such as the content of clay fraction, organic carbon, carbonates, and water-soluble salts on the accumulation of trace elements.

Using diffusive gradients in thin films (DGT) and DGT-induced fluxes in sediments model to assess the dynamic release of copper in sediment cores from the Three Gorges Reservoir, China

As one of the largest man-made reservoir, the Three Gorges Reservoir (TGR) brings great uncertainty and challenges regarding trace metal (e.g., Cu) remobilization in the sediment. Therefore, in this study, sediment cores were collected from the mainstream of the Yangtze River and its three tributaries in the TGR, with a focus on evaluating the Cu remobilization risk and release dynamics using conventional methods, diffusive gradients in thin films (DGT), and the DGT-induced fluxes in sediments (DIFS) model. The results showed that although total Cu concentrations were slightly higher than background values, Cu was mainly observed in the unreactive residual fraction. Additionally, assessment of sediment quality guidelines and the risk assessment code indicated low Cu contamination risk for all sampling sites. However, the results of DGT measurements at the sediment-water interface showed efflux of Cu from sediment to the overlying water at most sites, except for the upstream of the Meixi River and the mainstream of the Yangtze River. Interestingly, diffusion fluxes at the three tributaries displayed an increase trend from the upper to lower reaches. The DIFS model simulation further implied that the highest resupply capacity and desorption rate occurred in the Zhuyi River and the upstream of the Meixi River. In fact, the accumulation of Cu from the upper to the lower reaches of the Zhuyi River significantly elevated the Cu resupply capacity. Thus, more attention should be paid to Cu mobilization in the TGR, especially in the Zhuyi River and the upstream of the Meixi River.

The spatial and vertical distribution of heavy metal contamination in sediments of the Three Gorges Reservoir determined by anti-seasonal flow regulation

The contamination of heavy metals in sediments of the Three Gorges Reservoir (TGR) is increasingly concerned as a major issue for water quality since the full impoundment. In this study, the sediment profiles in the riparian and submerged areas from Fuling to Zigui in the middle-low TGR mainstream were collected to investigate the stratified distribution of heavy metals (Cd, Cr, Cu, Ni, Pb and Zn) and assess their contamination and potential eco-risk. The results showed that the concentrations of these metals in the riparian sediments did not present a marked spatial trend except the concentrations of Cd that increased towards the dam. However, the metal concentrations (except Cd) in the submerged sediments were generally higher near the dam. The concentrations of heavy metals in the riparian sediments did not show marked vertical variation, while in the submerged sediments they fluctuated dramatically with depth, indicating the metal accumulation processes in last few years. Sediment grain sizes as an indicator of hydrodynamic regimes dominated the vertical distribution of heavy metals over organic matters and Fe/Mn oxide/hydroxides. The sediments from both riparian and submerged areas of the TGR were contaminated by anthropogenic metals of Cd, Cu and Pb that were mainly from the ore mining, fossil fuel combustion, agricultural pollution and atmospheric deposition. Cadmium was a major metal pollutant in the sediments with a high contamination and potential eco-risk level. The results of this study indicate that the sorting of sediments with the anti-seasonal flow regulation determines the spatial and temporal distribution of heavy metal contamination in the sediments, and the impoundment stages of the TGR in history regulate the accumulation processes of the metals.

Geochemical and health risk assessments of antimony (Sb) in sediments of the Three Gorges Reservoir in China

Toxic metals in the sediments of the Three Gorges Reservoir (TGR) in China are a conspicuous scientific issue. However, compared with the commonly monitored metals, the geochemical behavior and potential risk of antimony (Sb) in TGR sediments remains unclear. Here, sediment samples were collected from the entire TGR. Multiple assessment approaches (i.e., geochemical baseline concentration of Sb (GBC_Sb), grain size normalization, potential ecological risk, and health risk assessment) were used to evaluate the Sb status in sediments. The average Sb concentration was 1.14 ± 0.20 mg/kg, ranging from 1.10 to 1.19 mg/kg. The spatial distribution of Sb exhibited a tendency to accumulate from upstream to downstream. Normalization by fine grain size revealed that fine grain particles mainly influenced the Sb spatial distribution in TGR sediments. The temporal variation of Sb revealed that the mean content of Sb was higher in the dry season than in the wet season. Moreover, GBC_Sb (1.10 mg/kg) was higher than the background value of Sb in Yangtze River sediments. Determination of the anthropogenic contribution rate gave a mean value of 11.43% using the GBC model, indicating that the main contribution of Sb in TGR sediments originated from natural sources. According to sediment loads during 2015 and 2016, the anthropogenic input deposited 2.74 and 3.32 t of sediments in 2015 and 2016, respectively. In addition, the assessment results of the potential ecological risk index based on GBC_Sb suggested that Sb presented a low ecological risk in TGR sediments. Sb accumulation in fish caused by the sediment resuspension was calculated by the environmental impact assessment model. Further assessment of target hazard quotients demonstrated that Sb was at a safe level for fish consumption.

Spatio-temporal dynamics, drivers and potential sources of heavy metal pollution in riparian soils along a 600 kilometre stream gradient in Central China

Riparian ecosystems are particularly prone to heavy metal (HM) contamination, acting as a sink for HMs coming from human activities upstream or on adjacent uplands. An advanced understanding of the spatio-temporal dynamics, environmental drivers and the likely sources of HM contamination in riparian soils will be necessary for the conservation of riparian ecosystems. Thus, we conducted a nine-year study across a 600 km stream gradient along the Yangtze river, which has come under immense pressure in recent years partly due to the establishment of the Three Gorges Dam (TGD), the largest hydropower dam in the world. Levels of soil As, Cr, Pb, and Cu in the TGD's water level fluctuation zone (WLFZ) have consistently increased since the TGD's establishment. This increase tended to be more rapid at the upstream reaches of the WLFZ, where most HMs (As, Cd, Pb, Cu, and Zn) also tended to be particularly high. Our analyses suggest that the spatio-temporal dynamics of these metals are strongly influenced by soil phosphorus (P), organic matter, texture and manganese. In many cases HM levels exceeded acceptable pollution levels according to multiple indices. However, from 2008 to 2010 Hg and Cd presented great threat to ecosystem health, but from 2011 to 2016 levels of As and Pb became the primary concern due to increases in their concentrations of 152 and 38%, respectively, relative to 2009 levels. Factor analysis indicated that the major identifiable anthropogenic sources of HMs were traffic exhaust, sources associated with organic matter output (e.g. sewage), and sources associated with P output (e.g. agricultural runoff), with the latter generally dominant in the upper and middle reaches of the TGD watershed. These results indicate that the prioritization of As and Pb pollution and control of agricultural runoff will play an important role in the ecological protection in the TGR's riparian ecosystems.

Anthropogenic Impact and Ecological Risk Assessment of Thallium and Cobalt in Poyang Lake Using the Geochemical Baseline

As the largest freshwater lake in China, water environmental pollution in Poyang Lake is increasingly attracting attention. However, to date, the occurrence of seldom-monitored trace elements (SMTEs) (such as thallium (Tl) and cobalt (Co)) and their potential ecological influences in the Poyang Lake area remain unclear. Here, geochemical baseline concentrations (GBCs) were employed to investigate anthropogenic inputs of these two metals (Tl and Co) and were used to assess the potential ecological risk in surface sediments of Poyang Lake. The results showed that the mean concentrations of Tl and Co were 1.11 mg/kg and 15.42 mg/kg, respectively, slightly higher than background values (BVs) in local soil. Spatially, the Co concentrations in surface sediments of Poyang Lake varied; Co concentrations in the west of the study area were significantly lower than in the middle of the lake. In contrast, no significant spatial variation of Tl was found in studied areas of Poyang Lake. Average GBCs were similar to the mean measured concentrations of Tl and Co in the lake, and both were higher than the BVs in local soil. In anthropogenic source analysis based on the GBCs, 47.22% samples and 41.67% samples were influenced by human activity for Tl and Co, respectively; the mean anthropogenic contribution rate was 5.90% for Tl (0.43 to 16.22%), and 13.01% for Co (0.82 to 31.59%). These results indicated that Tl and Co mainly originate from natural processes. However, the anthropogenic contribution should not be ignored. Moreover, the geo-accumulation index (Igeo) and potential ecological parameter (Er) generally indicated no contamination and a low potential risk for Tl and Co in Poyang Lake sediments, which used the GBCs as BVs. The results of this study suggest that GBCs are more scientifically robust and practical as a reference than BVs. They can also be used when BVs are lacking.

Available forms of nutrients and heavy metals control the distribution of microbial phospholipid fatty acids in sediments of the Three Gorges Reservoir, China

The construction of the Three Gorges Reservoir (TGR) as well as the development of local industry and agriculture not only had tremendous impacts on the environment but also affected human health. Although water, soil, and air in the TGR have been well studied for environmental risk assessment, very little information is available on benthic sediments and microorganisms. In this study, sedimentary samples were collected along the main stream of the TGR to examine microbial phospholipid fatty acids (PLFA) and relevant variables (e.g., nutrients and heavy metals) after the full operation of the TGR. The results showed that there were prominent trends (increase or decrease) of sedimentary PLFAs and properties from downstream to upstream. Bacteria-specific PLFA decreased toward the dam, while fungi-specific PLFA did not show any significant trend. The PLFA ratio of fungi to bacteria (F/B) increased along the mainstream. The total PLFA concentration, which represents the microbial biomass, decreased significantly toward the dam. Upstream and downstream sampling points were clearly distinguished by PLFA ordination in the redundancy analysis (RDA). That finding showed microbial PLFAs to have an obvious distribution pattern (increase or decrease) in the TGR. The PLFA distribution was markedly controlled by nutrients and heavy metals, but nutrients were more important. Moreover, among nutrients, Bio-P, NH₄⁺-N, NO₃^--N, and DOC were more important than TP, TN, TOC, and pH in controlling PLFA distribution. For heavy metals, Tl, V, Mo, and Ni were more important than Zn, Cu, Cd, and Pb. These findings suggested that Tl, V, Mo, and Ni should not be ignored to guard against their pollution in the TGR, and we should pay attention to them and make them our first priority. This study highlighted that the construction of the TGR changed riverine environments and altered microbial communities in sediments by affecting sedimentary properties. It is a reminder that the microbial ecology of sediment as an indicator should be considered in assessing the eco-risk of the TGR.

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.

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.

Comprehensive assessment of seldom monitored trace elements pollution in the riparian soils of the Miyun Reservoir, China

The South-to-North Water Diversion Project has aroused widespread concerns about the potential ecological risks posed by the project, especially for the Miyun Reservoir (MYR). The potential release risk of metals from the flooded riparian soils into MYR after water impoundment is one of key scientific problems. In this study, riparian soil samples were collected considering three vertical heights (130, 140, and 145 m) and four types of land uses in the MYR areas, namely, forestland, grassland, wasteland, and recreational land. We analyzed soils texture, the content and chemical fractionations of seldom monitored trace elements (SMTEs): Li, Be, B, V, Co, Ni, Ga, Sn, Sb, Tl, and Bi). Results showed that the four types of soils in MYR had the similar textures, while recreational land showed significantly higher contents of Ni and V. Additionally, there were no significant differences found for most SMTEs (except for V) at different vertical heights in each soil type, while the concentrations of V at 140 and 145 m in forestland and recreational land were significantly higher than those at 130 m. However, a comprehensive evaluation of potential ecological risk (contamination factor (CF), modified degree of contamination (mCd), and geoaccumulation factor (I _geo)) consistently indicated the insignificant contaminations of all SMTEs in MYR soils before water impoundment. The Community Bureau of Reference (BCR) sequential extraction results showed that the chemical fractionations of SMTEs were independent of land use patterns and vertical heights. Co in reducible fractions and Ni were identified as the candidates which had potential to release into MYR when the lands were submerged. Principal component analysis (PCA) and cluster analysis (CA) results suggested that a portion of V, Co, and Ni may originate from anthropogenic activities, and the coal combustion was possibly the main anthropogenic source. The findings of this work would provide valuable information on the environment management of MYR and offer a reference for the investigation on the effect of water impoundment on potential release risk of SMTEs in MYR.

Assessing the remobilization of Antimony in sediments by DGT: A case study in a tributary of the Three Gorges Reservoir

The Three Gorges Reservoir (TGR) is one of the world's largest man-made hydropower projects, which has posed great challenges to the aquatic environment of the Yangtze River since the impoundment of water. As a non-essential toxic metalloid, information on the bioavailability of Antimony (Sb) in TGR sediments is lacking. Four sediment cores were collected from a tributary and the mainstream in the TGR to investigate the distribution and remobilization of Sb using the diffusive gradients in thin films (DGT) technique. The results showed that the concentrations of Sb obtained by DGT (CDGT-Sb) at all of the sampling stations were low (below 0.30 μg/L), compared to the relatively high Sb concentrations in the sediments. The lateral and vertical distributions of CDGT-Sb revealed different tendencies in overlying water and sediments at all of the sampling sites in the TGR, which may be attributed to anthropogenic impacts, the heterogeneity of sediments and the unevenness of the sediment-water interface (SWI) during the deployment of DGT probes. In addition, CDGT-Sb in the surface sediments were lower than those in the overlying water, and concentration gradients were found near the SWI, demonstrating that Sb has the potential to diffuse from the overlying water into the sediment. In the sediment cores, different peaks were discovered in the DGT probes and the remobilization of Sb simultaneously appeared in the vicinity of -10 cm. Correlation analysis showed that CDGT-Sb had no or negative correlation with CDGT-Fe and CDGT-Mn in all of the DGT probes, suggesting that the release of Sb was unassociated with Fe and Mn in the sediments in the study area.

Current state, sources, and potential risk of heavy metals in sediments of Three Gorges Reservoir, China

Heavy metal (HM) contamination in sediments of Three Gorges Reservoir (TGR) is a particularly important issue for the safety of water quality due to the potential threats of metal toxicity to local and downstream human health. Surface sediments from riparian and submerged areas in the entire TGR mainstream were collected in 2014 to investigate the spatial distribution of HMs (Cd, Cu, Pb, and Zn), identify their possible sources, and assess their potential risk by multiple indices and metal fraction. Results showed that the concentrations of HMs in the sediments increased after the TGR operation, but were lower than those in other Chinese rivers of developed areas. The acid-soluble Cd accounted for more than 50% of total Cd in the sediments, whereas that of other HMs was very low. The Cd concentrations in the riparian sediments increased towards the dam; however, other metals in the riparian sediments and all HMs in the submerged sediments did not show any regular variation trend spatially. The stocks of HMs were significantly higher in the submerged sediments than in the riparian sediments. The high accumulation of HMs in the riparian sediments emerged between Fuling and Fengjie, and those in the submerged sediments existed in the near dam areas. Grain size and Fe/Mn oxides controlled the mobility and transfer of HMs in the sediments. Human activity in the catchment including industrial and agricultural production, shipping industry, mining, etc., increased inputs of HMs in the sediments, and altered their spatial distribution patterns. The sediments were moderately to highly contaminated by Cd, and slightly contaminated by other HMs. The results indicate the current priority of Cd contamination in the TGR, and will conduce to ecological protection in the TGR region.