Abstract
In order to estimate the state of anaerobic layer formation in the reservoir bottom and the resulting water quality problem in the JWA dam reservoirs, we examined the seasonal fluctuation of the vertical distribution of dissolved oxygen concentration and other parameters in each reservoir. We found that 66% of the dam reservoirs with no hypolimnion aerators (8 out of 12 reservoirs) form anaerobic layers. They are categorized into three groups: reservoirs having anaerobic layers from autumn to winter (four reservoirs), from summer to winter (two reservoirs), and throughout the year (two reservoirs). In addition, this study suggests the possibility that frequency of the anaerobic layer formation depends on not only total phosphate concentration of the reservoir (an eutrophication indicator) but also the vertical distance from the discharge equipment to the dam bottom. Moreover, it is likely that the downstream problem of hydrogen sulfide odor occurs when the dam has the spillway located at a relatively lower position.
