We investigate the flow rate dependency of solute transport within an undisturbed monolithic soil core, collected in an Inceptisol. Through a series of nine controlled steady-state solute breakthrough experiments, flow rate dependency of solute transport was elucidated using the general transfer function (GTF) modeling theory. We first observed that the apparent dispersion coefficient increases with depth and flow rate. We also observed that the flow regime is rather a convective–dispersive (CD) process at low flow rates and a stochastic–convective (SC) process at high flow rates. At intermediate flow rates, the flow regime could not be described with either CD or SC processes. To better understand the mechanisms of altering flow regime at intermediate flow rates, a dye tracer experiment was conducted. Results show that preferential flow is initiated at intermediate flow rates. We hypothesize that the mixing of solutes between stream tubes decreases when flow rate increases, due to the decrease of the tortuosity of solute flow paths and the initiation of preferential flow through macropores.

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