Abstract
This paper deals with the problem of groundwater pollution caused by seawater intrusion and fertilizer dissolution into phreatic aquifers in Gogo Island, Ehime Prefecture, where the mandarin orange is widely cultivated.
Groundwater levels, chloride ion concentrations and electric conductivities in shallow wells were measured in five districts on this island. In the Kitaura district, sulfate ion, nitrate nitrogen, nitrite nitrogen and phosphorus concentrations were also measured. In the case of the fertilizer dissolution, we propose a tank model simulating the concentration variation with time in the phreatic aquifer.
The main results are as follows : 1) High chloride ion concentrations of far greater than 200 mg·l-1, the standard of drinking water, were always found in coastal regions of the Yura district. 2) The concentration of total ion except chloride ion, T-Cl-, increases at one to four months after fertilization, and T-Cl- correlates well with SO42- and NO3N (the multiple correlation coefficient is 0.88). 3) The predicted temporal variations of concentration by the tank model agree well with the observed T-Cl-variations by adopting relevant initial conditions and an a value which controls the fertilizer dissolution rate on the ground surface.
