The region including Kumamoto City and neighboring municipalities is called “Kumamoto area”. The Kumamoto area is a quite unique urban region worldwide because the drinking water demand for over one million population in this area is covered by only groundwater. Although the water volume of the groundwater is still abundant and its water quality is also excellent, there is fear that the decreasing trend of groundwater level and the deterioration of water quality recently. The author has been working to study on the water quality conservation of the groundwater in this area for the past 15 years. In this article, water-quality characteristics of the groundwater are reviewed based on the origin and flow system of the groundwater in the Kumamoto area. In addition, the knowledge about groundwater contamination by nitrate nitrogen at current is illustrated, and finally the effect of the 2016 Kumamoto earthquake on the groundwater quality is explained.

 The water purification performance of mesocosm tidal flow constructed wetlands filled with sand were compared with 4 different flood drain ratios under constant flood drain cycle of 24 h. The highest removal efficiency for COD_Cr and T－N was obtained when the flood drain ratio was set to take place 3 : 1 （18 h drain and 6 h flood）. Then this flood drain ratio was adopted and the water purification performance limits of a mesocosm tidal flow constructed wetland filled with zeolite introducing partial saturation were evaluated under 5 different flood drain cycles where influent loading was increased gradually. The maximum removal loading of 243g－COD_Cr/m²d and 13.8g－N/m²d were obtained when the influent loading was increased by 5 times. To maintain a removal efficiency higher than 90%, these removal loading is likely maximum capacity of COD_Cr and T－N removal. On the other hand, the T－P removal performance was affected by shorten the flood drain cycle and 90% removal efficiency was attained only when the flood drain cycle was set to take place 24 h. The maximum removal loading of 0.472g－P/m²d was obtained when the influent loading was increased by 4 times. These results demonstrate the potential of tidal flow constructed wetland filled with zeolite introducing partial saturation to improve the removal loading of constructed wetlands, resulting in intensive reduction of land area needed for constructed wetlands.