Long-Term Water Quality Patterns in an Estuarine Reservoir and the Functional Changes in Relations of Trophic State Variables Depending on the Construction of Serial Weirs in Upstream Reaches

Water quality degradation is one of the major problems with artificial lakes in estuaries. Long-term spatiotemporal patterns of water quality in a South Korean estuarine reservoir were analyzed using seasonal datasets from 2002 to 2020, and some functional changes in relations of trophic state variables due to the construction of serial weirs in the upper river were also investigated. A total of 19 water quality parameters were used for the study, including indicators of organic matter, nutrients, suspended solids, water clarity, and fecal pollution. In addition, chlorophyll-a (CHL-a) was used to assess algal biomass. An empirical regression model, trophic state index deviation (TSID), and principal component analysis (PCA) were applied. Longitudinal fluctuations in nutrients, organic matter, sestonic CHL-a, and suspended solids were found along the axis of the riverine (Rz), transition (Tz), and lacustrine zones (Lz). The degradation of water quality was seasonally caused by resuspension of sediments, monsoon input due to rainfall inflow, and intensity of Asian monsoon, and was also related to intensive anthropic activities within the catchment. The empirical model and PCA showed that light availability was directly controlled by non-algal turbidity, which was a more important regulator of CHL-a than total nitrogen (TN) and total phosphorus (TP). The TSID supported our hypothesis on the non-algal turbidity. We also found that the construction of serial upper weirs influenced nutrient regime, TSS, CHL-a level, and trophic state in the estuarine reservoir, resulting in lower TP and TN but high CHL-a and high TN/TP ratios. The proportions of both dissolved color clay particles and blue-green algae in the TSID additionally increased. Overall, the long-term patterns of nutrients, suspended solids, and algal biomass changed due to seasonal runoff, turnover time, and reservoir zones along with anthropic impacts of the upper weir constructions, resulting in changes in trophic state variables and their mutual relations in the estuarine reservoir.

Application of the SWAT-EFDC Linkage Model for Assessing Water Quality Management in an Estuarine Reservoir Separated by Levees

Estuarine reservoirs are available for use in various water resource systems. In agriculture, supplying irrigation water that meets water quality standards is essential for food safety. This study focused on the Ganwol estuarine reservoir in the midwestern region of South Korea, which suffers from water quality deterioration problems. To explore the water quality improvement in an estuarine reservoir, it is essential to understand the characteristics of water quality changes in the reservoir following water pollution control management. Therefore, the purpose of this study is to evaluate the effects of water quality management on the estuarine reservoir, which is separated by levees, using the soil and water assessment tool (SWAT)-environmental fluid dynamics code (EFDC) linkage model. In this study, soil remediation by dredging the reservoir’s bottom soil and effluent control from public sewage treatment works were considered as the water management plans. The results of this study indicate that reducing the internal load of the reservoir increases internal resilience and reducing the external inflow load decreases the impact on the system. Hence, comprehensive measures are effective in improving water quality.