Influence of stream restoration on water quality in the gap stream, Daejeon, Korea

To study the influence of stream restoration on long-term chemical water quality, we downloaded the annual average water quality data for the Gap Stream, Korea, from 2003 to 2019. Dissolved oxygen, pH, total nitrogen and phosphorus (TN and TP), suspended solids, biochemical oxygen demand (BOD), and chemical oxygen demand (COD) were chosen. For this study, we compared the averages between pre- and post-restoration using analysis of variance after obtaining the annual volume-weighted averages. The BOD and TP decreased significantly upstream and downstream for seven years after the stream restoration project. The TN was significantly reduced only downstream. Thus, the project to restore the stream was successful in lowering the BOD, TN, and TP. In contrast, the restoration project did not affect the concentration of refractory organic matter; it merely decreased the amount of biodegradable organic matter. Therefore, it is vital to determine the source of refractory organic matter and to apply new solutions to prevent it from entering the stream. Based on the TN/TP ratio, before the restoration project, nitrogen was the limiting ingredient for phytoplankton growth downstream. Phosphorus, however, became the limiting element following the project. Our results indicated that monitoring long-term chemical water quality after a stream restoration assists with water quality management, such as identifying and obstructing polluting sources and giving project feedback.