An experimental investigation of variable density flow and mixing in homogeneous and heterogeneous media

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Abstract
This study is an experimental investigation of variable density groundwater flow in homogeneous, layered and lenticular porous media. At the scale of the experiments the flow of dissolved mass in water depends upon both forced and free convection. In addition, density differences as low as 0.0008 g/cm3 (1000 mg/L NaCl) between a plume of dense water and ambient groundwater in a homogeneous medium produces gravitational instabilities at realistic groundwater velocities. These instabilities are manifest by lobe‐shaped protuberances that formed first along the bottom edge of the plume and later within the plume. As the density difference increases to 0.0015 g/cm3 (2000 mg/L NaCl), 0.0037 g/cm3 (5000 mg/L NaCl), or higher, this unstable mixing due to convective dispersion significantly alters the spreading process. In a layered medium, reductions in hydraulic conductivity of the order of half an order of magnitude or less can influence the flow of the dense plume. Dense water may accumulate along bedding interfaces, which when dipping can result in plume migration velocities larger than ambient groundwater velocities. In a lenticular medium the combination of convective dispersion and nonuniform flow due to heterogeneities result in relatively large dispersion. Scale considerations, further, indicate that convective dispersion may provide an important component of mixing at the field scale.