Chemical Study of River and Ground Waters on Ryukyu Islands

Chemical and physical studies on 304 water samples collected from Iriomote-Island, Ishigaki-Island, Irabu-Island, Miyako-Island and northern part of Okinawa-Island during the last two years from 1977 to 1978 have been carried out for total 15 components such as water temperature, pH, electric conductivity, Ca^, Mg^, Na^+, K^+, Cl^-, HCO^-_3, SO^=_4,SiO_2 NH_3-N, N0_2-N, P0_4-P and COD. From the studies above, the following results were obtained: (1) High contents of chloride are observed in ground waters collected from wells located along with west side of Irabu-Island. The chloride concentrations are also high at north-west and southwest sides of Irabu-Island. They are, however, low at central part of the island. The wells with high chloride concentration are observed at Yonaha Bay at Shimoji-cho, north and west sides of Kadekaru areas of Miyako-Island, which would largely be affected by sea water. The central part of Miyako-Island is also low in chloride concentrations. The wells at central area of Kabira village at Ishigaki-Island are generally low in chloride contents and they increase toward the beach side of Kabira Bay. The reasons for the occurance of high chloride content in a shallow well at rather far area from the beach toward mountain side, however, has not yet been well known. (2) Key diagrams for chemical compositions on waters show that those on Iriomote-Island are composed of both Ca(HC0_3)_2 and NaCI types whereas the waters on Ishigaki-Island are made of not only Ca(HC0_3)_2 and NaCI types but also of intermediate of two types above. Most waters on Miyako-Island are grouped into Ca (HC0_3)_2 type with some scattering distributions of NaCI types of waters. It is characteristic for waters on northern part of Okinawa-Island that some part of waters are grouped into both Ca (HC0_3)_2 and NaCl types wheareas the majority of them are grouped into intermediate type of both above. These types of waters are somewhat resemble to waters on Ishigaki-Island. It will be said that all of waters studied are characteristic on islands isolated from the continents.(3) The average chloride content at Irabu-Island, the smallest island in the area among all, is 160 ppm with great influence of sea water as compared with that in other larger islands. Among individual island studied, the average calcium content of 83.8 ppm is the highest in Miyako-Island affected from Riukiu Limestone, whereas it is the lowest at non carbonate area of northern part of Okinawa-Island. The highest average content of magnesium is observed at Irabu-Island with the value of 15.9 ppm, which is also low in Mg content at northern part of Okinawa-Island in which may receive least influence of sea water. Bicarbonate content as well as calcium content are high in waters from islands of limestone fonnation and are largely concerned with degree of saturation by calcium carbonate in relation of changes in pH values. The average sulfate content is the highest at Irabu-Island with a value of 34 ppm and is the lowest at northern part of Okinawa-Island which may be said that the most part of sulfateions are brought from sea water. (4) Geographical dependence of land derived chemical species as well as the influences of sea water have been studied to evaluate a pattern of mass transportation cycle in each area. It is known from the study that the land derived materials in waters are decreasing at islands in the order of Irabu, Miyako, Ishigaki, Iriomote and northern part of Okinawa Islands. (5) COD values are largely dependent of absorption of ultraviolet radiation at 250 nm wave length, and the relation can be expressed as the following: COD(meq/l) = 3.81 x (Ab_) (6) From the study of correlation coefficients among chemical components measured, it is said that the chemical species such as Na+, Mg^, K^+, SO: and electric conductivity are highly correlated with chloride content and with each other, particularly in surface waters at many islands in which may receive large influences of sea water. The waters at northern part of Okinawa, however, receive least influence of sea water and almost no correlation between Cl^- and SO^=_4 in particular and lower correlations between CI^- and other components such as Mg^ and electric conductivity are observed