5:15 PM - 6:45 PM
[AHW25-P02] The long-term changes of shallow groundwater quality and its process in the Musashino Plateau, Tokyo
- based on the data in the Suginami Water Treatment Plant -
Keywords:Downtown Tokyo, Shallow groundwater, Water quality, Long term changes, Legacy contamination, Sewage leakage
The Suginami Water Treatment Plant of the Tokyo Metropolitan Government Bureau of Waterworks had pumped groundwater from the Musashino gravel layer for many years through shallow wells of about 15 m deep until 2016. Its water quality data have been published since 1935 by the Tokyo Metropolitan Government. In this presentation, the characteristics and causes of changes in shallow groundwater quality in the Musashino Plateau over the past 80 years will be discussed with reference to the progress of urbanization in the catchment, changes in sewage treatment methods, and the status of sewerage system development. We also examine the current shallow groundwater quality and its evolution process in the Musashino Plateau on the basis of the geochemistry of shallow groundwater sampled in the catchment in February 2024.
The groundwater map by Hosono (2003) revealed that the catchment of shallow groundwater at the plant is limited to the northeast part of Musashino City (about 4 km2 in area). The rapid urbanization of the Musashino City resulted in a decrease in farmland area from 50% in 1947 to 20% in the early 1970s, and to about 5% in the mid-2010s (Musashino City, 2023). Sewerage system development had started in earnest in the early 1970s, and its coverage rate reached 100% in the late 1980s. (Musashino City, 2018). Therefore, by 2016, when shallow groundwater intake was discontinued at the plant, more than 25 years had passed since the completion of the sewerage system in the catchment. For reference, when the sewerage system was not yet in place, domestic wastewater was disposed of by using the so-called 'infiltration pit' in the Musashino Plateau, and the shallow groundwater at that time was therefore significantly contaminated (Tsushima et al., 2008).
The Cl- concentration in shallow groundwater had increased from 5.3 mg/L in 1935 to a maximum of 21-23 mg/L from around 1967 to the late 1990s, and then it began to decrease. Since the early 2000s, however, its concentration has remained almost constant at 15-17 mg/L. The NO3- concentration had begun to increase around 1944 (9.0 mg/L), reached a maximum of 43 mg/L from 1980 to 1990, and then consistently decreased toward 27 mg/L of 2016. These long-term changes in the concentrations of both ions, which are indicators of anthropogenic pollution, clearly indicate that, in the long run, the quality of shallow groundwater in the Musashino Plateau has been improved with the sewerage system development. However, there is a difference of about 10 years between the time when the peak concentrations of both ions were reached and the time when the concentrations began to decline. In addition, the decreasing trends in the concentrations of both ions toward 2016 are clearly different.
It is noteworthy that even in 2016, more than 25 years after sewerage coverage rate had reached 100% in the late 1980s, shallow groundwater quality has not returned to that of pre-urbanization period of the 1930s and 1940s. This may be due to 'legacy contamination' from residual contaminants in the Kanto loam layer, originating from domestic wastewater that was disposed of by infiltration pits in the past. The difference in the observed long-term changes of concentration of each anion may be partly attributed to the difference in the behavior of these anions in the Kanto loam and Musashino gravel layers. On the other hand, it is also plausible that the addition of new contaminants due to sewage leakage from aging sewer pipes slows down the decline in the concentration of contaminants. The NO3- concentration and its δ15N and δ18 O isotopic ratios of shallow groundwater sampled in February 2024 strongly support this scenario. This is also consistent with the fact that the cause of the 2016 water intake discontinuation at the plant was the deterioration of its water quality, namely the detection of E. coli bacteria.
The groundwater map by Hosono (2003) revealed that the catchment of shallow groundwater at the plant is limited to the northeast part of Musashino City (about 4 km2 in area). The rapid urbanization of the Musashino City resulted in a decrease in farmland area from 50% in 1947 to 20% in the early 1970s, and to about 5% in the mid-2010s (Musashino City, 2023). Sewerage system development had started in earnest in the early 1970s, and its coverage rate reached 100% in the late 1980s. (Musashino City, 2018). Therefore, by 2016, when shallow groundwater intake was discontinued at the plant, more than 25 years had passed since the completion of the sewerage system in the catchment. For reference, when the sewerage system was not yet in place, domestic wastewater was disposed of by using the so-called 'infiltration pit' in the Musashino Plateau, and the shallow groundwater at that time was therefore significantly contaminated (Tsushima et al., 2008).
The Cl- concentration in shallow groundwater had increased from 5.3 mg/L in 1935 to a maximum of 21-23 mg/L from around 1967 to the late 1990s, and then it began to decrease. Since the early 2000s, however, its concentration has remained almost constant at 15-17 mg/L. The NO3- concentration had begun to increase around 1944 (9.0 mg/L), reached a maximum of 43 mg/L from 1980 to 1990, and then consistently decreased toward 27 mg/L of 2016. These long-term changes in the concentrations of both ions, which are indicators of anthropogenic pollution, clearly indicate that, in the long run, the quality of shallow groundwater in the Musashino Plateau has been improved with the sewerage system development. However, there is a difference of about 10 years between the time when the peak concentrations of both ions were reached and the time when the concentrations began to decline. In addition, the decreasing trends in the concentrations of both ions toward 2016 are clearly different.
It is noteworthy that even in 2016, more than 25 years after sewerage coverage rate had reached 100% in the late 1980s, shallow groundwater quality has not returned to that of pre-urbanization period of the 1930s and 1940s. This may be due to 'legacy contamination' from residual contaminants in the Kanto loam layer, originating from domestic wastewater that was disposed of by infiltration pits in the past. The difference in the observed long-term changes of concentration of each anion may be partly attributed to the difference in the behavior of these anions in the Kanto loam and Musashino gravel layers. On the other hand, it is also plausible that the addition of new contaminants due to sewage leakage from aging sewer pipes slows down the decline in the concentration of contaminants. The NO3- concentration and its δ15N and δ18 O isotopic ratios of shallow groundwater sampled in February 2024 strongly support this scenario. This is also consistent with the fact that the cause of the 2016 water intake discontinuation at the plant was the deterioration of its water quality, namely the detection of E. coli bacteria.