Shingo MASUKI, Hiroshi YAJIMA and Yasushi SEIKE 1 () (680-8550 4-101) 690-0046 340 2 () 680-8552 4-101 Centre for Water Research 3 () 690-8504 1060 A field test of supplying oxygen-rich water into the bottom of dredged pits in Lake Nakaumi was performed in 2009. We used recently developed hypolimnetic oxygenators called WEP. In Yonago Bay, we selected two adjacent dredged pits, then installed and operated the WEP system in one pit for the experimental purpose. The other pit was used for the control purpose. The vertical profiling data of water quality in the pits showed the dissolved oxygen concentration was increased to 15-29 mg/L nearby the outlet of WEP system, and was kept at 5-15 mg/L in the experimental pit, on the other hand it was less than 5 mg/L in the control pit for 6 months of the experimental period (May-Oct.) in spite of the existence of tidal current. Moreover, we performed 3-D numerical simulations using ELCOM to evaluate the flow characteristics in the pits. They revealed that the water in the bottom of the pit stayed still but the water in the shallower layer there sometimes flowed out to the surrounding area.

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Harmful cyanobacterial blooms are continuously formed in water systems such as reservoirs and lakes around the world. Geosmin and 2-methylisoborneol (2-MIB) produced by some species of cyanobacteria have caused odor problems in the drinking water of the Sanbe Reservoir in Japan. Field observations were conducted for four years (2015–2019) to investigate the cause of this musty odor. It was found that geosmin was produced by Dolichospermum crassum and Dolichospermum planctonicum (cyanobacteria), and 2-MIB was due to Pseudanabaena sp. and Aphanizomenon cf. flos-aquae (cyanobacteria). Changes in water temperature and pH caused by rainfall were correlated with changes in the concentration of geosmin and 2-MIB. In particular, geosmin and 2-MIB tended to occur under low rainfall conditions. When there was low rainfall, the reservoir changed to an alkaline state because the phytoplankton consumed CO2 for photosynthesis. In an alkaline reservoir, dissolved inorganic carbon mainly existed in the form of bicarbonate (HCO3−). Thus, the results suggest that under such conditions in reservoirs, cyanobacteria grew easily because they could use both CO2 and HCO3− for photosynthesis. Specifically, our study suggests that in order for the musty odor problem in the reservoir to be solved, it is important that the pH of the reservoir be controlled.

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