[A-HW27] 流域の水及び物質の輸送と循環－源流域から沿岸域まで－

[AHW27-18] 上水道水源自然湧水中のセシウムの地下水滞留時間に伴う減少

*中屋 眞司¹、ファン ホアン　ミン　ハ¹、岩井 慶貴¹、伊東 章裕¹、青木 秀斗¹、中野 孝教² (1.信州大学工学部土木工学科、2.総合地球環境学研究所)

キーワード：セシウム, 地下水, 滞留時間, 六フッ化硫黄, 八ヶ岳

It is known that the active Cs sorption onto colloidal-size clay minerals in groundwater is observed and the active Cs can be transported with the colloidal fraction of groundwater by water flows. However, the longtime behavior of radioactive and non-active Cs, contained in the flowing groundwater in the aquifers of groundwater source areas, is unknown in terms of the natural water cycle. Herein, we investigate the non-active Cs concentration in natural spring drinking water with the residence time in a groundwater source area of a mountainside composed of volcanic rock, compared with those of other trace elements. This investigation demonstrates that the observed Cs concentration in natural spring drinking water exponentially decreases slowly with the groundwater residence time (~ 45 yr), while several trace elements, namely, P, V, Ga, and Ge, increase in concentration with the groundwater residence time through chemical weathering. The findings suggest that active Cs, contained in flowing groundwater in mountain water source areas, may decrease exponentially at the rate of one-tenth in twenty-two years, by sorption onto the aquifer through rock-water interaction excluding radioactive decay. For the sustainable management of water sources and ecosystems, the long-term (~50 yrs.) monitoring of the active Cs in groundwater is needed in mountain water source areas where radioactive cesium has been dispersed at times of nuclear power plant accidents.