報告書番号

N05066

タイトル（和文）

地下水年代測定評価技術の開発（その2）－He濃度の深度方向分布を利用したオーストラリア大鑚井盆地におけるHeフラックスの評価－

タイトル（英文）

Research and development on groundwater dating -An evaluation of helium flux using profile of helium in Great Artesian Basin-.

概要 （図表や脚注は「報告書全文」に掲載しております）

オーストラリア大鑽井盆地において地表から帯水層へ向けてボーリングを実施した。得られたボーリングコア間隙水中のHe濃度・同位体比を明らかにし、He濃度・同位体比の深度方向への分布を得た。MarreeにおけるHe濃度の深度方向への分布では、帯水層下部の難透水層において、深度の増加と共に急激なHe濃度の増加が観察された。これはHeフラックスの存在について、直接の証拠を得た初めての結果である。帯水層上下のHe濃度分布から、帯水層へのHeの収支を算出し、帯水層においてのHe蓄積速度として、5.0×10^-11(ccSTP/gw/y)を得た。この値は。放射性元素とHe濃度を組み合わせることによって算出された、帯水層におけるHe蓄積速度とよく一致している。2つの独立した方法で得られた値が一致したことは、得られたHe蓄積速度の値およびHe蓄積速度算出のための2つの方法のそれぞれの妥当性を相補的に示す結果である。

概要 （英文）

In this study, a drilling to aquifer at GAB was carried out at 2 sites (Richmond and Marree) to investigate the depth profile of He concentration and their isotope ratio. At Marree drilling site, in the overlying aquitard (50-172m in depth) the concentration of He increased with the depth. In the underlying aquitard (deeper than 181m), the concentration of He also increased with depth. However, the gradient of He concentration in the vertical profile of He concentration is completely different between the overlying and underlying aquitard. The large gradient found in the underlying aquitard indicates that a large amount of He is in the deeper geological formation and such He in the deep geological formation migtrates into the aquifer by diffusion. The smaller gradient in the overlying aquitard indicates that most He comes from the deep geological formation accumulating in the aquifer and a little fraction of accumulated He that lost from the aquifer by diffusion to the overlying aquitaed. Isotope ratio of He is uniform at depth of 50 to 200m. This is due to that the vertical profile of He is mainly controlled by the He flux from the deep geological formation or source of He flux is same as for the aquifer and underlying aquitard, that is, radiogenic He in host rock. Thus, clear evidence of existence of the He flux in GAB is shown for the first time by this study.The amount of He flux from deep the geological formation to aquifer is calculated by using the vertical profile of He and He diffusion coefficient estimated in our previous study. As the result, He accumulation rate in aquifer at GAB is found to be 5.0x10^-11(ccSTP/gw). He accumulation rate also can be estimated by using relationships between He concentration and ^36Cl/Cl. He accumulation rate is estimated to be 1.7x10^-11~2.0x10^-10(ccSTP/gw/y) by this method and showed good agreement with that estimated from the He flux. This good agreement indicated the validity of estimated value and methodology for estimation of accumulation rate of He.

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