Pollution and mobility of heavy metals in the soils of a typical agricultural zone in eastern China.

The pollution of heavy metals (HMs) in agricultural soils profoundly threatens national food safety, and the mobility and environmental behaviors of HMs are closely implicated in crop safety. Here, we assessed the pollution level and mobility of ten HMs and explored their environmental behaviors in the soils of three different land uses from a main crop production zone in eastern China. The concentrations of HMs in the soils were higher in the farmland than the woodland and wasteland, and Cd showed a relatively higher pollution and ecological risk levels compared to other metals. Cadmium was dominated by the reducible (41%) and exchangeable (23%) fractions, and the rest of HMs were mainly in the residual fraction (> 60%). The significant correlation between the exchangeable and DGT-labile Cd indicates relatively higher mobility of Cd in the soils. Soil pH, organic matters and mineral elements had significant correlation with the exchangeable and reducible fractions of most of the HMs (e.g., Cd, Co, Mn, Ni, Pb and V; p < 0.05), indicating their good predictors of the HMs mobility. However, this was not the case for the DGT-labile fraction, which suggests a marked difference in the controlling mechanisms of the mobility versus potential bioavailability of HMs in the soils. The results of this study indicate that both the chemically extracted fractions and the bioavailable fractions of HMs need be considered when effectively assessing the safety of agricultural soils.

New perspectives on microbiome and nutrient sequestration in soil aggregates during long-term grazing exclusion.

Grazing exclusion alters grassland soil aggregation, microbiome composition, and biogeochemical processes. However, the long-term effects of grazing exclusion on the microbial communities and nutrient dynamics within soil aggregates remain unclear. We conducted a 36-year exclusion experiment to investigate how grazing exclusion affects the soil microbial community and the associated soil functions within soil aggregates in a semiarid grassland. Long-term (36 years) grazing exclusion induced a shift in microbial communities, especially in the <2 mm aggregates, from high to low diversity compared to the grazing control. The reduced microbial diversity was accompanied by instability of fungal communities, extended distribution of fungal pathogens to >2 mm aggregates, and reduced carbon (C) sequestration potential thus revealing a negative impact of long-term GE. In contrast, 11-26 years of grazing exclusion greatly increased C sequestration and promoted nutrient cycling in soil aggregates and associated microbial functional genes. Moreover, the environmental characteristics of microhabitats (e.g., soil pH) altered the soil microbiome and strongly contributed to C sequestration. Our findings reveal new evidence from soil microbiology for optimizing grazing exclusion duration to maintain multiple belowground ecosystem functions, providing promising suggestions for climate-smart and resource-efficient grasslands.

Spatial redistribution and enantiomeric signatures of hexachlorocyclohexanes in Chinese forest soils: Implications to environmental behavior and influencing factors.

Hexachlorocyclohexanes (HCHs) are a group of highly persistent pesticides. The concentrations of HCHs and the enantiomeric fractions of α-HCH in the O- and A-horizons from 30 mountains across China were analyzed in this study. The concentrations of total HCHs ranged from 0.061 to 46.9 ng/g (mean 2.12 ng/g) and 0.046 to 16.1 ng/g (mean 0.792 ng/g) in the O- and A-horizons, respectively. The HCH residues were mainly derived from the historical applications of technical HCH and lindane. Higher concentrations of HCHs were typically found in northern China, and no significant correlations were found between historical technical HCH usage and HCH isomer concentrations in either the O- or A-horizons (p > 0.05). Conversely, the concentrations of HCH isomers were significantly correlated with the environmental parameters (temperature and precipitation), thus indicating a typical secondary distribution pattern. Some HCH isomers tended to be transported northward under the long-term effect of monsoon. Chiral α-HCH was non-racemic in soils and showed preferential degradation of (-) α-HCH in both the O- and A-horizons. The transformation from γ-HCH to α-HCH might alter the enantiomeric signatures of α-HCH in soils. Moreover, the deviation from racemic of α-HCH was positively correlated with the C/N ratio in the A-horizon (p < 0.01), thus suggesting that the C/N ratio could alter the microbial activity and significantly affect the enantioselective degradation extent of α-HCH in soils.

Soil covering measure mitigates vanadium loss during short-term simulated rainfall in the vanadium titano-magnetite tailings reservoir.

Soil covering is an operative measure to decline pollutant release in tailings reservoirs and promote vegetation restoration, yet urgent research still needs to probe into pollutant leaching and migration in the artifact technology under extreme precipitation. Here, a soil column leaching experiment was designed to explore the migration and behaviors of vanadium (V) in the system of vanadium titano-magnetite tailings (VTMTs) covered by soils with different depths (5 cm, 10 cm, and 15 cm). Chemical fractions of V in the VTMTs and covered soils were analyzed to decipher the mechanisms underlying the V migration. We found a limited V leaching (0.26-0.52 µg/L, <0.01% of total V) in the columns during the experiments, and V in the VTMTs was not apt to be leached or migrate upward to the overlying soils. The soil volumes overlaid had nonsignificant effect on the V behaviors in the VTMTs (P > 0.05), because of the dominant and stable residual V (96.4% of total V) in the tailings. Although acid soluble V might be transformed to oxidizable V, it was resupplied by the fractions of weak-bound V in the solid phases during the leaching experiments. The mineral metal (hydr)oxides (e.g., aluminum, iron) determined the V behaviors in the VTMTs via absorption effect, and the high affinity of V to organic matters probably prevented its migration throughout the overlying soils. The results indicate that soil covering measure in the VTMTs reservoirs effectively reduces V migration or release from the tailings through leaching or upward migration, which provides a significant guidance for vegetation restoration in V-rich tailings reservoirs.

Novel ecological ditch system for nutrient removal from farmland drainage in plain area: Performance and mechanism.

The loading of nitrogen (N) and phosphorus (P) from agricultural drainage as the non-point sources is a worldwide environmental issue for aquatic ecosystem. However, how to remove these nutrients effectively from agricultural drainage remains a big challenge with increasing cemented ditches for better management. Here, we designed a novel ecological ditch system which integrated an earth ditch and a cemented ditch with iron-loaded biochar in the Chengdu Plain to reduce the loss of N and P from farmland. After a two-year monitoring, the removal efficiency of total N and total P reached 24.9% and 36.1% by the earth ditch and 30.7% and 57.8% by the integrated ditch system, respectively. The water quality was evidently improved after passing through the ditch system with the marked decrease in the concentrations of N and P. Dissolved organic N, nitrate, and particulate P became the dominant fractions of N and P loss. Rainfall soon after fertilization increased the concentrations of N and P in the ditch system and markedly affected their removal efficiency. The iron-loaded biochar effectively removed N and P from the drainage, especially at the high concentrations, which was mainly attributed to its high adsorption of the dissolved N and P fractions and the interception of the particulate nutrients. Our results indicate that the designed ecological ditch system has a high potential for alleviating agricultural non-point source pollution in the plain area and can be extended to other lowland agricultural ecosystems.

Dam construction attenuates trace metal contamination in water through increased sedimentation in the Three Gorges Reservoir.

Dam construction has a far-reaching impact on trace metal accumulation and the metal-induced quality of the aquatic environment. However, the long-term impacts of dam construction and impoundments on the spatial distribution of trace metals and water quality remain poorly understood. Here, we found that the concentrations of trace metals in the mainstream water of the world's largest reservoir, Three Gorges Reservoir (TGR), decreased after impoundment, while their concentrations and contamination in the sediments of the water-level fluctuation zone increased significantly, especially for anthropogenic sources of metals such as cadmium, lead, and zinc. The spatial and temporal variations of anthropogenic metals in the sediments revealed increased anthropogenic dominance in their distribution under current hydrological management, especially for the urban area of the upper TGR. Sediment fluxes, particle composition, and extreme climate modulated the distribution of trace metals in the sediments. The results demonstrate that human activities have increasingly determined the distribution and contamination state of trace metals in the mainstream TGR. However, in contrast to our previously thought, the anthropogenic discharge of trace metals did not adversely affect water quality. Our results indicate that dam construction in riverine systems attenuates trace metal contamination in water through sediment sorting and deposition.

Predicting soil cadmium uptake by plants in a tailings reservoir during 48-year vegetation restoration.

Plant uptake can reduce soil cadmium (Cd) pollution, while how to exactly predict plant Cd uptake in industrial or mining areas during vegetation restoration remains unexplored. Taking Heteropogon contortus as the object plant, we predicted plant Cd uptake in the Majiatian tailings reservoir during 48-year vegetation restoration by the methods of soil total Cd, DGT (diffusive gradients in thin films technique) and acetic acid (HAc) extraction. Meanwhile, we explored the effects of soil properties on the accuracy of the prediction. Total Cd concentrations in the soils exhibited a better prediction of plant Cd uptake relative to the methods of HAc extraction and DGT. However, the DGT method effectively predicted plant Cd uptake at low Cd supply (lower than 0.42 µg/L), probably because of the dominant diffusion limitation by plants. The prediction of plant Cd uptake by HAc extraction was improved when combined with soil pH. Our results indicate that with increasing external Cd inputs during the vegetation restoration, soil total Cd and traditional extraction method in combination with soil properties are effective ways to predict plant Cd uptake, especially when the Cd fractions cannot be measured by DGT. However, the DGT method works once plant Cd uptake dominated by diffusion limitation despite the interference in soil properties.

Vanadium accumulation mode of Heteropogon contortus and its driving factors in Majiatian tailing reservoir in Panzhihua, Southwestern China.

Native plants in metal pollution sites have great potentials for mine rehabilitation. In the presented work, we investigated Vanadium (V) concentrations of soils and plants (Heteropogon contortus) in Majiatian V-Ti magnetite tailing reservoir in Panzhihua, Southwestern China. The objectives were to explore the V accumulation mode of H. contortus and its driving factors, as well as the phytoremediation potential of H. contortus. As the results, H. contortus accumulated 37.53 mg/kg and 8.69 mg/kg of V in root and aerial part, respectively. With the increase of rehabilitation age, root V concentrations decreased, while aerial part V concentrations remained constant. The significant negative correlations between root V and soil V, acid-soluble V (VHAc) (P < 0.05) indicated that increasing soil V and VHAc concentrations drove the V accumulation mode of H. contortus. Soil properties had a little influence on the V accumulation mode of H. contortus. Therefore, H. contortus might be not the suitable plant extractant to remove V from mine tailing for its lower V accumulation capacity. On the other hand, it can tolerate high V stress through elimination and detoxification/isolation V. Furthermore, the settlement of H. contortus increased the content of soil organic matter and might thus improve the soil quality. The cover of H. contortus is also beneficial to reduce the dispersion of the tailings and prevent contaminating surrounding soil. Therefor it showed a great potential to serve as a pioneer plant in the remediation of V-rich tailing reservoirs and other V-contaminated sites with similar poor soil condition.

Contamination, sources and health risk of heavy metals in soil and dust from different functional areas in an industrial city of Panzhihua City, Southwest China.

The contamination of heavy metals in urban soil and dust is closely related to anthropogenic emissions, while to what extent the metal contamination varies among different functional areas in industrial cities remains unclear. In this study, the contamination and health risk of seven heavy metals in the soil and dust were assessed at different functional areas of Panzhihua City, Southwest China, and their sources were identified by the spatial divisions and Pb isotopes. The results showed that the contamination of V and Cr in the vanadium/steel plant (VsA) and the contamination of Cd, Pb and Zn in the iron-ore smelting (IsA) were significantly higher relative to other functional areas. The sources of the contaminated heavy metals in the soil and dust were mainly from smelting and manufacturing vanadium/steel products, coal combustion and traffic. Vanadium and Cr were the major metals primarily contributing to the noncarcinogenic and carcinogenic risks, despite the low contamination level of Cr. The results indicate that the VsA is the priority control area in the Panzhihua City, and besides V, more attention should be paid in the future to monitor Cr risk in the soil and dust because of its high contribution to the health risk.

A novel framework to predict water turbidity using Bayesian modeling.

High water turbidity in aquatic ecosystems is a global challenge due to its harmful impacts. A cost-effective manner to rapidly and accurately measure water turbidity is thus of particular useful in water management with limited resources. This study developed a novel framework aiming to predict water turbidity in various aquatic ecosystems. The framework predicted water turbidity and quantified the uncertainty of the prediction through Bayesian modeling. To improve model performance, a model-update method was implemented in the framework to update the model structure and parameters once more measured data were available. 120 paired records (an image from smartphone and a measured water turbidity value by standard turbidimeters for each record) were collected from rivers, lakes and ponds across China to evaluate the performance of the developed framework. Our cross-validation results revealed a well prediction of water turbidity with Nash-Sutcliffe efficiency (NS) >0.87 (p<0.001) during the training period and NS>0.73 (p<0.001) during the validation period. The model-update method (in case of more measured data) for the developed Bayesian models in the framework resulted in a decreasing trend of model uncertainty and a stable mode fit. This study demonstrated a high value of the Bayesian-based framework in predicting water turbidity in a robust and easy manner.

Characterizing the river water quality in China: Recent progress and on-going challenges.

Food production systems, urbanization, and other anthropogenic activities dramatically alter natural hydrological and nutrient cycles, and are primarily responsible for water quality impairments in China's rivers. This study compiled a 16-year (2003-2018) dataset of river water quality (161,337 records from 2424 sites), watershed/landscape features, and meteorological conditions to investigate the spatial water quality patterns and underlying drivers of river impairment (defined as water quality worse than Class V according to China's Environmental Quality Standards for Surface Waters, GB3838-2002) at a national scale. Our analysis provided evidence of a distinct water quality improvement with a gradual decrease in the frequency of prevalence of anoxic conditions, an alleviation of the severity of heavy metal pollution, whereas the cultural eutrophication has only been moderately mitigated between 2003 and 2018. We also identified significant spatial variation with relatively poorer water quality in eastern China, where 17.2% of the sampling sites registered poor water quality conditions, compared with only 4.6% in western China. Total phosphorus (TP) and ammonia-nitrogen (NH3-N) are collectively responsible for >85% of the identified incidences of impaired conditions. Bayesian modelling was used to delineate the most significant covariates of TP/NH3-N riverine levels in six large river basins (Liao, Hai, Yellow, Yangtze, Huai, and Pearl). Water quality impairments are predominantly shaped by anthropogenic drivers (82.5% for TP, 79.5% for NH3-N), whereas natural factors appear to play a secondary role (20.5% for TP, 17.5% for NH3-N). Two indicator variables of urbanization (urban areal extent and nighttime light intensity) and farmland areal extent were the strongest predictors of riverine TP/NH3-N levels and collectively accounted for most of the ambient nutrient variability. We concluded that there is still a long way to go in order to eradicate eutrophication and realize acceptable ecological conditions. The design of the remedial measures must be tailored to the site-specific landscape characteristics, meteorological conditions, and should also consider the increasing importance of non-point source pollution and internal nutrient loading.

Terrain-modulated deposition of atmospheric lead in the soils of alpine forest, central China.

Alpine ecosystem has a potential to intercept the transport of atmospheric metals, while the regulation mechanisms with variations in altitude and slope direction remain unclear. In this study, the soil and moss samples on the northern and southern slopes of Shennongjia Mountain were collected with altitude to quantitatively identify the sources of lead (Pb) and to decipher the regulation mechanisms of altitude and slope on the Pb distribution. The results showed that the concentrations of Pb decreased evidently with soil depth, and in the O (organic soils) and A (surface mineral soils) horizons they increased with altitude. The Pb isotopes and moss biomonitoring revealed that Pb was mainly from atmospheric deposition, and the sources included fossil fuel combustion, ore mining and smelting. Based on a binary mixing model of Pb isotopes, the percentage of atmospheric Pb in the O and A horizons and mosses averaged 58.8%, 43.7% and 71.0%, respectively. Atmospheric wet deposition strikingly controlled the distribution of soil Pb along the altitude. Canopy filtering and leaching also impacted the accumulation of Pb in the forest floor. The significant difference in the atmospheric Pb accumulation in the soils between the two slopes was not observed as expected, since atmospheric dry deposition from northwestern China contributed to the Pb accumulation on the northern slope according to the Pb isotopic ratios and air mass trajectories. The results of this study indicate that altitude determines the distribution pattern of atmospheric Pb, while slope direction screens the source region of Pb in alpine ecosystems.

Fractions, Contamination and Health Risk of Cadmium in Alpine Soils on the Gongga Mountain, Eastern Tibetan Plateau.

Anthropogenic cadmium (Cd) in alpine soils is mainly from long-range atmospheric transport. Because of the high toxicity and mobility, whether the accumulation of Cd in the soils threats to ecosystem safety remains unclear. The fractions of soil Cd along three altitudinal transects on Gongga Mountain were analyzed to decipher the drivers on its mobility, and its contamination and health risk were assessed. The concentrations of Cd in the organic (O) and mineral (A) horizons were significantly higher on the eastern and southern transects than the western transect. The Cd fractions in the two horizons dominated by acid-soluble and reducible Cd. Soil organic matter and pH modulated the mobilization of soil Cd. Cadmium reached a moderate contamination level on the eastern and southern transects, but no or slight contamination on the western transect. The soil Cd had a low non-carcinogenic risk and no carcinogenic risk despite of adults or children.

Altitudinal-modulated sediment inputs rather than the land-uses determine the distribution of lead in the riparian soils of the Three Gorges Reservoir.

Lead (Pb) as a toxic metal has potential ecological hazards for aquatic quality. However, the variation in the distribution patterns of Pb and its fractions in flooding soils with frequent and anti-seasonal water-level fluctuation and various human disturbances remains unclear. In this study, the distribution of Pb and its fractions in the riparian soils of the Three Gorges Reservoir (TGR) were delineated based on the differences in altitude and land-uses including farmland, orchard, forest and residential area. Then, we assessed the contamination and eco-risk of Pb in the soils and deciphered the key factors determining the distribution of Pb and its fractions. The results showed that the concentrations of Pb and its fractions in the soils decreased significantly with altitude, while the significant difference was not observed among the land-uses. The contamination of Pb in the soils reached a moderate level, and its eco-risk was very low by the potential eco-risk index and mobile Pb fraction. The source of soil Pb at the upper zone (> 160 m) was mainly from natural inputs, while the source at the lower zone (≤ 160 m) was attributed to anthropogenic contributions including ores mining, fossil fuel combustion, vehicle emissions and atmospheric deposition indicated by Pb isotopic ratios. With the limited effect of land-uses, the sediment inputs regulated by frequent water-level fluctuation determined the altitudinal distribution of Pb and its fractions in the flooding soils. The soil particle size dominated the migration and transformation of Pb over other soil properties such as pH and organic matters. The results of this study indicate that the anthropogenic Pb mainly exists in the soils of lower riparian zone in the TGR, and the frequent and anti-seasonal dry and rewetting alternation aggravates the potential for the Pb migration downstream due to the determinant of soil particles.

Fine sediment particle microscopic characteristics, bioavailable phosphorus and environmental effects in the world largest reservoir.

The transport and retention of sediments in fine grain sizes plays an important role in the cycles of phosphorus (P), and is closely related to the extent and potential for eutrophication in water reservoirs. In order to highlight the environmental indications for the transport of fine sediment particles and the associated bioavailable phosphorus (Bio-P) in the world largest reservoir, the Three Gorges Reservoir (TGR), the suspended and bed sediments were collected at 13 sections in 2016. The sediment physicochemical properties, micromorphology of sediment particles, distribution of elements on particle surface, P adsorption parameters, and P fractions in different grain sized sediments were analyzed. The results showed that the fine sediment particles had a strong P adsorption ability due to their micromorphology, mineral compositions, and the high contents of Fe/Al/Mn (hydr)oxides, which contributed a higher concentration of Bio-P in <16 µm sediment particles. The adsorption of P on the sediment particles occurred longitudinally along the TGR, and the fine sediment particles (<16 µm) dominated the transport and distribution of Bio-P in the TGR sediments. The reduced inflow and retention of fine sediment particles, caused by the construction of cascade reservoirs along the Jinsha River (upper reach of the Yangtze River), has resulted in the decrease in the retention of Bio-P in the TGR. Therefore, we conclude that the continuously decrease of inflow and retention of the fine sediment particles in the TGR, and with it a reduced sediment P buffer capacity, may enhance algal blooms occurrence also in view of the increased P discharge from the overall TGR catchment. The study results can contribute to improved management guidance on fine sediment particles and associated phosphorus for the operation and environmental protection of other large reservoirs in the world.

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.

Levels and enantiomeric signatures of organochlorine pesticides in Chinese forest soils: Implications for sources and environmental behavior.

We investigated the levels and distributions of organochlorine pesticides (OCPs) in 159 background soil samples collected from 30 forested mountain sites across China. The sum of DDT was the most abundant OCP, with the concentrations of 0.197-207 ng/g and 0.033-122 ng/g in the O-horizon and A-horizon, respectively. High concentrations of OCPs usually occur near agricultural regions or high consumption areas. The spatial distribution was mainly influenced by the emission sources and soil total organic contents (TOC). The chiral compounds were generally nonracemic in the soils and showed preferential degradation of (-) o,p'- dichlorodiphenyltrichloroethane, (+) trans-chlordane, and (-) cis-chlordane in both the O- and A-horizons. The enantiomeric fraction (EF) distributions of chiral OCPs displayed no differences across the forest sites in the O-horizon or the A-horizon. Comparing the deviation of EFs from racemic (DEVrac = absolute value of 0.500 - EF) with environmental parameters, we found that DEVrac of cis-chlordane demonstrated a strong positive correlation with TOC (p < 0.05) and the C/N ratio (p < 0.01). This relationship suggests that these factors could affect the microbial activity and significantly impact the extent of enantioselective degradation of chiral compounds in the soils. Fresh and historical applications of DDT and historical chlordane and endosulfan uses may be prominent sources of OCP accumulation in Chinese forest soils.

Impacts of atmospheric particulate matter pollution on environmental biogeochemistry of trace metals in soil-plant system: A review.

Atmospheric particulate matter (PM) pollution and soil trace metal (TM) contamination are binary environmental issues harming ecosystems and human health, especially in the developing China with rapid urbanization and industrialization. Since PMs contain TMs, the air-soil nexus should be investigated synthetically. Although the PMs and airborne TMs are mainly emitted from urban or industrial areas, they can reach the rural and remote mountain areas owing to the ability of long-range transport. After dry or wet deposition, they will participate in the terrestrial biogeochemical cycles of TMs in various soil-plant systems, including urban soil-greening trees, agricultural soil-food crops, and mountain soil-natural forest systems. Besides the well-known root uptake, the pathway of leaf deposition and foliar absorption contribute significantly to the plant TM accumulation. Moreover, the aerosols can also exert climatic effects by absorption and scattering of solar radiation and by the cloud condensation nuclei activity, thereby indirectly impact plant growth and probably crop TM accumulation through photosynthesis, and then threat health. In particular, this systematic review summarizes the interactions of PMs-TMs in soil-plant systems including the deposition, transfer, accumulation, toxicity, and mechanisms among them. Finally, current knowledge gaps and prospective are proposed for future research agendas. These analyses would be conducive to improving urban air quality and managing the agricultural and ecological risks of airborne metals.

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.

Divergent patterns of soil phosphorus discharge from water-level fluctuation zone after full impoundment of Three Gorges Reservoir, China.

Phosphorus (P) discharged from soils in the water-level fluctuation (WLF) zone becomes increasingly important to the water quality control of the Three Gorges Reservoir (TGR) as the decrease in P input from upstream reaches and point-source pollution. To investigate the amount of soil P discharge from the WLF zone since the full impoundment of the TGR in 2010, soil and sediment samples were collected along the altitudinal gradients (140, 150, 160, 170, and 180 m above sea level) in three transects in the middle reaches of the TGR. Soil P composition was determined by a sequential extraction procedure. Different amounts of P discharge from the WLF zone were found among three soil types because of their difference in the initial P content before impoundment, with an order of yellow earth (171.1 g m-2), fluvo-aquic soil (141.7 g m-2), and purple soil (73.8 g m-2). An altitudinal pattern of soil P discharge was observed with the maximum at the 170-m sites. The downward transport of exchangeable P and clay-bound P with runoff was the major path of the soil P discharge at the 170-m sites with a slope gradient > 15°. Considerable P discharge with erosion at the upper section of the WLF zone was facilitated by the longer exposure period compared with that at bottom section (150-m sites) because of the annual anti-seasonal impoundment-exposure cycles of the TGR. The transformation of Al/Fe-P and subsequent release to water was a main mechanism of the soil P discharge during the impoundment period. The altitudinal pattern of P discharge was a result of joint effects of slope gradient, soil P forms, and the anti-seasonal hydrological regime of the TGR. The results highlight the critical role of the upper section (165-175 m) in controlling the P output from the WLF zone into the water of the TGR.