Host: The Association of Japanese Geographers
Name : Annual Meeting of the Association of Japanese Geographers, Spring 2023
Date : March 25, 2023 - March 27, 2023
I. Introduction.
Miyakejima has experienced frequent eruptive disasters since the beginning of history: since the 20th century, four eruptions have occurred at a frequency of about 20 years, in 1940, 1962, 1983 and 2000. More than 20 years have already passed since the last eruption, and the ongoing expansion of the mountain's deep interior suggests that Miyakejima's volcanic activity is gradually increasing. Under these circumstances, it is important to understand the current water environment on Miyakejima from the perspective of preserving the island's water environment and protecting the livelihoods of the island's inhabitants. In this study, observation points are set up on the island and an analysis based on the results of observations at each point is carried out to clarify the current hydrogeographical environment of Miyakejima and to make a comprehensive study on the factors behind the situation. In addition, this laboratory also conducted surveys on Miyakejima in 2012 and 2013, with the aim of clarifying the actual state of water quality change from the post-eruption period in 2000 to the present by comparing the past data.
II Target area
The study area is Miyake Village, Tokyo. Due to the influence of the Kuroshio Current that flows around the village, the climate is warm and rainy, with an average annual temperature of 18°C and an average annual rainfall of 3024.7 mm. III Research methods
Field surveys were conducted once a month over a nine-month period from April 2022 to December 2022. A total of 35 sites were surveyed on the island, including three lakes (ponds), two streams, nine groundwater sites (of which eight were tap water sources), two springs, 14 tap water sites, three rainwater sites, one other hot spring site and one rainwater-supply site. Air temperature, water temperature, (thermometers and HANNA), electrical conductivity (EC), pH, RpH, chemical oxygen demand (COD) and flow rate were measured in the field. Rainwater was collected at three Miyake-mura water distribution points on the island with the cooperation of Miyakejima Waterworks staff, using a simple sampling device (Hosei type) consisting of a 2 L plastic bottle with a funnel attached to its mouth and a case filled with insulation material.
IV Results and discussion
The average values of electrical conductivity for all sites during the nine surveys (Fig. 1) showed that rainwater, streams and ponds in the north-west tended to have relatively low conductivity, while both groundwater and tap water had high conductivity in the southern area near Oji Pond. The tap water and groundwater in the eastern part of the island were found to have relatively low EC compared to those near Oji Pond in the southern part of the island. Tap water was measured at 14 locations on the island, mainly in public toilets and places of accommodation, and a marked difference in electrical conductivity values between the east and west was evident. In the west, values exceeding 800 µs/cm, which is extremely high for tap water, were often found, while in the east, most values were around 400 µs/cm (Fig. 2).
IV Conclusion.
Continued investigations will be carried out in the future and the results of seasonal variations in groundwater and analysis of the main dissolved constituents will be reflected in the research.
References Hama, T. (2014): Water quality characteristics of volcanic islands in the Izu Islands, Graduation Thesis, Department of Geography, Faculty of Letters, Hosei University.
