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

Abstract

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 Äspö 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 −3/2 (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.