Evaluating test conditions for in situ tracer migration test in fractured siliceous mudstone involving groundwater with dissolved gas

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  • ガスが溶存した地下水を含む泥岩中の割れ目を対象とした原位置トレーサー試験条件の設定に関する検討
  • ガス ガ トカ ソンシタ チカスイ オ フクム デイガン チュウ ノ ワレメ オ タイショウ ト シタ ゲン イチ トレーサー シケン ジョウケン ノ セッテイ ニ カンスル ケントウ

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Abstract

<p>  Fault zones and excavation damaged zones have the potential to act as flow paths, and the characterization of solute transport in such zones in mudstones is important for the safe geological disposal of radioactive waste. However, few in situ tracer migration tests have been conducted on fractures in mudstones. The Japan Atomic Energy Agency has conducted in situ tracer migration experiments using uranine, which is a non-sorbing tracer, for fractures in siliceous mudstone of the Wakkanai Formation. In the tests, preliminary hydrogeological investigations were performed to evaluate in situ groundwater flow in fractures, taking into account the presence of dissolved methane and carbon dioxide in the pressurized environment. These gases are degassed by pressure release due to tunnel excavation and borehole drilling. Subsequently, the test conditions were evaluated, and 18 experiments were conducted under various conditions, which resulted in tracer recovery ratios of ca. 60 % to 80%. Large injection flow rates as compared with pumping flow rates were utilized to reduce the effects of degassing in the groundwater, which is a common problem in mudstone formations. However, these large injection flow rates sometimes led to multiple peaks in the breakthrough curves and generally reduced the recovery ratios. Therefore, an injection flow rate that is slightly higher than the pumping flow rate is ideal for tracer migration experiments involving injection and pumping, as conducted in this study. In situ tracer migration experiments involving injection and pumping conducted in a groundwater environment with dissolved gases allow empirical evaluation of the relationship of the tracer recovery ratio and the groundwater degassing with the injection and pumping flow rate ratio. This evaluation is effective for the design of experimental conditions that account for degassing and ensure high levels of tracer recovery.</p>

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