HETEROGENEOUS FLOW AND SOLUTE TRANSPORT IN AN UNSATURATED STONY SOIL MONOLITH

Field studies of solute transport have shown that water flow velocity may vary tremendously across a field. To assess the effects of flow heterogeneity on solute transport in a stony subsoil, miscible displacement experiments were conducted in a large undisturbed gravel monolith (75-cm length, 30-cm diam.). The soil sample contained about 0.8 kg kg(-1) of gravel. Two consecutive pulses (runs) of a Cl- solution were leached through the monolith, each at matric pressures ranging from -2.0 to -2.2 kPa. The cumulated outflows, collected with 19 separate porous plates located at the bottom, ranged from 0 to 3000 mt. The outflow rates of the individual plates were constant, and highly correlated between the two runs (r = 0.95). The Cl- breakthrough curves (BTCs) of the total and individual BTCs could be well described by the classical convective-dispersive equation. Pore eater velocities, dispersion coefficients, and dispersivities of the two runs indicated a high correlation of the breakthrough behavior of the individual outlets (r = 0.995, 0.88, and 0.97, respectively). In addition, the iodine starch method revealed a heterogeneous flow pattern. Even though the column was resaturated and drained between the two runs, the results between the two consecutive runs were consistent. This suggests that the how paths of water remained invariant and might be an intrinsic feature of a soil medium for a given water content.