저수지 비점오염원 저감을 위한 인공습지의 설치효과 및 개선방안

Abstract

Abstract Improving Artificial Wetlands for Nonpoint Source Pollution Control Multi-purpose dam construction projects contributed to Korea’s industry and economy by spurring economic motivation. However, they are now being blamed for negative environmental impacts caused by the construction and operation of artificial reservoirs. In addition to point pollution sources, nonpoint pollution sources spreading in upstream areas raise diverse environmental issues such as the accumulation of sediments, abnormally turbid waters, floating garbage, and eutrophication. Especially, in cases of abnormal turbidity provoked by soil inflows from cultivated land upstream after heavy rainfalls, the possibility remains that it can seriously disturb the ecosystem and water quality of the entire watershed area. Recently, nonpoint sources in artificial reservoirs have caused more environmental issues because nonpoint sources are lightly regulated, while point sources are regulated strictly and controlled by wastewater treatment facilities. Thus, many mitigation methods are being considered to control contaminants from nonpoint sources, including detention ponds, complex contact oxidation facilities, nonpoint source treatment plants, and constructed wetlands. However, most mitigation methods lack practicality because there has been little feedback based on applications in the field. This study aims to analyze, in detail, the mechanisms of environmental issues in artificial reservoirs, evaluate the effectiveness of the suggested mitigation methods quantitatively, and propose some measures regarding the construction and operation of wetlands according to feedback from field applications. Furthermore, this study serves to improve the effectiveness of mitigation methods in the field in order to develop new and existing reservoirs in a sustainable manner. Constructing artificial wetlands, in spite of being induced originally to treat the discharge from point sources, has been suggested as one of the most applicable methods to control the quality of water flowing into artificial reservoirs. Artificial wetlands are favorable over other mitigations due to their low cost, easy operation, and large treatment capacity. In addition, artificial reservoirs are environmentally friendly as they provide an alternative habitat for wildlife. In Korea, artificially constructed wetlands around reservoirs are classified into several types based on their structure and function. They range from small marsh areas vegetated with aquatic plants to wetlands with large surface areas that include recreation areas and leisure facilities. While many previous studies show that artificial wetlands are effective in treating sewage and wastewater, there is still a debate whether the constructed wetlands could be effective when they are adapted upstream of artificial reservoirs. This is due, in part, because some constructed wetlands have been surveyed to be less-effective in controlling pollutants from nonpoint sources during certain periods, such as rainy or cold seasons. The main reason why existing constructed wetlands show less-effectiveness in certain periods can be summarized as follows. First of all, inflow amounts entering constructed wetlands show extreme seasonal variations, while inflow amounts are relatively stable for the constructed wetlands dealing only with sewage. Additionally, heavy rainfall affects the inflow quantity and effluence ratio of the contaminants from nonpoint sources. Secondly, because contaminants are discharged from a number of nonpoint sources and mixed in a wetland, the biochemical properties of the contaminants are hard to control. Thirdly, the chemical reactions in a constructed wetland proposed to treat nonpoint contaminants in reservoirs could be altered seriously because of poor operation and management. Many wetlands composed artificially upstream are exposed to external natural variations such as fallen leaves, decaying plant matter, overgrown vegetation, eutrophication, sedimentation, and inflow variations. Moreover, due to their large surface areas, their maintenance becomes a burden. This study found that many wetlands are operated without the control of inflow and that little care is taken to maintain the constructed wetlands as per their suggested technical designs. Thus, it is advised to design the types and technical specifications of each constructed wetland based on much consideration of the physical properties of the entire watershed area and biochemical properties of the pollution sources, including detailed surveys of the suggested sites, rather than adapting previous applications without any alterations. Secondly, it is important to monitor not only the quality, but also the quantity of the in- and outflow of the constructed wetlands regularly. The monitoring parameters should be decided carefully according to detailed surveys regarding the properties of the pollution sources, and it is desirable to survey the concentrations of the specific chemicals of the main pollutants such as Nitrogenous and Phosphorus compounds. Thirdly, it is necessary to find the best operating conditions and establish detailed operating instructions for each constructed wetland, which define the frequency and the scope of tasks to keep the wetlands in a desirable state. The tasks should include removing accumulated sediments, regular monitoring and inspection, and seasonal control of the hydraulic facilities. Lastly, adopting ordinary mitigation facilities such as constructed wetlands is desirable, though it is also necessary to directly control the nonpoint sources of pollution as well. For instance, a waterway should be installed near cultivated land in order to prevent the inflow of soil. Development activities upstream should be limited as they may potentially affect water quality.