Estimation of laboratory-scale dispersivities using an annulus-and-core device

SummaryThis paper investigates the use of two-dimensional radial column experiments to estimate longitudinal and transverse dispersivity at the laboratory scale. The experimental device is an “annulus-and-core” device: it is based on a classical column system, of which the inlet reservoir is divided into three independent concentric zones, allowing non-uniform tracer injection. The outlet reservoir is similarly adapted, so that information on the radial distribution of concentration becomes available through mean effluent concentration measurements in each annular zone. In this study, we only investigated continuous tracer injections through the central inlet zone. An analytical solution to a similar problem was available in the literature, and was adapted to compute effluent concentrations. The influence of the simplified boundary conditions of the solution was assessed by means of a numerical model. A general methodology is suggested to obtain transport parameters from breakthrough curve analysis, involving (i) the determination of effective porosity and longitudinal dispersivity from the full averaged breakthrough curve using classical one-dimensional tools and (ii) the determination of transverse dispersivity from the breakthrough curves recorded in the annular zones. Preliminary experiments were performed on a glass bead porous medium, on a gravel sand and on a natural medium sand. It is found that the rapidity of the test, its low cost, and the ability to simultaneously estimate three transport parameters comes at the price of potentially larger experimental errors. Transverse dispersivities were found to be higher than values previously reported in the literature, probably as a result of plume meandering, which cannot be detected nor corrected when using annulus-and-core devices.