5:15 PM - 7:15 PM
[AHW31-P02] Issues and challenges of a multi-tracer method for quantifying sewage leakage in urban areas
Keywords:urban area, sewarage leak, groundwater, multi-tracer
In recent years, road cave-ins have occurred in many parts of Japan, and leakage from sewer pipes has been pointed out as one of the causes. According to Tamoto et al. (2021), the number of road cave-ins caused by sewer pipes decreased from over 4,000 in FY2008 to about 3,000 in FY2018. However, according to the Ministry of Land, Infrastructure, Transport and Tourism, the number was still about 2,600 in FY2022 and remains high. In January 2025, a large-scale road cave-in accident occurred in Yashio City, Saitama Prefecture, and road cave-ins caused by sewage leaks were once again widely covered by the media.
Although sewage leaks that cause road cave-ins are more likely to occur on arterial roads where large-diameter sewer pipes are buried, the mechanism of sewage leaks, i.e., sewer pipe spoilage (hydrogen sulfide generated from sewage becomes sulfuric acid through microbial activity on the inner wall of the sewer pipe spoilage and damage to the pipe culvert (e.g., Iwamatsu et al., 1988)), the poor connections from households to sewer pipes, aging of sewer pipes, etc. (e.g., Yasuhara et al., 2014), suggest that sewage leaks themselves are likely to occur to a greater extent. Yasuhara et al. (2013, 2014, 2015) pointed out that high concentrations of SO42- are dissolved in shallow groundwater in a wide area of the Shakujii River basin, which flows over the Musashino upland located in the southwestern part of the Kanto Plain. In this case, SO42- concentrations were reported to be particularly high in the downstream area of the river basin, where urbanization has largely progressed. In a survey conducted by the authors, the contribution of sewage leakage has been widely found from the springs scattered in the western part of the Kanto Plain (e.g., Hayashi et al., 2024). In this study, PPCPs and artificial sweeteners were detected in the spring waters not only in suburban areas but also in urban areas with 100% sewerage coverage.
On the quantitative aspect of the water cycle, sewage leakage can be one of the major sources of groundwater recharge in urban areas where groundwater recharge is decreased due to surface coverage. On the other hand, from the viewpoint of quality, leaked sewers can contaminate not only groundwater but also surface water bodies recharged by springs. Therefore, it is essential to evaluate sewage leakage quantitatively in each part of urban areas to prevent the destruction of social infrastructure caused by road cave-ins, conserve the groundwater environment, and sustainably use groundwater resources. However, in many urban areas in Japan, water supply and sewage systems distribute and collect water over a wide area. In addition, the combined sewerage system in which sewage and rainwater flow in the same culvert makes it hard to quantify sewage leakage by region from data on water supply distribution, water revenue, and water volume received at sewage treatment plants. Therefore, it is difficult to quantify the amount of sewage leakage for each part from data on the amount of water distributed and received from waterworks and sewage treatment plants. Thus, it is essential to conduct a multi-tracer method that combines various elements, their isotopes, and chemical substances dissolved in groundwater. This presentation reports the results of our study on the issues and challenges for quantitative evaluation of sewage leakage focusing on SO42-.
In the above-mentioned mechanism of decay and breakage of sewer pipes, sulfur isotope fractionation occurs when SO42- is converted to hydrogen sulfide by sulfate-reducing bacteria in sewage (e.g., Hoefs, 2021). Therefore, isotope fractionation may be more pronounced in trunk sewer pipes and downstream sewers where a large amount of sewage gathers. On the other hand, when sewage leaks due to poor connection between households and sewer pipes or aging of sewer pipes, it is assumed that sewage leaks in which sulfate reductions have not progressed, and sulfur isotope fractionation also does not progress during this process. Given these points, to use SO42- and sulfur isotopes as indicators of sewage leakage, it is necessary to evaluate whether or not there is an effect of sulfate reduction before the leakage from the sewer pipes. For example, some kinds of PPCPs that are easily decomposed and not preserved in a reducing condition are the candidates of the indicator.
Although sewage leaks that cause road cave-ins are more likely to occur on arterial roads where large-diameter sewer pipes are buried, the mechanism of sewage leaks, i.e., sewer pipe spoilage (hydrogen sulfide generated from sewage becomes sulfuric acid through microbial activity on the inner wall of the sewer pipe spoilage and damage to the pipe culvert (e.g., Iwamatsu et al., 1988)), the poor connections from households to sewer pipes, aging of sewer pipes, etc. (e.g., Yasuhara et al., 2014), suggest that sewage leaks themselves are likely to occur to a greater extent. Yasuhara et al. (2013, 2014, 2015) pointed out that high concentrations of SO42- are dissolved in shallow groundwater in a wide area of the Shakujii River basin, which flows over the Musashino upland located in the southwestern part of the Kanto Plain. In this case, SO42- concentrations were reported to be particularly high in the downstream area of the river basin, where urbanization has largely progressed. In a survey conducted by the authors, the contribution of sewage leakage has been widely found from the springs scattered in the western part of the Kanto Plain (e.g., Hayashi et al., 2024). In this study, PPCPs and artificial sweeteners were detected in the spring waters not only in suburban areas but also in urban areas with 100% sewerage coverage.
On the quantitative aspect of the water cycle, sewage leakage can be one of the major sources of groundwater recharge in urban areas where groundwater recharge is decreased due to surface coverage. On the other hand, from the viewpoint of quality, leaked sewers can contaminate not only groundwater but also surface water bodies recharged by springs. Therefore, it is essential to evaluate sewage leakage quantitatively in each part of urban areas to prevent the destruction of social infrastructure caused by road cave-ins, conserve the groundwater environment, and sustainably use groundwater resources. However, in many urban areas in Japan, water supply and sewage systems distribute and collect water over a wide area. In addition, the combined sewerage system in which sewage and rainwater flow in the same culvert makes it hard to quantify sewage leakage by region from data on water supply distribution, water revenue, and water volume received at sewage treatment plants. Therefore, it is difficult to quantify the amount of sewage leakage for each part from data on the amount of water distributed and received from waterworks and sewage treatment plants. Thus, it is essential to conduct a multi-tracer method that combines various elements, their isotopes, and chemical substances dissolved in groundwater. This presentation reports the results of our study on the issues and challenges for quantitative evaluation of sewage leakage focusing on SO42-.
In the above-mentioned mechanism of decay and breakage of sewer pipes, sulfur isotope fractionation occurs when SO42- is converted to hydrogen sulfide by sulfate-reducing bacteria in sewage (e.g., Hoefs, 2021). Therefore, isotope fractionation may be more pronounced in trunk sewer pipes and downstream sewers where a large amount of sewage gathers. On the other hand, when sewage leaks due to poor connection between households and sewer pipes or aging of sewer pipes, it is assumed that sewage leaks in which sulfate reductions have not progressed, and sulfur isotope fractionation also does not progress during this process. Given these points, to use SO42- and sulfur isotopes as indicators of sewage leakage, it is necessary to evaluate whether or not there is an effect of sulfate reduction before the leakage from the sewer pipes. For example, some kinds of PPCPs that are easily decomposed and not preserved in a reducing condition are the candidates of the indicator.