Experimental Study of Non-Darcian Flow and Non-Fickian Transport In Variable Rough-Walled Fractures

We have experimentally studied solute transport in a single fracture under the non-Darcian flow condition in the laboratory. Non-Darcian flow has been observed at average velocities between 8.58E-03m/s to 8.42E-02m/s in single fractures whose apertures are between 2.8 mm and 4.9 mm. The breakthrough curves (BTCs) under the non-Darcian flow condition have some new features that cannot be explained using the Fickian type advection-dispersion equation. In general, all the BTCs show asymmetric distributions and have long tails. We have tried different alternative conceptual models to explain the observed BTCs. The mobile-immobile model is found to be less convincing for explaining the transport phenomena, particularly when the fracture aperture is relatively small (less than 4mm). The turbulent mixing concept, which accounts for the backward movement of solute caused by the turbulent eddies associated with non-Darcian flow, can qualitatively explain the delayed early arrival sections of the BTCs and the long tails as well. However, a quantitative description associated with such a conceptual model is still not yet available. The artificial neural networks model has the capability of simulating the transport process quite accurately after training without knowing the exact physical mechanism. This study will serve as a basis for investigating solute transport in more complicated fractured media under the non-Darcian flow condition in the future.