Host: The Association of Japanese Geographers
Name : Annual Meeting of the Association of Japanese Geographers, Spring 2023
Date : March 25, 2023 - March 27, 2023
1. Introduction
Among the Japanese islands, the Yaeyama Islands, which are located at the southwesternmost tip of Japan and belong to a subtropical climate, have a special marine environment, such as being surrounded by coral reefs. There is concern about the effects ofHosei University's Hydrogeography Laboratory began conducting surveys in 2007, and has conducted surveys of the entire island every three months for several years, fixed-point surveys every January for several years, and surveys of the impact of heavy rain several times, and reported them. Recently, there have been concerns about the impact of new resort developments and the construction of Self-Defense Forces bases.This time, after reorganizing all the survey results and deepening the contents of the hydrological journal of Ishigaki Island, we will clarify the transition, current situation, and issues of the water environment, and present guidelines for future surveys and research.
2. Research method
Fixed points were determined from the results of the preliminary survey in 2007, and field surveys and water sampling were conducted at 50 to 100 points every three months from May 2008 for about two years.During the approach of the typhoon in September 2010, 9 points were surveyed every 24 hours and 5 points were surveyed every 3 hours, and the changes before and after the event were intensively observed. At the site, air temperature, water temperature, pH, RpH, electrical conductivity (EC), and COD were measured.In the laboratory, after filtration, EC was measured again, and major dissolved components were analyzed by TOC and ion chromatography.
Furthermore, with the cooperation of local citizens, from May 2009 to October 2010, 15 locations were installed each month, mainly in the lower reaches of major rivers. We requested monthly sampling at the site and conducted the same analysis.
In addition, after sorting out information related to hydrological phenomena such as geology, topography, climate and weather, land use, agriculture, industry, etc., and creating a hydrological journal, the characteristics of the overall water environment of Ishigaki Island were shown.
3. Results and considerations
A. Whole island survey
The characteristics of each region and river were clarified by summarizing the results of surveys conducted at approximately 80 locations on average in various seasons for almost all rivers on the island. Water quality clearly shows differences in topography, geology, and land use.
The Arakawa River in northern Omoto is strongly influenced by granite. In addition, the Miyara River, Nagura River, Todoroki River, and Shinkawa River are strongly influenced by human activities such as land use.
B. Event research
After the rainfall, the EC dropped sharply, and after a minimum value of less than 250 μS/cm was observed, the EC began to increase, but it took several days to recover. The recovery rate of EC is faster upstream, and the recovery is slower toward downstream. Correspondingly, Ca2+, Mg2+, and Cl- also increased and decreased, and depending on the point, Na+ and SO42- also increased and decreased. EC fluctuations are caused by the outflow of dissolved substances due to rainfall, but the magnitude of the difference is different depending on the location, and it was found that there are places where soil components are outflow and places where they are not.
4. Conclusion
It is difficult to summarize the results of the long-term research at once, and it is difficult to say that the overall picture has been clarified. There is room for improvement in the hydrological journal, and I would like to clarify the current state of the water environment on Ishigaki Island while collecting more detailed and highly accurate data.
References
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