Studies on Environment in Small Islans

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Various water qualities, especially stable isotope ratios, of groundwater in shallow wells were measured, together with groundwater levels, in Gogo Island, Ehime Prefecture. In a coastal region of the Yura district, chloride ion concentration far greater than 200mg/l(the stadard value of city water) was always found and the delta D versus delta^<18>O diagram suggests that the groundwater in this region is a mixture of seawater and upstream groundwater. Non-dimensional values of deltaD, delta^<18>O and Cl^- of groundwater near the bottom of a well in a coastal region of the Yura district indicate almost the same value, and it was estimated that groundwater contained approximately 20% seawater. The vertical distribution of non-dimensional Cl^- is different from those of deltaD or delta^<18>O.The former indicates higher concentration in the upper part of the well. The higher concentration may be caused by the Cl source except seawater, e.g.fertilizer.The behavior of coastal and insular groundwaters was investigated by use of a sand tank. In the case of seawater intrusion into confined coastal aquifers, both the fresh-water/salt-water interfaces simulated by using an analytical solution adopting Dupuit's assumptions and by a boudary element method (BEM) have a relatively good agreement with the experimental ones, though further inland saltwater situation in lower hydraulic gradients. In the hydrodynamic dispersion model, isopleths of concentration in the case of velocity-dependent dispersion coefficient agree well with experimental ones than in the case of constant dispersion coefficient, and the Cauchy's boundary condition is rather realistic. In the case of freshwater lenses, both the fresh-water/salt-water interfaces simulated by using the analytical solution and by the hydrodynamic dispersion model exist slightly above the experimental visual interfaces.