Insight into groundwater quality change before and after the 2016 Kumamoto earthquake.

The Kumamoto Earthquake is named after the Kumamoto Prefecture where the series of earthquakes occurred in the city located on the island of Kyushu, in southwest Japan. In this area, 100 % of the population depends on groundwater for drinking water; hence, groundwater is an extremely important lifeline for people. The effect of earthquakes on the water quality of groundwater has been suggested based on monitoring data measured by public agencies once or twice a year. Frequent water quality monitoring of the groundwater in the Kumamoto area has continued once a month for six years since 2014. In this study, we aimed to statistically evaluate the changes in groundwater quality caused by the 2016 earthquake and provide further insights into its mechanisms. The Stiff diagrams using major ionic components showed no apparent overall water quality change before and after the earthquake for 14 groundwater samples in the area. However, statistically significant changes were indicated for 11 samples by k-medoids clustering (non-hierarchical cluster analysis), divided into two clusters before and after the earthquake. In addition, the samples showing water quality changes did not vary even over the four years after the earthquake, indicating that the change was not temporal. The magnitude and trend of the change in individual water quality parameters depended on the sampling sites, while some parameters, such as nitrate, showed seasonal fluctuations due to precipitation conditions more than the effect of the earthquake. No particular relationship was found between the distance from the sampling sites to the epicenters or active faults and the degree of water quality changes. Finally, it was considered that the aquifer breaching/fluid mixing model (AB/FM model), through the change in the bleeding channel of water due to seismic shaking, most likely explains the main mechanism of water quality change in groundwater in the Kumamoto area.