Solute transport in a filled single fracture under non-Darcian flow

Filled single fractures (FSF) are commonly seen in the subsurface but have received little attention in terms of flow and transport. This study uses well-controlled laboratory experiments to investigate flow and transport in a FSF under non-Darcian flow conditions. Flow in the experiments is well described by the Forchheimer or the Izbash equations. Non-Fickian transport is found to dominate with early arrival and long tails. The breakthrough curves (BTCs) of the solute transport are modeled by the conventional advection–dispersion equation (ADE), the ADE with retardation (ADE-R) and the single rate mobile–immobile (MIM) model. The ADE is used as a benchmark for comparison. The choice of ADE-R is to acknowledge the fact that mechanical filtration and intake of contaminant particles in dead or disconnected pore spaces can retard the transport. The choice of MIM is to acknowledge the early arrival and long tails of the BTCs. The ADE model poorly describes the transport; the ADE-R model is better than the ADE in fitting the peak concentration, but it is less satisfactory in fitting the tails of BTCs; the single rate MIM model shows the potential to model the transport behavior over a wide range of discharge. The large dispersivity values found in the experiments are consistent with the prediction of Taylor, who had shown that dispersion under non-Darcian flow condition was much stronger than that under Darcian flow.