Fig. 1
![Fig. 1](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2Fs12889-019-6934-7/MediaObjects/12889_2019_6934_Fig1_HTML.png)
Study area and the location of monitoring stations. Left panel shows map of Japan. Niigata and Kumamoto prefecture were shown in the right-upper panel and right-lower panel, respectively. Monitoring stations were presented as red circles. The original data of the map was obtained from http://d-maps.com/index.php?lang=en