Presentation Information
[O12-P13]Estimation of the temporal dynamics of sewage inflow into wells at Otomeyama Park, Shinjuku Ward
*Kohei Hara1 (1. Kaijo Earth Science Club)
Keywords:
tank model,groundwater,infrastructure
1. Background and Objectives
This study focuses on Otomeyama Park in Shinjuku, Tokyo, where rare spring water exists in an urban area. The Earth Science Club has monitored spring discharge for 15 years. A 2024 study showed that carbamazepine (CBZ), a persistent pharmaceutical, is an effective tracer of sewage contamination. Because CBZ was detected year-round, mixing of sewage-derived water was confirmed, suggesting possible sewer pipe damage and undetected deterioration.
2. Methods
Groundwater level and CBZ concentration were measured at a well in Otomeyama Park. CBZ is an antiepileptic drug that degrades slowly and is not naturally present. Groundwater levels were recorded every 30 minutes from March 7 to April 12 using a HOBO logger and averaged hourly.
CBZ samples were collected 19 times between January 11 and March 15 and analyzed using LC/MS/MS.
3. Results
Groundwater levels are shown in Figure 1. A tank model was constructed to simulate groundwater fluctuations. CBZ concentrations (Figure 2) remained nearly constant with minor rainfall effects, averaging 2.75 ng/L.
4. Discussion
A two-layer tank model was constructed with an upper loam layer and a lower Musashino gravel aquifer, with a larger lower tank. Parameters were optimized using Excel Solver (ΔT = 1 h).
Model performance was evaluated using the Nash–Sutcliffe efficiency coefficient, yielding E = 0.741, indicating acceptable accuracy. A composite runoff coefficient k = 1.77 × 10-5/h was obtained.
This study aims to estimate the onset of sewage mixing from groundwater CBZ concentrations and to evaluate sewer pipe service life using installation records. A previous 2025 study contained errors in the tank model structure, leading to incorrect interpretation. This study reconstructs the model and identifies appropriate parameters.
Two scenarios were tested.
(A) If concentrations are increasing toward equilibrium, the time constant (1/k) is 6.45 years. However, concentrations remained nearly constant over three months and matched previous observations, so this scenario is rejected.
(B) If concentrations are at equilibrium, the onset time is estimated by −(1/k)ln/k. With X = 0.99, the onset is more than 29.7 years ago, consistent with observations. Thus, this scenario is considered valid, though long-term verification is required.
Finally, sewer pipe service life was evaluated. The oldest pipe in the recharge area was installed in 1933. Assuming older pipes fail first, the maximum service life is about 63.4 years. This is consistent with the 50-year standard recommended in Japan.
5. Acknowledgments
We would like to express our sincere gratitude to Professor Tsuyoshi Murahashi (Nihon Pharmaceutical University) for his cooperation in observations and analysis, as well as his valuable advice. We also thank Mr. Mikio Asami (Chairperson of the Shimo-Ochiai Higashi Neighborhood Association) for lending us the key to the well. We are grateful to the Tokyo Metropolitan Government Sewerage Office for allowing us to investigate the installation dates of sewer pipes in the recharge area.
We also received valuable advice from the following individuals:
Mr. Naoki Yamada (Kaijo Junior and Senior High School)
Mr. Sho Iwade (Kaijo Junior and Senior High School)
Taichi Shimokobe (Massachusetts Institute of Technology)
Koki Sato (Tohoku University)
Katsuto Nakasute (Shinshu University)
5. References
Japan Meteorological Agency, “Past Weather Data Search”
Sato et al. (2012), Investigation of Spring Water in Otomeyama Park, Shinjuku, and Evaluation of Infiltration Facilities (unpublished)
Takano et al. (2015), Long-term Changes in Spring Discharge and Their Causes in Otomeyama Park, Shinjuku, Journal of Groundwater Hydrology
Shimokobe (2022), Understanding Groundwater Conditions Surrounding Springs Using Numerical Models: A Case Study of Otomeyama Park Area (unpublished)
Oyama (1991), Changes in Discharge of Ubako Hot Spring, Hakone
Uchida (2024), Investigation of E. coli and Source Estimation Using Pharmaceutical Analysis in Otomeyama Park (unpublished; presented at JpGU 2024 High School Poster Session)
University of Tsukuba Hydrological Science Laboratory (2009), Hydrological Science, Kyoritsu Publishing
Thomas Rodgers (2013), Chemical Reaction Engineering
Sugawara (1972), Runoff Analysis Methods
Kyoto City University Gene Information Database, “Carbamazepine”
This study focuses on Otomeyama Park in Shinjuku, Tokyo, where rare spring water exists in an urban area. The Earth Science Club has monitored spring discharge for 15 years. A 2024 study showed that carbamazepine (CBZ), a persistent pharmaceutical, is an effective tracer of sewage contamination. Because CBZ was detected year-round, mixing of sewage-derived water was confirmed, suggesting possible sewer pipe damage and undetected deterioration.
2. Methods
Groundwater level and CBZ concentration were measured at a well in Otomeyama Park. CBZ is an antiepileptic drug that degrades slowly and is not naturally present. Groundwater levels were recorded every 30 minutes from March 7 to April 12 using a HOBO logger and averaged hourly.
CBZ samples were collected 19 times between January 11 and March 15 and analyzed using LC/MS/MS.
3. Results
Groundwater levels are shown in Figure 1. A tank model was constructed to simulate groundwater fluctuations. CBZ concentrations (Figure 2) remained nearly constant with minor rainfall effects, averaging 2.75 ng/L.
4. Discussion
A two-layer tank model was constructed with an upper loam layer and a lower Musashino gravel aquifer, with a larger lower tank. Parameters were optimized using Excel Solver (ΔT = 1 h).
Model performance was evaluated using the Nash–Sutcliffe efficiency coefficient, yielding E = 0.741, indicating acceptable accuracy. A composite runoff coefficient k = 1.77 × 10-5/h was obtained.
This study aims to estimate the onset of sewage mixing from groundwater CBZ concentrations and to evaluate sewer pipe service life using installation records. A previous 2025 study contained errors in the tank model structure, leading to incorrect interpretation. This study reconstructs the model and identifies appropriate parameters.
Two scenarios were tested.
(A) If concentrations are increasing toward equilibrium, the time constant (1/k) is 6.45 years. However, concentrations remained nearly constant over three months and matched previous observations, so this scenario is rejected.
(B) If concentrations are at equilibrium, the onset time is estimated by −(1/k)ln/k. With X = 0.99, the onset is more than 29.7 years ago, consistent with observations. Thus, this scenario is considered valid, though long-term verification is required.
Finally, sewer pipe service life was evaluated. The oldest pipe in the recharge area was installed in 1933. Assuming older pipes fail first, the maximum service life is about 63.4 years. This is consistent with the 50-year standard recommended in Japan.
5. Acknowledgments
We would like to express our sincere gratitude to Professor Tsuyoshi Murahashi (Nihon Pharmaceutical University) for his cooperation in observations and analysis, as well as his valuable advice. We also thank Mr. Mikio Asami (Chairperson of the Shimo-Ochiai Higashi Neighborhood Association) for lending us the key to the well. We are grateful to the Tokyo Metropolitan Government Sewerage Office for allowing us to investigate the installation dates of sewer pipes in the recharge area.
We also received valuable advice from the following individuals:
Mr. Naoki Yamada (Kaijo Junior and Senior High School)
Mr. Sho Iwade (Kaijo Junior and Senior High School)
Taichi Shimokobe (Massachusetts Institute of Technology)
Koki Sato (Tohoku University)
Katsuto Nakasute (Shinshu University)
5. References
Japan Meteorological Agency, “Past Weather Data Search”
Sato et al. (2012), Investigation of Spring Water in Otomeyama Park, Shinjuku, and Evaluation of Infiltration Facilities (unpublished)
Takano et al. (2015), Long-term Changes in Spring Discharge and Their Causes in Otomeyama Park, Shinjuku, Journal of Groundwater Hydrology
Shimokobe (2022), Understanding Groundwater Conditions Surrounding Springs Using Numerical Models: A Case Study of Otomeyama Park Area (unpublished)
Oyama (1991), Changes in Discharge of Ubako Hot Spring, Hakone
Uchida (2024), Investigation of E. coli and Source Estimation Using Pharmaceutical Analysis in Otomeyama Park (unpublished; presented at JpGU 2024 High School Poster Session)
University of Tsukuba Hydrological Science Laboratory (2009), Hydrological Science, Kyoritsu Publishing
Thomas Rodgers (2013), Chemical Reaction Engineering
Sugawara (1972), Runoff Analysis Methods
Kyoto City University Gene Information Database, “Carbamazepine”