Host: The Association of Japanese Geographers
Name : Annual Meeting of the Association of Japanese Geographers, Spring 2020
Date : March 27, 2020 - March 29, 2020
I. Introduction
Miyakejima is a volcanic island with a population of about 2,400 and an area of 55.5 km2. Mt. Oyama in the center of the island has erupted repeatedly, most recently in 1983 and 2000, forcing islanders to evacuate the island. The geology is covered with basalt, but sediment runoff from the surface is likely to occur in the wasteland affected by the eruption. There are no permanent rivers on the island due to the high permeability of the soil. Therefore, groundwater is mostly used for daily life on the island. Our laboratory has been focusing on research in the Izu and Ogasawara Islands for about 10 years and has been conducting continuous research. In this paper, we report on the field survey in Miyakejima Island and attempt a comprehensive examination of the factors of water environment formation.
II. Research methods
Since September 2019, field surveys have been conducted once every 2 months on Miyakejima. At the site, air temperature, water temperature, pH, RpH, and electrical conductivity (EC) were measured. The sampled water was treated in the laboratory, and the major dissolved components (N +, K +, Ca2 +, Mg2 +, Cl-, NO3 −, SO 42 −) were analyzed using TOC and ion chromatography. Since the survey in November 2019, Mr. Hirose, who is in charge of water supply at the Miyake-mura Regional Development Division, has been asked to accompany the survey and conducted a survey of water sources.
III Results and Discussion
The ion balance of water quality on the island shows that all three water sources (Suzuki Suisan Canal, Okubo Water Source (3), and Okubo Water Source (2)) in the northern part of the island are affected by seawater. It is considered that this is because wind-blown salt accumulates in the spring water under the cliff. The electric conductivity (EC) of tap water sources ranged from 300 to 1,600 μS/cm, showing a characteristic water quality distribution in each region.
The ion balance of the other sites showed that the first and second sources of Miike, Yaema, and Lake Tairo were Ca-HCO3 groundwater. However, the fourth and fifth water sources of Lake Tairo Street are Ca-HCO3 type groundwater and Mg-SO4 type volcanic groundwater, which combine to form an ionic balance, and the water quality of Lake Tairo is thought to be formed by mixing the groundwater and sea salt. Comparing the quality of tap water with that of water sources, the concentrations of tap water are higher in many locations than those of water sources. There may be seasonal changes in water quality depending on the season, and it is necessary to continue the investigation and clarify the cause. In addition, the ion balance of tap water in the north was very similar to that of the fifth water source on the main street. Based on the results of past studies, it is assumed that the water supply situation in 1991 was caused by the supply of drinking water from the well in the south to the north.
IV Conclusion
As for tap water sources, although groundwater, seasonal changes in water quality are possible. The purpose of this study is to clarify the water quality after the eruption, including the past, and to continue to follow the changes in water quality and to deepen the consideration of the factors of water environment formation.
