Fluctuation of NO3-N in groundwater of the reservoir of the Sunagawa Subsurface Dam, Miyako Island, Japan

The Japanese government started to construct two subsurface dams on Miyako Island in 1988, and the project was completed in 2001. Before the construction of the dams, the NO₃-N concentration of groundwater on the island was about 10 mg/l, the upper limit for drinking water in Japan, owing to the application of fertilizer to sugarcane fields. Predicting the effect of these subsurface dams on the groundwater environment was difficult because they were probably the first mega-subsurface dams in the world. We measured the NO₃-N concentration in the groundwater at observation wells before and after construction of the Sunagawa Dam and after the groundwater began to be used. We also measured the NO₃-N concentration monthly at a typical observation well in the catchment over a period 14 years to evaluate the environmental impact of construction of the dam. The highest NO₃-N concentrations were downstream before completion of the subsurface dam, and a high NO₃-N concentration zone remained around the cut-off wall after its completion, but this high-concentration zone disappeared and the distribution of NO₃-N became uniform after pumping of the groundwater began. Overall, the NO₃-N concentration decreased gradually. These results show that the groundwater quality did not deteriorate as a result of the construction of the Sunagawa Subsurface Dam.