Effects of Fine‐Scale Surface Alterations on Tracer Retention in a Fractured Crystalline Rock From the Grimsel Test Site

Abstract

Fine‐scale alterations found on fracture surfaces and between fracture surfaces and rock matrix are key uncertainties that need to be considered when developing solute transport models for fractured crystalline rocks. This paper presents a comprehensive approach developed for coupling laboratory tests, microscopic observations, and modeling in order to understand and quantify tracer transport processes occurring in natural fracture surfaces, using a single‐fractured granodiorite sample from the Grimsel Test Site, Switzerland. Laboratory tests including through‐diffusion, batch sorption, and flow‐through tests using five tracers with different retention properties indicated that tracer retention was consistently in the sequence of HDO or HTO (deuterated or tritiated water) ≈ Se < Cs < Ni < Eu and as well as showing the existence of a diffusion‐resistance layer near the fracture surface, cation excess, and anion exclusion effects for diffusion. Microscale heterogeneities around the fracture were clarified and quantified by coupling X‐ray computed tomography and electron probe microanalysis. A three‐layer model incorporating as much data as possible including weathered vermiculite, foliated mica, and undisturbed matrix layers, and their properties such as porosity, sorption, and diffusion parameters, obtained from laboratory diffusion tests and microscopic observations, provided a much better interpretation for breakthrough curves and concentration distributions near‐fracture surface of all tracers, measured in flow‐through tests, than either a one‐ or two‐layer model. Mechanistic understanding and detailed modeling considering the effects of fine‐scale surface alteration around a natural fracture should improve confidence for the safety assessment in fractured crystalline rocks.

A Scaling Approach for Retention Properties of Crystalline Rock: Case Study of the In‐Situ Long‐Term Sorption and Diffusion Experiment (LTDE‐SD) at the Äspö Hard Rock Laboratory in Sweden

“…Therefore, to develop the conceptual model, it is important to consider near-surface heterogeneities for the fracture surface and the rock matrix. In the previous studies in the Grimsel LTD project, the effects of the borehole disturbed zone and microscale heterogeneities near the natural fracture surface were evaluated to be critically important for deriving D e and K d parameters (Tachi et al, 2015(Tachi et al, , 2018. In these studies, the effects of such surface heterogeneities were evaluated based on the diffusion profile Widestrand, Byegård, Kronberg, et al, 2010) analysis and microscopic observations, by considering their high porosity and high sorption capacity and their gradual changes at the near-surface zones.…”

“…The relationships between K d values and particle size or specific surface area have been widely recognized as critical factors for scaling sorption parameters from laboratory batch tests using crushed samples to in-situ intact rock conditions (Byegård et al, 1998;Crawford, 2010;Hakanen et al, 2014;Tachi et al, 2015Tachi et al, , 2018. However, such relationships may depend significantly on both the mineralogy of rock samples and the sorption mechanisms of RNs.…”

“…Further studies on RN transport in natural fractures at the GTS in Tachi et al. (2018) concluded that fine‐scale alterations found on fracture surfaces significantly affect RN migration; therefore, such fracture surface heterogeneities must be considered when developing RN transport models for fractured crystalline rocks.…”

“…To develop a realistic model and reliable parameters for long-term safety assessments, it is necessary to understand and quantify RN diffusion and sorption processes in the heterogeneous rock matrix and fracture systems.The different types and scales of heterogeneities that must also be considered for RN transport in fractured crystalline rocks include (a) heterogeneous distribution of minerals and pores in the rock matrix, (b) heterogeneity in mineral and pore distribution near fracture surfaces (e.g., fracture fillings and altered zones near fracture surfaces), and (c) heterogeneous flow distribution in the complex channel structures along fracture openings. As discussed in detail in Tachi et al (2018), the first heterogeneity (a) in a rock matrix affects sorption and diffusion at a relatively smaller scale. The domain pore spaces between mineral grains and intragranular secondary pores form water-filled, heterogeneous networks with varying diffusion-related geometric factors, such as tortuosity and constrictivity, and accessible reactive surfaces related to sorption (e.…”

Comparison of water phase diffusion experiments in laboratory and in situ conditions

“…In the simplest case, these experiments have been performed using through diffusion experiments where fluxes of radionuclides through the drill core samples are measured and the transport properties are determined from the breakthrough curves [7,8,9,10]. Slightly advanced measurements include advection in a fracture coupled with matrix diffusion [11,6,12] or even with sorption Hölttä et al [13], Tachi et al [14]. These experiments aim to demonstrate the retarding effects of matrix diffusion and sorption in cases comparable with groundwater flow in fractured rock.…”