In the Higashi-Yokobori and Dotonbori Rivers, water quality is controlled by the inflow of relatively clean river water from above the upstream floodgate and outflowing polluted water to below the downstream floodgate. However, a hypoxic water mass occurs during high-temperature periods; therefore, there is room for further water quality improvement. In this study, field surveys, laboratory experiments, and simulations were conducted to determine the water quality characteristics of these two rivers.The impacts of these water quality improvements resulting from the exchange of the water mass with water from outside the floodgates were evaluated, and more effective methods were proposed. It was shown that the main factor leading to hypoxia was dissolved oxygen (DO) consumption in the water. In order to maintain a satisfactory DO environment in the summer, it would be necessary to continuously allow water to enter from above the upstream floodgate at a rate of more than 2.0 m 3 /s.
The effects of operations to increase nitrogen discharge from sewage treatment plants to prevent oligotrophication at Harima-nada in the Seto Inland Sea on concentrations of organic matter, nitrogen, and phosphorus in surface water were evaluated. First, the sea waters around the Futami treatment center, which has been conducting the operation, were analyzed. The results show that no significant changes in water quality were observed during the operation. Next, the effects of the operations of 20 sewage treatment plants at Harima-nada on the water quality were simulated using a numerical simulation model. As a result, it was estimated that dissolved inorganic nitrogen and phosphorus were taken up by phytoplankton, and organic matter and nitrogen concentrations in the entire area of Harima-nada were increased by phytoplankton advection and diffusion. The observed values could not confirm the effect of the operation to increase nitrogen discharge at the Futami treatment center owing to dilution by seawater and the drainages of other nitrogen emission sources. However, it was shown that when the number of operations to increase nitrogen discharge from sewage treatment plants increased, the water quality in a wide area of Harima-nada may be affected.
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