TRANSPORT OF CONSERVATIVE CHEMICAL THROUGH AN UNSATURATED 2-DIMENSIONAL MILLER-SIMILAR MEDIUM WITH STEADY-STATE FLOW

Numerical simulation of water flow in a two-dimensional, macroscopical ly homogeneous, Miller-similar medium showed the existence of a networ k of flow channels with two complementary states separated by a critic al point [Roth, 1995]. The consequences of this for solute transport a re explored by numerical simulations using particle tracking. It is fo und that many experimentally observed features of transport through so il are reproduced qualitatively by these simulations. Analyzing the re sults reveals that in the corresponding effective medium the travel di stance for the transition to convection-dispersion and the effective d ispersivity depend on the water flux. In particular, the effective lon gitudinal dispersivity, which is often assumed to be a material consta nt of the porous structure, is found to vary by more than an order of magnitude with a minimum near the critical point. The simulations furt her demonstrate that the local structure of the velocity field and the subscale hydrodynamic dispersion are of minor importance for the fiel d-averaged transport process.