Use of multi-objective dredging for remediation of contaminated sediments: a case study of a typical heavily polluted confluence area in China

Release characteristics of arsenic from sediments and its source or sink competition with phosphorus: A case of a great lake with grass-algae alternation

The arsenic (As) release from sediments in great lakes is affected by various factors. In this study, the characteristics of As release from sediments was investigated, and the As sources and sinks with the strengths in sediments from different areas (grass-type, algae-type, and grass-algae alternation areas) in great shallow lakes (Taihu Lake, China) were analyzed, and the influence of P competition in the process of As release was also studied. The results showed that changing trend of the values of equilibrium As concentration in sediments were consistent with the regional changes (0 to 28.12 µg/L), and the sediments from algae-type areas had the higher values. The sediments from western lake and northwest lake bay were a strong As and a weak P source, and the north lake bay had the opposite trend of these two regions. Intense P source competition with As from the sediments occurred in algae-type areas. The grass-type areas had strong As and P retention capacities, indicating a sink role of sediment with high As and P sorption capacities. The degree of As and P saturation had similar trend in sediments, and the grass-type areas had the higher values, 18.3%-21.4% and 15.31%-20.34%, respectively. Contribution analysis results showed that most of As release contribution was from the bottom (30-50 cm) sediments, and the surface (0-10 cm) sediments from algae-type areas contributed more to the overlying water than other region.

Inhomogeneous antibiotic distribution in sediment profiles in anthropogenically impacted lakes: Source apportionment, fate drivers, and risk assessment

Antibiotic residues in lake ecosystems have been widely reported; however, the vertical distribution of antibiotics in lake sediment profiles have rarely been examined. This study systematically revealed the vertical distribution pattern, sources, and risks of antibiotics in sediments of four typical agricultural lakes in central China. Nine of 33 target antibiotics were detected with a total concentration range of 39.3-18,250.6 ng/g (dry weight), and the order of average concentration was erythromycin (1447.4 ng/g) > sulfamethoxazole (443.7 ng/g) > oxytetracycline (62.6 ng/g) > enrofloxacin (40.7 ng/g) > others (0.1-2.1 ng/g). The middle-layer sediments (9-27 cm) had significantly higher antibiotic detected number and concentration than those in the top layer (0-9 cm) and bottom layer (27-45 cm) (p < 0.05). Correlation analysis showed that significant relationships existed between antibiotic concentrations and the octanol-water partition coefficients (K_ow) of antibiotics (p < 0.05). Redundancy analysis indicated that Pb, Co, Ni, water content, and organic matter (p < 0.05) jointly affected the distribution of antibiotics in sediment profiles. Risk assessment showed that the highest potential ecological and resistance selection risks of antibiotics occurred in the middle-layer sediments, and oxytetracycline, tetracycline, and enrofloxacin had the most extensive potential risks in the sediment profiles. Additionally, the positive matrix factorization model revealed that human medical wastewater (54.5%) contributed more antibiotic pollution than animal excreta (45.5%) in sediment. This work highlights the inhomogeneous distribution of antibiotics in sediment profiles and provides valuable information for the prevention and control of antibiotic contamination in lakes.

A method of assessing the depth of contaminated sediments that should be removed in lakes: a case study of Tangxun Lake, China

The depth of contaminated sediments constrains the water environment of large shallow lakes and can affect shallow lake water quality through sediment resuspension and nutrient release. Although such effects can be avoided by sediment dredging methods, we still cannot precisely quantify the depth of sediment dredging. Therefore, we used organic index method, pollution index method and potential ecological risk evaluation to evaluate the contamination status of split samples of in situ sediments layer by layer, and established a comprehensive contamination index evaluation method for layer-by-layer sediments, then combined with the contamination release characteristics of split samples to assess the contamination degree of the sediments obtained. The results show that the content of nitrogen and phosphorus in the surface layer of Lake Townsend sediments is generally higher than that in the middle and bottom sediments, and the heavy metals also satisfy this pattern, which is consistent in the sediments of both east and west regions. We also simulated the release process of nitrogen and phosphorus nutrients in the in situ sediment of Tangxun Lake in 2019, and the experimental results showed that the risk of nitrogen and phosphorus nutrient release in the sediment was mainly concentrated in the surface and middle layers, and the risk of elemental nitrogen release was significantly greater than that of phosphorus release. Finally, a comprehensive evaluation was carried out to obtain the desilting depth of the sediments in Lake Townsend, and it can be determined that the recommended desilting depth is about 20 cm for West Lake and 30 cm for East Lake. The results show that the recommended dredging depth can be determined based on this method, which provides an important scientific basis for sediment dredging in Tangxun Lake and even provides a new paradigm for sediment dredging depth estimation in similar large shallow lakes.

Chronological deposition record of trace metals in sediment cores from Chaohu Lake, Anhui Province, China

Lakes located in the mid-low reaches of the Yangtze River watershed have been subjected to various degrees of human perturbation that would have resulted in toxic metal concentrations and would pose potential risk to the natural habitats. Therefore, in the present study, two sediment cores from Chaohu Lake were collected to determine any such concentration, expressed as the enrichment characteristics of major and trace metals (Al, Fe, Mn, Cu, Pb, Zn, Cr, Cd, As, Hg, and Ni) in response to natural and anthropogenic changes. An approximate 180-year (1840-2021) deposition record of trace metals in sediment cores was obtained on the basis of ¹³⁷Cs and ²¹⁰Pb dating. Enrichment factors (EFs) and the geo-accumulation index (I_geo) were adopted to evaluate the enrichment state and pollution status of trace metals. The results showed that Cu and Zn were persistently more enriched in sediment, whereas Mn, Pb, Cd, and Hg has shown remarkable increasing trends in the west lake since the 1970s. Evaluation of the I_geo confirmed that sediment was moderately polluted with Cu and Zn within the whole lake and with Cd in the west lake area, whereas it was uncontaminated with Cr and slightly contaminated with other metals. Furthermore, source identification based on multivariate statistical analysis including correlation analysis, principal component analysis, and cluster analysis suggested similar pollution sources for the studied metals. Combined with the natural and anthropogenic changes within the watershed, enhanced soil erosion due to population expansion and agricultural intensification was a major contributor to sedimentary metals before the 1970s while industrial wastewater, urban runoff, and domestic sewage were predominant inputs of trace metals after the 1970s.

Case study of the in-situ restoration of black-odorous water by combined process of forced aeration and biological contact oxidation

Black and odorous water bodies are an extreme phenomenon that impair ecological integrity, adversely affect the lives of residents and the town's image, and cause unpleasant sensory experiences. Herein, we consider a black and smelly river in Heshan City, Guangdong Province, as a case study. The proposed comprehensive governance process combines the use of pollution control and interception, sediment remediation, aeration oxygenation, a high-efficiency biological contact oxidation/denitrification pond, and ecosystem construction. The project operation results showed that the combined process can effectively improve water quality. The water quality of the river improved to the Class V standard. All indicators met the requirements of the 'China Surface Water Environmental Quality Standard' (GB3838-2002). River water quality indicators, monitored for four months, revealed that water transparency and dissolved oxygen increased by 5.9 times and 24.5 times, respectively. Dichromate index (CODcr), total phosphorus (TP), and ammonia nitrogen (NH₃-N) were reduced by 5.8, 4.17, and 5.17 times when compared to the values observed before treatment. The black-odor and eutrophication of the river were successfully eliminated, and the water quality improved significantly. In general, the combined process exhibits a high technical feasibility for implementation, providing a specific reference value for the treatment of black and odorous water bodies in urban settings.

Characterization, fractionation and mobility of trace elements in surface sediments of the Jequiezinho River, Bahia, Brazil

The Jequiezinho River is a temporary river. In the urban stretch it is impacted, with perennial flow coming from domestic sewage and rainwater. This study evaluated the geochemical distribution and potential mobility of some metals (Pb, Co, Ca, Cr, Mg, Cu, Fe, Mn, Ni and Zn) in sediments of the Jequiezinho River. Sediment samples were collected at ten different sites along the river. The silt/clay fraction was submitted to acid digestion and sequential extraction with analysis by FAAS. The results indicated that, in the more densely populated region, there was an increase in concentrations of Cr, Ni, Cu, Co, Pb, and Zn. The contents found were compared with the guideline values of TEL (Threshold Effect Levels) and PEL (Probable Effect Levels)​​, not exceeding the maximum reference limits. The results indicated that Ca, Mg, Mn and Fe presented greater susceptibility to mobility and bioavailability suggesting the geochemical origin responsible for these high concentrations. The multivariate analysis showed that Cr, Ni, Cu, Co, Pb, and Zn presented a similar behavior, especially in locations with higher population density and the discharge of non-discriminated effluents, reflecting the anthropogenic contributions as responsible for the concentration increase.

Dredging method effects on sediment resuspension and nutrient release across the sediment-water interface in Lake Taihu, China

Environmental sediment dredging is one of the most common methods for the remediation of contaminated sediments in lakes; however, debate continues as to whether the effectiveness of dredging methods contributes to this phenomenon. To determine sediment resuspension and nutrient release following dredging with a variety of dredging methods, four dredging treatments at wind speeds of 0-5.2 m/s were simulated in this study, namely suction dredging (SD), grab dredging (GD), ideal dredging with no residual sediments (ID), and non-dredging (ND). Field sediments from suction and grab dredging areas (including post-dredged and non-dredged sediments) of Lake Taihu were used to assess the release abilities of soluble reactive phosphorus (SRP) and ammonia nitrogen (NH₄⁺-N) from the sediment-water interface. The effects of residual sediments on nutrient concentrations in water were also evaluated. The results reveal that inhibition of resuspension of particulate matter and nutrients released through sediment dredging decreases with increasing levels of residual sediment. Total suspended particulate matter content in the mean water columns of ID, SD, and GD under wind-induced disturbance (1.7-5.2 m/s) decreased by 67.5%, 56.8%, and 44.3%, respectively; total nitrogen and total phosphorus in ID (SD) treatments were 19.8% (12.9%) and 24.5% (11.2%) lower than that in ND treatment. However, there were ~ 1.6 and 1.5 times higher SRP and NH₄⁺-N in the GD treatment compared with the ND treatment at the end of the resuspension experiment (0 m/s). A significant increase in the SRP and NH₄⁺-N release rates at the sediment-water interface was also observed in field sediments from a grab dredging area, indicating that GD may pose a short-term risk of nutrient release to the water body. Hence, dredging methods with less residual sediments both during and after dredging improves the dredging quality.

Immobilization of fluoride in the sediment of mine drainage stream using loess, Northwest China

Fluoride (F) is a necessary trace element in the human body, which would lead to some diseases if human body lacks or accumulates it excessively (1-1.5 mg d^-1). Fluoride contamination in sediments has become more and more serious, which has potential hazards to human body. In this paper, a novel sorbent (loess) was proposed to immobilize trace element F in sediment. The effectiveness of loess on F stabilization was evaluated by decreasing F bioavailability in contaminated sediment. The loess and the sediment were mixed at different proportions for stabilization. About 70 days after the application of loess, the soil column was subject to simulate acid rain leaching test to observe the leaching-migration of F, which can be used to predict the leaching migration of F in the study area. The results showed that when the loess dose was 5 kg, the loess converted highly effective fractions of F (i.e., water-soluble and exchangeable fractions) into a more stable state (i.e., residual state). After 30 days of leaching with HNO₃ solution with pH at 3.0, the lowest concentration of F was found in the leachate of soil column with 2 kg loess application. Correlation analysis showed that the F concentration in soil column profile was affected by CaCO₃, EC, pH, and OM, of which, pH and CaCO₃ have greater influence than other factors.

Responses of water quality and phytoplankton assemblages to remediation projects in two hypereutrophic tributaries of Chaohu Lake

Water shortages and the presence of point and diffuse source pollution have caused a serious deterioration in water quality in two tributaries (the Tangxi River and Shiwuli River) of Chaohu Lake, China. To reduce nutrient pollution and suppress harmful algal blooms (HABs), hard engineering and ecological remediation projects were implemented. A post-project investigation from 2013 to 2016 was carried out to evaluate the outcome of the remediation projects by monitoring the seasonal and spatial variations in water quality and the phytoplankton community. In the Tangxi River, the average total phosphorus (TP) concentrations in the four seasons were below 0.5 mg L^-1, with the lowest concentration (0.29 ± 0.12 mg L^-1) found in autumn. Remediation measures including sediment dredging, riparian buffer zone creation, downstream wetland park construction, and water augmentation using reclaimed water and filtered lake water might combine to promote P source mitigation. Moreover, the percentage of bloom-forming cyanobacteria (i.e., Microcystis, Aphanizomenon, Anabaena, Oscillatoria, Phormidium and Planktothrix) in the phytoplankton assemblage and the biomass of the dominant species indicated successful HAB control. In the Shiwuli River, water quality improvements and phytoplankton responses have been observed since 2015 after the upgrading of a local wastewater treatment plant (WWTP) with effluent that was used for flow augmentation. Nevertheless, there is still room for improvement via increasing the river self-purification ability (e.g., the creation of downstream wetlands and riparian buffer zones) and promoting water augmentation according to the experience gained in the remediation projects of the Tangxi River.

An overview of heavy metal pollution in Chaohu Lake, China: enrichment, distribution, speciation, and associated risk under natural and anthropogenic changes

An exhaustive overview of heavy metal pollution in Chaohu Lake illustrating enrichment intensity, temporal and spatial distribution, chemical speciation, and ecological risk under natural and anthropogenic changes was conducted. Low concentrations of heavy metals excluding Hg were found in water whereas high Hg might be ascribed to surrounding coal-fired power plants. Copper, Pb, Zn, Cd, and Hg were enriched in sediment whereas Cr and Ni were comparable to background values. Besides, As demonstrated an equal accumulation from natural and anthropogenic fluxes. Heavy metals were at a low level prior to the 1950s; it increased gradually during the 1950s-1960s owing to population growth and agricultural expansion; then it displayed abrupt increase since the late 1970s due to rapid modern urbanization and industrialization and agricultural intensification. Spatial distribution of heavy metals was a good indicator of natural and anthropogenic changes, where higher enrichment was found in the western lake. Apart from fluvial input, anthropogenic disturbances such as land use changes, atmospheric deposition, and algae-derived organic matter, along with natural stressors including climate change, hydrological alteration, and soil erosion, made significant contribution to the biogeochemical cycle of heavy metals in the lake. Heavy metals mainly from anthropogenic sources were dominantly partitioned in non-residual fractions, whereas those mainly from natural sources were predominantly distributed in residual form. Mercury and Cd were below the threshold effect concentration (TEC) indicating that adverse effects were excluded. However, result of chemical speciation demonstrated Cd would pose a considerable potential ecological risk. Besides, most of the heavy metals were in the range of TEC-PEC suggesting possible toxicity.

Sulfur Development in the Water-Sediment System of the Algae Accumulation Embay Area in Lake Taihu

Sulfur development in water-sediment systems is closely related to eutrophication and harmful algae blooms (HABs). However, the development of sulfur in water-sediment systems during heavy algae accumulation still remains unclear, especially in hyper-eutrophic shallow lakes. In this study, a quarterly field investigation was carried out for a year in the algae accumulated embay area of Lake Taihu, accompanied by a short-term laboratory experiment on algae accumulation. The results show that hydrogen sulfide and methanethiol dominated the volatile sulfur compounds (VSCs) in the water during non-accumulation seasons, whereas the concentrations of dimethyl sulfides increased during heavy algae accumulation, both in the field and the laboratory. An increase in the acid volatile sulfide (AVS) in the surface sediments was also discovered together with the increase in dimethyl sulfides. The depletion of oxygen in the overlying water and sediment–water interface during the heavy algae accumulation and decomposition was found to be closely related to both the increase in VSCs in the overlying water and increase in AVS in the sediment. The increased concentrations of these reductive sulfocompounds might aggravate the eutrophication and HABs and should be given more consideration in future eutrophication control plans for lakes.

Efficacy of dredging engineering as a means to remove heavy metals from lake sediments

Dredging is used worldwide to remove polluted sediments from water bodies. However, the dredging efficacy remains hard to identify. Here, we studied the efficacy of dredging engineering as a means to remove Cu, Cd, and Pb from polluted lake sediments, after six years of completion. Dissolved metals and DGT-labile metals were quantified in the non-dredged and post-dredged sediments by high-resolution dialysis (HR-Peeper) and diffusive gradients (DGT) in thin films techniques. April and July measurements showed that dredging was effectively remediate the polluted sediments. The dissolved Pb, Cd, and Cu contents decreased up to 30%, 44%, and 26%, and the DGT-labile contents decreased up to 51%, 27%, and 33% compared with the contents in the non-dredged zone. Dredging was thus proven efficient in decreasing the labile metal fractions, increasing the capacity of available solids to bind metals, and slowing the leaching of metals from available solids in the post-dredged sediments. In October and January, the dredging efficacy was counteracted by the decomposition of algae, which increased the dissolved and DGT-labile metal concentrations in the post-dredged zone.

Long-term effects of sediment dredging on controlling cobalt, zinc, and nickel contamination determined by chemical fractionation and passive sampling

Studies of dredging effectiveness, especially the ones that last for several years, are scarce. In this study, we evaluated effectiveness of dredging performed for six years on controlling cobalt (Co), zinc (Zn), and nickel (Ni) contamination of sediments. High-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) methods were applied to analyze the non-dredged and post-dredging sediments. The soluble and DGT-labile Co and Ni content declined by 22% and 44% (soluble) and by 16% and 26% (labile) in April, July and October in the post-dredging region. In contrast, their concentrations increased by 105% and 9% (soluble) and 322% and 27% (labile) in January. These changes in the dredging effects were caused by the corresponding changes in the reducible and residual fractions of Co and the residual fraction of Ni in sediments in the dredged site, respectively. Soluble and DGT-labile Zn decreased on average by 23% and 29% in July and October and increased on average by 151% and 52% in April and January in the post-dredging region. The different Zn mobility in the post-dredging region was controlled by the reducible fraction of Zn. The results revealed positive influence of dredging engineering in summer, autumn and/or spring and negative one in winter. Therefore, an accurate assessment of dredging effectiveness should take its seasonal variation into consideration.

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.

Exchanges of nitrogen and phosphorus across the sediment-water interface influenced by the external suspended particulate matter and the residual matter after dredging

Dredging is frequently implemented for the reduction of internal nitrogen (N) and phosphorus (P) loadings and the control of eutrophication. Residuals during dredging activities and external pollution loadings after dredging both commonly contribute to influence the effectiveness of dredging and have been widely discussed. In the current study, the exchanges of N and P across the sediment-water interface (SWI) to these two factors were compared in a six-month field incubation experiment. The results showed that the continuous deposition of external suspended particulate matter (SPM) led ammonium nitrogen (NH₄⁺N) and soluble reactive phosphorus (SRP) fluxes across the newly formed SWI to increase by factors of 4.16 and 12.71, respectively, while residual material caused the same fluxes to increase by factors of 2.06 and 5.06. Both the deposition of external SPM and the residual matter led to higher increase of the fluxes of P across the SWI than those of the fluxes of N across the SWI after dredging. The SPM easily adsorbed P in the water due to extensive adsorption of water soluble organic matter (consisting primarily of easily-decomposed humic-like substances), iron, and aluminum. However, the decomposition of organic matter in the SPM after the deposition on the dredged sediment accelerated the dissolution of redox-sensitive P and organic P across the SWI after dredging. Both the increase in the fluxes of N and P across the SWI would further increase the concentrations of N and P in the overlying water and thereby aggravate the eutrophication status in lakes. More frequent dredging operations might be necessary to reduce the fluxes of N and P from the sediment due to the continuous influence of the external SPM and the residual matter.

Effects of Dredging Season on Sediment Properties and Nutrient Fluxes across the Sediment–Water Interface in Meiliang Bay of Lake Taihu, China

The influence of dredging season on sediment properties and nutrient fluxes across the sediment–water interface remains unknown. This study collected sediment cores from two sites with different pollution levels in Meiliang Bay, Taihu Lake (China). The samples were used in simulation experiments designed to elucidated the effects of dredging on internal loading in different seasons. The results showed that dredging the upper 30 cm of sediment could effectively reduce the contents of organic matter, total nitrogen, and total phosphorus in the sediments. Total biological activity in the dredged sediment was weaker (p < 0.05) than in the undredged sediment in all seasons for both the Inner Bay and Outer Bay, but the effect of 30-cm dredging on sediment oxygen demand was negligible. Dredging had a significant controlling effect on phosphorus release in both the Inner Bay and Outer Bay, and soluble reactive phosphorus (SRP) fluxes from the dredged cores were generally lower (p < 0.05) than from the undredged cores. In contrast, NH4+-N fluxes from the dredged cores were significantly higher (p < 0.05) than from the undredged cores in all seasons for both sites, this indicates short-term risk of NH4+-N release after dredging, and this risk is greatest in seasons with higher temperatures, especially for the Inner Bay. Dredging had a limited effect on NO2−-N and NO3−-N fluxes at both sites. These results suggest that dredging could be a useful approach for decreasing internal loading in Taihu Lake, and that the seasons with low temperature (non-growing season) are suitable for performing dredging projects.

Successful control of internal phosphorus loading after sediment dredging for 6years: A field assessment using high-resolution sampling techniques

The effectiveness of sediment dredging for the control of internal phosphorus (P) loading, was investigated seasonally in the eutrophic Lake Taihu. The high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were used to measure the concentrations of soluble Fe(II) and soluble reactive P (SRP) as well as DGT-labile Fe/P in the non-dredging and post-dredging sediments. The P resupply kinetics from sediment solids were interpreted using DGT Induced Fluxes in Sediments (DIFS) modeling. The results showed no obvious improvement in water and sediment quality after dredging for 6years, due to their geographical proximity (a line distance of approximately 9km). However, dredging significantly decreased the concentrations of soluble Fe(II)/SRP and DGT-labile Fe/P in sediments, with effects varying at different depths below the sediment-water interface; More pronounced effects appeared in January and April. The diffusive flux of pore water SRP from sediments decreased from 0.746, 4.08 and 0.353mg/m²/d to 0.174, 1.58 and 0.048mg/m²/d in April, July and January, respectively. DIFS modeling indicated that the P retention capability of sediment solids was improved in April in post-dredging site. Positive correlations between pore water soluble Fe(II) and SRP as well as between DGT-labile Fe and P, reflect the key role of Fe redox cycling in regulating dredging effectiveness. This effect is especially important in winter and spring, while in summer and autumn, the decomposition of algae promoted the release of P from sediments and suppressed dredging effectiveness. Overall, the high-resolution HR-Peeper and DGT measurements indicated a successful control of internal P loading by dredging, and the post-dredging effectiveness was suppressed by algal bloom.

Nitrogen exchange across the sediment-water interface after dredging: The influence of contaminated riverine suspended particulate matter

Dredging has been widely implemented in shallow lakes to reduce internal nitrogen (N) loading. The suspended particulate matter (SPM) coming from polluted rivers usually contains high levels of N and ultimately deposits on the dredged sediment surfaces near the river mouth. To study the influence of the riverine SPM on N exchange across the sediment-water interface (SWI) after dredging, a 360-day experiment was carried out comparing un-dredged and dredged sediments from Lake Chaohu, China. Dredged treatments showed a significant increase (p < 0.01) in total N concentrations in the sediments, while the deposition of SPM had little influence on labile NH₄⁺-N concentrations. In addition, NH₄⁺-N concentrations in pore-water and NH₄⁺-N fluxes were significantly lower in dredged than in un-dredged sediments, despite the deposition of SPM. The oxygen production rates and the oxygen penetration depth in the dredged sediments were both higher than those in the un-dredged sediments. The increase of Nitrospira in dredged sediments was consistent with their decreased NH₄⁺-N concentrations and fluxes across the SWI. Therefore, the oxidizing condition, increased oxygen production/consumption rates and Nitrospira relative abundance across the SWI were believed to be correlated with the low N exchange rates in dredged sediments. Dredging for reducing internal N loading in a river mouth area is therefore feasible, although the influence of the riverine SPM should be taken into account when aiming to achieve a long-term internal N loading reduction.

Temporal occurrence and sources of persistent organic pollutants in suspended particulate matter from the most heavily polluted river mouth of Lake Chaohu, China

The Nanfei River is by many measures the most heavily polluted tributary to Lake Chaohu. In this study, the temporal occurrence and sources of four classes of persistent organic pollutants (POPs), including polycyclic aromatic hydrocarbons (PAHs), organochlorine pesticides (OCPs), polybrominated diphenyl ethers (PBDEs), and polychlorinated biphenyls (PCBs), in suspended particulate matter (SPM) from the river mouth were investigated monthly during 2014. Results show that concentrations of all four POPs in SPM were higher than those in the sediment of Lake Chaohu. PBDEs (26.7 ng g^-1 dry weight (dw)) were originated mainly from commercial deca-BDE mixtures. PCB concentrations (1.336 ng g^-1 dw) were lower than those of sediments in many other water bodies worldwide. PAHs (2597 ng g^-1 dw) and OCPs (57.38 ng g^-1 dw) were the most common POPs. PAHs mainly had high molecular weights and originated from pyrolytic sources, with a small proportion of petrogenic origin. The predominant OCPs were DDTs, heptachlorepoxides, dieldrin, hexachlorocyclohexanes, hexachlorobenzene, and chlordanes. Most OCPs originated from historical use, except lindane. Generally, of all the POPs studied, those of primary ecological concern should be acenaphthene, fluorene, DDTs, and chlordanes. Higher concentrations of POPs were detected during winter and spring than in summer and autumn, probably because of the high river flow during the rainy season. The high concentration of POPs in the riverine SPM and the fractionation of POPs in the water and SPM of the river should be a focal point in the future study of Lake Chaohu.

Fifteen-year study of environmental dredging effect on variation of nitrogen and phosphorus exchange across the sediment-water interface of an urban lake

Environmental dredging has been applied widely in Chinese lakes to reduce their internal nutrient loads. However, the efficacy of dredging to reduce internal loading of nitrogen (N) and phosphorus (P) and to improve water quality has been questioned by some researchers. In this study, the long-term (∼15 years) effects of dredging to reduce internal N and P loading in a closed, polluted urban lake were investigated. The results showed that the release of soluble reactive phosphorus (SRP) could be suppressed quickly after dredging, and that the dredging effect was sustained for about 18 months. A significant release of NH₄⁺-N was discovered during the first 2-8 months after dredging, followed by maintenance of low-level release rates for about 21-32 months. The continuous inflowing of external pollution loading led to the increase in the release rates of SRP and NH₄⁺-N. The external pollution loading was therefore reduced three years after dredging to strengthen the remediation effect. After that, high diffusive flux from the sediment was observed for both NH₄⁺-N and SRP during summer seasons for about six years, followed by a decreasing trend. The NH₄⁺-N concentration in the overlying water was reduced after the reduction of external loading, while a high concentration of SRP in the overlying water was still observed during summer seasons. In conclusion, the mid-term (<3 years) reduction of internal N and P loading could be achieved by dredging if the external pollution loading were not reduced. Achieving long-term control would require modification of external loading.

Application of plow-tillage as an innovative technique for eliminating overwintering cyanobacteria in eutrophic lake sediments

Surface sediment in eutrophic lakes is both a destination and a habitat for overwintering cyanobacteria. The resuspension and recovery of viable, overwintering cyanobacteria from the surface sediment during warm spring weather is usually the primary stage of cyanobacterial blooms (CBs) in shallow eutrophic lakes. Therefore, the elimination of overwintering cyanobacteria in sediment is vital to control CBs. In the present study, sediment plow-tillage (PT) was introduced as an innovative technique for eliminating overwintering cyanobacteria in sediments from Lake Chaohu. Four depths of PT (2, 5, 10, and 15 cm) were tested during the 42-day experiment. The results showed that rapid cell death during the first 0-7 d after PT was accompanied by high oxygen uptake rates. The viable cells in deeper sediment died more quickly and at a higher rate after PT. A PT depth of >10 cm effectively eliminated viable cyanobacteria (with a removal rate of 82.8%) from the sediment and prevented their resuspension. The activity of the viable cyanobacteria also decreased quickly as cyanobacteria were eliminated. It appears that the dark, anoxic environment of the deeper sediment after PT was responsible for the elimination of viable cells. Although high release rates of nitrogen and phosphorus were found to accompany the dying and decomposition of cyanobacteria during days 0-7 of the experiment, greater depth of PT was found to decrease nutrient concentrations in the overlying water. In conclusion, we recommend sediment PT as a new technique for eliminating overwintering algae in sediments. However, the release of nutrients from the sediment and the in situ control of CBs in lakes after PT should be further studied.

Effects of riverine suspended particulate matter on the post-dredging increase in internal phosphorus loading across the sediment-water interface

Dredging is frequently used in the river mouths of eutrophic lakes to reduce internal phosphorus (P) loading from the sediment. However, the accumulation of P-adsorbed suspended particulate matter (SPM) from the inflowing rivers negatively affects the post-dredging sediment-water interface and ultimately increases internal P loading. Here, a 360-d experiment was carried out to investigate the influence of riverine SPM on the efficacy of dredging in reducing internal P loading. SPM was added to dredged and undredged sediments collected from the confluence area of Lake Chaohu. Several parameters related to internal P loading, including oxygen profile, soluble reactive P, and ferrous iron across the sediment-water interface, organic matter, alkaline phosphatase activity, and P fractions, were measured throughout the experimental period. The results showed that the P content (especially mobile P) in the sediment increased to the pre-dredging level with the accumulation of SPM in the dredged sediment. In addition, the P flux across the sediment-water interface increased with the accumulation of SPM. Several characteristics of SPM, including high organic matter content, mobile P, high activity of alkaline phosphatase, and high biological activity, were considered correlated with the post-dredging increase in internal P loading. Overall, this study showed that the heavily contaminated riverine SPM regulates the long-term efficacy of dredging as a nutrient management option in the confluence area. Management is needed to avoid or reduce this phenomenon during dredging projects of this nature.