Identification of groundwater sources in the Ashigara Plain using temperature and environmental isotopes.

Four groundwater sources were identified in the Ashigara Plain by analysing δ 18 O contents, 3 H ratios and the temperature of the groundwater flow system. Groundwater in the Ashigara Plain is mainly recharged in the upper area by river water percolation and by infiltration of irrigation water in the paddy fields. According to the distribution of groundwater head, the direction of the dominant groundwater flow of the plain is southeastward and the groundwater flows from the upper area to the lower area. Local groundwater recharges were expected at the foot of Mount Hakone because the groundwater head contours decline to the east and at the coastal area where signs of saltwater intrusion were recognized. Investigating δ 18 O, 3 H and temperature, a recharge by surface water in the upper area, an intrusion of cool groundwater from Mount Hakone, and upconing of warmer and salty water in the coastal area, were recognized. In order to confirm the possible recharge at the foot of Mount Hakone, a two dimensional groundwater temperature simulation was carried out.