Chemical Water Quality in the Groundwater in the Ooyanojima Island and its Surrounding Islands, Kumamoto Prefecture

The purpose of this survey is to determine the chemical quality of the groundwaters in the Ooyanojima Island and its surrounding islands (Iwajima, Tobasejima, Yushima, and Nokamajima Island). These islands, a small island on the Ariake Sea and Yatsushiro Sea, is situated about fifty kilometers south west of Kumamoto City, Kumamoto Prefecture. In this article, 46 groundwater samples (38 shallow well waters and other samples) in this area were collected in September 1977 (partly during 1979 to 1983) and analyzed for the pH value, potassium permanganate consumed, total residue, major components and some ions dissolved in them. Based on the analytical results of many dissolved components, it was shown that the concentrations of the major components were widely varied in each well (table 1) . There were considerably a large amount of contents of Na+, Ca2+, HCO3—, Cl—, SO42—,and soluble silicate in the groundwaters in this area. The relations of the concentration of cations and anions in the groundwaters were Na+ )Ca2+ )Mg2+ )K+ and HCO3 )C1— )SO42—, respectively. By use of the key diagram method, the chemical composition of the groundwaters in this area was mainly devided into two different composition in each (fig. 2), namely noncarbonate hardness type (Ca—Cl•SO4 type) and carbonate hardness type (Ca—HCO3 type). The concentrations of chemical components dissolved in the shallow well waters were generally larger than that of dissolved in the deep well waters, except for total iron and soluble silicate. The concentrations of nitrate nitrogen in the shallow well waters in this area were found to be in the range of 0.2 — 39.1mg/1. Their arithmetic mean value (7.0mg/1) was markedly high and became about 10 times larger than that in the deep well waters, however, the source of that was not made to elucidate for this investigation. The concentration of soluble silicate in the groundwaters collected from the Cretaceos period