Current monitoring and sampling techniques in unsaturated fractured clay often fail to characterize fast preferential flow. To circumvent these problems, an isolated block (3.5 by 3.5 by 3.3 m) of unsaturated fractured clayey till was used for multiple tracing experiments. The setup allowed full control of the water balance in the block. Experiments at three different steady-state flow rates were performed. Multiple tracers with different diffusion coefficients were applied in each experiment to evaluate the influence of diffusive exchange between fractures and the matrix. The tracers included two halogen anions (Cl and Br), two fluorobenzoic acids (FBA) (2,3-DFBA and 2,6-DFBA), two fluorescent dyes (uranine and sulforhodamine B), and one colloidal tracer (0.5-μm latex particles). At high flow rates, the obtained tracer breakthrough showed a traditional asymmetrical behavior where a fast peak was followed by a long tailing period. At low flow rates, two of the applied tracers revealed a double peak breakthrough curve, whereas the tracer with the lowest molecular diffusion coefficient showed only one peak. The separation of the tracers was hypothesized to be influenced by extensive diffusion into stagnant areas by the tracers with the high diffusion coefficients. This was supported by the breakthrough curve obtained for the colloidal tracer, which showed earlier breakthrough and only one tracer peak.

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