Watershed Characteristics and Water Quality in Suburban River in Tokyo: Asakawa River

1. Introduction

In the Asakawa River, a suburban river in Tokyo, there are issues of water quality such as wastewater problems or substance runoff from forest ecosystem. To understand the water quality and characteristics of the river basin, not only field surveys but also comprehensive studies combining various methods are required. This study aims to clarify the characteristics of the Asakawa River watershed based on the results of field surveys, water quality analysis, and statistical analysis using the results.

2. Method

To understand the watershed characteristics and water quality of the Asakawa River, data analysis, field survey, water quality analysis, statistical analysis, and comparison with previous studies were conducted. Data analysis was conducted to obtain population trends and population density by township from the census results, changes in the watershed land use ratio from the National Land Numerical Data, and population trends by sewage treatment method from the Hachioji City Basic Plan for Domestic Wastewater Treatment 2014. Field surveys were conducted with monthly observations during a 17-month period from June 2020 to October 2021, and self-recording instrument observations from November 2021 to January 2022. Water quality analysis was conducted for total organic carbon and major dissolved constituents in July 2020, October 2020, January 2021 and September 2021. Ammonium ion, nitrite, nitrate and phosphate were measured of the samples of September,2021 . In the statistical analysis section, a cluster analysis was performed using the September 2021 data, which has the largest number of measured items. Comparison with previous studies was made between the electrical conductivity values of Ogura (1980) and Ota and Omori (2004) and the electrical conductivity values of the present field observations.

3. Results and Discussion

Monthly observations showed an increase in electrical conductivity (EC) during the winter months. The pH was low in winter, due to groundwater, and high in summer, possibly due to algal carbonate assimilation. In summer, pH was higher, because of carbon assimilation by algae. Nitrate ions were detected upstream in many locations, probably due to nitrogen saturation in the forest ecosystem. High concentrations of nitrate were detected in the Yamadagawa Riv., where wastewater from a sewage treatment plant flows in. It indicates that the wastewater from the plant has not been completely treated. Ammonium and nitrite were also detected upstream, indicating the effluent from the septic tank may have had an effect. Cluster analysis produced five clusters. In the Yudonogawa Riv., the upstream and downstream observation points were classified into different clusters, suggesting that water quality changes as the river flows downstream.

4. Conclusion

From this study, four issues in the Asakawa Riv. watershed were identified: the pollution caused by septic tank effluent in upper stream, nitrate runoff due to nitrogen saturation in the forest ecosystem upstream, pollution caused by the inflow of sewage treatment plant effluent into the small tributary named Yamadagawa Riv. and pollution caused by domestic wastewater from the Yudonogawa Riv. watershed which locates in southern part of its basin. To solve these problems, improvement of the watershed environment is required.