Evidence of non-Darcy flow and non-Fickian transport in fractured media at laboratory scale

Abstract

Abstract. Accurate predictions of solute propagation in fractured rocks are of particular importance when assessing exposure pathways through which contaminants reach receptors during a risk assessment procedure, as well as when dealing with cleanup and monitoring strategies. The difficulty in modeling fractured media leads to the application of simplified analytical solutions that fail to reproduce flow and transport patterns in such complex geological formations. A way for understanding and quantifying the migration of contaminants in groundwater systems is that of analyzing tracer transport. Experimental data obtained under controlled conditions such as in a laboratory allow to increase the understanding of the fundamental physics of fluid flow and solute transport in fractures. In this study laboratory hydraulic and tracer tests have been carried out on an artificially created fractured rock sample. The tests regard the analysis of the hydraulic loss and the measurement of breakthrough curves for saline tracer pulse inside a rock sample of parallelepiped (0.60 × 0.40 × 0.8 m) shape. The effect of the experimental apparatus on flow and transport tests has been estimated. In particular the convolution theory has been applied in order to remove the effect of acquisition apparatus on tracer experiment. The experimental results have shown evidence of a non-Darcy relationship between flow rate and hydraulic loss that is best described by Forchheimer's law. The observed experimental breakthrough curves of solute transport have been modeled by the classical one-dimensional analytical solution for advection–dispersion equation (ADE) and the single rate mobile–immobile model (MIM). The former model does not fit properly the first arrival and the tail while the latter provides a very decent fit.