EXPERIMENTAL INVESTIGATION OF SOLUTE TRANSPORT IN LARGE, HOMOGENEOUS AND HETEROGENEOUS, SATURATED SOIL COLUMNS

Laboratory tracer experiments were conducted to investigate solute tra nsport in 12.5-m long, horizontally placed soil columns during steady saturated water flow. Two columns having cross-sectional areas of 10 x 10 cm(2) were used: a uniformly packed homogeneous sandy column and a heterogeneous column containing layered, mixed, and lenticular format ions of various shapes and sizes. The heterogeneous soil column gradua lly changed, on average, from coarse-textured at one end to fine-textu red at the other end. NaCl breakthrough curves (BTC's) in the columns were measured with electrical conductivity probes inserted at 50- or 1 00-cm intervals. Observed BTC's in the homogeneous sandy column were r elatively smooth and sigmoidal (S-shaped), while those in the heteroge neous column were very irregular, nonsigmoidal, and exhibited extensiv e tailing. Effective average pore-water velocities (v(eff)) and disper sion coefficients (D-eff) were estimated simultaneously by fitting an analytical solution of the convection-dispersion equation to the obser ved BTC's. Velocity variations in the heterogeneous medium were found to be much larger than those in the homogeneous sand. Values of the di spersivity, alpha = D-eff/v(eff), for the homogeneous sandy column ran ged from 0.1 to 5.0 cm, while those for the heterogeneous column were as high as 200 cm. The dispersivity for transport in both columns incr eased with travel distance or travel time, thus exhibiting scale-depen dency. The heterogeneous soil column also showed the effects of prefer ential flow, i.e., some locations in the column showed earlier solute breakthrough than several locations closer to the inlet boundary. Spat ial fluctuations in the dispersivity could be explained qualitatively by the particular makeup of the heterogeneities in the column.