17:15 〜 18:30
[AHW22-P03] Hydro-geographical Study on the Water Environment of Asakawa River in Tamagawa River System
キーワード：浅川、生活排水、降水量、藻類、地下水
Ⅰ.Introduction
In the Asakawa River, a part of the Tamagawa River system, the deterioration of water quality due to domestic wastewater infrow had been advanced, and to solve this problem, various studies have been carried out(Ogura, 1980). However, most of the studies have conducted on the Minami-Asakawa River, the largest tributary of this river, and there were few comprehensive studies on the entire Asakawa River system. In this study, we will clarify the water environment characteristics of the whole Asakawa River system and examine the influence of the Minami-Asakawa River on the mainstream.
Ⅱ.Regional Summary
The source of this river is located at the Mt.Jimba in Hachioji City, Tokyo, and flows through Hachioji City before joining the Tamagawa River in Hino City. It has the Minami-Asakawa River, Kawaguchi River, Yudono River, Shiroyama River, Yamada River and other tributaries. The basin is surrounded by mountains and hills, such as the Kanto Mountains to the west, the Kasumi Hills to the north, and the Tama Hills to the south. The upstream area is mainly coniferous forest with low population density. The downstream area is relatively flat, with urban development and a high population density.
Ⅲ.Reserch Methods
After reviewing the results of previous studies, we started the field survey on May 3, 2020, and have been conducting continuous monthly observations at 34 sites in Asakawa river system. The items to be measured in the field are air temperature, water temperature, electrical conductivity(EC), pH, RpH, COD, and flow rate. The sampled river water was brought to the laboratory for analysis of major dissolved components using TOC and ion chromatography.
Ⅳ.Results and Discussion
The EC of the mainstream of the Asakawa River averages around 74-84μS/cm in the upper reaches. It increases from the point where the river passes through the mountainous area and enters the city, reaching an average of 132 μS/cm at Matsue Bridge. In the upstream of the Minami-Asakawa River, the values were higher than those in the upper reaches of the mainstream, averaging 109-115μS/cm. There was a difference of about 15μS/cm in the average EC between the value at the Matsue Bridge before convergence and at the Asakawa Bridge after the confluence of the Minami-Asakawa River, indicating that the mainstream EC increased with the confluence of it. In the lower reaches of the mainstream, the average EC was higher than 190 μS/cm, and values of 240 μS/cm or higher were observed in November and December. The pH values tended to decrease in the upper reaches of the mainstream, and was 7.2 in the uppermost reaches. The coefficient of variation was small in the upstream and large in the downstream, especially at the Arai Bridge at the downstream end, where the maximum value is 8.7 and the minimum is 7.6, a value gap of more than 1. The maximum values were observed in August at most of the sites, and this is because the low carbon dioxide concentration caused by the increased photosynthesis of algae due to the retention of water and high temperature. In November and December, when there was little precipitation, the values were lowest at many sites, indicating the influence of groundwater inflow and hence the RpH-pH values were mostly high.
Ⅴ.Conclusion
In the Asakawa River basin, water quality is different between urban and mountainous areas, and the spatial distribution of EC and pH values are also different. The seasonal changes in values of water quality index are influenced by the amount of precipitation, the level of alges photosynthesis, the presence of groundwater inflow and other factors. Along with confluence of Minami-Asakawa River to mainstream, the values are change many time, therefore it has significant effect on mainstream water quality. Hereafter, it will be necessary to clarify the characteristics of these changes through further observations and analyses.
In the Asakawa River, a part of the Tamagawa River system, the deterioration of water quality due to domestic wastewater infrow had been advanced, and to solve this problem, various studies have been carried out(Ogura, 1980). However, most of the studies have conducted on the Minami-Asakawa River, the largest tributary of this river, and there were few comprehensive studies on the entire Asakawa River system. In this study, we will clarify the water environment characteristics of the whole Asakawa River system and examine the influence of the Minami-Asakawa River on the mainstream.
Ⅱ.Regional Summary
The source of this river is located at the Mt.Jimba in Hachioji City, Tokyo, and flows through Hachioji City before joining the Tamagawa River in Hino City. It has the Minami-Asakawa River, Kawaguchi River, Yudono River, Shiroyama River, Yamada River and other tributaries. The basin is surrounded by mountains and hills, such as the Kanto Mountains to the west, the Kasumi Hills to the north, and the Tama Hills to the south. The upstream area is mainly coniferous forest with low population density. The downstream area is relatively flat, with urban development and a high population density.
Ⅲ.Reserch Methods
After reviewing the results of previous studies, we started the field survey on May 3, 2020, and have been conducting continuous monthly observations at 34 sites in Asakawa river system. The items to be measured in the field are air temperature, water temperature, electrical conductivity(EC), pH, RpH, COD, and flow rate. The sampled river water was brought to the laboratory for analysis of major dissolved components using TOC and ion chromatography.
Ⅳ.Results and Discussion
The EC of the mainstream of the Asakawa River averages around 74-84μS/cm in the upper reaches. It increases from the point where the river passes through the mountainous area and enters the city, reaching an average of 132 μS/cm at Matsue Bridge. In the upstream of the Minami-Asakawa River, the values were higher than those in the upper reaches of the mainstream, averaging 109-115μS/cm. There was a difference of about 15μS/cm in the average EC between the value at the Matsue Bridge before convergence and at the Asakawa Bridge after the confluence of the Minami-Asakawa River, indicating that the mainstream EC increased with the confluence of it. In the lower reaches of the mainstream, the average EC was higher than 190 μS/cm, and values of 240 μS/cm or higher were observed in November and December. The pH values tended to decrease in the upper reaches of the mainstream, and was 7.2 in the uppermost reaches. The coefficient of variation was small in the upstream and large in the downstream, especially at the Arai Bridge at the downstream end, where the maximum value is 8.7 and the minimum is 7.6, a value gap of more than 1. The maximum values were observed in August at most of the sites, and this is because the low carbon dioxide concentration caused by the increased photosynthesis of algae due to the retention of water and high temperature. In November and December, when there was little precipitation, the values were lowest at many sites, indicating the influence of groundwater inflow and hence the RpH-pH values were mostly high.
Ⅴ.Conclusion
In the Asakawa River basin, water quality is different between urban and mountainous areas, and the spatial distribution of EC and pH values are also different. The seasonal changes in values of water quality index are influenced by the amount of precipitation, the level of alges photosynthesis, the presence of groundwater inflow and other factors. Along with confluence of Minami-Asakawa River to mainstream, the values are change many time, therefore it has significant effect on mainstream water quality. Hereafter, it will be necessary to clarify the characteristics of these changes through further observations and analyses.