This two day course presents an introduction to reactive transport modeling as a promising tool for critical zone research issues. The course will be based on the HYTEC software, developed at MINES ParisTech. It will consist in a variety of example problems so that the participants will be able to use the code and set up models corresponding to their research questions. The example problems will address hyporheic zone as well as vadose zone environments. Topics will include processes such as oxygen dynamics, organic matter mineralization, microbially-mediated redox reaction, CO2 and carbonates dynamics, stable isotope fractionation, bioclogging…