Long-Term Water Quality Fluctuations in the Seomjin River System Determined Using LOWESS and Seasonal Kendall Analyses

Analysis of pollutant emission, water quality, and flow rate trends and the influence of water quality of the primary stream of the Seomjin River system using measurement data provides important information for formulating water quality management policies. In this study, we identified specific characteristics based on changes in water quality using graphs of long-term (2011–2020) water quality data from the Seomjin River system. We conducted correlation and factor analyses of the following water quality survey parameters: temperature, pH, dissolved oxygen (DO), electrical conductivity (EC), biochemical oxygen demand (BOD), chemical oxygen demand (COD), suspended solids (SS), total phosphorus (TP), total nitrogen (TN), and total organic carbon (TOC), and flow rate. Furthermore, we analyzed BOD and TP, which are designated as total maximum daily load management parameters, to examine the intrinsic properties of the group. In addition, seasonal Kendall tests and LOWESS analysis were conducted using long-term monitoring data to analyze the long-term trends of changes in water quality. According to the analysis results, at the target points, BOD was highly correlated with COD and TOC, and TP with SS, COD, and BOD, whereas pH and DO show low correlations. The fluctuation characteristics of BOD showed an overall “decreasing” trend. On the other hand, TP concentration showed an “increasing” trend. The characteristics of the water quality environment were identified using long-term water quality data. The results may contribute to developing a basic framework for proposing logical grounds and objective measures for long-term water quality management policies.