Geochemical study on the ground and surface waters of some Miyako Islands in Okinawa

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  • 沖縄県宮古諸島における陸水の水質形成に関する研究
  • オキナワケン ミヤコ ショトウ ニ オケル リクスイ ノ スイシツ ケイセイ ニ カンスル ケンキュウ

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Seventy samples of groundwater an d five samples of surface water from Miyako, Ikema and Irabu Islands were collected during 28, Jul. to 1, Aug.1994 and analyzed for chemical compositions.<BR>Spring water at the western and southern coast of Miyako Island was identified as Na-Cl type. Well water in Ikema and well and cave waters in Irabu Island and nearb y western and southern coast of Miyako Island were also identified as Na-Cl type.<BR>Equilibrium PCO2 in the groundwater was 10-2.86 to 10-1.36 atm with a n arithmetic average of 10-1.79 atm. This value was nearly equal to that of the groundwater in Okinawa Island. High concentrations of Ca2+ and HCO3- in the groundwater could be caused by the reaction of limestone with CO2. Equilibrium relationship with respect to feldspar and clay minerals of the groundwater showed that almost all samples except No.2 (Stagnant water) were belonged to the stability field of kaolinite.<BR>Difference of the NO3-N concentrations i n the groundwater was distinct among the residential area, farm village, farm and forest areas. This difference might be caused by the degree of influence of chemical fertilizer and human activities. High concentrations of excess SO42- (SO42- of non sea salt origin) were probably caused by chemical fertilizer. The influence of the chemical fertilizer on quality of the groundwater was higer in some islands of Miyako than in Hateruma and Yonaguni Islands of Yaeyama.<BR>Relationship between PO43-P and excess K+ was clear i n some groundwaters. This trend suggests that these waters were contaminated by waste water from human activities.

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