Coupling stable isotopes and river networks connectivity to identify the linking between hydrology and water quality in a urban river-lake water system

Accelerating urbanization development has a great impact on water environment with altered hydrological cycle process, deteriorating water quality and redistributed water quantity. In addition, urban river-lake networks have emerged as a blend of natural and engineered water environment under artificial transformation. However, water exchange relationships and pollution patterns in such water networks are complicated and poorly understood due to various disturbances from both nature and social activities. This study investigated water chemistry and stable isotopic tracers in the Dongsha River-Lake Networks (DRLN) in Wuhan, China, to enhance the understanding of hydrological connectivity and controlling factors of urban water pollution. The results indicated that domestic wastewater was the main pollution source in DRLN, significantly contributing to high nutrient levels, particularly in urban streams. Point sources such as domestic wastewater and industrial effluents exerted a more considerable influence on water quality in dry season with lower discharge. In contrast, during the wet season, non-point pollution increased due to rainfall-runoff carrying various pollutants on the landscape. Taking urban streams as the research object and considering precipitation, tap water, Yangtze River water and East Lake as water sources, the MIXSIAR model was adopted to analyze the differences in water sources of urban streams in different months and locations. Moreover, the variation of stable isotopes suggested that pollution patterns in river-lake networks were also shaped by interactions between waterbodies, and water exchanges were more frequent in summer. The water diversion from the Yangtze River (YR) effectively enhanced water quality in urban streams, but the improvement on East Lake was insufficient to affect the water quality in central areas. Therefore, more rational water management strategies in DRLN were urgently needed considering the impact of hydrological connectivity on water pollution distribution. Integration analysis of water chemistry and stable isotopes contributes to understanding the mechanisms of pollutant generation and transportation in complex urban water networks, and provides insights into anthropogenic impacts on urban water networks.