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22 nd Salt Water Intrusion Meeting: Salt Water Intrusion in Aquifers: Challenges and Perspectives2. METHODS2.1 Study AreaOur study area locates coastal area in the northern part of Hokkaido, Japan. An thousandmeters borehole was drilled, and core samples were taken to extract pore water. Watersample was analyzed to find contents of salt, stable isotopic ratios of hydrogen, oxygen.Electromagnetic investigation was also undertaken to obtain distribution of specificresistance to understand distribution of salt concentration comparing its concentration ofpore water. From the analysis, fresh groundwater could be found under the seabed. Classicaltheory about formulation of salt water / fresh water interface could not explain completely,and consideration of long-term geochemical process (e.g., sea level fluctuations) is needed tounderstand this mechanism.2.2 Groundwater flow modelingFigure 1. Location map of study area.Groundwater modeling was undertaken on three dimensional geological model with the finitedifference, variable-density flow and transport code SEAWAT 2000 (Guo & Langevin, 2002).Moreover, zeroth-order reactions are conducted in direct simulation of groundwater ages(Goode, 1996). The simulated area is discretised in 39 columns and 27 rows each with awidth of 2.5 km. In vertical direction the model is subdivided in 50 layers each with athickness of 0.1 km. A no-flow and no-diffusion boundary conditions were applied for thesides and the bottom boundaries. Top boundary is set Dirichlet boundary as time depend onsea level. Sea level in Japan was cyclically varied by 0 to -120 m to modern condition duringthe 120,000-year oscillations; regression periods are 100,000 years and transgressionperiods are 20,000 years (Japan Nuclear Cycle Development Institute, 2005). For initialconditions, heads at the land boundaries are applied to the top of the model and heads of theocean boundaries are zero. Salt concentrations at the top of the entire model are set at 100-%seawater salt concentration. Hydraulic conductivity varies between 10 -4 and 10 -9 cm/s, andeffective porosity varies between 0.069 and 0.215 (ex. Ito et al., 2010).242