Host: The Association of Japanese Geographers
Name : Annual Meeting of the Association of Japanese Geographers, Spring 2021
Date : March 26, 2021 - March 28, 2021
Ⅰ Introduction The Yaeyama Islands are composed of 32 islands. Among them, Ishigaki Island is the central island because it has the largest population and the tourism industry is prosperous. In the central part of the island, there is Mt. Omoto at an altitude of 526m, and mountains are connected. In addition, because it belongs to the subtropical climate, many primeval forests such as coral reefs and mangroves are distributed around it. However, Ishigaki Island has a low sewerage system, and the rivers that flow through the city area are becoming more polluted. Ⅱ Research method From September 2019 to October 2020, monthly fixed-point observations were conducted at about 15 points. The investigators were local residents who had the samples transported on a regular basis and analyzed the samples at the university. At the site, air temperature, water temperature, pH, RpH, and EC (electrical conductivity) were measured. In the laboratory, EC was analyzed again. In October 2019, a whole island survey was conducted at about 98 sites. Ⅲ Results and discussion There is the Ishigaki Shinkawa River, which is a second-class river that flows through the city from the area around the former Ishigaki Airport. A total of 10 fixed-point observations were made from Abunabashi, which is located upstream, and the average EC was 1052. The highest EC value was 1660 and the lowest EC value was 490. However, of the 10 water samples, the EC value exceeded 1000 in 7 times. (Figure 1). The average pH was 7.5 and the average water temperature was about 33 degrees. In addition, the reason why the EC value of Akashita Bridge observed in May is high is thought to be saltwater run-up. In the whole island survey in October 2019, the EC value of Abunabashi was 1068. However, the water sampling point with the highest EC value in the whole island survey was Kandabashi, a tributary of the Nagura River, which is also affected by saltwater run-up. (Fig. 2) The maximum water temperature of Abunagawa is 38.1 degrees Celsius, and the reason why the water temperature is high is thought to be the influence of wastewater from the adjacent factory, and the EC value is also high. References Takeshi Hayashi, Yohei Uchida, Takemasa Ishii (2003): Predictive Hydrological Environmental Studies in the Nansei Islands, Journal of the Japanese Society of Hydrological Sciences, 33-1, 13-18.
