Evaluating sewer exfiltration in groundwater by pharmaceutical tracers after the 2016 Kumamoto earthquakes, Japan

Highlights

• Sewer exfiltration after the Kumamoto earthquakes (M 7.3) was investigated.

• Carbamazepine, crotamiton, and gadolinium were used as co-tracers in groundwater.

• Estimated sewer exfiltration ranged from 0.11% to 0.66% of sewage dry weather flow.

• Sewer exfiltration did not increase after the earthquakes.

Abstract

In April 2016, a series of earthquakes (M 7.3 on the Japan Meteorological Agency scale) occurred in Kumamoto, Japan causing serious damage to underground sewerage networks. In this study, we evaluated sewer exfiltration in groundwater in the Kumamoto area after the earthquakes by using multiple tracers. We used 14 pharmaceuticals, including carbamazepine and crotamiton, and anthropogenic Gd as tracers, and we measured concentrations of these tracers from September 2016 to November 2017 seasonally. The detection frequency of caffeine, carbamazepine, crotamiton, ibuprofen, and anthropogenic Gd ranged from 29% to 45%, and the concentrations of the pharmaceuticals in the groundwater were lower than those in previous studies. The median of all pharmaceutical concentrations did not decrease, whereas the median of the sum of crotamiton and carbamazepine concentrations, which are quantitative sewage markers, decreased with time. The sewer exfiltration rates in September 2016 estimated using carbamazepine, crotamiton, and anthropogenic Gd were 0.59 ± 0.27%, 0.66 ± 0.47%, and 0.11 ± 0.18% of sewage dry weather flow, respectively, indicating that the effect of the earthquakes on sewer exfiltration was small, probably because the damaged sewers were quickly repaired. This study demonstrated that a multiple-tracer approach is useful for evaluating sewer exfiltration after major earthquakes.

Introduction

Groundwater is an important drinking water resource globally; thus, its conservation is vital. Sewer systems are a crucial part of water quality conservation and hygiene, and maintenance and replacement of old sewer pipes are required to prevent contamination of groundwater through sewage leakage from sewer pipes, a process called sewer exfiltration (Reynolds and Barrett, 2003, Roehrdanz et al., 2017).

Major earthquakes, which measure more than magnitude (M) 7, occur frequently in East Asia, including Japan, China, and Indonesia. In April 2016, a series of earthquakes (M 7.3 on the Japan Meteorological Agency scale) occurred in Kumamoto Prefecture in southwestern Japan, killing at least 50 people and causing serious damage to buildings, fields, and social infrastructure, including water supply and sewerage systems. Changes in the groundwater level and flow rate of spring water in the area were observed after the earthquakes (Hosono et al., 2018, Ide et al., 2020, Kagabu et al., 2020). Furthermore, 2.1% of sewer pipes (52 km of a total of 2544 km) in Kumamoto City were damaged, and breakages, cracks, and slack were found (Civil Engineering Department, Kumamoto Prefecture, 2017). Although most of the damaged sewer pipes were repaired within about a month (Kumamoto City Waterworks and Sewage Bureau, 2016), potential contamination of groundwater by sewer exfiltration has been a great concern because almost 100% of the drinking water originates from groundwater in the Kumamoto area. Evaluating the change in sewer exfiltration after a major earthquake would provide important insights into the extent of sewage contamination in groundwater.

To evaluate sewer exfiltration in groundwater, contaminants of emerging concern (CEC) have been proposed as tracers (McCance et al., 2018), including pharmaceuticals such as carbamazepine and crotamiton (Kuroda et al., 2012, Nakada et al., 2008, Ruzicka et al., 2011), perfluoroalkyl acids (Kuroda et al., 2014), artificial sweeteners (Buerge et al., 2009, Tran et al., 2014), and anthropogenic gadolinium (hereafter, Gd-anth), which is used as a contrast media for magnetic resonance imaging (Brünjes et al., 2016, Kuroda et al., 2016). Tryptophan-like fluorescent dissolved organic matter has also been suggested as a tracer (Roehrdanz et al., 2017). Regional-scale sewer exfiltration rates have been reported in Tokyo, Japan (Kuroda et al., 2012), Leipzig, Germany (Musolff et al., 2010), and Nottingham, UK (Yang et al., 1999) based on these tracers. Suitable tracers may differ by region (Kahle et al., 2009), and thus multiple tracers are more useful than a single tracer for assessing groundwater pollution. In addition, these previous studies were performed in urban areas, and sewer exfiltration in areas with paddies has not been studied. In the Kumamoto area, irrigation water for paddies is important in recharging groundwater, and thus the input of tracers from irrigation, which originate from river water, must be considered to estimate the exfiltration rate precisely. Furthermore, long-term monitoring of tracers in groundwater is important for tracking groundwater pollution, especially after an earthquake. However, there are few long-term studies of CECs in groundwater (Drewes et al., 2003, Ternes et al., 2007).

In this study, we evaluated sewer exfiltration in groundwater in the Kumamoto area based on long-term monitoring of multiple tracers after the 2016 Kumamoto earthquakes. We used 14 pharmaceuticals, Gd-anth, and tryptophan-like fluorescence as tracers, and seasonally investigated concentrations of these tracers as well as general water quality parameters from September 2016 to November 2017. To our knowledge, this is the first study of multiple pharmaceuticals in groundwater affected by major earthquakes.