Generation of dense plume fingers in saturated–unsaturated homogeneous porous media

• We numerically simulate variable-density flow in a lab-scale sand tank experiment.
• We introduce pore-scale heterogeneity by small perturbations in the numerical model.
• Existing and new perturbation mechanisms are applied to variable-density flow.
• Quality of results is evaluated by comparison with those from a physical experiment.
• Application of perturbation mechanisms improved results of the numerical model.

Flow under variable-density conditions is widespread, occurring in geothermal reservoirs, at waste disposal sites or due to saltwater intrusion. The migration of dense plumes typically results in the formation of vertical plume fingers which are known to be triggered by material heterogeneity or by variations in source concentration that causes the density variation. Using a numerical groundwater model, six perturbation methods are tested under saturated and unsaturated flow conditions to mimic heterogeneity and concentration variations on the pore scale in order to realistically generate dense fingers. A laboratory-scale sand tank experiment is numerically simulated, and the perturbation methods are evaluated by comparing plume fingers obtained from the laboratory experiment with numerically simulated fingers. Dense plume fingering for saturated flow can best be reproduced with a spatially random, time-constant perturbation of the solute source. For unsaturated flow, a spatially and temporally random noise of solute concentration or a random conductivity field adequately simulate plume fingering.