기후변화 대응을 위한 수질 제어 및 관리방안 I

Abstract

Water Quality Management Strategy in the Context of Climate Change I Human activities associated with the burning of fossil fuels have propelled global climate change, increasing air temperature as well as changing evaporation, precipitation, and runoff. Climate change has the potential to significantly alter not only available water resources but also water quality, with consequent impacts on biogeochemical cycling and ecosystems of natural bodies of water. Although there are still many uncertainties in forecasting the extent and impact of climate change, it is necessary to assess scientific understanding of the impact and vulnerabilities of climate change on water quality when considering policy implications and directions. To this end, this study investigated all historical data of surface water temperature to quantify potential climate change impacts on water temperature, and assessed possible impacts on water quality of rivers and reservoirs. Long-term water temperature data from 649 stations over the 20-year period from 1989 to 2008(n=144,765) reveal that 14 mesoscale watersheds of a total of 97 show a significant trend towards higher water temperatures. In the Nakdong watershed, for example, 48 stations out of 176 show a significant trend towards higher water temperatures. And 43.4% of variance of water temperatures in said watershed is directly correlated with air temperatures. The top-two Eigen vectors identified by the EOF analysis correspond closely to the air temperature and other metrological data (precipitation, humidity, cloud cover, hours of sunshine, etc.). Moreover, the spatial pattern of EOF reveals that water temperature is more sensitive to air temperature in river systems than in reservoir systems, but it is not possible to separate the effects of variations in water temperature from those of human interventions in the catchment such as land use change. For evincive purposes, 3 case studies were chosen to demonstrate that rising water temperatures are likely to yield lower dissolved oxygen and higher amount of chlorophyll, but at the same time are not associated with bacteria concentrations. As discussed, climate change is likely to exert both direct and indirect influence on water quality and the ecology of natural bodies of water due to increased water temperature and changes in water volume. Higher water temperature is projected to exacerbate certain water quality problems by reducing dissolved oxygen (likely to extend and intensify thermal stratification) and promoting algal bloom, which may become more eutrophic. Thus, it is important to secure appropriate instream flow for conserving water quality and ecology. In addition, a more systematic water quality monitoring system should be built to collect and analyze data, and collecting high-quality data should be priority. Although waterborne diseases induced by pathogens are decreasing gradually due to better hygiene practices, accelerating climate change will cause increased damages from pathogens due to rising temperatures, floods, and concentrated torrential rains. Adaptation to the changed environment is thus of critical importance, and to this end, it will first be necessary to establish and manage various indicator pathogens for the purposes of accurate pathogen management. Management of pathogens in domestic rivers is mostly virus management that uses total coliform and fecal coliform bacteria as indicators. These indicator bacteria may not be reliable, however, as they do not reflect each target waterborne disease that receives great impact from environmental factors, i.e., temperature and humidity, etc. As such, accurate pathogen management that expands the range of indicator pathogens through development of early diagnostic methods and basic research on root causes is first and foremost necessary. Second, it is essential to build sustainable systems for monitoring pathogens. This will involve management of not only direct pathogens in drinking water (as with recent expansions of access to rivers as part of efforts to create water-friendly space), but the building of monitoring systems for each watershed. Third, self-purification capacity of rivers must be improved and maintained through systematic integrated watershed management that embraces the control structure for management of nutrients, sediment, and pathogens together. Fourth, effective policies for securing public health over and above management of pathogens in response to climate change will require cooperation between the Ministry of Environment and the Ministry of Health and Welfare, as well as among other government departments and various experts. In particular, pathogen management will entail policy and technological cooperation among various experts in, as mentioned previously, establishing indicator pathogens, watershed management, monitoring, etc., as well as efficient division of labor and steady cooperation among government departments to be effective. Climate change will also affect the hydrological cycle of water on the earth. Such changes may increase the residual concentrations of micropollutants in the water environment. As water reuse is expanded, people may pay more attention to residual micropollutants of the treated water. Because the presence of any hazardous chemical in drinking water can directly affect human health, quality of the drinking water is regularly evaluated for controlled biological and chemical substances as well as unregulated micropollutants. Recently, river and lake water was also examined for micropollutants, but it still remains difficult to understand their occurrence in water because the sample number and the sampling frequency were limited in such studies. We cannot optionally and freely increase the sample number and sampling frequency because of financial concerns. Hence the target analysis and sampling period can be managed with the results from the drinking water monitoring programs. Korean Ministry of Environment already established the guidelines, which provide countermeasures for the occurrence of a new micropollutant in water. Such measures include the management of emission sources and the establishment of emission standards for a target micropollutant. Unfortunately, the management on the emission source lacks legal cogency because it depends on the voluntary actions, and the establishment of a new emission standard requires significant time and labor. This will create a gap between the detection of a new micropollutant in water and the establishment of the official restriction. Therefore, this study recommends the management of micropollutants based on the preventive principles. The existing regulations on drinking water source are of great importance because they are also valid for micropollutants, and technical reviews are required on the alternative drinking water source, the improvement of drinking water, and wastewater treatment plants, etc. In case of the Nakdong River where water pollution accidents have frequently occurred during dry seasons, for example, an adequate quantity of river water with satisfactory quality is essential in preventing such accidents in the future. In this study, a water quality modeling system has been developed for the replication of the flow and water quality in the Nakdong River, the projected changes and their impacts on water quality simulated in response to climate change stressors. Two Global Circulation Model (GCM) simulations (CSMK and CT63) on the A1B scenario are converted to regional scale data using the statistical downscaling method known as MSPG (Multi-Site Precipitation Generator), and applied to Soil and Water Assessment Tool (SWAT) model to simulate rainfall-runoff and pollutant loading in the Nakdong watershed. The results demonstrate that precipitation in the future (2011~2100) will increase 11~20% as compared to the last 30-year average. This phenomenon is more pronounced in the upper basin during winter season. Runoff also shows similar patterns to the precipitation, increasing by 23~34%. Accordingly, the runoff increase results in escalation of pollutant loading by 12~18% for TN and 10~17% for TP. More specifically, the rate of pollutant loading increase is expected to continue its acceleration until 2040. The 'Four Major Rivers Restoration Project' has been envisioned as water quality management plans investing in environmental infrastructure such as wastewater treatment plants, advanced wastewater treatment processing, treatment of livestock manure, and sewage pipelines etc. As such, the SWAT model was developed to quantify the impact of land management practices and the pollutant loading in the Nakdong drainage basin. The simulation results call for about 7.3% decrease of Total-P loading in the Nakdong drainage basin through project implementation, and most of the loading reductions are caused by large sewage treatment facilities in the lower basin (Geumho, Nakdong estuary, and Suyoung region). Subsequently, the changes of flow rate and water quality resulting from the project and climate changes were numerically simulated. The hydrodynamic model (EFDC) and water quality model (WASP) were linked for the simulation of the flow and water quality in the Nakdong River. The simulation during the flood season shows that the flood levels will likely be lower in general after the project, and the dredging and small to mid-sized submersible dams will likely increase retention time to 2~3 times its current value. Also, the water quality modeling in the Nakdong River shows that BOD5, TN, and TP are slightly lower or almost stable after the project. However, chlorophyll-a concentrations are increased in the upper basin and become lower in the lower basin, a phenomenon potentially caused by the inhibition effect on photosynthesis (not reduction of nutrient input) as a factor of increasing water depth by waterway dreading.