Effect of Rainfall and pH on Musty Odor Produced in the Sanbe Reservoir

Abstract

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.

Keywords

• musty odor
• reservoir
• geosmin
• 2-MIB
• pH