This report describes modifications to a U.S. Geological Survey (USGS) three-dimensional solute-transport model (MODFLOW-GWT), which is incorporated into the USGS MODFLOW ground-water model as the Ground-Water Transport (GWT) Process. The modifications were made to create compatibility between the Multi-Node Well (MNW) Package for MODFLOW and the MODFLOW-GWT model. This compatibility improves the capability of MODFLOW-GWT to represent accurately solute transport in simulations that include multi-node wells because long-screen wells or long open boreholes that extend through multiple model layers can provide fast pathways for solutes to move from one location to another in a ground-water flow system. For nonpumping multi-node wells (used to simulate open boreholes or observation wells, for example), a simple routing and local mixing model was developed to calculate nodal concentrations within the borehole. A depth-averaged concentration is calculated for such nonpumping wells. For pumping multi-node wells (either withdrawal or injection) in which the flow between the well and the ground-water system is in the same direction at all nodes, the average concentration in the well is calculated as a flux-based mean assuming complete and instantaneous mixing in the wellbore of all inflows. For pumping multi-node wells (either withdrawal or injection) in which the flow between the well and the ground-water system is not unidirectional, the concentration distribution within the well is calculated using the same routing and local mixing model used for a nonpumping multi-node well, with the added assumption that the flux pumped in or out of the well is added or removed above the first well node.
