| Nansi Lake is the largest freshwater lake in North China and plays a significant role in such aspects as flood control,irrigation,fishery production,shipping,maintaining the ecological balance of the river basin and the South-to-North Water Diversion.Nansi Lake Basin densely populated,frequent economic activities,the resulting environmental pollution problems are more prominent.In 1960,the Erji dam and other water conservancy projects were completed and used to divide Nansi Lake into two major lakes,the Shangji and Xiaji lakes,which greatly changed the sedimentary environment of Nansi Lake.Therefore,it is an urgent issue to discuss the regularity of the migration and transformation of pollutants after the completion of the second dam and to reveal the extent and consequences of human activities on the basin environment.Based on the dating of radioactive isotopes(137Cs,241Am and 210Pb),the sedimentary age of sediments in Nansi Lake was determined by systematically arranging sampling points on both sides of the Erji dam of Nansi Lake.On this basis,the spatial and temporal evolution of environmental proxy indicators such as grain size,nutrient elements and heavy metal pollutants in Nansi Lake were analyzed.Analysis of human activities(especially the construction of water conservancy projects)on the impact of the degree of lake sedimentation for the rational development and utilization of Nansi Lake and provide a theoretical basis.The research has obtained the following achievements:1)137Cs caused by delayed input of eroded soils in Nansi Lake basin are the main causes of the 137Cs profile deviating from the globally accepted distribution pattern in each sedimentary column of Nansi Lake.It is difficult to determine the depositional age of sediments by 137Cs time-scale alone,Which needs to be combined with other methods of dating;as a useful time scale for the construction of secondary dam water conservancy project(1960),it is proved that the 210Pb chronological CRS model is applied to sedimentary dating of Nansi Lake with higher reliability.In 1960,the deposition of Nansi Lake was divided into two periods:before 1960,the sedimentation rate of Xiaji lake was higher than that of Shangji lake;from 1960 to 2010,the sedimentation rate of Shangji lake was higher than that of Xiaji lake.2)Nansi Lake sediment particle size distribution in the upper and lower lake there are differences.The Shangji lake>63 um grade fine sand and 16～63 um grade middle and coarse silt accounted for the average proportion higher than the Xiaji lake,and 4～16 um fine silt and<4 um grade clay average content Is lower than the Xiaji lake;average particle size(Mz)of the average,the Shangji lake is higher than the Xiaji lake.Through the comparison and analysis of the relationship between the average grain size of multi-sedimentary rocks and the annual precipitation in the Shangji and Xiaji reaches of Nansi Lake,the results show that in the study of environmental change on the centennial time scale,the thickness fluctuation of sediment particles accords with long time scale and low resolution the rate of change of particle size indicates the law of climate change.However,the abnormal increase of coarse particles could indicate that the watershed encountered an abnormal flood or even indicated a secondary rainstorm.The year of abnormal precipitation can be used as a useful time scale to verify the accuracy of 210PbexCRS chronological results.3)The average content of TOC,TN and C/N in the nutrient indicators(TOC,TN,C/N,TP)of Nansi Lake Shangji lake was higher than that of Xiaji lake,and the changes of TOC and TN were in good synchrony.The content of TP has increased rapidly since the 1980s.The increase of TOC and TN synchronously is mainly caused by the pollution caused by human activities in the basin,which is related to the drainage of agricultural production and industrial wastewater in the basin.4)The elemental geochemical characteristics of the sediments in Nansi Lake are analyzed and the history of the weathering of Nansi Lake Basin for more than 100 years is well reflected.The Nansi Lake CIA average of 65.7,equivalent to the loess,the average ’shale CIA value,in the middle chemical weathering stage.Elements such as CIA,Rb/Sr,Saf.Ba and other geochemical indicators can confirm each other,which better reflects the natural evolution history of the basin.5)The profiles of five heavy metals(Cr,Cu,Zn,As.Pb)were analyzed.The pollution of these five heavy metals was evaluated by using human activity coefficient and geo-accumulation index.The results showed that these five kinds of heavy metal elements were at the moderate to moderate pollution level;Shangji lake with Cr,Cu and Zn pollution>Xiaji lake,Xiaji lake with As and Pb pollution>Shangji lake.6)The human activities in the basin,especially the second-level dam constructed by mankind,are the important reasons that cause the differences of environmental proxy indexes of sediments in Nansi Lake. |
PDF Full Text Request