Mt. Zao Proper has been the active center of the Zao volcanic group in historic tunes. At present, near the entrance of the Jigokuzawa, not far from G oshikidake (the central cone of Zao Proper), several fumaroles are still found. The Sugawa and the Nigorigawa, the too river systems around Zao Proper, have been well known for their poisonous acidic water. The Sugawa, including such branches as the Sukawa, Zaogawa, Miyakawa, Shirakawa, etc., flows into the Sea of Japan. The Nigorigawa has its origin in the vicinity of a crater Jake, Ohania, just west of Goshikidake and, after being joined by various branches, flows into the Pacific Ocean. Both of the systems seem to have a close relation to the volcanic activity. From the limnological point of view, these systems have already been investigated to some extent by Yoshida and others, but many problems are still unsolved. In order to gain further knowledge of the rivers and to study the change s in their chemical constituents, the present research has been carried on since 1951.
So far as Sugawa system. is concerned, in 1951. the author examined the temperature and the chemicall ingredients of the water at 18 stations in the drainage system. The results of the chemical analyses are summarised as follows:-
1) The Sukawa, the Zaogawa, and the Sugawa (the main stream) are strongly acidic. The Sukawa is joined by waters from. Zao hot spring (an acidic alum vitriol spring) and the Zaogawa is contaminated by poisonous acid water from a sulphur mine now in operation. As is shown in Figure 3 of the text, the acid content of the Sukawa has kept the same value for the last twenty years, but, on the contrary, the pH values of the Sugawa have become higher during these years.
2) It must be noted that the sulphate content of the Sukaw-a as well. as of the Zaogaw a is remarkably high, but the chloride content of tie latter is negligibly small as compared with that of the former. This may be explained by the fact the Sukawa contains n-iagniatic juvenile water while the Zaogawwa is not influ-enced by such water. Though running through adjacent areas composed of volcanic mud flows, the two branches show remarkable differences in their chemical properties.
3) As for calcium, magnesium, manganese, iron, and silicate, the water of the two rivers show abnormality in. their content and differ fromm. other branches of the system. The Sukawva especially shows a high concentration of aluminum owing to the fact that the water from the Zao hot spring joins it.
4) At every station in the Sukawa-Sugawa system, the ratio Ca/Mn and Cl/SO₄ keep the same values respectively.
In addition, the writer has been examining the chemical constituents every month since January, 1951, at six stations in the system. Front the. results of the observations, it is found that there is a negative correlation between the sulfate content of the Sugawa and the precipitation except at the snowmeting season.
In the middle of July, 1953, the author explored the Nigorigawa valley andd analysed water taken from. 16 stations in the drainage system. According to the results of these chemical analyses, he has reached the following conclusions:
1) As in the case of the Sugawa system, the Nigorigaw a system has long been famous for poisonous acidic water which originates from Mt. Zao Proper. The present investigation has shown that the pH value of the water in the Nigorigawa-Matsukaiva system increases gradually from 2.9 to 5.9 while the streams are running down grade.
2) Sulfur deposits which had once been mined are distributed in the vicinity of Paikokuten. Being influenced by those deposits, as well as by the crater lake Okanma, the water of the upper reaches of the Nigorigawa is rich, in sulfate content. Waters taken from the upper stream of the Nigorigawa and the Jigokuzawa are characterized by a large content of iron, calcium, and magnesiulit. 3)