Interplay between resident and infiltrating water: Estimates from transient water flow and solute transport

This study investigates the interplay between resident ('old') water and incoming ('new') water in homogeneous, partially saturated sand undergoing infiltration, using a series of laboratory column experiments. It was hypothesized that old water pockets, defined both spatially and temporally, may be established during infiltration. This in turn, may affect the flow pattern of the infiltrating new water and/or contravene current geochemical reactions. Sands with three different particle size distributions and three initial water contents were employed. The upper end of each column was irrigated with water containing a conservative tracer, at three different flow rates, while free-drainage conditions were employed at the lower end. Analysis of the resulting 27 infiltration events was based on the Richards equation to describe fluid flow. Subsequently, the mobile-immobile model (MIM) was employed to describe the solute transport; the measurements were reproduced satisfactorily by these models. Results were further analyzed using mass balance considerations. Two regimes were identified: an initial piston-like mechanism that displaces old water, followed by a slow mixing/entrainment of the remaining old water. The relative contributions of these regimes appear to depend on the initial water content and the average grain size. In some cases, up to one-third of the old water fraction remained in the system following five flowthrough pore volumes. Comparison between the measured fractions of old water remaining in the system at the end of each infiltration event (i.e., after five flowthrough pore volumes) and the immobile water content (optimized from the MIM) relative to the initial water content, exhibited a significant linear correlation, with values about threefold higher for the optimized immobile fraction.