Water quality pollution assessment and source apportionment across wetland types: A case study in the middle reaches of the Tumen River

The Middle Reaches of the Tumen River Basin, China Scientific assessment of water quality and quantitative identification of pollution sources are crucial for improving the management of regional surface water resources. However, research on water quality characteristics and pollution source identification across different wetland types remains limited. The Water Quality Index (WQI) and the Expectation–Threshold Index (ETI) methods were applied to comprehensively evaluate water quality. Principal Component Analysis/Factor Analysis (PCA/FA) and the Absolute Principal Component Score-Multiple Linear Regression (APCS-MLR) model were employed to detect and quantify pollution sources and their contribution rates. The results revealed significant differences in pollution characteristics among wetland types, driven by multiple factors including hydrological conditions, ecological processes, and anthropogenic activities. Overall, the water quality of the wetlands was classified as average. In swamp wetlands, the contributions of pollution sources were ranked as agricultural non-point sources (28.21 %) > natural ecological process sources (25.18 %) > domestic sewage and livestock manure sources (19.70 %). In river wetlands, the major contributing sources followed the order of combined effects of water mineralization and runoff scouring (29.29 %) > composite nitrogen sources (24.27 %) > combined influences of agricultural non-point sources and urban surface runoff (20.6 %). In constructed wetlands, the contributions were ranked as eutrophication–biological assimilation factors (26.43 %) > wetland biological treatment processes (25.91 %) > agricultural non-point sources (22.68 %). • PCA/FA and APCS-MLR models were applied for the quantification and analysis of wetland pollution sources. • WQI and ETI were used for the comprehensive assessment of wetland water quality. • Significant differences in pollution characteristics exist among wetland types. • The overall water quality grade of wetlands in the basin is at a “average” level.