Nitrogen accumulation in a big lake along wetland evolution affected by groundwater and surface water interaction in central Yangtze River Basin

Nitrogen pollution is an important cause of lake eutrophication, and the deterioration of lake water quality caused by nitrogen pollution has attracted wide attention worldwide. Honghu Lake is a large lake located in the middle and lower reaches of the Yangtze River in China, which has been threatened by nitrogen pollution for a long time. We studied the evolution of Hong Lake from 1990 to 2020. From 1990 to 2005, the natural water area of Hong Lake decreased by 72%, and most of the natural water was converted into aquaculture water, resulting in the rapid deterioration of lake water quality. Farming was gradually banned after 2005 and the area of natural water has been restored, but the lake's water quality has not improved, indicating that the lake is also threatened by other sources of pollution. According to the results of two groundwater surveys in 2011-2012 and 2019-2020, we found that the nitrate nitrogen content in groundwater in the study area increased from 1.29mg/L to 3.58mg/L. We also collected sediment samples from the lakeshore zone, and the test results showed that a large amount of exchangeable nitrogen was adsorbed on the sediment. Groundwater is one of the important factors causing nitrogen pollution in lake.