The Research Of The Sediment Pore Structure And Heavy Metal Pollution Assessment In Chaohu Lake

Since 1980, the water quality in Chaohu was deteriorated seriously. The sediments, served as a vital component of water environment, are the primary reservoir of heavy metals. When the environmental condition had changed, the pollutants in the sediments would have entered into the overlying water and caused endogenous pollution through diffusion, resuspension of the sediments or bioturbation and other ways. Contaminants in sediment transport and transformation processes are influenced by sediment microscopic physical structure. The quantification of sediment pore structure in three-dimension has important theoretical and practical value for understanding of pollutant transport and transformation processes and evaluate the risk of endogenous contamination. This study validate and improve a series of three-dimensional physical structure of sediments research method setting contains sample preparation method, data acquisition method, image interpretation method and parameter calculation method have been applied in Chaohu, combined with the analysis of sediment grain size and spatial distribution of heavy metals(Cu、Ni、Zn、Pb、Cr、Cd), as well as discussing the correlation between sediment physical pore structure and chemical contaminants(heavy metals) in Chaohu Lake. The results of the study are as follows:(1) In respect of physical structure, the constituents of the sediments’ pores in Chaohu Lake are not that different from each other, which by and large are comprised of clay and silt. Nonetheless, the different depositional condition and material source in each lake areas generated the difference of grain size parameters and the porosities of the overlying deposit are different. The connecting degree of the pore in the superficial sediments(which the deepness being within 1-2 cm differ) in the eastern lake area is higher than that in the middle or western part. The porosity is 0.75 in the eastern region and its interconnected pore volume is accounted for 67.47% of the whole, while the porosity in the western region is only 0.28. The pore curvature of the sediments varies from 2.83-9.31 and in addition the explicit otherness is shown in each direction. The sediments in the western region have bigger curvatures in the direction of Z, while those in the middle and eastern parts are having bigger curvatures in the direction of X and Y. Moreover, the volume of isolated pore, connectivity and other parameters looks much different. In vertical section, the porosity was decreased with the increase of depth. When the depth varies from 0-10 cm, the porosity has no obvious change which decreased from 0.51 to 0.44. But when it was greater than 10 cm, the rangeability enlarged markedly(the porosity being 0.01 at the depth of 17-18 cm).Those facts shows that the exchange between the sediments and overlying water material mainly centralize in the upper layer of the sediments. Yet the rangeability of S/V(the specific value of the isolated pore area and the total volume) is quite a little on the section, which distinctly is less than the change of it in its surface layer of sediments in different lakes. The isolated pore number, the volume and the superficial area have the tendency to decrease with the increase of depth. But the abnormity appeared at the depth of 13-14 cm, which might be effected by the benthonic animals and depositional condition at that time.(2) On the distribution of pollutants(heavy metal), besides Ni and Cr in the superficial layer of the sediments in Chaohu Lake, four heavy metals including Cu, Zn, Pb and Cd had exceeded background value in different degree, which is 1.40, 2.09, 1.37 and 4.46 times more than it. As the patchy distribution, heave metal pollution had a clear difference, which presented that the degree of the pollution were high in the east and west region and low in the middle region. In addition to the potential ecological hazard of heave medal was medium ecological risk, the RI(Potential Ecological Risk Index) value the other districts were all less than 150, which was a lower-potential ecological risk and wouldn’t pose a threat to the nearby environment in the short term. Sediment vertical profile data show that the overall content of heavy metals in sediments of Chaohu Lake all increased over time and the ecological risk of heavy metal trended to go up and the highest content was at the superficial layer of the sediments(0-5 cm).The order of the enrichment factors of the 6 heavy metal in the sediments is: Cd>Zn>Cu≈Pb>Cr>Ni. The enrichment factors of chemical element Cd were between 2.47- 7.71, description human input has become a major source of the elements Cd; the enrichments didn’t occur to the heavy elements Ni and Cr( the enrichment factor being less than 1.5), which were the natural resources; the elements Cu, Pb and Zn enrichments chiefly concentrated in the west lake region.Combing with the evaluation of the ecological risk, prevention and cure against the accumulated total of heavy metal Cd, Pb and Zn in the sediments in Chaohu Lake is the crucial goal to govern the heavy metal pollution in Chaohu Lake.(3)Chaohu sediment physical pore structure and chemical contaminants(heavy metals) correlation analysis found that there is a very notably positive correlation between the organic matter content in the sediments and porosity(P <0.01) while there is a highly significantly negative correlation between it and the curvature(P <0.01).It shows that with the increase of the organic matter content of the sediment will lead the porosity increase and curvature reduce, which availed the migration and transformation between the matter of the sediments and the overlying water; the constituents of different grain size has little effect to the porosity and the curvature. There is a significant positive between the content of Pb and the curvature in the sediments(P <0.05), probably because of the sediment with a large curvature in the vertical direction will inhibit the release of Pb overlying water upward.