Numerical assessment of the origin of deep salinity in a low permeability fractured medium

低透水性亀裂性岩盤中の塩水系地下水の起源に関する数値解析的評価

Guimer, J.*; Ruiz, E.*; Luna, M.*; Arcos, D.*; Domnech, C.*; Jordana, S.*; 三枝 博光; 岩月 輝希  

Guimer, J.*; Ruiz, E.*; Luna, M.*; Arcos, D.*; Domnech, C.*; Jordana, S.*; Saegusa, Hiromitsu; Iwatsuki, Teruki

東濃地域で観測されている塩分濃度の高い地下水について、その起源に関する複数の仮説及び概念モデルを構築した。それらの妥当性を確認することを目的とした数値解析を実施した。この数値解析においては、密度流を考慮した解析及び反応輸送解析を実施した。複数のモデルを統合することによって、ボーリング孔で観察された塩分濃度の高い地下水分布を定性的に説明できることが示された。

Many possible origins have been proposed for the saline groundwater observed in granite show total dissolved solids increasing to 50 mmol/L at depths below 800 m. Different hypothesis have been formulated to explain the observed fluid composition, among them, long-term water-rock interaction, mixing with residual fluids of magmatic origin and relict seawater dating from Miocene times. A review of the hydrochemical and isotopic data suggest that the three above hypotheses may be valid, at least to different degrees, or that processes acting over more recent geological times may be involved. The origin of the salinity was assessed by simulating land emersion by means of changing the upper recharge boundary. In this manner the Miocene seawater was modelled as being continually mixed with fresh water until the present time. The effects of different retardation processes were considered by varying factors such as matrix diffusion and fracture conductivity. Finally, geochemical reactions reproduced trends in major ions and master variables.