Traditional site investigations have focused on collecting both chemical concentrations and subsurface flow characteristics. These data have typically been analyzed separately, such as when chemical concentrations are compared to regulatory standards. When the two types of data are combined, it is typically for use in solute transport models. Recently there has been increased emphasis on combining chemical and flow data into a single metric to provide estimates of contaminant mass flux. This flux is the rate that a contaminant passes through a defined cross section in units of mass per time. Although the strict definition of mass flux is mass discharge per unit area (in units of mass per time per area), many researchers and practitioners use the term mass flux to denote the rate of mass discharge or total mass discharge . This document will use the term mass flux to represent data in units of mass per time passing by a plume transect perpendicular to groundwater flow. The Mass Flux Toolkit is an easy-to-use, free software tool that gives site personnel the capability to compare different mass flux approaches, calculate mass flux from transect data, and apply mass flux to manage groundwater plumes. The Mass Flux Toolkit attempts to assist site managers and site consultants perform mass flux calculations more quickly and cheaply, permitting their inclusion in more evaluations of groundwater plumes. With mass flux data, the progress of natural attenuation and remediation systems can be demonstrated more vividly and directly to regulators, making the entire remediation process clear, effective, and efficient.