5:15 PM - 6:45 PM
[AHW24-P02] A preliminary study on the geochemical characteristics and origin of shallow groundwater in Meguro-dai, southeastern Musashino Upland, downtown Tokyo
Keywords:Urban shallow groundwater, Downtown Tokyo, Musashino Upland, Origin of groundwater, Water quality evolution, Isotopes
Recently, urban shallow groundwater has been attracting attention as an emergency water source for the large cities. Municipalities are focusing their efforts on the development and maintenance of shallow well networks. On the other hand, information on the water quality, origin, and availability of urban shallow groundwater is still scarce, which is an obstacle to its effective and efficient utilization. In this study, we conducted a survey to understand the current status of shallow groundwater quality, and to elucidate the quality evolution process and origin of shallow groundwater in the Meguro-Dai (M1b surface according to Endo et al., 2019), which occupies the southeastern part of the Musashino Upland, downtown Tokyo. The preliminary results of the survey are to be reported here.
The survey was conducted in November 2023 and covered approximately 35 shallow wells with hand-pumps located in the Shinagawa Ward area of the Meguro-Dai. The depths of the wells are generally 10-15 m, and the Musashino gravel layer, which is located beneath the Itabashi clay layer, is the main source of water. Some of the wells may be drawing water from the sandy layers in the Tokyo Formation, which underlie the Musashino gravel layer.
The shallow groundwater in the Meguro-Dai is of the Ca-HCO3 type, characterized by low dissolved oxygen concentration (mostly, <1.5 mg/L) and low redox potential (lowest value of -73 mV). While an observed maximum NO3- concentration is 19.5 mg/L, about half of the wells had concentrations below the lower limit of quantitation (<0.1 mg/L). NH4+ and NO2- with highest concentrations of 1.7 mg/L and 2.7 mg/L, respectively were detected in several wells, and SO42- showed a remarkably wide range of concentration of 4.1-90.1 mg/L. This indicates that the shallow groundwater in the Musashino gravel layer at the Meguro-Dai is in a reducing environment and that denitrification and sulfate reduction reactions are likely to proceed in the groundwater. A three-component mixing analysis of precipitation, water leakage, and sewage leakage based on Cl- concentration and oxygen isotopic ratio of water revealed that the contribution of sewage leakage in groundwater recharge reached a maximum of 30% in some wells. Considering the existence of the Itabashi clay layer, several meters thick and located on the Musashino gravel layer, the mixing process of the sewage leakage into shallow groundwater will be an issue for further investigation.
In order to compare the shallow groundwater quality in the Meguro-Dai with that in the Yodobashi-Dai (three wells) to the north and the Ebara-Dai (two wells) to the south, both of which correspond to the so-called S-surface, we conducted a similar investigation of shallow groundwater in the Tokyo Formation under the Shibuya clay layer. Although the ratio of HCO3- to total anions in the shallow groundwater at the Yodobashi-Dai and the Ebara-Dai was slightly higher than that at the Meguro-Dai, there was no clear difference in the concentration of the respective ions and water composition among the sites.
The survey was conducted in November 2023 and covered approximately 35 shallow wells with hand-pumps located in the Shinagawa Ward area of the Meguro-Dai. The depths of the wells are generally 10-15 m, and the Musashino gravel layer, which is located beneath the Itabashi clay layer, is the main source of water. Some of the wells may be drawing water from the sandy layers in the Tokyo Formation, which underlie the Musashino gravel layer.
The shallow groundwater in the Meguro-Dai is of the Ca-HCO3 type, characterized by low dissolved oxygen concentration (mostly, <1.5 mg/L) and low redox potential (lowest value of -73 mV). While an observed maximum NO3- concentration is 19.5 mg/L, about half of the wells had concentrations below the lower limit of quantitation (<0.1 mg/L). NH4+ and NO2- with highest concentrations of 1.7 mg/L and 2.7 mg/L, respectively were detected in several wells, and SO42- showed a remarkably wide range of concentration of 4.1-90.1 mg/L. This indicates that the shallow groundwater in the Musashino gravel layer at the Meguro-Dai is in a reducing environment and that denitrification and sulfate reduction reactions are likely to proceed in the groundwater. A three-component mixing analysis of precipitation, water leakage, and sewage leakage based on Cl- concentration and oxygen isotopic ratio of water revealed that the contribution of sewage leakage in groundwater recharge reached a maximum of 30% in some wells. Considering the existence of the Itabashi clay layer, several meters thick and located on the Musashino gravel layer, the mixing process of the sewage leakage into shallow groundwater will be an issue for further investigation.
In order to compare the shallow groundwater quality in the Meguro-Dai with that in the Yodobashi-Dai (three wells) to the north and the Ebara-Dai (two wells) to the south, both of which correspond to the so-called S-surface, we conducted a similar investigation of shallow groundwater in the Tokyo Formation under the Shibuya clay layer. Although the ratio of HCO3- to total anions in the shallow groundwater at the Yodobashi-Dai and the Ebara-Dai was slightly higher than that at the Meguro-Dai, there was no clear difference in the concentration of the respective ions and water composition among the sites.