Evaluation of the Effect of Paddy Irrigation on the Groundwater Quality

The objective of this study is to evaluate the effect of the paddy irrigation on the groundwater quality. The groundwater and the canal water qualities are surveyed in a 600ha study site, which is located in Kyushu Island, southwest of Japan. Total nitrogen (TN) of groundwater, irrigation canal water, and the drainage canal water are measured. The TN values of the groundwater are higher than the canal water. The measurement results indicate that the pollutant of the groundwater is transported from the outside of the study site. The groundwater flow simulation and particle tracking are conducted to clarify the pollution source of the study site. The results indicate that the groundwater quality is affected by the flow from the outside of the study site. The contaminant transport with the groundwater flow is simulated using the simulation model to predict the contaminant expansion during a year. To clarify the effect of the paddy irrigation of the contaminant transport, the pollutant expansion is simulated under present land use condition, and the land use condition assuming that all paddy fields are used as the upland field. The simulation result indicated that the paddy irrigation prevents the diffusion of pollutant expansion