In early February 2016, tap water originating from Agano River was reported to have an unusual odor. In order to elucidate the source of the compound and its cause, the Agano River Basin was investigated from downstream to upstream following the specific odor. As a result, we found that the odor was derived from the branches of the Aga River in Fukushima Prefecture, and the maximum odor concentration reached 240,000. By measuring a water sample with the maximum odor intensity, we identified the odor compound using: 1) a search system for unknown mass spectrum using gas chromatography mass spectrometry (GC/MS); 2) sensory gas chromatography with flame ionization detector and 3) chemical constitutions deducted from the precise mass using gas chromatography time-of-flight mass spectrometry. The results of these measurements suggested that the odor compound was 2-methoxy-3,5- dimethylpyrazine (2-MDMP), which was confirmed by measurement of a 2-MDMP standard using GC/MS. Purge and Trap GC/MS was adopted as an easy and rapid method to determine the 2-MDMP concentration at the odor threshold level in river water. Using this method, 2-MDMP concentrations ranged from ＜0.5 to 29,000 ng/L. High correlation was observed between concentrations of 2-MDMP and odor concentrations, which shows that 2-MDMP was the main pollutant producing the unusual odor in the Agano River Basin.
