Coupling stable isotopes and hydrochemical parameters to elucidate the linkages between hydrological connectivity and water quality in an urban river-lake system.

Accelerating urbanization exerts a profound impact on the aquatic environment, altering the hydrological cycle, deteriorating water quality, and redistributing water resources. However, water exchange relationships and pollution patterns in such networks are complex and poorly understood due to disturbances resulting from natural processes and anthropogenic activities. This study coupled hydrochemical analysis with stable isotopes in the Dongsha River-Lake Networks (DRLN), China, to elucidate the spatiotemporal dynamics of urban water cycling and the response of water quality to hydrological connectivity. Isotopic signatures effectively traced diverse water sources, revealing a clear distinction between evaporation-dominated lakes and urban rivers, with the latter exhibiting higher sensitivity to precipitation. Driven by regional rainfall and seasonal pumping regulations, water exchange and hydrological connectivity were significantly enhanced during the wet season, as evidenced by isotopic homogenization and MixSIAR results. Hydrological connectivity plays a crucial role in shaping water pollution patterns by governing the transport pathways of pollutants. A pronounced "dual-effect" of hydrological connectivity on urban water quality was identified. Artificial diversion from the high-quality Yangtze River source effectively diluted anthropogenic pollutants in proximal receiving rivers, but this buffering capacity diminished with increasing spatial distance. Conversely, enhanced seasonal connectivity acts as a pollutant vector, facilitating the intrusion of contaminated local water into urban rivers and exacerbating downstream water quality degradation. The use of stable isotopes provides a critical and feasible tool to diagnose hydrological regimes and water exchange in urban systems, offering valuable insights into the water quality management related to connectivity in complex urban river and lake systems.