r3t - software package for numerical simulations of radioactive contaminant transport in groundwater. The software package r3t can realise numerical simulations of very large systems of coupled partial and ordinary differential equations that arise from the modelling of radioactive contaminant transport in porous media. It can solve precisely nontrivial mathematical problems like advection-dominated system with different retardation of transport for each component and with nonlinear Freundlich sorption and/or pre- cipitation. Additionally, long time simulations on complex 3D geological domains with locally refined unstructured grids can be realised