MFIT is a Windows-based computer software package for the analytical modeling of concentration breakthrough curves (BTCs) from tracer experiments in fractured and karst aquifers. The software integrates four transport models built on a multiflow approach that assumes that the spatial heterogeneity of the aquifer can be approximated by a combination of independent one-dimensional channels. The four transport models are referred to as MDMi (multi-dispersion model, instantaneous injection), MDMed (multi-dispersion model, exponentially decaying injection), MDP-SFDM (multi-double porosity, single-fracture dispersion model), and MDP-2RNE (multi-double porosity, two-region nonequilibrium model). The MDMi, MDMed, MDP-SFDM and MDP-2RNE models are implemented as independent codes that can be run either as console applications or using the MFIT program as a graphical user interface (GUI). MFIT provides a GUI for (i) importation and graphic visualization of user-provided BTC data, (ii) parameterization, direct running, and graphical output of the analytical transport models, (iii) inversion (automatic calibration) of model parameters for optimal curve fitting, and (iv) assessment of the uncertainty of calibrated parameter values. Both the optimization and uncertainty analysis of model parameters are carried out using routines from the PEST software package (http://www.pesthomepage.org/).

This version fixes a bug that could occur when displaying the results of an "Automatic Multiple Channels" optimization.