EXTENSION OF THE METHOD OF CHARACTERISTICS FOR SIMULATION OF SOLUTE TRANSPORT IN 3 DIMENSIONS

The method of characteristics (e.g., Garder et al., 1964; Konikow and Bredehoeft, 1978) has been widely used in the simulation of solute transport in ground-water systems because of its effectiveness for handling advection-dominated problems. However, the method requires a large amount of computer memory and execution time when it is necessary to track a large number of moving particles, especially in three dimensions. The method can also lead to large mass balance discrepancies under certain circumstances. This paper describes a number of changes which may be made in the traditional method of characteristics approach to mitigate these difficulties. These changes include a dynamic particle allocation procedure, which often reduces dramatically the total number of moving particles needed in a simulation; and a consistent velocity interpolation scheme and higher-order particle tracking techniques, which, coupled with effective handling of particle insertion and deletion at sources and sinks, can provide improved mass balance results. The accuracy of a method-of-characteristics model incorporating these changes is evaluated by comparison with analytical solutions for (1) three-dimensional advection and dispersion in a spherical flow field, and (2) concentration change in a well during an injection/pumping cycle.