[A-HW28_30PM2] 流域の水及び物質の輸送と循環－源流域から沿岸域まで－

[AHW28-22] 沖縄本島南部の琉球石灰岩帯水層の溶解に伴う地下水の水質の長期（50年以下）挙動

*中屋 眞司¹、安元 純²、ファン・ミン ハ¹、青木 秀斗¹、中野 拓治² (1.信州大学、2.琉球大学)

キーワード：地下水, 石灰岩, 溶解, 滞留時間, 六フッ化硫黄, 沖縄本島

Dissolution of a terrestrial limestone layer by chemical weathering is one of the most important factors affecting the carbon cycle and the transport of calcium from the land to the ocean. Residence times of sulfur hexafluoride (SF₆) and chlorofluorocarbons (CFCs), as well as their chemical composition in the groundwater, were investigated to estimate the longtime behavior of field dissolution of the Ryukyu-limestone aquifer on Okinawa Island, Japan. The Ca, (HCO₃+SO₄) and Pco₂ increase with groundwater residence time. The field dissolution of Ca was estimated to be 0.090 mM(Ca)/L/yr, with groundwater Ca ranging from 1.75 to 4.0 mM/L. The increase observed in groundwater alkalinity and SO₄ over time (0.170 meq(HCO₃+SO₄)/L/yr; 16 to 34 yr) implies that the groundwater acts as a CO₂ sink through chemical weathering of the Ryukyu-limestone aquifer when groundwater CO₂ (gas) concentrations range from 1.0% to 4.5%(logPco₂=-2 ~ -1.35 atom). The (Ca + Mg) content of groundwater was also affected by groundwater alkalinity (HCO₃), SO₄ and NO₃ derived from fertilizers used on Okinawa Island. These findings imply that the influence of fertilizer and the high partial pressure of groundwater CO₂ on the dissolution of Ryukyu-limestone aquifer may not be negligible. pH decreases with dissolution of the Ryukyu-limestone aquifer.