Effects of multifunctional weir construction on key water quality indicators: a case study in Nakdong River, Korea

Abstract

In this study, changes in water quality characteristics were compared before and after the construction of eight multifunctional weirs in the Nakdong River Basin in South Korea. A cluster analysis of spatial variation showed that the locations classified as Cluster 1 pre-construction (sites ND7 and ND8) shifted into Cluster 2 post-construction, indicating that spatial cluster types changed after the construction of multifunctional weirs. Principal component and factor analyses revealed that nitrogen and phosphorous nutrients at the pre-construction stage (total nitrogen, total phosphorous, and phosphate-phosphorus) and electrical conductivity were extracted in principal component 1, while biochemical oxygen demand, chemical oxygen demand, pH, and chlorophyll-a were extracted in principal component 2. However, biochemical oxygen demand, chemical oxygen demand, total organic carbon, and chlorophyll-a at the post-construction stage were extracted in principal component 1, while total nitrogen, nitrate-nitrogen, and dissolved oxygen were extracted in principal component 2. The key water quality indicators transitioned from nutrients to organic pollutants after the construction of the multifunctional weirs. A water quality trend analysis with seasonal Mann–Kendall testing and locally weighted scatter plot smoothing methods showed that organic pollutant indicators such as chemical oxygen demand and total organic carbon tended to increase after the construction of multifunctional weirs. Therefore, the key quality indicators for water quality improvement and management should be chemical oxygen demand and total organic carbon.