1985 年 27 巻 4 号 p. 115-127
This paper deals with the problem of the groundwater pollution caused by both the seawater intrusion into phreatic and confined aquifers and the fertilizer dissolution into a phreatic aquifer in Gogo Island, Ehime Prefecture.
Groundwater levels, chloride ion concentrations and electrical conductivities in shallow wells were measured at five districts in this island during a period from August 1983 to December 1984. At Yura district, groundwater levels and chloride ion concentrations in deep wells as well as tide levels were measured.
The main results are as follows: 1) Chloride ion concentration profiles show the rapid increase of the concentration in the neighborhood of the deep well bottom.2) Using a modified Kishi & Inouchi's model, the variation of the groundwater level and fresh water-salt water interface in response to the tide was analysed, which is applicable to the confined aquifer with an exponentially oaring thickness. It was found that the thickness of the confined aquifer estimated from this analysis is in fairly good agreement with the one obtained by boring examination.3) Very high chloride ion concentrations far greater than 200 mg⋅ 1-1, the standard value of city water, were found in coastal regions at Yura district.4) The concentration of total ions estimated by the electrical conductivity of groundwater minus the chloride ion- concentration increases at one to two months after fertilization.