報告書番号

U95044

タイトル（和文）

地球化学的手法を用いた地下水流動調査法の提案－六ヶ所サイトにおける地下水流動特性の把握－

タイトル（英文）

PROPOSAL OF GROUNDWATER SURVEY METHOD USING GEOCHEMICAL TECHNIQUES-CHARACTERISTICS OF GROUNDWATER FLOW AT THE ROKKASHO SITE-

概要 （図表や脚注は「報告書全文」に掲載しております）

低レベル放射性廃棄物埋設施設のある六ヶ所サイトの中深層までの地下水の流動を把握するために，地球化学的手法（環境同位体分析，溶存希ガス分析，トリチウム分析，一般水道分析）を用いて地下水起源，地下水年代，地下水流動区分ならびに地下水形成過程について推定を行った。その結果，ＥＬ－４０ｍまでの浅層地下水は，最近の降水を起源としておりその流動性は高い。ＥＬ－４０ｍ～－１００ｍまでの地下水は，年代的には１０＊＊２～１０＊＊３年程度でありその流動性は，低い。またＥＬ－１００ｍ以深には，非常に古い時代の降水を起源とする地下水にその後の海進によって地下に浸入した海水が混合して出来た地下水が存在する。その地下水年代は，１０＊＊３～１０＊＊５年である。

概要 （英文）

A MAJOR DRY WASTE WHICH CONSISTS OF METALS IS PLANNING TO BE DISPOSED OF IN THE NEXT DISPOSAL PRGRAMME AT THE ROKKASHO SITE. METALLIC WASTES ARE SUPPOSED TO HAVE LESS PERFORMANCE TRAPPING RADIONUCLIDES INTO A WASTE PACKAGE THAN A HOMOGENEOUS PACKAGE SOLIDIFIED WITH CEMENT. CONSEQUENTLY,IF WE WOULD LIKE TO DUMP THE DRY WASTE INTO THE SAME SHALLOW UNDERGROUND FACILITIES AS IN THE FIRST DISPOSAL PROGRAMME,A NATURAL BARRIER IS REQUIRED FOR ENHANCING THE TRAPPING PERFORMANCE OF RADIONUCLIDES. THE PRECISE UNDERSTANDING OF GROUDWATER FLOW AT THE SITE IS INDISPENSABLE FOR THE ENHANCEMENT OF THE NATURAL BARRIER PERFORMANCE. HOWEVER,THERE ARE LACKS OF THE IN-SITU GEOHYDROLOGICAL DATA AT THE SURROUNDING CANDIDATE SITE FOR CONSTRUCTING THE NEXT DISPOSAL FACILITIES AND OF THE DEEP REGIONAL DATA EXCEPT THE SHALLOW AND LOCAL DATA CALLECTED IN THE FIRST DISPOSAL PROGRAMME. IN THIS STUDY,WE FOCUSED ON PRECISELY UNDERSTANDING THE GROUNDWATER MOVEMENT AT THE SITE TO FABRICATE THE CONCEPTUAL MODEL OF MACRO GROUNDWATER FLOW. THE MAJOR SURVEGING SUBJECTS ARE THE DATING GROUNDWATER AND THE ORIGIN OF GROUNDWATER. WECAN CONCLUDE AS FOLLOWS; A). THERE ARE TWO GROUNDWATER BOUNDARIES LYING AT THE DEPTH OF EL-40M AND EL-100M BELOW THE SITE. THE VERY OLD GROUNDWATER DATED MORE THAN 50000 YEARS STATES BENEATH THE LOWER BOUNDARY AND THE SHALLOW GROUNDWATER CIRCULATES ABOVE THE UPPER BOUNDARY. THE SHALLOW GROUNDWATER IS, THEREFORE,EXPECTED TO BE COMPLETELY SEPARATED FROM THE DEEP GROUNDWATER CIRCULATION. B). THE MIXING BETWEEN DEEP GROUNDWATER AND SHALLOW GROUNDWATER IS OBSERVED UP TO DEPTH OF EL-30M. CONSEGUENTLY,GROUNDWATER WITH AGES 0 40 YEARS ABOVE THE UPPER BOUNDARY DOMINATESTHE SHALLOW CIRCULATION BY ONLY INFILTRATION OF RAINWATER ON THE SITE.C). SIGNIFICANT FLOW OF GROUNDWATER WAS PREDICTED TO THE DEPTH OF EL-100M FROM THE VERTICAL PROFILES OF THE DISSOLVED HE CONCENTRATION MEASURED AT THE SITE. HOWEVER,THE VELOCITY OF GROUNDWATER IS VERY SMALL,WHICHWAS ESTIMATED TO BE NOT MORE THAN THE

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