A steady-state, three-dimensional flow model coupled with a particle-tracking algorithm was developed to assess the complex interaction of hydrogeologic conditions affecting ground-water flow and contaminant transport --including aquifer heterogeneities, hydrologic boundaries such as ponds and streams, ground-water withdrawals, and aquifer recharge to characterize the migration of contaminants emanating from beneath the Massachusetts Military Reservation, Cape Cod, Massachusetts. The known extent of contaminant plumes were used to assess the effects of changes in model input parameters on model-calculated water-table and pond altitudes, streamflow, and water-particle pathlines. Although changes in selected simulated hyodrogeologic conditions resulted in minor changes in model-calculated water-table altitudes, pond altitudes, and streamflow, these changes had significant effects on the projection of model-calculated water-particle pathlines. Therefore model calibration based simply on water levels and streamflows may be inadequate for the prediction of contaminant migration. However, model calibration that includes the sensitivity of water-particle pathlines to changes in hydrogeologic conditions can provide a valuable tool for optimizing data collection, characterizing hyodrogeology, and remediating ground-water contamination.