This paper focuses on the understanding of the controlling factors of breakthrough curves in fractured systems. DFN.lab is used to define DFN models derived from the characteristics of a sub-part of the Forsmark repository and to perform simulations of flow and of inert transport with a particle tracking algorithm. We show that particles potentially follow two kind of paths: fast paths, composed by a limited number of highly transmissive, well oriented fractures, and slow paths, where particles go through a larger number of fractures. These paths have different characteristics: fast paths are controlled by the transmissivity structure of some large fractures and produce “peak” arrival times while slow paths are controlled by the network organization and the transmissivity structure, resulting in arrival times obeying an inverse gamma distribution. In addition, those late arrival times are controlled by the presence of "traps" where the velocity is significantly slower.

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