| Lake sediment recorded pollution history, human impact and environmental changes. Sediment interacts tightly and strongly with the water column. It is vital and significant to study on the pollution status and dynamic of sediment and further to assess the effect of internal pollution on water quality and predict water quality change and eutrophication. Based on large literature, the pollution characteristics of nutrients and heavy metals in sediments in the upperstream and downstream Nansi Lake, potential release risk, environmental behaviour, release rule and pattern, interaction between sediment-water interface and corresponding water quality response related the sediments were studied comprehensively and systimatically by the means of field monitoring, laboratory experiment analysis and dynamic modeling and multivariate statistical analysis. The main results and concusion were listed as follow:1. Nutrient accumulation degree were discussed by the vertical profile of sediment nutrients, and the results showed that Nansi Lake suffered quite human impact in recent years, nitrogen (N), phosphorus (P) and organic matters (OM) accumulated constantly, and the contents range were 925.66～4695.82mg/kg,176.37～1281.37 mg/kg and 1.21%～16.65%, respectively. Surface sediment total phosphorus (TP) accumulation in inflows was heavier than that in lake area. In the lake sediment, TP load followed the order: Nanyang Lake>Dushan Lake>Weishan Lake>Zhaoyang Lake.2. P speciation was determined by the sequential extaction method to evalue the mobility, bioavailability and potential release risk. The results indicated that inorganic phosphorus was predominant both in the upperstream and the downstream lake, while, organic phosphorus just accounted for 3.98%～27.59% of total phosphorus. Among inorganic phosphorus, reactive P, like Ex-P and Fe-P, was predominant speciation, with a lare percentage, and was high bioavailable, so it is extremely susceptible to release into water column and brought large risk to environment.3. The correlation between phosphorus speciations suggested that, except for De-P, other phosphorus forms showed a significant correlation with total phosphorus, especially for Fe-P and Ex-P, displaying an extremely significant correlation. The change of bioavailable P accorded with the increase of TP in the sediment due to human activities. Fe-P manifested a quite high correlation with Ex-P, Oc-P, ACa-P and Or-P. except De-P, other P forms showed a good correlation with sediment OM, especially, Or-P, is highly related to OM.4. Comparation the TP contents of sediment and water of winter and thoseof summer at 12 monitoring sites in the downstream Mansi Lak, the results showed that water TP tended to increase in summer, however, most sediment TP decreased in summer, indicating part of P was susceptible to release in summer. Inaddition, the downstream lake had a relatively low sediment TP load, but a high water TP load, suggestin a moer stronger exchange among pollutants in sediment and water.The analysis revealed that there was no correlation between sediment TP and water TP. Water quality response to the sediment P pollution is not reflected by the sediment P content, but, in most extent, dependent on the exchange and interaction between the solid and the liquid phase, ie., sediment and water, as well as the status of sediment and water.5. In order to clarifying the exchange and interaction of pollutant between sediment and water, sediment-water interface dynamic modeling experiment was performed, which, to some degree, reflected the water response to sediment pollution when environmental condition varied, and can further to determine the exchange rate and flux in certain time. The results indicated that P release can occure in both anerobic and oxic condition, the release rate and flux of TP and SRP in anerobic condition were higher than those in oxic condition. Sediment tended to adsorb P in oxic condition, while, when the overlying water P is quite low, sediment can relese P into overlying water. Sediment was more susceptible to release P in anerrobic status.6. The kinetics and isotherm of adsorption/desorption can well decriblethe capacity of adsorption/desorption, and further to predict the exchange dymamic and rule. The results showed that Pseudo-first and Pseudo-second order rate equation can well describe the adsorption and release course, however, the pseudo-second-order rate model was more proper to elucidate sediment P sorption and release because adsorption/adsorption capacity predicted by the pseudo-second-order rate was more close to the measured results Pseudo. It suggested that the adsorption and release of P were chemically rate controlled, and the rate depended on the sorption capacity rather than the concentration of the sorbate. The results indicated that the P sorption isotherms on three sediment samples were better described by the modified Langmuir model than by Freundlich or the linear model because of its higher R2 value, which suggested that the adsorption process was single molecular layer adsorption within a low concentration range, and the maximum adsorption contents could be determined to assess the capacity of adsorption and release.7. The EPCo method can effectively predict sediment status. At the sample site S1, S3, S4, S5, and S7, i.e. at the sites Baima, Xinxue, Xiaoni, Shizi River and Dajuan, EPC0> SRP, the sediments had potential to release P to the water column. These sites need continual concern; while at S2, S6, S8, and S11, i.e. at the sites Dajuanwa, Aihu and North of Weishan Island, EPC0 SRP, the bed sediment and water were approximately in equilibrium with respect to SRP, and it was likely to release P into water, therefore, these site need to be monitored further.8. EFs for all metals from each sampled site can determinate the enrichment degree and the anthropogenic metals. According to the EFs, Cd suffered the most heavy accumulation, up to dozens of times of background value;then, Zn, Cu and Pb showed a medium pollution, about 2-6 times background values; followed by Cr and Mn, were slightly higher than background values. Last, Fe was just close to its background value.the upperstream lake suffered more heavily pollution of metas than the downstream did. In the upper reaches, the most serious pollution in lake area was founded in Dushan Lake, while, in the lower reaches, found in Aihu Lake. As for inflows, Guangfu River and Shizi Rigver were the mos large pollution spots.9. Compared the mean of total metal the Nansi Lake sediments with LEL and the SEL, the result indicates that Nansi Lake sediments can be severely impacted by Cr and Zn, meanwhile, the impact from Cd, Cu, Pb and Mn may be considered moderate, while, there is just very low impact from Fe. Based on the above analysis from metal distribution, in the whole, the upstream lake suffered more severe pollution from heavy metal. This is in accordance with the more industries, more developed cities and such high-dense population in the upstream.10. Comparing the percentages of 7 heavy metals contributed to different risk levels from 26 sites in Nansi Lake with the Risk Assessment Code (RAC) was used to assess the potential risk of sediment metals. According to the RAC, there was considerable enrichment of metals at inflows and lake area. Amongst the different metals, Cd concentration was the lowest but a major portion (30%-65%) was contained in mobile fraction (either exchangeable or carbonate bound), therefore, it was of great risk to enter the food chain. About 46%-50% of Pb in sediment existed in exchangeable or carbonate fraction and thus exert a potential risk for the aquatic environment. Followed this, Cr and Zn were relatively easy to mobile. Most Cu and Mg were in immobile fraction in Nansi Lake, just little part was found in carbonate fraction thus posing medium risk for the aquatic environment. Fe concentration was the highest but with lowest enrichment, thus showed only low risk for the aquatic environment. The potential risk of these 7 metals followed the order:Cd>Pb>Cr>Zn>Cu>Mn>Fe.11. The kinetics of heavy metal adsorption/release can explain the capacity of adsorption and release. The study results indicated that Pseudo-second-order model can more better fit the kinetics of heavy metal adsorption/release than Pseudo-first-order model and Elovich model。The capacity of desorption of five heavy metals followed in the order:Cr>Cu>Pb>Zn>Cd. |
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