報告書番号

N12005

タイトル（和文）

堆積岩地域の溶存有機物を用いた地下水の流動履歴評価 －励起蛍光マトリクスと多変量解析による地下水起源の推定－

タイトル（英文）

Evaluation of origins and hydrologic flow patterns of groundwater in a sedimentary rock by using dissolved organic matter with excitation-emission matrix fluorescence and multivariate analysis

概要 （図表や脚注は「報告書全文」に掲載しております）

本研究では、電力施設等の保守・維持管理および地下空間利用に必要な地下水流動評価のため、十勝平野東部の地下水を対象に励起蛍光マトリクス（EEM）および多変量解析法を用いて溶存有機物の起源を推定し、地下水流動履歴指標としての評価を行った。その結果、地下水、および地下水から抽出したフミン酸、フルボ酸のEEMは、4つの腐植様成分でモデル化できた。採水地層が陸成～浅海成層ではフミン酸、海成層ではフルボ酸が主な成分であったが、浅部地下水（300m以浅）および十勝中央断層（TCF）以西の深部地下水では土壌・泥炭等由来成分が多かったため、浅部とTCF以西での地下水流動は、地層内の移動のみと考えられるTCF以東の深部地下水より相対的に速いと解釈できた。これは主要元素・同位体比による解釈と調和的であることから、本手法は地下水の流動履歴の推定方法となりうることが示された。

概要 （英文）

Evaluation of origins and hydrologic flow patterns of groundwater in a sedimentary rock is necessary from the point of view of maintenances and environmental assessment at various electric power facilities as well as utilization of underground spaces such as gas storage and waste disposal in future. In addition to major elements and stable isotope ratios, which have been employed to examine groundwater flow successfully, we focused on a new index of excitation-emission matrix (EEM) fluorescence, which needs only a small amount of samples, in order to understand the groundwater flow history better. In this study, origins of chromophoric dissolved organic matter (CDOM) in groundwater samples with varying depths in the eastern Tokachi sedimentary basin including isolated humic (HA) and fulvic (FA) acids from the groundwater were investigated with the EEM spectroscopy and subsequent multivariate analyses. PARAFAC modeling for the corrected EEM identified four humic-like components in groundwater, HA and FA in the Tokachi basin area and subsequent principal component analysis statistics of the PARAFAC ratios showed that HA is dominant in groundwater sampled from terrestrial/shallow-marine formations and FA is dominant in those from marine formations. The ratios of the four PARAFAC components varied gradually in the descending order of the sedimentary rocks and increasing depth except for the samples in the west of Tokachi Central Fault (TCF). Shallow groundwater and deep groundwater in the west of TCF showed higher ratio of the component due possibly to CDOM from soil and peat, while deep groundwater in the east of TCF showed higher ratios of the other components due to microbial CDOM; degraded HA for the terrestrial/shallow-marine formations and degraded FA for the marine formations with higher ratios in increasing depth. This result implies that shallow groundwater and deep groundwater in the west of TCF flow faster than deep groundwater in the east of TCF, which possibly flows within the formation only. The interpretation agrees well with the origins of the groundwater and flow patterns in this area previously proposed from the results of major elements and stable isotope ratios, thus, the EEM with the multivariate analyses can be another index to estimate origins of groundwater.

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