海岸平野における塩水化地下水の水・たい積物相互作用と化学組成

Abstract

Large-scale intrusion of saltwater into freshwater aquifers under overdraft conditions has occurred in the coastal areas of Fuji, Hamamatsu and Iwata in Shizuoka Prefecture. The origin of saltwater was taken to be sea water from the isotopic study of waters. The correlation coefficients of the chloride concentration to the δD and δ18O were 0.97 and 0.96, respectively, throughout these areas. Contrasting with this, the chemical characteristics of the salinized groundwater were quite different from area to area. In the Fuji and Hamamatsu areas, the contents of alkaline earth in the water greatly exceeded those values estimated by the simple mixing of sea water and groundwater, so that the water character shows Ca-Na-Cl type in Fuji, and Ca-Mg-Cl type in Hamamatsu area. In the Iwata area, on the other hand, the water character of Na-Cl type reflects simple mixing of sea water and groundwater. In the experiments of ion exchange reactions between sea water and sediments using the boring core samples in these areas, the clay particles in the Fuji and Hamamatsu areas had calcium and magnesium as the exchangeable cations, and their contents exceeded 90% of the total exchangeable cations. On the contrary, clay particles in the Iwata area had sodium as the predominant exchangeable cation that equilibrates with sea water. The different mode of cation exchange reactions induced by the sea water intrusion into the aquifers, caused the different chemical evolution of water.