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Profiles of chloride in matrix porewater as natural tracer for matrix diffusion in crystalline rocks
Applied Geochemistry ( IF 3.4 ) Pub Date : 2020-07-01 , DOI: 10.1016/j.apgeochem.2020.104635
F. Eichinger , T. Gimmi , A. Möri , J. Rüedi

Abstract Matrix porewater from low permeable Grimsel granodiorite was successfully characterised using indirect methods applied to originally saturated core samples. Core samples were taken from a 17 m long borehole originating from a tunnel of the Grimsel Test Site into the crystalline bedrock intersecting a tectonic shear zone with a water-conducting fracture. Matrix porewater chloride profiles on the meter scale were determined on both sides of the water-conducting fracture. To evaluate transport processes within the bedrock formation, a series of diffusive model calculations were performed, which to fit the porewater data. Boundary and initial conditions were varied according to the geological conditions, whereas other required parameters such as the connected porosity and pore diffusion coefficients were determined by laboratory experiments on the cores and extrapolated to in situ conditions. The main conclusions can be summarized as follows: (1) Chloride porewater profiles at the meter scale can be simulated using diffusive transport models. This provides evidence that diffusive exchange with active fractures occurs over a range of a few meters in the low-permeable crystalline bedrock; (2) the best fit of the diffusion profile was achieved by a model approach, which takes asymmetric initial Cl-concentrations into account. This indicates that prior to the activation of the present water-conducting fracture, the porewater system in the bedrock was already active showing a concentration gradient in chloride; (3) the water-conducting fracture was activated at least between 850 and 1700 years before present, with a best-fit 1200 years before present; and (4) the hydraulic were affected by the construction of the rock laboratory 20 years ago, resulting in a rapid dilution of the fracture groundwater by advection.

中文翻译:

作为结晶岩中基质扩散的天然示踪剂的基质孔隙水中氯化物的分布

摘要 对原始饱和岩心样品采用间接方法,成功表征了来自低渗透格里姆塞尔花岗闪长岩的基质孔隙水。岩心样品取自一个 17 m 长的钻孔,钻孔起源于 Grimsel 试验场的隧道,进入结晶基岩,该钻孔与构造剪切带和导水裂缝相交。在导水裂缝的两侧确定了米级的基质孔隙水氯化物分布。为了评估基岩地层内的输运过程,进行了一系列扩散模型计算,以拟合孔隙水数据。边界和初始条件根据地质条件而变化,而其他所需的参数,如连通孔隙率和孔隙扩散系数,是通过实验室对岩心的实验确定的,并外推到原位条件。主要结论可概括如下: (1) 米尺度的氯化物孔隙水剖面可以使用扩散输运模型进行模拟。这提供了在低渗透结晶基岩中几米范围内发生与活动裂缝的扩散交换的证据;(2) 扩散曲线的最佳拟合是通过模型方法实现的,该方法考虑了不对称的初始 Cl 浓度。这表明在目前的导水裂缝活化之前,基岩中的孔隙水系统已经活跃,显示出氯化物的浓度梯度;(3) 导水裂缝至少在850-1700年之前被激活,最适合在1200年前;(4)水力受20年前岩石实验室建设的影响,导致裂缝地下水被平流迅速稀释。
更新日期:2020-07-01
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