Solute mixing during imbibition and drainage in a macroscopically heterogeneous medium

Abstract

[1] We studied the flow of water and the transport of solutes in a heterogeneously packed sand tank by performing transport experiments with a fluorescent tracer. The packing consisted of an inclined, layered sand bedding characterized by a large number of thin layers of three quartz sands of different grain sizes. The purpose was to evaluate to which extent the preceding wetting-draining history affects the mixing of solutes in presence of textural heterogeneities. We irrigated the surface at stepwise increased flow rates (imbibition) and afterward at stepwise decreased flow rates (drainage). At each step the flow rate was kept constant to monitor the transport and spreading of several tracer plumes in the stationary flow field. The results show that during imbibition, mixing at the textural boundaries is hindered and the solutes are funneled through connected preferential flow paths. During drainage we observed enhanced mixing resulting in a definitely less heterogeneous transport regime. These results stress the role of the preceding wetting-draining history for determining the characteristics of solute transport in heterogeneous media. We demonstrate this using the integral mixing characteristics "dilution index" and "reactor ratio," which quantify the degree of preferentiality of flow and transport. Copyright 2007 by the American Geophysical Union.