Mass transfer between recirculation zone and main flow domain in fractures: Is the first order rate law valid?

•Recirculation zones (RZs) are present and grow with Reynolds number (Re) in fractures when inertial forces are significant.•Mass transfer between RZ and main flow channel results in a late time tailing in residence time distributions.•The first order rate law is robust for describing mass transfer between RZs and main flow channel.