The interaction between irrigated water, groundwater and sea water in the Ise Plain, Japan

A rice field is one of the most popular land utilization at the plains and delta in Japan. The large amounts of irrigated water used and stored at the rice field growing the rice, expanding the surface water areas, the irrigated water recharges strongly to the shallow groundwater during on-season for irrigation. So in this season, it is possibly considering that the water qualities and the water level of the shallow groundwater will change, and the interaction between shallow groundwater and seawater will be also changed by shallow groundwater discharge. The rice crop is one of the most important human activities that effect to the surface water-groundwater interactions. So it is important to understand surface water-groundwater interactions in order to plan for land utilization and water management on groundwater at Ise Plain and Ise Bay. The purposes of this study have been made (1) to evaluate the actual condition of the groundwater flow system in the inland area, (2) to evaluate the effect of irrigated water at the rice field due to seasonal changes in land utilization and water management on groundwater and Ise Bay. The pore-water resistivities were measured with using resistivity cable to evaluate seawater-freshwater interface in February, 2004 during off-season for irrigation, when the amounts of recharge irrigated water is essentially zero. So we can evaluate the almost natural condition of groundwater flow in this area. The other field surveys were carried out at 40 observation wells in each month from November 2003 through August 2004 to make clear the seasonal change of groundwater qualities and groundwater level. Water samples were analyzed for major cations and anions and NO3. Observation results show the fresh water discharges at the coast and the bottom of the sea, and groundwater level in on-season for irrigation is higher than that in off-season. In addition, chemical ions concentration in on-season for irrigation was lower than that in off-season. These suggest that the fresh water is recharged by the irrigated water, and effect to the chemical component of the seawater, when the fresh water discharges to the sea especially in on-season for irrigation.