Assessment of metal toxicity and development of sediment quality guidelines using the equilibrium partitioning model for the Three Gorges Reservoir, China

Equilibrium partitioning approach for metal toxicity assessment in tropical estuarine sediment of Bandon Bay, Thailand

An equilibrium partitioning approach (EqPA) was employed to evaluate the metal toxicity and define sediment quality guidelines (SQGs) for arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), zinc (Zn), and mercury (Hg) in the cockle cultivated areas located in Bandon Bay, Thailand. An assessment of metal toxicity using the [∑SEM]-[AVS] and [∑SEM]-[AVS]/foc models indicated no adverse effect on benthic organisms. The normalized total metal concentrations in this area were below the established SQG values for As, Cd, Cu, Ni, Pb, Zn, and Hg, namely respectively 21.3, 0.8, 84.6, 36.0, 34.6, 440.9 mg/kg dry weight, and 49.3 μg/kg dry weight on sand and calcium carbonate free with 1 % total organic carbon basis, suggesting low metal toxicity. This study provides locality adapted SQG values for supporting sediment quality management specifically in Bandon Bay, potentially serving as a model for other coastal areas.

Spatial and temporal distribution of Mo in the overlying water of a reservoir downstream from mining area

This study aimed to evaluate the spatial and temporal variations of molybdenum (Mo) in the downstream water body of a Mo mine during three hydrologic periods (wet, dry and medium seasons). The physical properties in Luhun Reservoir reflected seasonal variations in different hydrological periods. The redox potential (ORP) and dissolved oxygen (DO) increased in the dry season. The concomitant decrease in temperature (T), conductivity (COND) and total dissolved solids (TDS) were lowest in the wet season. The pH value did not change significantly during the three hydrologic periods. The distribution of Mo in the dry season was high in upstream and low in downstream areas, which was significantly different from that of the wet and medium seasons. The total Mo concentration in wet (150.1 µg/L) and medium season (148.2 µg/L) was higher than that in the dry season, but the TDS (288.3 mg/L) and the percentage dissolved Mo (81.3%) in overlying water was lowest in the wet season. There was no significant relationship between the dissolved Mo and the total Mo with TDS. In the dry season, the mean total Mo concentration was 116.3 µg/L, which was higher than the standard limit value (70 µg/L) for drinking water (US EPA-United States Environmental Protection Agency recommended value 40 µg/L). Non-point source pollution is the main characteristic of mining area pollution, which was closely related to rainfall. Thus, the Luhun Reservoir contains substantial Mo pollution, which was a significant concern given that it is used as a source of drinking and irrigation water.

Assessing the remobilization and fraction of cadmium and lead in sediment of the Jialing River by sequential extraction and diffusive gradients in films (DGT) technique

Cadmium (Cd) and lead (Pb) are two typical heavy metals of the Jialing River, and their threat to the river has been considered by the government in recent years. In this study, the diffusive gradient in thin films (DGT) technique and sequential extraction were employed together to analyse the remobilization and fraction of Cd and Pb in the sediments. The total concentration of Cd and Pb in four sampling sites both followed the order S3>S4>S2>S1. The sequential extraction results indicated that large amounts of Cd and Pb (over 50% of the total concentration) were bound to the exchangeable and reducible fraction. The DGT results showed that both Cd and Pb presented a significant increasing trend at the bottom of the DGT probe (-10 cm to -12 cm) and that the two metals had a significant positive correlation (r = 0.831, p < 0.01). The apparent diffusive flux result indicated that Cd and Pb had a potential risk of release from surface sediments. A significant correlation was observed between the DGT-labile fraction and sequential extraction at the surface sediments. A further correlation analysis found that the concentration of labile Cd/Pb measured by DGT (C_DGT-Cd and C_DGT-Pb) had a strong negative correlation with C_DGT-Fe, and this process was mainly mitigated by the iron oxides in the sediments. In addition, the correspondence of a "dark area" of AgI gel with corresponding "hotspots" of Chelex gel also proved that the release of Cd and Pb may regulate the dissolved sulfide in the sediments.

Attempt of basin-scale sediment quality standard establishment for heavy metals in coastal rivers

Heavy metal sediment quality standards (SQSs) derived from sediment quality guidelines (SQGs) are crucial in risk evaluation and environmental management. However, the establishment of SQSs is quite complex, especially for heavy metals. This study attempted to establish basin-scale SQSs for Cd, Cu, Pb and Zn based on SQGs combined with water quality standards in two coastal rivers in North China, named Jiaolai River (JL) and Jiahe River (JR), respectively. The spatial distribution, fraction, partition coefficients and environmental risk of heavy metals in sediments-porewater were investigated. The results showed that most heavy metals in sediments in JH were higher than those in JL, however, in the porewater, it exhibited an opposite trend. The geochemical fraction showed that most heavy metals in sediments were dominated by residual fraction. The partition of heavy metals between sediment and porewater were mainly affected by both sediment and porewater properties, and exogenous input of heavy metals. Contamination factors showed that Cd in sediment posed high pollution degree; the interstitial water criteria toxicity units and Nemerow Indexes suggested that heavy metal toxicities in porewater were low. The basin-scale heavy metal SQGs were calculated based on porewater quality derived from surface water quality standards using the modified equilibrium partitioning approach. The basin-scale heavy metal SQGs was classified with different grades to deduce the SQSs. Evaluated results of heavy metals in sediments based on SQSs showed lower potential bio-toxic effects in two rivers. In total, basin-scale SQGs for heavy metals were feasible for basin-scale SQSs establishment in coastal rivers.

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.

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.

Multiple assessments of trace metals in sediments and their response to the water level fluctuation in the Three Gorges Reservoir, China

Response of trace metals to the consecutive water level fluctuation in the Three Gorges Reservoir (TGR) sediments remains unclear. Here, we evaluated the influence of consecutive stages of water level fluctuation on trace metal pollution using multiple analytical approaches. The spatio-temporal distributions of trace metals in TGR sediments were investigated for five consecutive water impoundment stages from 2015 to 2017. Anthropogenic contributions and trace metal stocks in the TGR were quantitatively estimated using a combination of a regional geochemical baseline (RGB) and annual sediment load. Results showed that trace metals were accumulated after the construction and impoundment of the TGR. However, after the TGR operated normally for more than five years, trace metals concentrations stabilized in sediments. Trace metal concentrations in the mainstream were slightly higher than those in the tributaries. In the mainstream, metal concentrations in the upstream were lower than those in the midstream and downstream except for Cd. Anthropogenic contributions of trace metals ranged from 8.51 to 24.86% and were highest for Hg and Cd. The sediment load was the main factor influencing trace metal stock in TGR sediments. Although the total Cd stock amount was relatively low, its potential ecological effects are of great concern due to its high mobile fraction percentages and toxicity. The RGB-based geo-accumulation index and potential ecological risk index showed that TGR sediments were uncontaminated, and were subject to low ecological risk from trace metals. This result differs from traditional assessment results, indicating that previous assessments may overestimate the ecological risks of the trace metals in the TGR. The health risks posed by trace metals bio-accumulated in fish, stemming from sediment resuspension, were assessed using an environmental impact assessment model. Results suggested that residents should not experience significant health risks from the intake of individual metals through fish consumption.

A Comparative Evaluation of Different Sediment Quality Guidelines for Metal and Metalloid Pollution in the Xiangjiang River, Hunan, China

To evaluate intensively the quality of the sediments contaminated by heavy metals and metalloids in the Xiangjiang River, 52 surface sediment samples were collected at 13 sites and different combinations of empirical and theoretical sediment quality indexes, the consensus-based sediment quality guidelines, sediment toxicity degree, and equilibrium partitioning method were applied. The average contents of Cd, Pb, Cu, Zn, Hg, Cr, and As in the sampled surface sediments were significantly higher than the background values of trace elements in soils of Hunan Province, China. Moreover, speciation fraction analyses revealed that Cd, Hg, and Pb in the sediments were dominated by the more bioavailable organic or exchangeable fractions, whereas the major species of As and Cr were the less bioavailable residual fractions after strong acid treatment. In addition, all indexes showed that these metals posed a median-high degree of toxic risk to benthic organisms in sediments from nearly all of the sampling sites along the Xiangjiang River. Cd, followed by Cu and Pb, erected the most severe ecological risk. Pearson correlation and linear regression analyses between the mean PEC quotients, sediment toxicity degree, interstitial water criteria toxic units, and sediment pollution index showed that these indexes were relatively consistent to assess the quality of sediments contaminated by heavy metals and metalloids in the Xiangjiang River. Our results will facilitate the proposal of proper sediment quality guidelines for the Xiangjiang River.

Heavy metal pollution in sediments of the largest reservoir (Three Gorges Reservoir) in China: a review

The Three Gorges Dam in China is the world's largest dam. Upon its completion in 2003, the Three Gorges Reservoir (TGR) became the largest reservoir in China and plays an important role in economic development and national drinking water safety. However, as a sink and source of heavy metals, there is a lack of continuous and comparative data on heavy metal pollution in sediments. This study reviewed all available literatures published on heavy metals in TGR sediments and further provided a comprehensive assessment of the pollution tendency of these heavy metals. The results showed that heavy metal concentrations in TGR sediments varied spatially and temporally. Temporal variations indicated that Hg in tributaries, as well as As, Cd, Cr, Cu, Ni, Pb, and Zn in the mainstream, exhibited a higher probability to exceed background values after the impoundment of TGR. Pollution assessments by contamination factor, geoaccumulation index, and potential ecological risk were similar. High Cd and Hg concentrations in both the mainstream and tributaries are a cause for much concern. However, sediment quality guidelines produced different results, as most previous studies adopted different sampling and measurement strategies. The data inconsistencies and lack of continuity regarding the reservoir confirm the need for a continuous monitoring network and the development of quality criteria relevant to the sediments of the TGR in the future.

Characteristics of cadmium remobilization in tributary sediments in Three Gorges Reservoir using chemical sequential extraction and DGT technology

The Three Gorges Reservoir (TGR) is the largest reservoir in China. Cadmium (Cd) is a primary pollutant in the TGR, and its speciation and bioavailability have attracted extensive attention since TGR submergence. In this study, Chelex-100 DGT (diffusive gradient in thin films) and the sequential extraction method were used to investigate the bioavailable Cd in sediments obtained from a typical tributary (Meixi) and mainstream (Yangtze) in the TGR. The total Cd concentrations in sediments of the four stations were also determined. In comparison to the concentrations of labile Cd measured by DGT (C_DGT-Cd) in four profiles, CJ and MX-upstream/downstream were at potential risk for Cd release from surface sediments using the apparent diffusion flux across the interface numerical model. The order of C_DGT-Cd in surface sediments was as follows: CJ > MX-downstream > MX-upstream > MX-midstream. Additionally, a positive correlation was demonstrated between C_DGT-Cd and Cd in the exchangeable fraction (F1) in the surface sediments, indicating that Cd in the exchangeable fraction was readily captured by DGT. A negative correlation was observed between C_DGT-Cd and C_DGT-Fe, C_DGT-Mn in the sediment-water-interface (SWI), suggesting that Fe/Mn oxides did not control the release of labile-Cd from sediments. Furthermore, a positive correlation existed between the C_DGT-Cd in the surface sediments and Cd in the oxidizable fraction (F3), illustrating that Cd sorbed or bound with organic matter or sulfide was labile and released into the water phase from the surface sediments. A dark area was found in the AgI gel, which further demonstrated that Cd simultaneously was released with sulfide in this area.