This study focuses on a rapidly urbanizing coastal plain where river networks serve as critical pathways for pollutant transport to nearshore waters. Under frequent sluice control and sluggish hydrodynamics, pollutants accumulate in channels and are subsequently flushed during intense rainfall or sluice-opening events, increasing pollutant loads in downstream estuaries. Based on 2017–2024 water quality monitoring data, integrated multi-source environmental factor analysis and unmanned patrol boat technology, systematic water quality assessment and pollution source identification were conducted. Significant spatial heterogeneity was observed: phosphorus and nitrogen pollution dominated in the eastern region, whereas the permanganate index was more prominent in the western part of the network. Identification of abrupt water quality change sections revealed industrial wastewater as the primary contributor to phosphorus and nitrogen, whereas permanganate index pollution originated widely from aquaculture, agriculture, and industrial discharges. Atmospheric deposition likely provides a non-negligible contribution to phosphorus and nitrogen input, with fluxes strongly correlated to rainfall. Sediment release posed internal risks of carbon and phosphorus, with intensity positively linked to pollution levels. This study elucidates the water quality characteristics and multi-source pollution mechanisms in typical coastal river networks under rapid economic development. Therefore, it provides a scientific basis for precise regional water environment management and coastal water quality protection.

In November 2023, surface sediments were collected at 46 sites around the main islands of the Zhoushan Archipelago (Dinghai, Daishan, Qushan, and Shengsi) in the East China Sea. The concentrations of Cu, Zn, Cr, Pb, Cd, Hg, and As were determined, together with sediment TOC and Eh. Pollution and ecological risks were evaluated using the single-factor index (Pi), Nemerow pollution index (PN), and Hakanson’s potential ecological risk index (RI). Source apportionment was investigated using FA–PC and EPA PMF 5.0. Mean concentrations (mg/kg) were Zn 77.58, Cr 70.08, Cu 28.44, Pb 18.92, As 9.40, Cd 0.09, and Hg 0.073, with higher levels generally observed near Dinghai, Daishan, and Shengsi. The overall risk was low, whereas Cd and Hg contributed disproportionately to RI. FA–PC suggested two major source groups, and PMF resolved three factors related to (i) agriculture/aquaculture (As), (ii) industrial and domestic effluents (Hg), and (iii) port and ship-related activities (Cd, Cu, Cr, Pb, Zn). The results support targeted management focusing on Cd, Hg, Cu, and As in identified hotspots.

The plain river network area is strongly influenced by human activities, with flat terrain, weak hydrodynamics and poor water quality. Previous studies have mostly focused on water quality categories, lacking an overall study of the pollution characteristics of water bodies and their changing trends. In order to comprehensively grasp the water pollution situation of the river network in the northern Yu Plain, this study scientifically analysed the water quality characteristics of the river network in the northern Yu Plain and its correlation with rainfall through regression analysis and MK mutation test based on the long series of water quality and rainfall data of the Dongjin River and Unity River in the northern Yu Plain from 2017-2021. The results show that the correlation between water quality and rainfall in the river network of the northern Yu Plain is not significant, reflecting that the pollution in the watershed is still controlled by point sources, but there is a trend for the Unity River to change to non-point source control; the CODMn, NH3-N and TP of the Dongjin River and Unity River all show a high positive correlation, and the correlation of the Dongjin River is higher than that of the Unity River; the MK mutation test shows that there is no significant mutation point in the Dongjin River in the last five years, and the Unity River has no significant mutation point in the last five years. The MK mutation test shows that there is no significant mutation point in Dongjin River in the past 5 years, and there is a sudden change in water quality in Unity River from 2017 to 2018, and the water quality indexes show a decreasing trend in general, but remain stable in recent years. The study aims to academically reveal the dominant factors, pollutant characteristics and water quality trends of pollutants in rivers in the northern Yu Plain, and to propose scientific references for the pollution prevention and control of other plain river networks.