COUPLING OF THE TIME-DOMAIN RANDOM-WALK METHOD WITH THE FINITE FRAGMENT METHOD TO SIMULATE FLOW AND TRANSPORT IN 1-D HETEROGENEOUS MEDIA

A new method to simulate flow and solute transport in 1-D heterogeneou s media is presented. This method integrates the time domain random wa lk method (TDRW) and the finite fragment method (FFM). The TDRW, simil ar in concept to the classical random walk method, calculates the arri val time of a particle cloud at a given location and provides the solu te breakthrough curve. The FFM, which can be seen as an enhancement of the finite difference scheme, allows a heterogenous media to be divid ed into homogeneous zones to which the TDRW can be applied. The main a dvantage of the resulting coupling is that the restrictions can be avo ided on the space increments and the time steps which exist with the c lassical methods of finite differences and random walk. In a homogeneo us zone of soil, the breakthrough curve can be calculated directly at a given distance with a reasonable number of particles. A 1-D heteroge neous domain to be simulated is then split into homogeneous zones in w hich the hydraulic heads and the velocities are calculated by the FFM and at the end of which the temporal solute distributions are calculat ed by the TDRW. A few hundred particles are generally sufficient to cl early define the breakthrough curve. Comparisons with analytical and n umerical solutions and experimental data shows the reliability and adv antages of this new method. (C) 1997 Elsevier Science B.V.