Changchun South Lake Sediment Shu Ling Engineering Effect

Environmental effects of sediment dredging events have been uncommonly reported for shallow lake in China. A project of bottom sediment dredging has been carried at the Lake Nanhu since 2000 in order to control its eutrophication. During the period from May to October monthly in 2002～2003, the characteristic of the water physical-chemical, the plankton, zoobenthos and the pollutants in sediment had been investigated from 4 sample sites. The objective of this paper was to determine the effects of hydraulic sediment dredging at the Lake Nanhu prior to and after dredging. The effects on surface water pH and Secchi disk depth (SD) were negligible but dissolved oxygen (DO) and chemical oxygen demand by K2Cr2O7 method (CODcr) was increased. While total nitrogen (TN), ammonia nitrogen (NH4-N), nitrate nitrogen (NO3-N), nitrite nitrogen (NO2-N), total phosphorus (TP) and biological oxygen demand for five days (BOD5) were decreased. Factor Analysis of water quality indicated that the first factors were TP and CODcr, the second were TN and NH4-N prior to dredging. However, the first factors were pH value and BOD5, the second were suspended solids (SS) after dredging. The result of Paired Sample T test prior to and after dredging showed that the water quality had not been changed significantly.After dredging, the species number of phytoplankton was 285 and 334 respectively, between 2002 and 2003. The dominated species was stable in 2002-2003. The phytoplankton community took succession from chlorophyta-cyanophyta-diatom of prior to dredging to chlorophyta-diatom-cyanophyta of post dredging. The species numbers of zooplankton were 134 and 117 respectively, between 2002 and 2003. The dominated species was stable in 2002-2003. The zooplankton community changed. The zooplankton community was mainly consisted of protozoa and rotifer, and the number of cladocera and copepoda were less than 1 percent. The phenomena, which the number of zooplankton was up and the biomass was down, demonstrated miniaturization trend of the zooplankton was obvious. The sediment dredging had significant impact on mollusca. The Margalef Index of benthic animal community decreased from 3.45 to 1.87. Investigation of the sediment showed that the mean concentrations of organic carbon and the mean total nitrogen decreased 17 percent and 36 percent respectively, compared with prior to sediment dredging. But the concentrations total phosphorus only decreased 3.33 percent. The discrepancy of total phosphorus between different sample sites was significant. The mean concentrations of TP sample site I was significant higher than that of three other's. The concentrations of the Cd in sediment decreased than that of prior to dredging. While the concentrations of Zn and Cu increased than that of prior to dredging. The cause attributed to the point source situated in sample site I. The heavy metal in sediment contamination factors were very high for Zn (18.15), Hg (6.67), Pb (5.64) and considerable for Cd (2.6), Cu (2.54), Ni (1.72), C r (1.02), As (0.098). C r and As The pollution of Zn was most serious in all eight heavy metals analyzed, its concentration did not meet Soil Environmental Quality Standard(GB15618-1995).