Sorbing tracer experiments in a crystalline rock fracture at Äspö (Sweden): 2. Transport model and effective parameter estimation

Abstract

[1] Transport and retention of sorbing tracers in a single, altered crystalline rock fracture on a 5 m scale is investigated. We evaluate the results of a comprehensive field study (referred to as Tracer Retention Understanding Experiments, first phase (TRUE-1)), at a 400 m depth of the Ä spö Hard Rock Laboratory (Sweden). A total of 16 breakthrough curves are analyzed, from three test configurations using six radioactive tracers with a broad range of sorption properties. A transport-retention model is proposed, and its applicability is assessed based on available data. We find that the conventional model with an asymptotic power law slope of À3/2 (one-dimensional diffusion into an unlimited rock matrix) is a reasonable approximation for the conditions of the TRUE-1 tests. Retention in the altered rock of the rim zone appears to be significantly stronger than implied by retention properties inferred from generic (unaltered) rock samples. The effective physical parameters which control retention (matrix porosity and retention aperture) are comparable for all three test configurations. The most plausible in situ (rim zone) porosity is in the range 1%-2%, which constrains the effective retention aperture to the range 0.2-0.7 mm. For all sorbing tracers the estimated in situ sorption coefficient appears to be larger by at least a factor of 10, compared to the value inferred from through-diffusion tests using unaltered rock samples.

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

“…It should be noted that the case of heterogeneous porosity distribution gives sorption coefficients which are very close to the K d values as stated in part 2 [ Cvetkovic et al , 2007, Table 5], for effective porosity in the most probable range 0.01–0.02. Thus the introduction of heterogeneity in porosity may be an important link in the joint interpretation of the laboratory and in situ data.…”

“…A summary of the range in results of porosity and sorption measurements is given in Tables 4a and 4b. In part 3 of the article series (Cvetkovic et al, manuscript in preparation, 2007), the independent information of microheterogeneity as summarized in this work will be used for interpreting the effective retention parameters as obtained in part 2 [ Cvetkovic et al , 2007].…”

Flow‐dependence of matrix diffusion in highly heterogeneous rock fractures

“…For relatively slow groundwater movement, diffusive mass transfer is another important transport mechanism, as demonstrated in the field on a variety of scales [Shapiro, 2001;Reimus et al, 2003;Cvetkovic et al, 2007;Reimus and Callahan, 2007;Cvetkovic, 2010;Cvetkovic and Frampton, 2010]. A controlling parameter for tracers subject to reactions or retention that involve diffusion into the rock matrix, is the transport resistance b [T/L] [Cvetkovic et al, 1999;Cvetkovic and Frampton, 2010].…”

The tritiated water and iodine migration in situ in Grimsel granodiorite. Part I: determination of the diffusion profiles

“…matrix diffusion) and sorption onto the mineral surfaces are the most significant processes that retard RNs flowing through fractures [6][7][8]. Diffusion and sorption of RNs have been studied extensively in the laboratory [9][10][11][12] but very few studies have been carried out using in situ experiments, especially with RNs [13][14][15]. One of the main reasons for this is that these experiments are time consuming, cost intensive and require special legislation if RNs are used.…”