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Managing river water quality requires the accurate analysis and evaluation of the achievement of various policies and systems. Here, we targeted total maximum daily load unit basins in the Yeongsan River. Various water quality evaluation methods were investigated along with their advantages, limitations, and applicability. Based on the findings, linear interpolation, arithmetic mean, and converted mean were applied to Yeongbon C, where the flow rate was controlled by beams; percentile evaluation method to Yeongbon D, where the variations in water quality was significant due to the influence of the flow rate; and percentile evaluation method to Yeongbon D, where the influence of water quality by flow rate was marginal due to estuary banks. The results were derived differently depending on the characteristics of the target site (water quality and flow rate, river shape, and natural conditions) and evaluation method. The results suggested that a careful approach is required in selecting and applying a water quality evaluation method. Application of an appropriate water quality evaluation method for each study site through a multifaceted approach was confirmed to be more reliable than the application of a single water quality evaluation method. This approach helps accurately analyze water quality by identifying the cause.

The annual amount of water pollution discharge load is utilized to assess its implementation of the load allotment on each unit watershed in the management of total maximum daily loads (TMDLs). The amount of the discharge load might be changed as the rainfall changes in the area with the combined sewer system. In order to evaluate the implementation properly, the effects of rainfall changes should be considered in the estimation of the discharge load. It is necessary to standardize the annual discharge load to be calculated at the same rainfall condition as that of the reference year. But the calculation process is very difficult and might have some limitations. This study investigated and developed two methods in order to estimate the discharge load in a relatively simple way. The load conversion method (LCM) is designed to calculate the differences in discharge due to rainfall changes and to convert the discharge load of the current rainfall condition into that of the reference rainfall condition. The multi-regression equation method (MREM) is to predict the discharge load directly on reference rainfall conditions using a multi-regression equation. These methods were applied to examine the calculation results. LCM showed a more precise result with an error of –0.06%, while MREM –0.18%. Judging from the application results of this study LCM may be useful as a tool for estimating the discharge load for evaluation preferably in the TMDL process.

This study focused on improving the regression equations for the cumulative rainfall ratio (CRR), cumulative pollutant load ratio (CPR), and average pollutant load reduction efficiency of non-point source (NPS) pollution load reduction facilities to facilitate the calculation of NPS pollution load cutback used in South Korea’s technical guidelines for the total maximum daily load. The recently updated hourly rainfall data from the Korea Meteorological Administration and NPS pollution load monitoring data from the Ministry of Environment (MOE) were used. Furthermore, the updated rainfall patterns and data together with the existing regression equation were used to formulate an improved regression equation for the CRR, which was used to calculate the CRR. Additionally, the improved regression equation for the CPR was used to determine the area type weighted CRR based on (i) CRR of land cover and (ii) public data provided by the MOE for 2008– 2014. Also, the pollutant load reduction efficiency of the NPS pollution reduction facilities was updated using data of the inflow and outflow monitoring of NPS pollution reduction facilities obtained from various pilot projects or research on NPS pollution load by the MOE and by adding efficiency data of previously excluded low impact development techniques.

Rapid urbanization in recent years has increased the area of impervious surfaces in South Korea. Increasing impervious areas increases the direct runoff when rain falls, which adversely affects the health of watersheds through increased pollution from nonpoint source pollutants. However, few studies have investigated the relationship between hydrological phenomena in broad watersheds and nonpoint sources of pollution. In this study, a watershed model to simulate the flow and water quality of rivers, with different impervious area coverages, was developed using the Hydrological Simulation Program – Fortran. The effect of different impervious area reduction scenarios on direct runoff and loading of nonpoint source pollution in the Geum River watershed, South Korea, was analyzed and the water circulation rate was used as a proxy for the health of the watershed. The impervious area coverage in the medium influence area of Gapcheon, where Daejeon Metropolitan and Sejong Cities are located, was 23.17%, approximately two times higher than that of other areas. Based on the impervious area reduction scenarios, the direct runoff, and a load of nonpoint source pollution showed maximum reductions of 51% and 41%, respectively. The water circulation rate showed a maximum improvement of 21%. Realistically, it is impossible to reduce the impervious area rapidly in regions with high proportions of urban development or built-up land. Therefore, low-impact development methods should be applied when conducting the development of public facilities or projects larger than a certain size.

A nonpoint source is not easy to take countermeasures against point sources flowed from land during rainfall. In order to establish systematic management of these nonpoint sources, a nonpoint source management area was designated in Article 54 of the Water Environment Conservation Act in Korea since 2007. Once an area is designated as a nonpoint source management area, management objectives, target substances, and concrete reduction measures are established for the management policies and implementation plans and the nonpoint reduction effect is analyzed based on the assessment of the achievement of management objectives. This study was conducted on the watersheds of Saemanguem showed both urban and rural land use and Goljicheon showed rural land use to be designated and reported as nonpoint source management areas. Accordingly, this study presented the concrete conditions of the two areas and the achievement results of the management objectives in each flow management section, which was the same as in the implementation plan. Based on these results, this study aimed to propose a monitoring method for nonpoint source management areas. The proposed method encompasses the improvement in setting management objectives, which could control the influence of nonpoint pollution, and a monitoring method for nonpoint sources.

This study presents the principles and methods used in set target water quality values of total phosphorus (TP) in three major rivers of South Korea, such as Nakdong, Geum, and Yeongsan/Seomjin River. The QUALKO model (modified QUAL2E) was applied to establish the target water quality. According to the findings, while determining total maximum daily load (TMDL) targets, the intended use of water, current water quality, load density, and future pollutant elimination plans should be considered. If a watershed fails to meet the target quality standard, then the appropriate plan for the area should be established. In areas where small amounts of pollutants are released, development should be allowed within the limits of compliance with the water quality standards, targeting both development and water quality protection, which is the basic concept of TMDLs. Heavily polluted areas tend to continue releasing large amounts of pollutants, whereas others release only small amounts. The TMDL system is designed to address the problem by encouraging heavily polluted areas to reduce pollutant emission while allowing others to release some for development. The recommendations of this paper provide a reference to the development of a methodology for determining TP water quality considering TMDL.