In this study, we investigated the interrelationships between organic matter and water quality indices in the total maximum daily load basins, namely, Churyeongcheon and Yocheon of the Seomjin River system, and identified trends. Churyeong A and Yocheon B, the basins being analyzed, have high proportions of nonpoint pollution sources and pollutant loads from terrestrial sources. During the study period, biochemical oxygen demand (BOD) decreased in both basins, whereas chemical oxygen demand (COD) and total organic carbon (TOC) increased in Churyeong A and decreased in Yocheon B. The increase in organic matter in Churyeong A correlated with the flow rate, whereas organic matter in Yocheon B showed little correlation with flow rate. Variations in organic matter (BOD, COD, and TOC) in Churyeong A exhibited seasonality under the influence of increased flow rate. Organic matter in Yocheon B was affected by increased flow rate, wherein with time, BOD decreased and COD and TOC increased. This study provides basic data that can be used as a reference to facilitate continuous water management and appropriate strategy implementation by analyzing the influencing factors and trends of organic matter using long-term measurement data.

The total maximum daily load (TMDL) system divides the watershed into unit basins for implementation and evaluates water quality by assessing whether targets have been achieved based on investigated data through continuous monitoring. River water quality is influenced by the amount and type of pollutants entering the river, making continuous monitoring, along with analysis and evaluation, essential for the ongoing development of policies and systems aimed at improving water quality. In this study, basic data for water quality management were gathered by analyzing the pollution contributions of the main river (the Seomjin River) and its tributaries, identifying major pollutant sources, and conducting trend analyses. The delivery pollution load of the Seombon D unit basin, one of the main watersheds of the Seomjin River in South Korea, shows a rapid increasing trend (BOD, 1.2–2.4, 2020), which is different from the trend in the B unit watershed of the Boseong River, also a tributary. The rapid increase is presumed to be due to the characteristics of Seombon D, including the inflow of pollution sources of Seombon C, an upstream point. The D unit basin of Seombon is located in the middle of the unit watersheds that divide the main stream of the Seomjin River in Korea into A, B, C, D, E, and F. This increase is thought to be due to the inflow of pollutants specific to Seombon D’s characteristics and the influence of the upstream Seombon C unit basin. In the pollution load contribution rate analysis of Seombon D, it was found that the contribution rate from Seombon C, the upstream area (BOD, 38.42–120.08%), was higher than that of the Boseong B unit basin tributary. The self-purification capacity of Seombon D is believed to have contributed to the improvement in its water quality. It is essential to manage the upstream Seombon C unit basin to sustainably improve the water quality of the Seombon D unit basin. Therefore, managing Seombon C is deemed necessary to further enhance the water quality of Seombon D.

Water quality in the four major river basins in South Korea has been managed by introducing total maximum daily loads (TMDLs) to achieve or maintain a target water quality (TWC); TMDLs can be obtained by discharging the allocated pollutant loads from the watershed. The Nakdong River basin is the most important source of water supply among these four river basins and consists of 41 watershed units. The current method for evaluating the rate of load management involves calculating the measured values over the past 3 years through a log transformation process and assessing whether the TWC is achieved after comparing the values with the reference TWC. The objective of this study is to compare and analyze the results of a flow duration curve (FDC) and load duration curve (LDC) evaluation based on the current method for TWC evaluation and the daily flow data measured at 8-day intervals (weekly) to present an efficient TWC assessment method for more relevant flow conditions. The daily flow data were from 1995 to 2004, and the flow data measured at 8-day interval were from 2005 to 2015. The target study areas included the 41 watershed units on the Nakdong River basin (14 main streams and 27 tributaries). Biochemical oxygen demand (BOD) and total phosphorus (TP) constituted the water quality indicators studied. LDCs were prepared using the estimated daily flow rate, and the measured flow data at 8-day intervals based on the BOD and TP revealed similar results. Thus, even the data measured at 8-day intervals are considered applicable for creating FDCs and evaluating LDCs without requiring complex tests, calibrations, and prediction processes using a watershed model based on accumulated data over many years.