09MI21-4 agreed with the corresponding peaks of the 09MI21-2 and 3. On the other hand, the third peak of the 09MI21-4 was dis- tinct, located around 22 nm with a relatively large width. The size distributions of organic colloids measured by fluorescence in Fig. S5 were more or less similar to the corresponding size distributions by UV/vis in Fig. 4, except for the 09MI21-1 and 2. For 09MI21-1 the peak position by fluorescence was smaller than that by UV/vis and coincided with those of the other groundwater measured by fluorescence.
The comparison of the size distributions of organic colloids in the 09MI21 groundwater samples suggested the presence of dif- ferent organic colloids or their aggregates. As these results were for the different sections of single borehole, it was concluded that