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

Abstract Harmful cyanobacterial blooms (HCBs) are continuously formed in water systems such as reservoirs and lakes around the world. In particular, geosmin and 2-methylisoborneol (2-MIB), which are made by cyanobacteria, have caused serious musty-odor problems in the drinking water of the Sanbe reservoir in Japan. To investigate the cause of this musty odor, we conducted field observations for five years (2015–2019). As a result, it was determined that the geosmin was caused by the cyanobacteria as Dolichospermum crassum and Dolichospermum planctonicum, and 2-MIB was caused by the cyanobacteria as Pseudanabaena sp and Aphanizomenon cf. flos-aquae. It was also determined that the occurrence of geosmin and 2-MIB are influenced by changes in water temperature and pH due to the inflow of rainfall. Especially, the geosmin and 2-MIB tended to occur in low rainfall conditions. When there is low rainfall, the reservoir changes to an alkaline state due to the consumption of CO2 by the photosynthesis of phytoplankton. In an alkaline reservoir, CO2 exists in the form of bicarbonate (HCO3-); thus, under these condition cyanobacteria will increase because it uses both CO2 and bicarbonate (HCO3-) for photosynthesis. Our study suggests that for solving the musty-odor problem in the reservoir, it is important to control the reservoir’s pH. Moreover, more focus should be place in managing the water quality problem of the Sanbe Reservoir, especially, during low rainfall years. The results of this study will help in addressing odor related water quality problem in other reservoirs.