Why to model variable viscosity in groundwater flow? Why should we take into consideration this phenomenon in the conceptualization of groundwater flow and transport? Variable viscosity is key when we want to model not only water but other fluids as salinity, heat or dense or light non aqueous phase liquids (LNAPL or DNAPL).

In order to improve the accuracy of a groundwater flow simulation we need a strong conceptual model, a high quality observation dataset and a numerical code that can handle specific characteristics of the groundwater flow we want to evaluate. MODFLOW 6 can deal with variable density flow with the BUY package and also with variable viscosity with the VSC package that is implemented on the latest versions of Model Muse. This tutorial covers an applied case of variable viscosity modeling due to the injection of salinity and heat to a model with regional groundwater flow. The main features for the implementation of the VSC package are explained and the resulting concentrations on two observation points are evaluated with Flopy codes.