Environmental Behavior And Ecological Risk Of Heavy Metals In Sediment Dredging And Hydraulic Reclaiming

Wenruitang River is the important coastal plain river network of Wenzhou City. With the fast economic development of Wenzhou City, the ecological environment of Wenruitang River has been seriously damaged, especially the water quality and sediment which was extremely polluted. In order to control the pollution, the sediment was dredged from the polluted river and transported to Linkun Shoal for land reclamation.With the support of the National Major Project (2009ZX07317-006), this dissertation took Wenruitang River as the research object and carried out research on the environmental behaviors of heavy metals in the whole process of sediment dredging and hydraulic reclamation, hopefully to provide a theoretical basis and technical support for the scientific assessment and reasonable planning of pollution control and resource disposal of polluted sediment. The main investigations of this research were as follows:1) monitoring physicochemical factors and heavy metal contents of surface water and sediment in the process of sediment dredging of Niuqiaodi River in Wenzhou City to determine the effect of dredging on water qulity and heavy metal removal of sediment;2) investigating temporal and spatial variation of heavy metal contents and speciations in core soil samples from sediment reclamation district in Linkun shoral and distribution of heavy metals in adjacent soils and plants to assess the effect of sediment reclamation on adjacent area;3)analysis of ecological risk of sediments in the process of dredging and hydraulic reclamation by the ecological risk assessment model of heavy metal and toxicity test of Luminescent bacteria and Xenopus tropicalis embryos;4) carring out research on the tolerance and phytoremediation of heavy metals by two dominant plants in reclamation district-Suaeda australis and Phragmites australis.The major results of this dissertation were listed as follows:(1) After the sediment dredging of Niuqiaodi River, the CODCr concentration in surface water was significantly reduced, while the DO content and transparency were enhanced. In different stages of dredging, the contents of NH4+-N, TN, TP and heavy metal in surface water were first increased, then decreased, while the contents of TP, TOC and heavy metal in sediment exhibited the opposite trend. The heavy metal contents in sediment reached the lowest level in one to three months after dredging, The contents of the pollutants in the river were significantly reduced after sediment dredging, but the improvement of water quality and sediment only lasts three to nine months. Therefore, the exogenous pollution control, endogenous pollution control and in situ remediation should be combined to achieve complete treatment of the river.(2) The acid-soluble fraction of heavy metals in sediments before dredging was Ni(34.8%)>Mn(33.7%)>Zn(30.6%)>Cd(16.7%)>Pb(13.4%)>Cu(9.8%)>Cr(4.4%), showing the potential hazards of Ni, Zn and Mn were relatively greater. After dredging, the oxidation reduction potential and the contents of organic matter and sulphide were decreasing, which resulted in the decrease of reducible fraction and oxidizable fraction and the increase of acid-soluble fraction, indicating greater potential ecological risk of heavy metals.The acid-soluble fraction of heavy metals was reduced again in9months after dredging.(3) The soils of sediment reclamation district and peripheral area were polluted by Cd and Hg. The contents of heavy metal Cd, Cr, Cu, Mn, Ni, Pb and Zn in soils of sediment reclamation were higher than that of peripheral area, with the contents of Cd, Hg, Cu, Zn and Ni higher than the second standard of soil environtmental quality. In general, the average contents of heavy metals in vertical direction decreased throughout the study from July2012to April2013, while the heavy metal contents in NO.4sample exhibited the most significant reduction, with the drop order of Hg(23.0%)>As(17.7%)>Cd(10.9%)>Zn(5.6%)>Mn(5.4%)>Cr(4.7%)>Cu(4.2%)>Pb(2.4%)>Ni(1.0%). The reduced heavy metals might spread to the adjacent soil with rainwater leaching solution and caused pollution to adjacent environment. The heavy metal contents in soils and plants of the adjacent area decreased with increasing distance from sediment reclamation district, while the heavy metals contents of the adjacent area were highest in25cm range of sediment reclamation district and near the drainage of remaining water. The bioavailable state of heavy metals in surface soils show a rise throughout the study from July2012to April2013, indicating greater ecological risk, while the oxidizable fraction of heavy metals in soils of40-60cm depth increased at the same time.(4) The evaluation results of the geoaccumulation index (Igeo) showed that heavy metal Cd, Zn and Pb in sediment of Niuqiaodi River before dredging belonged to serious pollution. The results of the potential ecological risk evaluation indicated the sediments of Niuqiaodi River were seriously polluted by heavy metals, mainly due to heavy metal Cd and Hg. The pollution index and ecological risk index both significantly decreased after the sediment dredging, with reduction of pollution levels of heavy metals in sediment. The soils of sediment reclamation district were seriously polluted, while most of sampling sites showed extremely strong ecological risk, mainly account for Cd, Hg and Cu. The geoaccumulation index and ecological risk index both illustrated that the pollution level and ecological risk of heavy metals in soils was sediment reclamation district> adjacent area> peripheral area, which was attributed to the pollution of sediment reclamation.(5) The toxicity test of Luminescent bacteria showed the surface water before sediment dredging belonged to medium toxicity, while the sediment belonged to great toxicity. Both the Luminescent bacteria and Xenopus tropicalis embryos tests indicated the toxicity of surface water and sediment firstly weakened, then enhanced in different stages of dredging. Generally speaking, sediment dredging were helpful for alleviating toxicity of surface water and sediment. The relative luminous intensity of sediment in different stages of dredging increased by11.7%-18.9%after the addition of EDTA for chelating heavy metals, while the survival rate of Xenopus tropicalis embryos increased from14.8%-50.0%to27.4%-65.5%, indicating the addition of EDTA reduced the toxicity of sediment. The soils of sediment reclamation district mainly belonged to medium toxicity. The soils of adjacent area belonged to slight toxicity, while the soils of peripheral area exhibited no toxicity. According to the toxicity test of Xenopus tropicalis embryos against soil leaching, the average hatching rate, survival rate, teratogenic rate and length was86.5%,80.4%,13.2%and3.92mm, respectively. The toxicity of sediment reclamation district was greater than peripheral area.(6) The results of pot experiment indicated Suaeda australis showed more vigorous growth than Phragmites australis under the treatment of low concentration of heavy metals. But Phragmites australis exhibited stronger tolerance to high concentration of heavy metals. The removal of heavy metals by Suaeda australis and Phragmites australis increased with the decreasing sediment ratio. Suaeda australis and Phragmites australis showed better removal of Cd, with removal rate of21.2%and25.2%respectively in the control group. The two plants had a positive effect on the bioavailability of Cu. The accumulation of heavy metals by two plants increased with the increasing sediment ratio, while Cd was mainly distributed in leaves of Suaeda australis and roots of Phragmites australis, with the maximum accumulation of70.96mg/kg and133.12mg/kg respectively. The bioaccumulation factors of two plants reached the highest value in the control group, while the two plants had stronger accumulation ability of Cd and Cu. The transportation factors (TF) of Suaeda australis for Cd and Zn and TF of Phragmites australis for Cr were higher than one.