In the traditional method of characteristics for groundwater solute-transport models,advective transport is represented by moving particles that track concentration. This approach can lead to global mass-balance problems because in models of aquifers having complex boundary conditions and heterogeneous properties, particles can originate in cells having different pore volumes and (or) be introduced (or removed) at cells representing fluid sources (or sinks) of varying strengths. utilize of volume-weighted particles means that each particle tracks solute mass. In source or sink cells, and the changes in particle weights will match the volume of water added or removed through external fluxes. This enables the novel method to conserve mass in source or sink cells as well as globally. This approach also leads to potential efficiencies by allowing the number of particles per cell to differ spatially—using more particles where concentration gradients are high and fewer 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. The novel model determines whether solute mass added by fluid sources in a cell should be represented by (1) novel particles having weights representing appropriate fractions of the volume of water added by the source,or (2) distributing the solute mass added 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 novel method works well and conserves mass. The method is incorporated into a novel version of the U.
S. Geological Survey’s MODFLOW–GWT solute-transport model.
Source: usgs.gov