Large‐scale sandbox experiment on longitudinal effective dispersion in heterogeneous porous media

Abstract

In heterogeneous media the breakthrough curves observed at single points may differ significantly from those integrated over a control plane. While standard macrodispersion describes the spread of concentration averaged over the entire cross section, effective dispersion quantifies the average spread in the locally observed breakthrough curves. We perform a conservative tracer test in a 14‐m‐long sandbox filled heterogeneously with four types of silica sand. The filling resembles natural heterogeneities including the distribution of facies and micro‐structures within the various sand types. We use fluorescein as tracer, which we measure at 126 measurement points directly within the porous medium and at 19 levels in the outlet of the domain. We use fiber‐optic probes with a point‐like measurement tip for fluorescence intensity measurements. From the second central temporal moments of the locally obtained breakthrough curves we compute apparent effective dispersion coefficients, and from the average over all probes within a measurement plane we derive apparent macrodispersion coefficients. The effective dispersion coefficient is about two thirds of the macrodispersion coefficient. Considering the typical length scales of the heterogeneities, the experimental findings are qualitatively in agreement with linear stochastic theory.