Evolution and restoration of water quality in the process of urban development: a case study in urban lake, China

Urban development has positive and negative effects on the evolution of enclosed lake water quality. This study aims to quantitatively analyze the water quality evolution of a typical urban lake, the Sha Lake, in the process of urban development. The land use degree comprehensive index (I) was calculated to reveal the level of urban development; water quality index (S_mid) and eutrophication index (T_mid) were used to evaluate the water quality changes by fuzzy comprehensive-quantifying assessment (FCQA) method. The urban construction process and the water quality changes in 2000-2018 in the Sha Lake Basin were divided into three stages: (1) in 2000-2006, with the slow urban development, water quality remained stable and the degree of eutrophication improved slightly; (2) in 2007-2009, I increased rapidly to reach 300, S_mid and T_mid increased from 90.62 to 92.83 and 75.06 to 87.52, respectively. Water quality deteriorated because of the failure to implement environmental protection measures in time; (3) in 2010-2018, although urban development reached a high level (I > 300), the water network connection project, dredging project, exogenous pollutant control, and sewage pipe network renovation since 2009 were critical measures to improve water quality for a long time. Due to the lag effect on improving water quality, the implementation of environmental protection measures should be synchronized with or even before urban construction. The research results can provide a scientific basis for the urban lake water environment protection in the process of urban development.