A new computer post-processing program called ZONECONC, has been created by the U.S. Geological Survey (USGS) to help analyze concentration data from the USGS Ground-Water Flow and Solute Transport Model (MODFLOW-GWT). ZONECONC, written in FORTRAN-95, tabulates zonal (userdefined three-dimensional clusters of model cells) concentration statistics over time by reading the concentration data produced from the ground-water transport (GWT) process of MODFLOW. Zones can be created interactively in ZONECONC or from an externally created file of zone information. The concept of analyzing data by zones was patterned after the USGS program ZONEBUDGET. Statistical parameters of concentration, calculated per zone and time, include the highest maximum concentration of any cell within the zone, the flow-model row, column, and layer location of the maximum concentration cell, the minimum concentration for the zone, the mean concentration of the zone, total (cumulative) concentration, and the standard deviation. ZONECONC also counts the number of cells per zone with positive-computed concentrations, which can be used to track plume size. Statistical parameters from ZONECONC can be used to qualitatively assess solute-transport properties, such as dispersion and attenuation. ZONECONC has been tested on a number of MODFLOW-GWT simulations from a variety of models and has been shown to provide an effective and efficient way to analyze concentration results.