Sorbing tracer experiments in a crystalline rock fracture at Äspö (Sweden): 1. Experimental setup and microscale characterization of retention properties

Abstract

[1] Mineralogical and retardation properties of rock materials responsible for water-rock interaction in in situ migration experiments with sorbing radioactive tracers were studied in laboratory experiments. The porosity was studied by water saturation measurements and the PMMA method was used for detailed porosity characterization of heterogeneity distributions and porosity profiles toward the fracture surface. Mylonite and altered diorite sampled in the rim zone of the fracture and representative bulk rock types were investigated by batch sorption measurements with crushed materials and through-diffusion and in-diffusion experiments in intact rock pieces. Autoradiography was used for visualization of in-diffusion profiles of sorbing tracers. The use of detailed porosity information and quantitative data on heterogeneity in porosity is shown to significantly improve the interpretation and evaluation of laboratory-scale diffusion experiments. We show through the combined approach of detailed porosity characterization and laboratory sorption and diffusion investigations that we can distinguish retention properties of bulk rock and altered rock and provide qualitative and quantitative data of heterogeneous rock properties that expand the possibility for including relevant processes in the interpretation of the results of in situ tracer tests.

Matrix diffusion and sorption of Cs+, Na+, I– and HTO in granodiorite: Laboratory-scale results and their extrapolation to the in situ condition

“…Such dual profiles have been observed in granitic rocks by many researchers and interpreted by different models (e.g., Byegård et al, 1998;Idemitsu et al, 1992;Widestrand et al, 2007;Xu and Wörman, 1999). Near and far profiles were simultaneously fitted by coupling reservoir depletion (RD) and breakthrough (BT) curves, respectively (solid lines in Fig.…”

“…An advantage of laboratory experiments is that they can provide reliable data and mechanistic understanding over a wide range and welldefined boundary conditions. On the other hand, in situ experiments can play important roles providing site-specific data under partial real conditions, although other uncertainties such as saturation and stress state must also be carefully evaluated depending on site conditions (e.g., Missana et al, 2006;Smith et al, 2001;Vilks et al, 2003;Widestrand et al, 2007). Diffusion and sorption parameters are typically derived from laboratory experiments using drilled or crushed rock samples, which have different properties in porosity, poreconnectivity and reactive surface area in comparison with in situ conditions.…”

Review: The state-of-art of sparse channel models and their applicability to performance assessment of radioactive waste repositories in fractured crystalline formations

“…Recent observation has also un-6 veiled an often occurring zone of altered rock at the interface of the fracture and rock matrix, and this altered 119 zone has a higher diffusivity coefficient (Polak et al, 2003;Widestrand et al, 2007). Effective matrix diffusion 120 coefficients are also found to increase with testing scale (Zhou et al, 2007), and Zhang et al (2006) …”

Quantitative mapping and statistical evaluation of fracture minerals in the granitic bedrock at Forsmark, Sweden

“…As chlorite often covers the entire fracture surface, it could have a great impact on solute exchange between flow paths and the underlying rock matrix, should its porosity and effective diffusivity be sufficiently low (e.g. Widestrand et al 2007). On the other hand, should its porosity be sufficiently large, it could enhance the retardation of non-sorbing solutes in short-term tracer tests.…”

“…It is often assumed, and on occasions demonstrated, that fracture minerals and the altered rock adjacent to hydraulically conductive fractures have a higher effective diffusivity than the undisturbed rock matrix (e.g. Siitari-Kauppi et al 2003;Widestrand et al 2007). However, this assumption needs to be strengthened for the fractures of low transmissivity in rock volumes outside larger deformation zones, which are of primary importance for radionuclide retardation in geological disposals for spent nuclear fuel in crystalline rock.…”