The U.S. Geological Survey, in cooperation with the Air Force Civil Engineer Center, developed a numerical, steady-state regional model, using MODFLOW-2005, to evaluate current (2010) conditions and the potential effects of future (2030) groundwater withdrawals on water levels, streamflows, hydraulic gradients, and advective transport near the Joint Base Cape Cod (JBCC). Two numerical models were used in this analysis. An existing two-dimensional (one layer) model (termed the “coast model”), developed as part of a previous investigation of the potential effects of sea-level rise on the Cape Cod aquifer system (https://doi.org/10.3133/sir20165058), simulates the position of the freshwater/saltwater interface by using MODFLOW-2005 with the SWI2 module. The regional model is a steady-state, three-dimensional (32 layers) model, which incorporates that interface position, and is capable of representing hydrologic conditions on the Sagamore flow lens and advective transport in and near the JBCC. Particle-tracking analysis using the MODPATH program (https://pubs.er.usgs.gov/publication/ofr94464), was done to represent the advective transport of conservative solutes in the aquifer. A number of underlying assumptions inherent in the model, including observations and weights used in the calibration, representation of local-scale heterogeneity, and simulation of the freshwater/saltwater interface, could affect model calibration and predictions; these assumptions were evaluated with alternate models and alternate inverse calibrations. Alternate calibrations were performed in which different, but reasonable, observations and weights were used. In addition, a set of alternate models, in which silts were represented with different correlation distances and hydraulic conductivities, were done to evaluate if explicitly representing silt lenses could affect model calibration. Finally, two alternate models representing silty and sandy sea bottoms and their associated interface positions were developed to test the importance of the assumed coastal-boundary condition. This USGS data release contains all of the input and output files for the simulations described in the associated model documentation report (https://doi.org/10.3133/sir20185139).