Study On The Distribution, Evaluation And Evolution Of Nutrients And Heavy Metals In Hongze Lake Sediments

Sediment samples of Hongze Lake at 10 representative sampling sites were collected In June, 2008. Sediment nutrients (nitrogen, phosphorus, organic matter) and heavy metals (Cu, Zn, Pb, Cd, Fe, Al,Cr, Hg, Mn, As) content were measured, the patterns of nitrogen and phosphorus in the surface sediments from all sampling sites were also determined. The spatial distribution of heavy metals and nutrients were analyzed and their pollution assessment were carried out. Three core sediment samles (one was near the Huaihe River inlet of Hongze Lake, one was western and the other came from the lake center) were selected and their contaminant vertical distribution characteristics, resources and influencing factors were analyzed. The result showed that:The accumulation of nutrients and heavy metals in the surface sediments of Hongze Lake belonged to a typical flood washed type stacking mode (Turbidity Flood Model).Organic matter and total nitrogen in surface sediments had a very good correlation, and the organic nitrogen accounts 90% of total nitrogen, indicating that the carbon and nitrogen are homology; the inorganic phosphorus in sediment accounted about 70% of total phosphorus, in which calcium - phosphorus accounted a larger proportion of inorganic phosphorus; the spatial distribution of organic matter in surface sediment was very similar to total nitrogen, organic nitrogen, and total phosphorus, the C/N rate showed that the main source of organic matter in sediments should be biological settlement. Aaccording to organic matter index evaluation standards and organic nitrogen evaluation standards, the pollution levels of sediment at each sampling sites in the lake were classified as clean category, except that at north part of Longji, which was polluted by organic nitrogen.Influenced by the hydodynamical and topographical condition, the contents of heavy metals in the inflow river mouth areas were significantly lower than that of open water area in the eastern part of the lake, and that of bays and outflow river mouth areas. These heavy metals were highly positively related with each other, indicating that they were from the same pollution source. The geo-accumulation index (Igeo) and the Hakanson potential ecological risk index were applied for assessing the status of sediment heavy metal enrichment and the extent of potential ecological risk. The pollution extent of heavy metals followed the order: Cd>As>Cu>Cr>Zn>Hg>Pb, while the single potential ecological risk of heavy metals followed the order: Cd>As>Hg>Cu>Pb>Cr>Zn. The pollution extent and potential ecological risk of Cd were most serious than those of other heavy metals. The distribution pattern of Cd individual potential ecological risk indices was exactly the same as that of general potential ecological risk indices for all heavy metals, indicating the important contribution of Cd in the general indices. The analyse results showed that heavy metal pollution and the potential ecological risk existed in three risk regions: the central open water area and outflow river mouth region (S9, S1 and S8), the northern bay region (S6) and the western bay region (S5).The sources and factors of core sediment samples indexs were discussed, which revealed the nutrients and nutrient evolution rule and pollution history. Huaihe River estuary sediment environment influenced more by the Huaihe River Basin, and nutrients had no change potential, Zn, Cd, Al, Fe and Mn affected less than the rest heavy metals by human activities,.Western Lake sediment environment was affected by the Huaihe River, the inlet river and the cities surrounding western lake. The pollution degree of TN, OM and heavy metal elements was exacerbated since the late 1970s, but the contamination situation was improved since the 1990s. As and Cr were influenced by human activities greater than rest heavy metals. Main pollution source of the lake center were clastic material. The correlation of nutrients and heavy metals was obvious, and their vertical distribution was similar with the content raised from the bottom to the surface.Different nutrients of the core sediment samples had good correlation. The majority of organic matters came from zooplankton and phytoplankton. The natural sources of heavy metals were relatively stable, and the disturbance of human activity may be the main reason for fluctuations in vertical distribution of heavy metals. Nutrients and heavy metals pollution had the consistent characteristics with close relationship with the river basin development stage.The sediment environment in Hongze Lake was fitted in Turbidity Flood Model. This project was supported by the key research programme of Ministry of Environemntal Protection, guaranteeing scheme for ecological safty of the important lakes and reservoirs in China(WFLY-2009-1-01,WFLY-2009-1-04).