Water Environment Quality Of Rivers Flowing Into The South Four Lakes And Its Response To The Spatial Pattern Of Land Use In The Basi

Nansihu Lake is the largest freshwater lake in north China,which receives water from more than 30 counties and cities in shandong,Jiangsu,Henan and Anhui provinces,and it is an important transfer station for the eastern route of South-to-North Water Diversion Project,the largest commodity grain base and freshwater fishery base in Shandong Province.With the development of the urbanization process in the basin,the urban construction land area is increasing in recent years,and leads to the decrease of cultivated land area.The amount of nitrogen,phosphorus fertilizer and pesticides is increasing to maintain the total grain,and the unused materials are washed into rivers by rainfall runoff and eventually into lakes,causing serious non-point source pollution in the water environment of Nansihu Lake.At the same time,Nansihu Lake basin also contains rich mineral resources,the heavy metal pollution problem is prominent in Nansihu Lake for unreasonable development and utilization.This study is divided into three sampling periods,and namely dry season,normal season and wet season,to investigate the environmental pollution status of the inlet water of 51 major rivers in Nansihu Lake through field monitoring and indoor analysis.There are three evaluation methods,namely single factor index method,comprehensive water quality index method and comprehensive label index method,are used to comprehensively evaluate the 18 water quality indexes investigated.The responses of water environmental quality to land use and landscape patterns at different spatial scales were analyzed in order to provide theoretical and data support for the water environment protection of Nansihu Lake basin by using Correlation Analysis and Redundancy Analysis（RDA）.The main conclusions are as follows:（1）According to the Environmental Quality Standard for Surface Water（GB3838-2002）,it was found that there were 5 key water quality indexes that affected the deterioration of water environmental quality of the rivers entering Nansihu Lake.They are Total Nitrogen（TN）,Total Phosphorus（TP）,Chemical Oxygen Demand（COD）,Ammonia Nitrogen（NH₃-N）and heavy metal Nickel（Ni）,among which TN and Ni are the main water quality indexes that cause water quality deterioration in most rivers entering the lake.At present,however,there are serious pollution problems of surface water in the inflow rivers of Nansihu Lake for TN,TP,COD,NH₃-N and Ni,the water quality evaluation shows that the overall water quality is good,and can be reaching the classⅢwater standard.According to the evaluation results of water quality comprehensive index method and comprehensive identification index method,the water quality in normal season is the best,followed by dry season and wet season.（2）The variation trend of the average mass concentration of the 18 water quality indexes in the three sampling periods shows two situations:one is dry season?normal season?wet season,mainly including TN,NO₃-N,Zn,Mn;the other is wet season?dry season?normal season,mainly including TP,NH₃-N,COD,Cu,Fe,Pb,Cr.（3）The analysis results of land use in Nansihu Lake basin,2020 shows that the basin is a typical agricultural basin,and the arable land area accounts for more than 75%of the total area,and the area of construction land,forest and grassland accounted for21.02%,0.45%,0.72%,respectively.And the unused land area was the smallest,accounting for only 0.01%of the total area,indicating that the overall land development and utilization degree of Nansihu Lake basin was high,and the whole basin was strongly affected by human activities.（4）The analysis results of landscape pattern index at different spatial scales shows that the landscape fragmentation degree of 100 m and 300 m buffer zone is large,the physical connectivity between patches is poor,and the agglomeration degree between different types of patches is low.At sub-watershed scale,the landscape fragmentation degree is low,the physical connectivity between patches is good,the agglomeration degree between different patches is high,and the landscape structure is reasonable.（5）The correlation analysis between water quality and land use status in the surrounding buffer zone shows that there is a significant negative correlation between arable land and TP at 300 m and 500 m buffer scale（p<0.05）,indicating that arable land blocks the input of phosphorus pollutants in the rivers at 300 m and 500 m buffer scale.At the 1 km buffer scale,arable land has a significant positive correlation with Ni（p<0.05）,indicating that arable land has a positive effect on Ni input at the 1 km buffer scale.As the area with the most frequent human activities,the 300 m,500 m and 1 km buffer scale of construction land in the estuary of the lake has a significant positive correlation with water quality indexes such as TP,NH₃-N,COD and Ni（p<0.05）,indicating that within the buffer zone of 1 km and below,construction land is the main type of land use that causes water pollution of the rivers in the Nansihu Lake basin.The frequency of forestland and grassland is low in the buffer zone of 1km and below in the estuary of the lake,and have little effect on water environmental quality.However,at the sub-watershed scale,there is a significant negative correlation between grassland and unused land with Ni（p<0.05）,indicating that grassland and unused land blocked Ni input at the sub-watershed scale.（6）The RDA analysis of landscape pattern index and water quality index at different spatial scales shows that LPI,CONTAG and COHESION have higher explanatory ability to TN,TP,NH₃-N and Ni at 300 m,500 m and 1 km buffer scales at the estuary of the lake,and the results show that the area proportion of the largest patches,the degree of aggregation of different patches and the physical connectivity between patches promoted the input of nitrogen and phosphorus pollutants and heavy metal Ni in rivers.At the sub-watershed scale,SHDI has higher explanatory ability to NO₃-N and TN in dry season,COHESION has higher explanatory ability to COD and NH₃-N,and CONTAG and LPI have higher explanatory ability to TP in wet season,indicating that the heterogeneity of various patch types promotes the input of nitrogen pollutants in rivers in dry season,the physical connectivity between patches promoted the input of nitrogen and organic pollutants,and the aggregation degree between different patches and the complexity of landscape shape promoted the input of phosphorus pollutants.（7）The results of correlation analysis and RDA analysis show that the 1 km buffer scale is the spatial scale that has the greatest impact on the water environmental quality of the rivers in the Nansihu Lake basin.Therefore,1 km buffer scales at the estuary of the lake,within the scope of the condition of land use and landscape pattern is the key of the Nansihu lake water environment quality impact factors,and we should focus on into the lake from the estuary area of 1 km buffer scales to management and protection the water quality of lakes,and set up vegetation buffer,construction into the lake in the estuary wetland ecosystem,and intercept or purification of pollutants to protect the water environment of lakes.