Seasonal and spatial dynamics of fecal coliforms and water pollutants in the Nakdong River, South Korea: implications for climate adaptive water quality management strategies.

This study examines five years (2015 to 2019) of water quality data from 44 national monitoring sites along South Korea's Nakdong River to identify key environmental factors and pollutants affecting the spatial and seasonal distribution of total coliform bacteria (TC) and fecal coliform bacteria (FC). Using principal component analysis and cluster analysis, we analyzed the relationships between 18 key pollutant indicators and coliform distribution, focusing on seasonal and spatial variations. Our results revealed distinct spatial gradients in organic pollutants, including total organic carbon (TOC), biochemical oxygen demand (BOD), and chemical oxygen demand (COD), with higher concentrations downstream correlating with increased fecal coliform levels. Elevated total nitrogen (TN) and total phosphorus (TP) concentrations in the middle and lower streams also influenced fecal coliform (FC) distribution patterns. Seasonal variations in electrical conductivity (EC), total nitrogen (TC), and nitrate-nitrogen (NO-N) were notable during heavy rainfall, indicating the impact of land use and human activities on fecal coliform fluctuations. The distribution of total coliform bacteria during peak fecal coliform periods was closely related to five-day cumulative precipitation and was influenced by sewage treatment efficiency and manure management practices. Cluster analysis identified two main pollution groups, emphasizing the role of urbanization and agricultural activities in water quality degradation. Additionally, we forecast substantial increases in water temperature and fecal coliform concentrations exceeding Korean water quality standards by more than twice after 2032 at a hotspot site, primarily due to climate change. To address these challenges and ensure sustainable water quality, we recommend adaptive management strategies tailored to regional and seasonal characteristics, such as enhanced real-time monitoring, upgraded sewage treatment systems, and stricter land-use regulations. Our findings highlight the urgent need for regional and seasonal water quality management to mitigate climate change impacts, while providing insights for other regions facing similar water contamination challenges.