| Dilution and sediment dredging have been widely usedin eutrophication waterbody remediation. Dilution and flushing refer to diluting or flushing the eutrophicwater with which has a lower nutrient concentration. Sediment dredging is defined asremoving the upper sediment which contains a large amount of nutrient, reducing theinternal load, and remediating the eutrophication lakes.Although dilution anddredging can improve the water eutrophication situation rapidly, the short-term andlong-term effect of them on nutrient distribution, release and bioavailability of heavymetals in the sediment are still unknown. Towards these problems, the verticaldistributions and seasonaldynamics of total organic carbon (TOC), total nitrogen (TN),total phosphorus (TP), acid volatile sulfides (AVS), and simultaneously extractedmetals (SEM) of sediment in Donghu Lake were investigatedbefore and after theconnecting project.Besides, the verticaldynamicsof nutrient and the fraction ofphosphorus in the sedimentsof Shuangqiao River before and after dredging projectwere also studied. The main results were summarized as follows:The response of pollutants in the sediment to water body disturbance has alag.The content of pollutants in the sediment of Donghu Lake did not decrease rapidlyafter the connecting project finished. The TOC and TN contents in the sediments ofShuiguohu and Guozhenghu decreased significantly(P<0.05) compared with thosebefore the connecting project. The TP content in Shuiguohu decreasedsignificantly(P<0.05), while increased slightly in Guozhenghu which means therising trend of TPis under control. It demonstrated that Donghu and Shahu connectingproject was beneficial to reduce the internal nutrient load in the sediment and curb therising trend.The content of nutrients in Shuiguohu is highest, followed by Tanglinhuand the connecting site, and Guozhenghu is the lowest.The value of AVS increased with depth at the upper sediment and then decreased,and were relatively higher in summer, which was similar to those before theconnecting project. There was no seasonal significant difference in the sediments ofShuiguohu and Guozhenghu after the connecting project(P>0.05), while the AVSvalue showed a significant difference between summer and winter in Tanglinhu (P<0.05). Meanwhile, the AVS value of Guozhenghu and Tanglinhu have a similartrend with those before the connecting project, and the maximum of AVS decreased42.8%and30.4%, respectively. The SEM value showed a trend of decrease with thedepth increased and was much higher in summer. The SEM/AVS value shows a trendof rise first and then reduceswith the depth increased, and the SEM/AVS value inShuiguohu is higher in summer, while is much higher in winter in Guozhenghu andTanglinhu. The SEM/AVS value was much higher in the upper sediment than those of1999, while was much lower in depth of10to20cm. While the AVS/SEM value inGuozhenghu and Tanglinhu were lower than before. It indicated that the bioactivity ofheavy metal in Shuiguohu was higher than before, while was lower in Guozhenghuand Tanglinhu, which means should think twice when usingdilution to restore aeutrophic lake.It is possible to reduce the internal load by dredging in a short time inShuangqiao River. The nutrient content in river sediment rises again two years afterthe dredging due to the external load input is still there and the residue of aquaticorganisms accumulation in the sediment. The labile phosphorus in the surfacesediment is much higher than before, but the absolute value is still very small. Ca-Pand OP took up the largest proportion of the total phosphorus, accounts for about60%and20%, respectively. Two years after the dredging, Lab-P did not changesignificantly with depth, and Al-P content in deep sediment was stable. A surfaceenrichment phenomenon of Fe-P was observed, which means potential risks ofinternal nutrients releasecaused by dredging. It was suggested that the effective wayto improve the water quality should combine dredging with external source pollutioncontrol as a whole. |
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