Analysis On The Trend And Driving Forces Of Water Quality In Hongze Lake,Jiangsu Province

Hongze Lake is located in Huai’an and Suqian cities in the northern part of Jiangsu Province(33°06′～33°40′N,118°10′～118°52′E).It is the largest lake in the Huai River Basin,an important industrial,agricultural and living source of water,and also the hub of the East Route of the South-to-North Water Diversion Project.However,with the development of industrial and agricultural production and population growth,the loading of industrial and domestic sewage is increasing,coupled with reclamation,aqualcture,and sedimentation,the environmental capacity of the lake area is continuously reduced,and water pollution is increasing.There have been many pollution accidents in the lake for a long time.Several researchers,relevant departments and scholars have done a lot of work on Hongze Lake,but they lack in-depth and long-term research and analysis.This thesis examined the trends and drivers for the long-term dynamics of the water quality of Hongze Lake,based on Ensemble Empirical Mode Decomposition(EEMD),MannKendall(M-K)test,generalized additive models(GAM),Construction of water quality index(WQI)of rivers entering Hongze Lake,pollutant flux calculations,combined with data of temperature,precipitation,hydrology,water level and water quality of rivers and lakes entering the lake from 2008 to 2018 in Hongze Lake.The main results as follow:1)From 2008 to 2018,the annual average temperature in Hongze Lake region was15.29℃,and the average cumulative annual precipitation was 989.08 mm.The annual average water level in Jiangba station was 12.95 m,and the annual average runoff of Wujiadu was 22.434 billion m~3.The linear fitting results showed that the temperature,precipitation,run off and water level of Hongze Lake had increased.The M-K test results showed that the upward trend of monthly average temperature,cumulative month precipitation,and Jiangba water were not significant.In the time series decomposition(EEMD)results,temperature’s fluctuation period is 12 months;while cumulative monthly precipitation’s fluctuation period is 4 or 12 months.Water level’s fluctuation cycle is 4 or 12 months,and runoff’s fluctuation cycle is 4 or 8 months.2)The average concentration of dissolved oxygen in rivers entering Hongze Lake was 8.15 mg/L(Ⅰ),the average concentration of permanganate index was 5.78 mg/L(Ⅲ),and the average concentration of ammonia nitrogen was 1.53 mg/L(Ⅴ).The average concentration of nitrogen was 3.57 mg/L,the average concentration of total phosphorus was 0.23 mg/L(IV).The M-K trend test method showed that 83.3% of the rivers entering lake had a significant decline in the permanganate index(Z<0 p<0.05),44.4% of the rivers had a significant decline in ammonia nitrogen concentration,and the proportion of rivers with a significant decline in total nitrogen and phosphorus were 33.3% and 27.8%,respectively;while 44.4% of rivers had a significant upward trend in WQI(Z>0,p<0.05).The estimation of the long-term total nitrogen and total phosphorus in the Huaihe River showed that they are closely related to the runoff’s changes,which was obvious.Correspondingly with the amount of water,the nutrient fluxes of the Huaihe River entering lake have increased since 2013.3)The multi-year average value of dissolved oxygen concentration in Hongze Lake was 8.95 mg/L(Ⅰ),the average permanganate index concentration was 4.33 mg/L(Ⅲ),and the average concentration of ammonia nitrogen was 0.25 mg/L(Ⅱ).The average concentration of nitrogen was 1.72 mg/L(Ⅴ),the average concentration of total phosphorus was 0.08 mg/L(IV),and the average concentration of chlorophyll a was 0.008mg/L.As a result of M-K test,the overall changes of total nitrogen and total phosphorus in Chengzi Lake,Lihe Basin and the water-passing area were not obvious.Correlation analysis results showed that the water quality of Hongze Lake and the water quality of rivers entering lake had a high correlation coefficient and strong correlation.Different lake areas were affected by changes in the water quality of rivers entering the lake.4)The chlorophyll a concentration of Hongze Lake decreased significantly from2011 to 2018.Mutation points where the chlorophyll of Hongze Lake dropped significantly were concentrated in 2014-2015.The GAM model simulated the relationship between chlorophyll a concentration and water quality indexs,hydrological and meteorological factors in different lake reachs.The comparison found that the factors that affect the change of chlorophyll a concentration in each lake reachs were slightly different,and similarly,they were affected by the concentration of permanganate index.