Runoff of acidic substances that originated from atmospheric deposition onYakushima Island, a world natural heritage site

In this paper we present monitoring data of stream waters that may reflect acidic impacts on the island as well as the rainwater qualities. The pH ran ges of the river water in the Kawara streams in the western part of the isl and and the Yodogo stream in the central part of the island were 5.71-6.35 and 5.85-6.12 during 1992-1999, respectively. The concentrations of SO42- a nd NO3- in the river water were lower than those in the rainwater. Many dif ferences were observed among the sampling sites. Higher concentrations of a cid substances are found in the stream waters of the western area compared to the other areas. On the other hand, sulfuric acid is the major acid in t he rainwater, snow and rime ice. No differences were observed in the ion co nstituents of the rainwater collected in the areas. These results suggested that the densely growing canopy may play a role in holding air pollutants, and acidic substances deposited on the canopy would be discharged as a thr ough-fall and a stem flow. Furthermore, the water mass containing high ioni c substances in the western area has been held in the groundwater layer, co ntinuously supplying the stream waters during dry weather days. On the othe r hand, part of the basic runoff will be diluted with a surface runoff duri ng the rainy days. As a result, the concentrations of the ionic substances in the stream waters during rainy days decreased.