Experimental Investigation of Solute Transport in Large, Homogeneous and Heterogeneous, Saturated Soil Columns

Laboratory tracer experiments were conducted to investigate solute transport in 12.5-m long, horizontally placed soil columns during steady saturated water flow. Two columns having cross-sectional areas of 10 x IOcm’ were used: a uniformly packed homogeneous sandy column and a heterogeneous column containing layered, mixed, and lenticular formations of various shapes and sizes. The heterogeneous soil column gradually changed, on average, from coarse-textured at one end to fine-textured at the other end. NaCl breakthrough curves (BTC’s) in the columns were measured with electrical conductivity probes inserted at 50or 100-cm intervals. Observed BTC’s in the homogeneous sandy column were relatively smooth and sigmoidal (S-shaped), while those in the heterogeneous column were very irregular, nonsigmoidal, and exhibited extensive tailing. Effective average pore-water velocities (tr,rr) and dispersion coefficients (&r) were estimated simultaneously by tilting an analytical solution of the convection-dispersionequation to the observed BTC’s. Velocity variations in the heterogeneousmedium were found to be much larger than those in the homogeneous sand. Values of the dispersivity, (Y = Derr/ecrrr for the homogeneous sandy column ranged from 0.1 to 5.0cm, while those for the heterogeneous column were as high as 200cm. The dispcrsivity for transport in both columns increased with travel distance or travel time, thus exhibiting scaledependency. The heterogeneous soil column also showed the effects of preferential flow. i.e., some locations in the column showed earlier solute breakthrough than several locations closer to the inlet boundary. Spatial fluctuations in the dispersivity could bc explained qualitatively by the particular makeup of the heterogeneities in the column.