Pollution Characteristics And Risk Of Persistent Toxic Substances In The Sediments From Northern Taihu Lake

Since reform and opening up, with the rapid development of industry and agriculture and the increase of population of Taihu lake watershed, the water consumption and wastewater discharged from industries increased year by year. Meanwhile, water loss and soil erosion made the agricultural chemical and fertilizer into the Lake region, causing significant water quality pollution, especially in Zhushan, Meiliang and Gonghu Bay located at northwest of Taihu Lake. The emission of pollutant brought about deposit accumulation and more pollution. However, when the hydrodynamic condition(wave, lake flow, transport and bioturbation) changed, the pollutant in sediment released into water body, that leaded to secondary pollution of the water quality directly. Therefore, after taking effectual control of external contamination, it seems more important to study the internal contamination of Taihu lake watershed. In our research, according to the water quality and function of Bay located at North of Taihu lake, taking the middle point of the lake as contrast, we selected the Zhushan, Meiliang, Gonghu bay and mid-lake region as our study object to analyze the heavy metal(Cd, Cr, Cu, Ni, Pb and Zn) and polycyclic aromatic hydrocarbons(PAHs, 16 species). Furthermore, the pollution characteristics of Persistent Toxic Substances(PTS) in sediment were analyzed in order to assess their ecological risk. Additionally, the vertical distribution, deposition flux distribution and source apportionment were studied to understand the historical emission. What’s more, in order to estimate the internal release of the heavy metals, the diffuse fluxes of heavy metals at sediment-water interface were also listed in our study. The following conclusions have been made:(1)There was a certain degree of enrichment of heavy metals at north Taihu lake and mid-lake region, indicting slight ecological risk. The average concentrations of Cd, Cr, Cu, Ni, Pb and Zn were 0.34, 89.33, 36.53, 49.29, 15.20 and 87.86 mg?kg-1, respectively, which were 1.3, 1.1, 1.9, 1.5, 0.9 and 1.5 times of the background level of Taihu sediment. From the spatial distribution, the average concentrations of heavy metals in surface sediment had the following trend: Zhushan Bay> Meiliang Bay> Gonghu Bay> mid-lake region. 1980 years ago, the concentrations and deposition fluxes of heavy metals almost unchanged along with time. Since 1980, the concentrations and deposition fluxes of heavy metals increased firstly and then decreased, and peaked in 2004 and 2000, respectively. The enrichment coefficient indicated that there was a difference in the enrichment of Ni and Cu while there was no enrichment phenomenon of Cd, Cr, Pb and Zn. Associated with the principal component analysis(PCA), Cd, Cr, Cu, Ni and Zn were probably derived from the discharge of industrial wastewater and domestic sewage. The ecological risk degree of 6 heavy metals was: Cd>Ni>Cu>Pb>Cr>Zn, and none of them had the ecological risk. The RI of heavy metals showed that Taihu lake was at a low ecological risk in the whole. However, the Zhushan Bay was at a moderate ecological risk. 1980 years ago, the ecological risk almost unchanged along with time. Since 1980, the ecological risk had a gradual increased trend, and peaked in 2000, then experienced a decrease. Among all the heavy metals, the contribution of Cd was the largest, reaching the stronger ecological risk.(2)In the sediment-water interface of Bays in northern Taihu lake(Zhushan Bay,Meiliang Bay and Gonghu Bay) and mid-lake region, the heavy metals of As, Cd, Cr, Cu, Ni, Pb and Zn all had a release trend from sediment pore water to overlying water, laying a potential release risk. The average concentrations of As, Cd, Cr, Cu, Pb and Zn above-mentioned in the overlying water of 0-20 cm above the sediment-water interface of in Bays northern Taihu lake and mid-lake region were 5.11, 3.19, 13.86, 14.25, 12.12 and 62.66 ug ? L-1, respectively. Which were in accordance with the environmental quality standards for surface water(GB3838-2002) class III water quality standard. The concentration of heavy metals in pore water fluctuated with the variation of the deep of these Bays. The diffuse fluxes of sediment-water interface of Taihu lake demonstrated that Cd, Ni and Zn in Zhushan Bay had a released characteristic from overlying water to sediment pore water, functioning as the sink. Among which the diffuse flux of Zn was the highest, could be reached to 11.65 ug?m-2?d-1. At the same time, As, Cd, Cr, Cu, Ni, Pb and Zn in Meiling, Gonghu Bays and mid-lake region all had the internal release trend, posing a certain ecological risk. The IWCTU value of surface sediment pore water demonstrated that Cd, Cu and Pb had potential toxicity to biology. The average value of NI index was 3.06, which showed that there was strong pollution of the metals in surface sediment pore water.(3)The polycyclic aromatic hydrocarbon(PAHs) in northern Bays of Taihu lake and mid-lake region sediment were at a light pollution level, so that there was a low ecological risk. The total concentration of 16 PAHs in surface sediment of Taihu lake ranged from 356.11 to 880.61 ng·g-1, with an average concentration of 548.67 ng·g-1, indicting a light pollution level. Omong the 16 PAHs, middle and high ring PAHs dominated, Furthermore, Benzo [b] fluoranthene, Benzo [k] fluoranthene, Philippines, Fluoranthene and Pyrene accounted for 68% of the total PAHs. From the spatial distribution, the average concentrations of PAHs in surface sediment had the following trend: Zhushan Bay> Gonghu Bay> Meiliang Bay> mid-lake region. 1980 years ago, the concentrations and deposition fluxes of PAHs almost unchanged along with time. The concentration and deposition fluxes of PAHs in sediment increased firstly and then decreased after the year of 1980, and reached the maximum in 2000. The PAHs in surface sediment of Taihu lake were maily derived from wood and coal burning, gasoline and diesel combustion and coke incineration. Although PAHs in surface sediment of Taihu lake were at low ecological risk, the average TEQBaP of PAHs was 52.82 ng·g-1, suggesting a certain potential toxicity.