HR: 0830h
AN: H11G-0927    [PDF]
TI: Use of Volume-Weighted Particles to Improve the Method of Characteristics
AU: * Konikow, L F
EM: lkonikow@usgs.gov
AF: U.S. Geological Survey, 431 National Center, Reston, VA 20192
AU: Hornberger, G Z
EM: gzhornbe@usgs.gov
AF: U.S. Geological Survey, 431 National Center, Reston, VA 20192
AB: In groundwater solute-transport models based on the traditional method of characteristics, advective transport is represented by moving particles that track concentration. This approach often leads to global mass-balance problems (even if the grid spacing is uniform) because in heterogeneous groundwater systems, particles can originate in cells having vastly different pore volumes and (or) be introduced (or removed) at cells representing fluid sources (or fluid sinks) of varying strengths. Use of weighted particles, in which the weights represent an appropriate proportional volume of water, implies that each particle, in effect, tracks solute mass. This assures that the new method will produce a negligible mass-balance error. This approach also leads to potential efficiencies by allowing the number of particles per cell to vary spatially--using high numbers of particles where concentration gradients are high and low numbers where gradients are low. The approach also eliminates the need for the model user to have to distinguish between "weak" and "strong" fluid source (or sink) cells. This is because the model automatically decides whether solute mass added by fluid sources in a cell should be represented by (1) new particles having weights representing appropriate fractions of the volume of water added by the source, or (2) distributing the solute mass added by the fluid source over all particles already in the source cell. The first option is more appropriate for the condition of a strong source. The latter option is more appropriate for a weak source. At sinks, decisions whether or not to remove a particle are replaced by a reduction in particle weight in proportion to the volume of water removed. A number of test cases demonstrate that the new method works well and conserves mass; largest improvements occur for cases involving strongly diverging or converging flow fields, such as are generated by one- and two-well tracer tests. The new algorithm is implemented as a solver option in the U.S.G.S. MODFLOW-GWT solute-transport model.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 3210 Modeling
SC: Hydrology [H]
MN: 2003 Fall Meeting